Question

Consider, for example, the reaction

2NO(g)+O2(g)⇌2NO2(g)

Then the standard heat of reaction for the overall reaction is

ΔH∘rxn=ΔH∘f(products)−−ΔH∘f(reactants)

ΔH∘rxn=2(33.2)-[2(90.2)+0]= -114kJ

Part A

For which of the following reactions is ΔH∘rxn equal to ΔH∘f of the product(s)?

You do not need to look up any values to answer this question.

Check all that apply.

Na(s)+12Cl2(l)→NaCl(s)

2Na(s)+Cl2(g)→2NaCl(s)

Na(s)+12Cl2(g)→NaCl(s)

H2(g)+12O2(g)→H2O(g)

2H2(g)+O2(g)→2H2O(g)

H2O2(g)→12O2(g)+H2O(g)

Part B

The combustion of heptane, C7H16, occurs via the reaction

C7H16(g)+11O2(g)→7CO2(g)+8H2O(g)

with heat of formation values given by the following table:

Calculate the enthalpy for the combustion of 1 mole of heptane.

Answer 1

Enthalpy of formation --> must have all elements in their standard state at STP, also, must produce only 1 mol of product required

Na(s)+12Cl2(l)→NaCl(s) --> incorrect, Cl2(g) is the correct std. state

2Na(s)+Cl2(g)→2NaCl(s) --> not 1 mol of NaCl produced

Na(s)+12Cl2(g)→NaCl(s) --> correct

H2(g)+12O2(g)→H2O(g) --> correct

2H2(g)+O2(g)→2H2O(g) --> not 1 mol of H2O produced

H2O2(g)→12O2(g)+H2O(g) --> incorrect, must produce a single mol

B)

HRxn = Hproducts - Hreactants

HRxn = (7CO2(g)+8H2O(g)) - (C7H16(g)+11O2(g))

HRxn = -393.5 + 8*-285.4 - (187.6 + 11*0)

HRxn = -2867.5 kJ per mol of heptane