Enthalpy Of Formation Of N2o4

And then we have our heat of formation of B-- delta heat of formation, let me call it, of B. Find out: a) The change of the Gibbs free energy at 25 °C. 20 mol/L x 0. Let's put it this way: > H2(g)+1/2O2(g)→H2O(g) H2(g)+1/2O2(g)→H2O(l) Although the reactants of the reactions are the same, pay attention to the products. Use uppercase for the first character in the element and lowercase for the second character. 6 Ag+ aq 105. To balance a chemical equation, enter an equation of a chemical reaction and press the Balance button. So the enthalpy of formation of this reaction is - 341. The standard enthalpy (∆H rxn = -57. (3) (Total 16 marks) 2. Limiting Reagent: A reactant is considered as limiting reagent when it limits the product formation which the product formed depend on the amount of that reactant. 02 ║ 11 ║ ╠══════════╬═══════╬════════╣ ║HNO ║ 31. Molecular parameters. C (s) + 2F2 (g) →CF4 (g) ∆H = −680 kJ Target is 2CF4, so double this. Chemistry 12 Unit 2- Equilibrium Notes It's important to know that many chemical reactions are reversible. 13 The Effect of a Change in Temperature To determine the effect of a change in temperature on equilibrium, heat is considered a component of the system. C(s) + 2H2 (g)---( CH4(g) Emphasise use of state symbols to represent states at 1 atmosphere of pressure and 298K. The new enthalpy of formation of gas-phase hydrazine, based on balancing all available knowledge, was determined to be 111. Asked in Chemistry , Elements and. As I remember, first the formation enthalpies at the non-standard conditions should be computed. At equilibrium, 0. 2006 Visiting Professor, University of Arizona. Constantieux, in Comprehensive Heterocyclic Chemistry III, 2008. The standard molar enthalpy of formation of NO2(g) is 33. At 25°C and 1 atm (101. So it's heat, change in enthalpy of formation was the same thing as heat of formation. Hess' Law Because enthalpy is a state function (at constant pressure), Hess' law states that the enthalpy change from one state to another is the same, regardless of how many or which steps the process takes. 1) Standard Molar Enthalpy of Formation: Enthalpy change of formation of 1 mole compound from its elements is called standard molar enthalpy of formation and expressed in kcal/mol or kjoule/mol. The form below provides you with blanks to enter the individual enthalpies or free energy d ata points for a given reaction. But ΔH°form of an element in its standard state is set at 0. Chemistry 12 Unit 2- Equilibrium Notes It's important to know that many chemical reactions are reversible. CIC Specific Heat 1 W 008 5. The standard enthalpy of formation, Gibbs energy of formation, entropy and molar heat capacity are tabulated for more than hundred organic substances. Calculate ΔG at 298 K if the partial pressures of NO2 and N2O4 are 0. 2 Expert Answer(s) - 84014 - Enthalpies of formation of CO(g), CO2(g), N2O(g) and N2O4(g) are –110, – 393, 81 and 9. We can look up that in a table, and let's say that that's some number. Thus, for the formation of FeO(s), Note that now we are using kJ/mol as the unit because it is understood that the enthalpy change is for one mole of substance. I solved for the pressure as 1. a) What is the energy of combustion at 1 atm and 298 K? E comb = kJ/mol b) Use Thermodynamic Properties to determine its heat of formation. The standard enthalpy of formation refers to the enthalpy change when one mole of a compound is formed from its elements. The standard enthalpies of formation are: NO (g) = +90. ΔH f (N2O) = 81kJ mol-1. 75 kJ 2NO2(g) N2O4(g) H=-145. The rate of the production of N2O4 is occurring at a faster rate than the rate of production of NO2, which is why the tube color turns from orange to clear. 23 is the reverse of the formation reaction for C 3 H 8 (g). Standard Enthalpy of Formation. Standard Enthalpies, Free Energies of Formation, Standard Entropies. Yep, its a law not a rule. Thermodynamic properties of the chemically reactive system N2O4 ⇌ 2NO2 ⇌ 2NO + O2 have been evaluated over a pressure range of 0. LeChatelier’s principle talks about the equilibrium and the changing of conditions of the reaction. 7 CO2(g) -393. Estimate the standard enthalpy of formation of hydrazine, $\ce{N2H4(g)}$, from the following data. Lattice thermodynamics. 47 kJ/mol at 298. N2O4 --> 2NO2; So the concentration of N2O4 decreases. 7 kJ mol-1 re. 11 ammonium chloride NH 4 Cl(s) -314. This information was acquired from the University of Wisconsin at Madison. 2 Expert Answer(s) - 84014 - Enthalpies of formation of CO(g), CO2(g), N2O(g) and N2O4(g) are -110, - 393, 81 and 9. formation of 1 mol of gas from 1 mol of one reactant gas and 1 mol of another reactant gas. Using standard heats of formation, calculate the standard enthalpy change for the following reaction. 0 comments. Equation 5. In an exothermic reaction, heat is a product. Enthalpy of reaction=-393. Since the elements and the compound from which they are made will have the same products of combustion we can set up an energy cycle. 78 x 10^2 -0. Enthalpy of formation of CO2. ) The standard enthalpy of formation of hydrazine, N 2 H 4 (g) is +96 kJ mol -1. N2O(g) + 3/2 O2(g) N2O4(g) ANS: B 10. 1,1,2,2-Tetrachloroethane is a manufactured, colorless, dense liquid that does not burn easily. calculate the standard enthalpy change for each of the following reactions. Let us consider. CIC Specific Heat 1 W 008 5. Reference: 1. Also called heat of formation. 