CK-12-Chemistry Intermediate

http://www.ck12.org Chapter 17. Thermochemistry

TABLE17.3:(continued)

Substance ∆Hf° (kJ/mol) Substance ∆Hf° (kJ/mol)

Br 2 (g) 30.91 NH 3 (g) −46.3

C (s, graphite) 0 NO(g) 90.4

C (s, diamond) 1.90 NO 2 (g) 33.85

CH 4 (g) −74.85 NaCl(s) −411.0

C 2 H 5 OH(l) −276.98 O 3 (g) 142.2

CO(g) −110.5 P(s, white) 0

CO 2 (g) −393.5 P(s, red) −18.4

CaO(s) −635.6 PbO(s) −217.86

CaCO 3 (s) −1206.9 S(rhombic) 0

HCl(g) −92.3 S(monoclinic) 0.30

CuO(s) −155.2 SO 2 (g) −296.1

CuSO 4 (s) −769.86 SO 3 (g) −395.2

Fe 2 O 3 (s) −822.2 H 2 S(g) −20.15

H 2 O(g) −241.8 SiO 2 (s, quartz) −910.7

H 2 O(l) −285.8 ZnCl 2 (s) −415.89

An application of Hess’s law allows us to use standard heats of formation to indirectly calculate the heat of reaction
for any reaction that occurs at standard conditions. An enthalpy change that occurs specifically under standard
conditions is called the standard enthalpy (or heat) of reaction and is given the symbol∆H°. The standard heat of
reaction can be calculated by using the following equation.

∆H◦=Σn∆H◦f(products)−Σn∆H◦f(reactants)

The symbolΣis the Greek letter sigma and means “the sum of.” The standard heat of reaction is equal to the sum
of all the standard heats of formation of the products minus the sum of all the standard heats of formation of the
reactants. The symbol “n” signifies that each heat of formation must first be multiplied by its coefficient in the
balanced equation. Sample Problem 17.8 illustrates the use of this equation.

Sample Problem 17.8: Calculating Standard Heat of Reaction

Calculate the standard heat of reaction (∆H°) for the reaction of nitrogen monoxide gas with oxygen to form nitrogen
dioxide gas.

Step 1: List the known quantities and plan the problem.

Known

• ∆Hf° for NO(g) = 90.4 kJ/mol


• ∆Hf° for O 2 (g) = 0 (pure element in its standard state)


• ∆Hf° for NO 2 (g) = 33.85 kJ/mol

Unknown

• ∆H° =? kJ

First, write the balanced equation for the reaction. Then, apply the equation to calculate the standard heat of reaction
from the standard heats of formation.

Step 2: Solve.

The balanced equation is: 2NO(g)+O 2 (g)→2NO 2 (g)