6.6 Standar Enthalpies of Reactions

Question 1

standard steps (definition)

Answer 1

set of conditions to study/compare reactions

Question 2

standard steps (list of steps)

Answer 2

1. for a GAS, the standard state is 1 atm and ideal behavior
2. for a substance in AQUEOUS SOLUTION, the standard state is 1 M concentration
3. for a pure substance (element or compound), the standard state is usually the most stable form of the substance at 1 atm and the temperature of interest

Question 3

formation equation

Answer 3

when 1 mol of a compound forms from its elements

Question 4

standard enthalpy of formation (ΔH°rxn)

Answer 4

enthalpy change for the formation equation when all the substances are in their standard states:

1. for an element in its standard state ΔH°f = 0
2. most compounds have a negative ΔH°f; under standard conditions, heat is released when most compounds form from their elements

Question 5

Steps to determine ΔH°rxn from ΔH°f values for Reactants and Products (2)

Answer 5

1. each reactant decomposes to its elements, so standard enthalpy change = -ΔH°f
2. each product form from its elements, so the standard enthalpy change = ΔH°f

Question 6

Standard enthalpy of reaction

Answer 6

the sum of the standard enthalpies of formation of the products minus the sum of the standard enthalpies of formation of the reactants