2NO2(g) → N2O4(g) ∆Hrxn = qp = − 57. What is the standard enthalpy of formation, f H , of NH 3 (g)? -47 kJ mol-1 (Note: the reaction in the question produces 2 mol of NH 3 so the enthalpy of formation is half of the enthalpy change of this reaction. Here, reactants are , CO and N 2 O 4 while products are N 2 O and CO 2, The value of enthalpy of formation of N 2 O is not given here which should be 81 KJ/mol. Estimate its value at 100°C. Find out: a) The change of the Gibbs free energy at 25 °C. You use the standard enthalpy of the reaction and the enthalpies of formation of everything else. In this case, the enthalpy of the reaction is small and negative and the Gibbs free energy is strongly influenced by entropy and temperature. Calculate the amount of heat required to raise the temperature of 10. Enthalpy is the thermodynamic function that describes heat flow. Asked in Chemistry , Elements and. At the temperatures over 570 °С, the ferrous alloys oxidize, as ferric oxide (wustite) with simple cubic lattice appears on the metal surface. The standard enthalpies of formation are: NO (g) = +90. You complete the calculation in different ways depending on the specific situation and what information you have available. At equilibrium at 373 K, 0. Previous studies of heterogeneous NPAH formation used simple inorganic was sterilized by dry heat at 453 K for 2 h. A negative result indicates that heat is lost when the reaction takes place, and a positive result indicates that heat is gained. 2NO=N2O4 The enthalpy is -58. 51 carbon monoxide CO(g) -110. Enthalpy of formation : 2. Dinitrogen tetroxide, commonly referred to as nitrogen tetroxide, and sometimes, usually among ex-USSR/Russia rocket engineers, as amyl, is the chemical compound N 2 O 4. THERMOPHYSICAL PROPERTIES 2 4. 5 kj/mol, and water vapor = -241. NO 2 is an intermediate in the industrial synthesis of nitric acid, millions of tons of which are produced each year for use primarily in the production of fertilizers. Oxides of Nitrogen. The difference between heat of formation and heat of reaction is that the heat of formation is the change in enthalpy during the formation of a mole of a substance at standard conditions whereas the heat of reaction is the change in enthalpy during a chemical reaction that occurs at a constant pressure. You can turn the second reaction around to get: Cl2O(g) + 3F2O(g) → 2 ClF3(g) + 2 O2(g) , but in so doing the enthalpy of formation is the negative of that given. 2 kJ/mol If we reverse the reaction so that 1 mol N2O4 is produced rather than consumed, the same amount of heat is released in the reaction as was absorbed in the original. The standard pressure value p ⦵ = 10 5 Pa (= 100 kPa = 1 bar) is recommended by IUPAC, although prior to. Now consider the following set of reactions: 2NO2→2NO + O2 ,ΔH=109 kJ/mol N2O4→2NO + O2 ,ΔH=172 kJ/mol Part A The equations given in the problem introduction can be added together to give the following reaction:. C (s) + 2F2 (g) →CF4 (g) ∆H = −680 kJ Target is 2CF4, so double this. The I2 bond energy is 151 kJ/mol. Yep, its a law not a rule. A) Ca(s) + C(s) + 3/2 O2(g) → CaCO3(s) B) C(s) + O2(g) → CO2(g) C) NO(g) + ½ O2(g) → NO2(g) D) N2(g) + 2 O2(g) → N2O4(g). save hide report. Hence as the temperate of the reaction increases, the reaction moves towards the forward reaction to minimize the effect of temperature change of system. 8 kJ/mol, "the standard enthalpy of formation of water vapor will not be an endothermic process". 8 Express your answer to four significant figures and include the appropriate units. N2O4(g)+4H2(g)=N2(g)+4H2O(g) asked by karen on March 4, 2012; Chemistry. First, write out the standard enthalpies of formation equivalents of the following reactions listed:. The new enthalpy of formation of gas-phase hydrazine, based on balancing all available knowledge, was determined to be 111. To balance a chemical equation, enter an equation of a chemical reaction and press the Balance button. 20 (ii) N2(g) + 2O2(g) = N2O4(g) +9. ∆ f G°: Standard Gibbs free energy of formation (kJ/mol). Shields-Maxwell under a Creative Commons Attribution-NonCommercial-ShareAlike 2. of N 2 O 4. Since enthalpy of reactions change with temperature and pressure, pressure and temperature must be constant. 5 kJ/mol at 1 atm and 298 K. K(s) + Cl2(g) + O2(g) → KClO3(s). a) What is the energy of combustion at 1 atm and 298 K? E comb = kJ/mol b) Use Thermodynamic Properties to determine its heat of formation. 2N2O(g) → 2N2(g) + O2(g) ΔHrxn = 2ΔH°form(N2) + 2ΔH°form(O2) - 2ΔH°form(N2O) =-163 kJ mol^-1. How are the heat of combustion of scandium and the heat of formation of Sc2O3 related? The heat of combustion is the heat absorbed when 1 mole of scandium reacts: 1 Sc + 3 /4O 2 → 1 /2 Sc 2O 3 The heat of formation is defined as the heat absorbed when 1 mole of Sc 2O 3 forms: 2 Sc + 3 /2 O. Using the data given in the problem, solve this for the unknown value of \(ΔH^o_f\) for benzene , the enthalpy of formation of benzene. Back to the Purdue AAE Propulsion main page. This little naught sign tells us it's a standard heat the formation. Enthalpy and Gibbs Free Energy Calculator Introduction : the purpose of this calculator is to calculate the value of the enthalphy of a reaction (delta H) or the Gibbs free energy of a reaction (delta G). 12 Enthalpies of formation of CO(g), CO2(g), N2O(g) and N2O4(g) are -110 kJ mol-1, - 393 kJ mol-1,…. Calculate ΔHo f for ammonia. N2O4 (g,red) 2NO2 (g,colorless): N2O4 (g,red) 2NO2 (g,colorless) Sample problem: Consider a mixture of N2O4 and NO2 at a total pressure of 1. Molecular weight and standard enthalpy of formation Molecular Weight And Enthalpy of Formation at 298K ╔══════════╦═══════╦════════╗ ║ ║ M. Rotational Constants : 6. Here, reactants are , CO and N 2 O 4 while products are N 2 O and CO 2, The value of enthalpy of formation of N 2 O is not given here which should be 81 KJ/mol. The Organic Chemistry Tutor 272,326 views 1:04:50. A negative result indicates that heat is lost when the reaction takes place, and a positive result indicates that heat is gained. Standard Enthalpy of NO2(g) = 33. Download as PDF. Publications Authored by Yoshihito Shiota. 2015–present Senior Instructor II, University of Oregon. Character Tables. 72 J/K (increase in entropy). Once entered. Mallard, Eds, NIST Chemistry WebBook, NIST Standard Reference Database. Remember, the bond enthalpy value is the amount of energy needed to break a bond. Chemistry Stack Exchange is a question and answer site for scientists, academics, teachers, and students in the field of chemistry. Could you please show working on how u got your answer Excuse the symbology the C is to mean degrees celsius and the numbers are subscripts in order to avoid any confusion. Estimate the standard enthalpy of formation of NH3(g) at 400 K from the data in the Data section. 