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Chapter 9 Chemical Quantities in Reactions

9.5

Energy in Chemical Reactions

Cold packs use an endothermic reaction.

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Heat of Reaction

The heat of reaction,• is the amount of heat absorbed or

released during a reaction at constant pressure

• is the difference in the energy of the reactants and the products

• is shown as the symbol ΔH ΔH = Hproducts − Hreactants

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Endothermic Reactions

In an endothermic reaction,• heat is absorbed• the sign of ΔH is +• the energy of the products is

greater than the energy of the reactants

• heat is a reactant

N2(g) + O2 (g) + 181 kJ 2NO(g)

ΔH = +181 kJ (heat added)

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Exothermic Reactions

In an exothermic reaction,• heat is released• the sign of ΔH is -• the energy of the products is

less than the energy of the reactants

• heat is a product

C(s) + O2(g) CO2(g) + 394 kJ

ΔH = –394 kJ/mol (heat released)

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Summary

Reaction Energy Change Heat Sign of

ΔH

Endothermic Heat absorbed Reactant +

Exothermic Heat released Product ─

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Learning Check

Identify each reaction as

(Ex) exothermic or (En) endothermic.

A. N2(g)+ 3H2(g) 2NH3(g) + 92 kJ

B. CaCO3(s) + 556 kJ CaO(s) + CO2(g)

C. 2SO2(g) + O2(g) 2SO3(g) + heat

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Solution

Identify each reaction as

(Ex) exothermic or (En) endothermic.

(Ex) A. N2(g)+ 3H2(g) 2NH3(g) + 92 kJ

(En) B. CaCO3(s) + 556 kJ CaO(s) + CO2(g)

(Ex) C. 2SO2(g) + O2(g) 2SO3(g) + heat

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Calculations Using Heat of Reaction

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Heat Calculations for Reactions

In the reaction

N2(g) + O2 (g) 2NO(g) ΔH = +181 kJ

181 kJ is absorbed when 1 mol of N2 and 1 mol of O2

react to produce 2 mol of NO.

N2(g) + O2 (g) + 181 kJ 2NO(g)

This can be written as conversion factors.181 kJ 181 kJ 181 kJ

1 mol N2 1 mol O2 2 mol NO

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Heat Calculations for Reactions (continued)

N2(g) + O2 (g) + 181kJ 2NO(g)

If 15.0 g of NO is produced, how many kJ was

absorbed?

1) 1400 kJ

2) 90 kJ

3) 45 kJ

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Solution

STEP 1 List given and needed data for the equation.

Given: 15.0 g of NO produced

ΔH = 180 kJ/2 mol of NO

Need: kJ absorbed

STEP 2 Write a plan using heat of reaction and any molar mass needed.

Plan: g of NO moles of NO kJ

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Solution (continued)STEP 3 Write the conversion factors

including heat of reaction.

2 mol of NO = 180 kJ

180 kJ and 2 mol NO

2 mol NO 180 kJ

1 mol of NO = 30.01 g of NO

1 mol NO and 30.01 g NO

30.01 g NO 1 mol NO

STEP 4 Set up the problem.

15.0 g NO x 1 mol NO x 180 kJ = 45 kJ (3)

30.01 g NO 2 mol NO

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Learning Check

How many grams of O2 react if 1280 kJ is released

in the following reaction?

CH4(g) + 2O2(g) CO2(g) + 2H2O(l) ΔH = -890 kJ

1) 92.0 g of O2

2) 46.0 g of O2

3) 2.87 g of O2

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Solution

STEP 1 List given and needed data for the equation.

Given 1280 kJ Need grams of O2

STEP 2 Write a plan using heat of reaction and any molar mass needed.

kilojoules moles of O2 grams of O2

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Solution (continued)

STEP 3 Write the conversion factors including heat of reaction.

2 mol of O2 = 890 kJ

890 kJ and 2 mol O2

2 mol O2 890 kJ

1 mol of O2 = 30.01 g of O2

1 mol O2 and 32.00 g O2

32.00 g O2 1 mol O2

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Solution (continued)

STEP 4 Set up the problem.

1280 kJ x 2 mol O2 x 32.00 g O2 = 92.0 g of O2 (1)

890 kJ 1 mol O2