Chemical energy is released or absorbed from reactions in various forms. The most easily measurable form of energy comes in the form of heat, or enthalpy. The enthalpy of a reaction can be calculated from the heats of formation of the substances involved in the reaction:
ΔH∘rxn=ΔH∘f(products)−ΔH∘f(reactants)
Entropy change, ΔS∘, is a measure of the number of energetically equivalent microstates introduced into the system during the reaction. The degree of spontaneity of a reaction is represented by the Gibbs free energy, ΔG∘. The Gibbs free energy depends on both the enthalpy and entropy changes that take place during the reaction:
ΔG∘=ΔH∘−TΔS∘
where T is standard temperature, 298 K.
Part A
Calculate the standard enthalpy change for the reaction
2A+B⇌2C+2D
where the heats of formation are given in the following table:
Substance | ΔH∘f (kJ/mol) |
A | -241 |
B | -397 |
C | 209 |
D | -477 |
Express your answer in kilojoules.
Standard enthalpy change = 343 kJ
Explanation
The balanced reaction is : 2 A + B 2 C + 2 D
Standard enthalpy change, Horxn = Hof products - Hof reactants
Horxn = [2 * Hof C + 2 * Hof D] - [2 * Hof A + Hof B]
Horxn = [2 * (209 kJ/mol) + 2 * (-477 kJ/mol)] - [2 * (-241 kJ/mol) + (-397 kJ/mol)]
Horxn = 418 kJ - 954 kJ + 482 kJ + 397 kJ
Horxn = 343 kJ
Chemical energy is released or absorbed from reactions in various forms. The most easily measurable form of energy comes...
Chemical energy is released or absorbed from reactions in various forms. The most easily measurable form of energy comes in the form of heat, or enthalpy. The enthalpy of a reaction can be calculated from the heats of formation of the substances involved in the reaction: ΔH∘rxn=ΔH∘f(products)−ΔH∘f(reactants) Entropy change, ΔS∘, is a measure of the number of energetically equivalent microstates introduced into the system during the reaction. The degree of spontaneity of a reaction is represented by the Gibbs free...
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