2 NO2(g) ----> N2O4(g) Equations Change in Heat Energy N2(g) + 2 O2(g) ---> 2 NO2(g) 67. For many calculations, Hess’s law is the key piece of information you need. The addition of the Cl ions to the cobalt complex ions will turn the solution blue because there will be more of the cobalt chloride than there is of the Cobalt in the water. And one final comment about enthalpy changes of formation: The standard enthalpy change of formation of an element in its standard state is zero. Co(H2O)6^+2 + 4Cl^-1 <---> CoCl4^-2 + 6H2O. Answer to: Use standard enthalpies of formation to calculate Delta Hrxn for the following reaction: N2O4(g) + 4H2(g) N2(g) + 4H2O(g) By signing. The heat of combustion of liquid cumene (C 9 H 12) is -5215. Consider the following equilibrium: N2O4(g) ⇄ 2 NO2(g) 1. The standard enthalpy of formation of gaseous water at 25°C is −241. It only takes a minute to sign up. 08 kJ/mol for N2O4(g) 95. The reference state is the most stable state of an element at the specified temperature and 1 bar. Calculate ΔHo f for ammonia. Yes you'll have to draw a Hess's cycle for that In your overall equation, N 2 O is the product, and N 2 O is present in one of those cyclesbut it's a reactant rather than a product, so that reaction effectively goes in a the opposite direction, so you have to reverse the sign to +193. Chapter 18: Thermodynamics Renee Y. CIC Specific Heat 1 W 008 5. 7 kJ mol-1 respectively. So the addition of heat will favor endothermic reaction. The standard enthalpy of formation, Gibbs energy of formation, entropy and molar heat capacity are tabulated for more than hundred organic substances. Each D H f º corresponds to a special thermochemical equation with the following features. 20 mol/L x 0. The reference state is the most stable state of an element at the specified temperature and 1 bar. Enthalpy of reaction at standard conditions Data from NIST Standard Reference Database 69: NIST Chemistry WebBook The National Institute of Standards and Technology (NIST) uses its best efforts to deliver a high quality copy of the Database and to verify that the data contained therein have been selected on the basis of sound scientific judgment. 2013–2015 Morrill Professor, Iowa State University. Click here 👆 to get an answer to your question ️ Question 6. K(s) + Cl2(g) + O2(g) → KClO3(s). Worksheet 8 I. First, write out the standard enthalpies of formation equivalents of the following reactions listed:. How would you calculate the standard enthalpy of formation of N2O5(g) in kJ/mol from the following data (all species are gases): 2 NO + O2-----2 NO2 ∆H° = -114. 00 moles CO2, & 6. IE: Ionization energy (eV). When NO2 is placed in a closed cylinder under pressure, it reacts in part to form N2O4 molecules, and an equilibrium is established. 00 L flask at equilibrium at 20oC. The equation for the formation of glucose is 6CO2+6H2O=C6H12O6+6O2. The thermodynamic variable used to describe the heat of a reaction at constant pressure, qP ; The potential thermodynamic energy of a reacting system ; The potential energy stored (as heat) in chemical bonds. This molecule has a very long N-N bond, 0. Dinitrogen tetraoxide is a nitrogen oxide. 4 kJ Calculate the enthalpy of reaction for 2NO + O2 ==> 2NO2" 1. with N2O3 and N2O4, as a gas or in solution, not with nitrite, and so can occur extensively at the pH of malt. How to Use Hess's Law to Calculate Enthalpy Changes. Standard free energies of formation values are. If you are breaking a Q-Q bond, then you are inputting 200 kJ/mol energy into the system (endothermic process). Vaporizes readily to give NO2, also an oxidizing agent. Consider frozen N2O4: only white solid is present. [], and was also used for the initial development of high-accuracy ANLn composite electronic structure methods []. 66 kJ/mol 0 kJ/mol 0 kJ/mol -241. Use the data to calculate the standard Gibbs free energy change for the reaction N2O4(g) → 2 NO2(g) at 25 C. The exact composition of the mixture depends on the temperature with higher temperature favoring conversion to NO2. At 25C for the polymerisation of acetylene C2H2(g) to form benzene C6H6 (l) Their heats of formation at 25 C are 227 kJ/mol and 49. molecular weight and standard enthalpy of formation Molecular weight and standard enthalpy of formation 73. N2O4(g) + 2N2H4(l) 3N2(g) + 4H2O(l). Could you please help me better understand the formation of aluminum hydroxide --- for instance: Again, lets assume we have a generally weak acid solution to which we add aluminum sulfate. dissolution of crystals in a solution. Question: 2. For each of the propellant combinations shown above, four graphs have been provided. Dear student ! We know that, Δ r H 0 = [sum of enthalpies of formation of products]- [sum of enthalpies of formation of reactants]. 83 J) of reaction can be calculated from the following standard-state enthalpies of formation and standard-state entropies:. Thermodynamic properties of the chemically reactive system N2O4 ⇌ 2NO2 ⇌ 2NO + O2 have been evaluated over a pressure range of 0. 96 kJ moll at 298. The standard enthalpy of reaction \(\Delta{H_{rxn}^o}\) is the enthalpy change that occurs when a reaction is carried out with all reactants and products in their standard states. [N (NO2)O2] Nitrogen Tetroxide O2NNO2 N2O4. In a dry ice - acetone bath, the N2O4 will crystallizes as a white solid. Option A is out bc it does not contain N2 and O2. Heat of Neutralization: The initial temperature of 50. 0 M HCl and a 3. Ideal Gas Enthalpy of Water Vapor (H2O) Enthalpy of Formation: -241,826 (kJ/kmol) Molecular Weight: 18. 3 kJ S = 160. Find out: a) The change of the Gibbs free energy at 25 °C. Add the reactions together, add the enthalpies together. 