Thermochemistry

Chapter 17:4 Hess’s Law and

Standard Enthalpies of Formation

Pages 527 – 532

A. Hess’s Law

Hess’s law states that if you can add two or more thermochemical equations to produce a final equation for a reaction, then the sum of the enthalpy changes for the individual reactions is the enthalpy change for the final reaction

2S(s) + 3O2(g) 2SO3(g) H = ?

A. Hess’s Law

A. Hess’s Law

Known: N2 (g) + O2(g) 2NO(g) H = -180 kJ

2NO(g) + O2(g) 2NO2(g) H = -112 kJ

N2(g) + 2O2(g) 2NO2(g) H = ?

Build your equation:

-292 kJ

A. Hess’s Law

Known: S(s) + O2(g) SO2(g) H = -297 kJ

2SO3(g) 2SO2(g) + O2(g) H = 198 kJ

2S(s) + 3O2(g) 2SO3(g) H = ?

Build your equation:

If you need to multiply your coefficients, then multiply

the ΔH as well

If you reverse the reaction, then reverse

the sign for ΔH

-792 kJ

A. Hess’s Law

Known: 2H2(g) + O2(g) 2H2O(l) H = -572 kJ

H2(g) + O2(g) H2O2(l) H = -188 kJ

Build your equation:

2H2O2(l) 2H2O(l) + O2(g) H = ? -196 kJ

A. Hess’s Law

Known: 2B (s) + 3/2O2(g) B2O3(s) H = -1273 kJ

B2H6(g) + 3O2(g) B2O3 (s) + 3H2O (g) H = -2035 kJ

H2 (g) + ½ O2(g) H2O (l) H = -286 kJ

H2O (l) H2O (g) H = 44 kJ

2B(s) + 3H2(g) B2H6(g) H = ?

Build your equation:

36 kJ

B. Standard Heats of Formation

Standard heat of formation, ΔHf, of a compound is the change in enthalpy that accompanies the formation of one mole of the compound from its elements

All substances at standard pressure and 25oC

ΔHf of free elements (and diatomic molecules) in standard state is set at 0

B. Standard Heats of Formation

Provides an alternative to Hess’s Law of calculating heat of reaction indirectly

For reactions occurring at standard conditions, calculate heat of reaction, ΔHo , using standard heats of formation (given to you on appendix copies)

ΔHo = ΔHf (products) – ΔHf (reactants)

Write the thermochemical equation for the formation of sodium oxide.

Na(s) + O2(g) Na2O(s)

B. Standard Heats of Formation

4 2 ΔH=

2 ½ -416 kJ

B. Standard Heats of Formation

What is the standard heat of reaction of CO(g) with O2(g) to form CO2 (g)?

2CO(g) + O2(g) → 2CO2(g) ΔH = ?

2(ΔHfCO2) – (2(ΔHfCO) + ΔHfO2) =

2(-393.5) – (2(-110.5) + 0)

ΔH =-566.0 kJ/mol

B. Standard Heats of Formation

CaCO3(s) → CaO(s) + CO2 (g) ΔH = ?

(ΔHfCaO + ΔHfCO2) – (ΔHfCaCO3) =

(-635 + -393.5) – (-1207)

ΔH =179 kJ/mol

B. Standard Heats of Formation

What is the standard heat of reaction for the combustion of methane?

CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) ΔH = ?

(2(ΔHfH2O) + ΔHfCO2) – (ΔHfCH4 + 2(ΔHfO2)) =

(2(-242) + -393.5) – (-75 + 2(0))

ΔH =-803 kJ/mol