384 J/g· C; lead, 0. The basis of this model is the saturation point of metal nitrate in N2O4. Standard Enthalpies, Free Energies of Formation, Standard Entropies. We use the thermodynamic data (constant pressure heat capacities and enthalpies and energies of formation) given in the excellent book by S. Sponsored Links ΔH° f : The standard enthalpy of formation at 25°C (298,15 K) for 1 mol of the substance in its given state (g= gas and l= liquide) from its elements in their standard state. The rate of the production of N2O4 is occurring at a faster rate than the rate of production of NO2, which is why the tube color turns from orange to clear. 42 kJ mol-1. Question: 2. CHEMICAL EQUILIBRIUM Chapter 16 a A + b B 'c C + d D K = [C]c [D]d [A]a [B]b conc. The standard free energies of formation of SO2(g) and SO3(g) are -300. So the reactions that involves only N2, O2, and NO2 would be considered to find the enthalpy of formation of NO2 as it will be same as the enthalpy of reaction. 9 K+(aq) −251. 12 Enthalpies of formation of CO(g), CO2(g), N2O(g) and N2O4(g) are –110, – 393, 81 and 9. It means when NO2 converts into N2O4 during a chemical reaction, certain amount of heat is released. 2 kcal/mol for ACF-gas-N2O4. 2 HBr(g) -36. The free energy is ∆G = ∆H −T∆S where T∆S is the heat that must appear to compensate the change in entropy owing to the reaction. Le Châtelier's Principle predicts that when the. We can look up that in a table, and let's say that that's some number. 2NO2 (g) →N2O4 (g) at 30 °C b. A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Blurry but cool pic of some n2o4 crystals that I made when cooling in dry ice bath. Using the data given in the problem, solve this for the unknown value of \(ΔH^o_f\) for benzene , the enthalpy of formation of benzene. ΔH°rxn = ΔH°f(products) - ΔH°f(reactants) You're essentially subtracting the energy held in the bonds of the reactants minus the energy held in the bonds of the products. 2NO2 --> 2NO + O2, /\ H = 109 kJ/mol 2. Back to list of reactions. Don't you need to add heat to water for it to turn into water vapor?[/quote]. Log in to reply to the answers Post; Still have questions? Get answers by asking now. I believe the reply is e becuase a standard enthalapty alternate of formation implies that one mole of product is being fashioned from all the elemts that make it up. Get an answer for 'What is the standard heat of formation of Fe2O3 (s) ? Fe2O3 (s) + 2Al (s) -> Al2O3 (s) + 2Fe (l) ΔH= -800. This little naught sign tells us it's a standard heat the formation. Join Yahoo Answers and get 100 points today. (i) H 2 (g) + I 2 (g) 2HI (g) (formation of HI) (ii) SO 2 (g) + Cl 2. save hide report. C5h12 Empirical Formula. At equilibrium, 0. The standard enthalpy (delta H° = -57. of N 2 O 4. 7 AgCl s −127. Is heat of formation the same thing as enthalpy? I have to calculate the enthalpy but my teacher only showed us how to calculate heat of formation. The products of the reaction are H2O, N2, and CO2. Standard Enthalpies of Formation at 298. Low-boiling (boiling point 21. Enthalpies of formation of CO2, CO, and N2O and N2O4 are -110, -393 and 81 and 9. 1,1,2,2-tetrachloroethane is a member of the class of chloroethanes that is ethane substituted by chloro groups at positions 1, 1, 2 and 2. Standard Enthalpy Formation 1. 2015–present Senior Instructor II, University of Oregon. The rate of the production of N2O4 is occurring at a faster rate than the rate of production of NO2, which is why the tube color turns from orange to clear. Linstrom and W. The heat of vaporisation at 18° C. CHEM 100 Thermochemistry Questions Question 11 • The complete combustion of ethanol, C2H5OH(l), to form H2O(g) and CO2(g) at constant pressure releases 1235 kJ of heat per mole of C2H5OH. Calculate the enthalpy of reaction for NO(g)+O(g)-->NO2(g)? Hess's Law simply states that enthalpy is a state function (something that depends only on the state of a system) and it can be used to add the enthalpies of reactions together. 4 kJ Calculate the enthalpy of reaction for 2NO + O2 ==> 2NO2" 1. DA: 2 PA: 21 MOZ Rank: 50. Have a question that asks to calculate the dissociation enthalpy of the N-N bonds in N2O4 given that: Formation of NO2 = 33. You complete the calculation in different ways depending on the specific situation and what information you have available. Advances in the application of N2O4/NO2 in organic reactions. Download as PDF. Yep, its a law not a rule. 2 Expert Answer(s) - 84014 - Enthalpies of formation of CO(g), CO2(g), N2O(g) and N2O4(g) are -110, - 393, 81 and 9. 3 moles of n2o4 and 2. Reactant or product ΔH∘f (kJ/mol) C2H6(g) -84. None of the above _____ 4. 9 K+(aq) −251. Determining ΔG⁰ from Standard Free Energy of Formation. 118 of the Thermochemical Network This version of ATcT results was partially described in Ruscic et al. The standard enthalpy of reaction \(\Delta{H_{rxn}^o}\) is the enthalpy change that occurs when a reaction is carried out with all reactants and products in their standard states. The reaction is endothermic; therefore, we can imagine heat as a reagent on the reactant side of the equation. Equilibrium 1. logP oct/wat. Oxides of Nitrogen. 67kJ/mol 2NO2 <-> N2O4 33. You can turn the second reaction around to get: Cl2O(g) + 3F2O(g) → 2 ClF3(g) + 2 O2(g) , but in so doing the enthalpy of formation is the negative of that given. This molecule has a very long N-N bond, 0. The thermodynamic variable used to describe the heat of a reaction at constant pressure, qP ; The potential thermodynamic energy of a reacting system ; The potential energy stored (as heat) in chemical bonds. Thus, standard enthalpy of formation of NH 3 gas = -46. Shields-Maxwell under a Creative Commons Attribution-NonCommercial-ShareAlike 2. Solution for Using a standard-state enthalpy of formation and absolute entropy data table for the reaction2NO2 == N2O4 Hfo(kJ/mol) So(J/mol-K)NO2(g)…. 4 N 2O(g) +82. Heat shield:A device which protects people or equipment from heat, such as a shield in front of a reentry capsule. Nitrous oxide (dinitrogen oxide), N2O, is formed when ammonium nitrate, NH4NO3, is heated. The standard enthalpy of formation, Gibbs energy of formation, entropy and molar heat capacity are tabulated for more than hundred organic substances. Calculate ΔHo f for ammonia. In the case of carbon, the graphite modification is the more stable form. Question: 2. Chemistry: Post your doubts here! N2O4-----2NO2( reversible reaction) D enthalpy change of formation of PbO and enthalpy change of formation of Pb3O4. Consider the following equilibrium: N2O4(g) ⇄ 2NO2(g) A 1. Helio-:Prefix referring to the Sun. NO2 gas exists as a mixture of NO2 and N2O4 at equilibrium (giving it a yellow-brown color). Means of reaching the saturation point are examined, and a relationship is made for the reaction/formation rate and diffusion rate. 67kJ/mol 2NO2 <-> N2O4 33. 15K) and at 1bar pressure. NOTATION AND UNITS 1 3. Enthalpy (DH), Lets Review. The standard enthalpy of formation, Gibbs energy of formation, entropy and molar heat capacity are tabulated for more than hundred organic substances. the standard heat of formation of NO2 and N2O4 are 8and 4 kcal/ mol respectively the heat of dimerisation of NO2 in kcal is - 5555930. 12 Enthalpies of formation of CO(g), CO2(g), N2O(g) and N2O4(g) are –110 kJ mol–1, – 393 kJ mol–1, 81 kJ mol–1 and 9. 1) A substantially more negative value, -466. Consider the following equilibrium: N2O4(g) ⇄ 2 NO2(g) 1. Enthalpy of formation (ΔH f (CO)) = -110 kJ mol-1. 2 Br−(aq) −120. at 690oC at equilibrium. The enthalpy of formation of propene is +20. 013 (kg/kmol). Worksheet 8 I. The standard enthalpy (∆H rxn = -57. It is used to calculate the material's properties under different. 5 atm… resulting from the dissociation of N2O4. Molecular parameters. Question 6. 5 atm… resulting from the dissociation of N2O4. The enthalpy of formation is the standard reaction enthalpy for the formation of the compound from its elements (atoms or molecules) in their most stable reference states at the chosen temperature (298. The final temperature of the resultant solution is measured. 4 kJ I reversed the first reaction so that I would have 2NO ==> N2 + O2. 42 kJ mol-1. Using standard enthalpies information, calculate the standard enthalpy change for this reaction: a) (thermite reaction) 2Al(s) + Fe2O3(s) = Al2O3(s) + 2Fe(s) b) Mg(OH)2(s) = MgO(s) + H2O(I) c) N2O4(g) + 4H2(g) = N2(g) + 4H2O(g) d) SiCl4(I. The standard enthalpy of reaction \(\Delta{H_{rxn}^o}\) is the enthalpy change that occurs when a reaction is carried out with all reactants and products in their standard states. Technical Note 270-6. Thermodynamic properties of the chemically reactive system N2O4 ⇌ 2NO2 ⇌ 2NO + O2 have been evaluated over a pressure range of 0. I don't understand why it can't be an endothermic process. Since H > 0 (endothermic), adding heat favors the forward reaction, i. This Letter examines the enthalpy of formation for 12 transition metal diatomic molecules and 23 transition metal complexes from the viewpoint of effect of the relativistic effect by using the infinite-order Douglas–Kroll–Hess method with the local unitary transformation and three types of pseudopotentials for several levels of theory. The formation of N2O4 from NO2 is an exothermic process, thus heat is emitted when two 2NO2 molecules form an N2O4 molecule. Standard Enthalpy of Formation -- ' Hq. Circular 500. The enthalpy of formation is the standard reaction enthalpy for the formation of the compound from its elements (atoms or molecules) in their most stable reference states at the chosen temperature (298. 2006 Visiting Professor, University of Arizona. ║ kJ/mol ║ ╠══════════╬═══════╬════════╣ ║H ║ 1. 5 C to 225C. To calculate the enthalpy of a chemical reaction, first balance the chemical equation. 2015–present Senior Instructor II, University of Oregon. Δ f H° gas: Enthalpy of formation at standard conditions (kJ/mol). Basically what you do is flip around and change the coefficients of the two reactions they give you until you get the products and reactants of the top equation, 2NO + O2 -->N2O4. 3 kJ S = 160. Yep, its a law not a rule. 38^2 then R= 8. Δ vap H°: Enthalpy of vaporization at standard conditions (kJ/mol). CHEM 100 Thermochemistry Questions Question 11 • The complete combustion of ethanol, C2H5OH(l), to form H2O(g) and CO2(g) at constant pressure releases 1235 kJ of heat per mole of C2H5OH. Molecular parameters. In view of the coronavirus pandemic, The enthalpy of formation of sulphuric acid at 298 K will be: 1 Verified Answer. 2 calories per gram, which corresponds to -2960 to -3420 calories per mol. Standard Enthalpies of Formation and Calculating Enthalpy of Reaction () By Shawn P. Join Yahoo Answers and get 100 points today. 05 kJ K−1kg−1, as given in the problem statement. 1 Electrochemical Cells 64 3. 15 K in kJ/mol for Nitric Acid (HNO3) is -174. •The standard heat of formation of a compound is always taken in reference to the forms of the elements that are most stable at 25°C and 1 atm pressure. Which of the following chemical equations does not correspond to a standard molar enthalpy of formation? a. 0 comments. Write the specific chemical reaction defining the enthalpy of formation of butane: Just checking to make sure this is correct: 4C + 5H2 —> C4H10. 4 Specific Enthalpy of the Saturated Liquid 3. Calc equilibrium constant at 25'c Calculate the standard enthalpy of formation of unknown reaction using Hess's Law. On the reactants we have N2O4 with -35. 0kJ/mol Explain your observations of the reversals in terms of LeChatelier’s principle. Show class gas tubes. Under certain nonstandard conditions, oxidation by O2 (g) of 1 mol of SO2 (g) to SO3 (g) absorbs 89. Heat Capacity at 298. A method and apparatus are disclosed for a large scale, electrochemical production of anhydrous nitric acid and N2O5. Constantieux, in Comprehensive Heterocyclic Chemistry III, 2008. 2 kJ Calculate the enthalpy of the reaction. The heat of combustion of liquid cumene (C 9 H 12) is -5215. 00 L container is initially filled with 0. Standard enthalpy of formation: It is the heat required when one mole of substance formed from its pure elements in standard state. It is used to calculate the material’s properties under different. Consider frozen N2O4: only white solid is present. a heat of formation of zero. Calculate the enthalpy of reaction for NO(g)+O(g)-->NO2(g)? Hess's Law simply states that enthalpy is a state function (something that depends only on the state of a system) and it can be used to add the enthalpies of reactions together. Vaporizes readily to give NO2, also an oxidizing agent. At 25C for the polymerisation of acetylene C2H2(g) to form benzene C6H6 (l) Their heats of formation at 25 C are 227 kJ/mol and 49. The standard state of a material is a reference point for the material’s thermodynamic state properties such as enthalpy, entropy, Gibbs free energy, etc. The standard enthalpies of formation are given in the chart on the bottom of page 271. and a temperature range of 200° to 900°K. of N 2 O 4. Hess' Law: two equations and their enthalpies - Problems 1 - 10 That is because of the subscripted 'f' on the enthalpy. It is a useful reagent in chemical synthesis. The energy required to melt a solid at its melting point ___D__ enthalpy of formation F. The standard pressure value p ⦵ = 10 5 Pa (= 100 kPa = 1 bar) is recommended by IUPAC, although prior to. 2 Galvanic Cells 65 3. Mallard, Eds, NIST Chemistry WebBook, NIST Standard Reference Database. 51 carbon monoxide CO(g) -110. Ask question + 100. 3 moles of no2 in a 5 liters container. Log in to reply to the answers Post; Still have questions? Get answers by asking now. 0396 mol The enthalpy change for the reaction can now be calculated. Thermal Gloves. The heat of formation of SO3 (g) is -204. 6 ║ ║N2O ║ 44. If you are breaking a Q-Q bond, then you are inputting 200 kJ/mol energy into the system (endothermic process). The observed value is −(466. Linstrom and W. 1,1,2,2-tetrachloroethane is a member of the class of chloroethanes that is ethane substituted by chloro groups at positions 1, 1, 2 and 2. PMID: 28727457. Hence as the temperate of the reaction increases, the reaction moves towards the forward reaction to minimize the effect of temperature change of system. Nitrogen (N) forms oxides in which nitrogen exhibits each of its positive oxidation numbers from +1 to +5. Enthalpy of formation (Hfg), Entropy, Integrated heat capacity (0 K to 298. 5 Cl−(aq) −167. NOTE: the ΔH°f value for an element in its natural state is 0 kJ/mol. 66 kJ/mol 0 kJ/mol 0 kJ/mol -241. Yep, its a law not a rule. The rate of the production of N2O4 is occurring at a faster rate than the rate of production of NO2, which is why the tube color turns from orange to clear. of products conc. 2 calories per gram, which corresponds to -2960 to -3420 calories per mol. 159 J/g· C; water, 4. N2O4 --> 2NO2; So the concentration of N2O4 decreases. N2O4 (g,red) 2NO2 (g,colorless): N2O4 (g,red) 2NO2 (g,colorless) Sample problem: Consider a mixture of N2O4 and NO2 at a total pressure of 1. Δ fus H°: Enthalpy of fusion at standard conditions (kJ/mol). Standard Enthalpy of Formation -- ' Hq. ΔH f (CO2) = -393 kJ mol-1. the standard heat of formation of NO2 and N2O4 are 8and 4 kcal/ mol respectively the heat of dimerisation of NO2 in kcal is - 5555930. Heats of formation and chemical compositions * 15 * heat of 16 * formation density 23 * in cal/g lbm/in^3 24 acan mae chan 9c 16h 10o 8n -776. asked by AJ on March 26, 2017 chemistry. For either reaction. N2O4 2NO2 Since, the heat is absorbed in the reaction, therefore Δ HfNO2 must be greater than zero as reaction is endothermic. You can turn the second reaction around to get: Cl2O(g) + 3F2O(g) → 2 ClF3(g) + 2 O2(g) , but in so doing the enthalpy of formation is the negative of that given. This tool relates the enthalpy change of a system at constant pressure with initial and final temperatures and the heat capacity at constant pressure of the system. 43 carbon dioxide CO 2 (g) -393. 82 kJ mol−1. Reactant or product ΔH∘f (kJ/mol) C2H6(g) -84. Join Yahoo Answers and get 100 points today. Structure, properties, spectra, suppliers and links for: Dinitrogen tetroxide, 10544-72-6, O2NNO2. org The standard enthalpy of formation or standard heat of formation of a compound is the change of enthalpy during the formation of 1 mole of the substance from its constituent elements, with all substances in their standard states. So with this reaction we have 2NO2 with a reverse reaction to N2O4. 18J/g⋅ ∘ C as the specific heat capacity. The N2 elements would cancel out -- I understand that much. 7 KJ/mol respectively asked Oct 6, 2017 in Chemistry by jisu zahaan ( 29. 2 ║ ║N2O5 ║108. 11 ammonium chloride NH 4 Cl(s) -314. 9 H 2PO 4 −(aq) −1302. the standard heat of formation of NO2 and N2O4 are 8and 4 kcal/ mol respectively the heat of dimerisation of NO2 in kcal is - 5555930. The initial rate of formation of products and disappearance of reactants is greater at higher temperatures. Enthalpy of formation of solid at standard conditions Data from NIST Standard Reference Database 69: NIST Chemistry WebBook The National Institute of Standards and Technology (NIST) uses its best efforts to deliver a high quality copy of the Database and to verify that the data contained therein have been selected on the basis of sound. Heliopause:The boundary theorized to be roughly circular or teardrop-shaped, marking the edge of the Sun's influence, perhaps 100 AU from the Sun. The enthalpy of formation (ΔH° f ) is defined as the heat change associated with the formation of one mole of a compound from its elements in their standard states. Standard Entropies Alan D. Standard enthalpies of formation, H f, are measured under standard conditions (25°C and 1. Answer to: Use standard enthalpies of formation to calculate Delta Hrxn for the following reaction: N2O4(g) + 4H2(g) N2(g) + 4H2O(g) By signing. 00-L flask is charged with 0. 66 kJ/mol 0 kJ/mol 0 kJ/mol -241. Solvent data (including Kf,Kb) Solubility data. I believe the reply is e becuase a standard enthalapty alternate of formation implies that one mole of product is being fashioned from all the elemts that make it up. Calculate the enthalpy of reaction for NO(g)+O(g)-->NO2(g)? Hess's Law simply states that enthalpy is a state function (something that depends only on the state of a system) and it can be used to add the enthalpies of reactions together. is -4300 calories. Since H > 0 (endothermic), adding heat favors the forward reaction, i. Determine whether enthalpy of reaction is exo- or endothermic from the sign of ∆H. Title: Enthalpy and Hess Law 1 Enthalpy and Hess Law. (i) H 2 (g) + I 2 (g) 2HI (g) (formation of HI) (ii) SO 2 (g) + Cl 2. 1, ∆Hovap (. Back to list of reactions. How much heat will be given off if molar quantities of cyclobutane react according to the following equation?. logP oct/wat. To obtain the enthalpy change for the reaction, you need to calculate the moles of HCl that reacted. 2 N(g) + 4 O(g) N2O4(g) b. ║ kJ/mol ║ ╠══════════╬═══════╬════════╣ ║H ║ 1. The enthalpy of formation of CO(g) CO2(g), N2O(g), N2O4(g) are -110, -393, 81, and 9. the formation of blue CoCl42-. For most chemistry problems involving ΔHo f, you need the following equation: where p = products and r = reactants. Standard Enthalpies, Free Energies of Formation, Standard Entropies. Dinitrogen tetroxide is a powerful oxidizer that is hypergolic upon contact with various forms of hydrazine, which has made the pair a common bipropellant for rockets. 2NO2(g) → N2O4(g) ∆Hrxn = qp = − 57. 2 kJ/mol at 25 C and that of N2O4(g) is 9. Atomic and ionic radii. 15 K from calorimetric measurements of dissolution of solid magnesium in aqueous perchloric acid using a bomb reaction-solution calorimeter. 800 moles H2O, 4. 7 AgCl s −127. Hydrazine (N2H4) is a fuel used by some spacecraft. ΔH f (N2O4) = 9. The standard heat of formation of PI is 334 kJ/mol, and that of I2(g) is 62 kJ/mol. logP oct/wat. can u go step by step how to answer the following question given the standard enthalpy changes, deltaHdegrees for the following reactions;@ 298K deltaHdegrees(KJmol-1) (I) 2NO2(g) = N2O4(g) -57. Solution for Using a standard-state enthalpy of formation and absolute entropy data table for the reaction2NO2 == N2O4 Hfo(kJ/mol) So(J/mol-K)NO2(g)…. The reason for choosing the elements is because substances are formed by the elements. 5 C to 225C. 1/2N2 (g) + O2 (g) → NO2 (g), ΔH°A = 33. In an exothermic reaction, heat is a product. CIC Specific Heat 1 W 008 5. Δ f G°: Standard Gibbs free energy of formation (kJ/mol). Note, too, by definition, that the enthalpy of formation of an element is exactly zero because making an element from an element is no change. From the parameters obtained from the graph, compute α, K p, ΔGo, and ΔSo at 127oC and at 25oC. 0396 mol-2133. Problems of Thermochemistry: Gibbs free energy and spontaneity 1) Consider the following reaction: N2O4 (g) → 2 NO2 (g) The standard enthalpy of this reaction is 57. Use standard enthalpies of formation to calculate Delta H for the following reaction. 5 kJ/mol at 1 atm and 298 K. 1, ∆Hovap (. Here is the question: "Given the following reactions: N2 + O2 ==> 2NO enthalpy = +180. The standard enthalpy of reaction \(\Delta{H_{rxn}^o}\) is the enthalpy change that occurs when a reaction is carried out with all reactants and products in their standard states. 3 kJ/mol If the standard heat of formation of Al2O3 (s) is -1675. Use tabulated data for other heats of formation. Δ f H° gas: Enthalpy of formation at standard conditions (kJ/mol). For each of the propellant combinations shown above, four graphs have been provided. The standard heat of formation (standard enthalpy of formation) of a compound is defined as the enthalpy change for the reaction in which elements in their standard states produce products. reactants convert into products and products also convert into reactants under similar conditions of pressure and temperature, the reaction is said to be in chemical equilibria. N2 + O2 ==> 2NO enthalpy = +180. Thermodynamic properties of the chemically reactive system N2O4 ⇌ 2NO2 ⇌ 2NO + O2 have been evaluated over a pressure range of 0. Under certain nonstandard conditions, oxidation by O2 (g) of 1 mol of SO2 (g) to SO3 (g) absorbs 89. The new enthalpy of formation of gas-phase hydrazine, based on balancing all available knowledge, was determined to be 111. 15 K to obtain the enthalpy of the. Ammonia is produced commercially by the Haber reaction: N 2(g) + 3 H 2(g) ↔ 2 NH 3(g) + heat The formation of ammonia is favored by (A)an increase in pressure (B)a decrease in pressure (C)removal of N 2(g) (D)removal of H 2(g) 2. N2O4(g)+4H2(g)=N2(g)+4H2O(g) asked by karen on March 4, 2012; Chemistry. Equilibrium 1. For example, H 2 (g) → H 2 (g) ΔH f = 0. The standard enthalpy of formation ΔH f o (Mg 2+) of the aqueous magnesium ion has been determined at 298. Calculate the heat released upon formation of 35. ΔH°rxn = ΔH°f(products) - ΔH°f(reactants) You're essentially subtracting the energy held in the bonds of the reactants minus the energy held in the bonds of the products. Problem: The standard molar enthalpy of formation of NO2(g) is 33. Learning Strategies. Electrostrictive energy conversion of polyurethane with different hard segment aggregations. Now consider the following set of reactions: 2NO2→2NO + O2 ,ΔH=109 kJ/mol N2O4→2NO + O2 ,ΔH=172 kJ/mol Part A The equations given in the problem introduction can be added together to give the following reaction:. 2NO2(g) N2O4(g) d. Molecular parameters. At the temperatures over 570 °С, the ferrous alloys oxidize, as ferric oxide (wustite) with simple cubic lattice appears on the metal surface. 🤓 Based on our data, we think this question is relevant for. 2NO2 --> 2NO + O2, /\ H = 109 kJ/mol 2. 2NO=N2O4 The enthalpy is -58. So the enthalpy of formation of this reaction is - 341. 5 Al3+(aq) −524. The difference between heat of formation and heat of reaction is that the heat of formation is the change in enthalpy during the formation of a mole of a substance at standard conditions whereas the heat of reaction is the change in enthalpy during a chemical reaction that occurs at a constant pressure. H2O(g) and H2O(l) are different. For example, H 2 (g) → H 2 (g) ΔH f = 0. I got -964kJ, but it says my answer is wrong. PMID: 28727457. So the addition of heat will favor endothermic reaction. Use the following equations to calculate the standard heat of formation of magnesium nitrate in kilojoules 8Mg(s) + Mg(NO3)2(s) -> Mg3N2(s) + 6MgO(s) ΔH= -3884 KJ Mg3N2(s) -> 3Mg(s) + N2(g) ΔH = +463 KJ 2MgO(s) -> 2Mg(s) + O2(g) ΔH = +1203KJ 2. 00 moles CO2, & 6. If you are breaking a Q-Q bond, then you are inputting 200 kJ/mol energy into the system (endothermic process). C (s) + 2F2 (g) →CF4 (g) ∆H = −680 kJ Target is 2CF4, so double this. 7 kJ mol -1. Heat shifts the equilibrium in favor of NO2 and the tube becomes darker. 3 moles of n2o4 and 2. Determine whether enthalpy of reaction is exo- or endothermic from the sign of ∆H. where n and m are the coefficients in the balanced chemical equation of the reaction. Determining ΔG⁰ from Standard Free Energy of Formation. Standard Enthalpy Standard Enthalpies of Formation of Formation The change in enthalpy that accompanies the formation of one mole of a compound from its free elements in their standard states. 1, ∆Hovap (. Enthalpy of reaction at standard conditions Data from NIST Standard Reference Database 69: NIST Chemistry WebBook The National Institute of Standards and Technology (NIST) uses its best efforts to deliver a high quality copy of the Database and to verify that the data contained therein have been selected on the basis of sound scientific judgment. ∆ f G°: Standard Gibbs free energy of formation (kJ/mol). 9 HF(g) -272. a) What is the energy of combustion at 1 atm and 298 K? E comb = kJ/mol b) Use Thermodynamic Properties to determine its heat of formation. The thermodynamic variable used to describe the heat of a reaction at constant pressure, qP ; The potential thermodynamic energy of a reacting system ; The potential energy stored (as heat) in chemical bonds. Join Yahoo Answers and get 100 points today. Show class gas tubes. For either reaction. The value calculated from the depression of the freezing-point is. 1 Vapour Pressure 3 4. Mg(s) + C(s) + 3/2 O_2(g) rightarrow MgCO_3(s) b. 015 (kg/kmol). The combination of HTPB or PMMA solid fuel and N2O is benign, non-toxic, and non-explosive. (a) Using the standard heats of formation on page 4 of the Data Booklet, determine the heat of formation of C2Cl4(g). Use those values and the one given for liquid pentane to solve for the standard enthalpy of formation for the reaction. C (s) + 2F2 (g) →CF4 (g) ∆H = −680 kJ Target is 2CF4, so double this. [Answer: −48. 2NO2 (g) →N2O4 (g) at 30 °C b. Dear student ! We know that, Δ r H 0 = [sum of enthalpies of formation of products]- [sum of enthalpies of formation of reactants]. I solved for the pressure as 1. The reference state is the most stable state of an element at the specified temperature and 1 bar. Under certain nonstandard conditions, oxidation by O2 (g) of 1 mol of SO2 (g) to SO3 (g) absorbs 89. Answer to: Use standard enthalpies of formation to calculate Delta Hrxn for the following reaction: N2O4(g) + 4H2(g) N2(g) + 4H2O(g) By signing. 47 kJ/mol at 298. INTRODUCTION 1 2. There are many ways to find the boiling point of a substance. Standard enthalpy of formation - Wikipedia. The standard heat of formation (standard enthalpy of formation) of a compound is defined as the enthalpy change for the reaction in which elements in their standard states produce products. Try Me! Overall: N2O4(g)=>N2(g) + 2O2(g) Reaction Mechanism: NO2(g) ½ N2(g) + O2(g) H=-84. 1032 25 acetamide. N2(g) + O2(g) → N2O4(l. 2NO2(g) N2O4(g) d. Standand Enthalpies of Formation & Standard Entropies of Common Compounds Substance State ∆H f S (kJmol) (Jmol·K) Ag s 0 42. Standard Enthalpies, Free Energies of Formation, Standard Entropies. Calculate the heat released (in kilojoules) per gram of the compound reacted with oxygen. 05║ ║N2O3 ║ 76. 0 mole of N2O4 was allowed to reach equilibrium at 400K. 5 moles of Al from 30. What is the enthalpy of reaction, given the following enthalpies of formation: methane gas = -74. 8 Solution: −2800. Becker Valencia Community College. Heliopause:The boundary theorized to be roughly circular or teardrop-shaped, marking the edge of the Sun's influence, perhaps 100 AU from the Sun. 5 N 2H 4(g) +95. meaning it always works! 2. 7 CO2(g) -393. The balanced equation will appear above. Calculate the enthalpy of reaction for NO(g)+O(g)-->NO2(g)? Hess's Law simply states that enthalpy is a state function (something that depends only on the state of a system) and it can be used to add the enthalpies of reactions together. 1990-1998 Associate Professor, Iowa State University, 1988-1990 Associate Professor of Chemistry and Director of Freshman. Draw the enthalpy diagram for the reaction. Line up the 3 beakers in a row. (a) Use bond enthalpies to estimate the enthalpy change for the reaction of hydrogen with ethylene. The objective is to precipitate aluminum hydroxide per the following reaction which I believe to be correct. 83 J) of reaction can be calculated from the following standard-state enthalpies of formation and standard-state entropies:. Enthalpy of formation of CO2. Unit 3 Electrochemistry 63 3. Which of the following chemical equations corresponds to the standard molar enthalpy of formation of N2O4? a. 8 kJ/mol, "the standard enthalpy of formation of water vapor will not be an endothermic process". The heat of combustion of liquid cumene (C 9 H 12) is -5215. 2NO2(g) → N2O4(g) ∆Hrxn = qp = − 57. Unit 4- Equilibrium. Standard Enthalpy of Reaction (ΔH rxn­) is the amount of heat absorbed (+ΔH value) or released (-ΔH value) that results from a chemical reaction. The free energy is ∆G = ∆H −T∆S where T∆S is the heat that must appear to compensate the change in entropy owing to the reaction. The products of the reaction are H2O, N2, and CO2. Mallard, Eds, NIST Chemistry WebBook, NIST Standard Reference Database. Liquid nitrous oxide (N2O / dinitrogen monoxide / 'laughing gas') is the oxidizer of choice for hybrid rocket motors because it is storable, and self-pressurizing to 48 atmospheres at 17 deg C. ∆ f H° gas: Enthalpy of formation at standard conditions (kJ/mol). Since the elements and the compound from which they are made will have the same products of combustion we can set up an energy cycle. 8 Express your answer to four significant figures and include the appropriate units. The enthalpy for that reaction would be 2x the enthalpy of formation of NO2 minus the enthalpy of formation of N2O4. formation reactions? Explain.