1. Chapter 9 Chemical Quantities in Reactions
9.5 Energy in Chemical Reactions
Learning Goal Given the heat of reaction (enthalpy change), calculate the loss or gain of heat for an exothermic or endothermic reaction.
© 2014 Pearson Education, Inc.

2. Energy Units, Chemical Reactions
Most reactions involve the loss or gain of energy.
To determine the change in energy for a reaction, we examine the energy of the reactants and products.
The SI unit used for representing energy is the joule (J) or kilojoules (kJ).
© 2014 Pearson Education, Inc.

3. Heat of Reaction, Enthalpy Change
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

4. 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
H2(g) + Cl2(g)  2HCl(g) kJ
ΔH = –185 kJ/mol (heat released)

5. Exothermic Reactions
The energy of the products is less than the energy of the reactants.

6. 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) kJ  2NO(g)
ΔH = +181 kJ (heat added)

7. Endothermic Reactions
The energy of the products is greater than the energy of the reactants.

8. Exothermic, Endothermic Reactions

9. Learning Check
Identify each reaction as exothermic or endothermic.
A. N2(g) + 3H2(g)  2NH3(g) kJ
B. CaCO3(s) kJ  CaO(s) + CO2(g)
C. 2SO2(g) + O2(g)  2SO3(g) + heat

10. Identify each reaction as exothermic or endothermic.
Solution
Identify each reaction as exothermic or endothermic.
N2(g) + 3H2(g)  2NH3(g) kJ
Exothermic
CaCO3(s) kJ  CaO(s) + CO2(g)
Endothermic
C. 2SO2(g) + O2(g)  2SO3(g) + heat

11. Calculations of Heat in Reactions

12. Calculations of Heat in Reactions
In the reaction
N2(g) + O2(g)  2NO(g) ΔH = kJ
how much heat (kJ) is absorbed when 1.65 grams of nitrogen monoxide gas is produced?
Step 1 State the given and needed quantities.
Given: grams of NO,
ΔH = kJ
Need: heat absorbed in kJ

13. N2(g) + O2(g)  2NO(g) ΔH = +181 kJ 1.65 g ? kJ
Learning Check
N2(g) + O2(g)  2NO(g) ΔH = kJ
1.65 g ? kJ
Step 2 Write a plan using the heat of reaction and any molar mass needed.
grams of NO
moles of NO
kilojoules of
energy
Molar Mass
Heat of Reaction

14. Calculations of Heat in Reactions
N2(g) + O2(g)  2NO(g) ΔH = kJ
1.65 g ? kJ
Step 3 Write the conversion factors, including heat of reaction.
1 mole of NO = g of NO

15. Calculations of Heat in Reactions
N2(g) + O2(g)  2NO(g) ΔH = kJ
1.65 g ? kJ
Step 3 Write the conversion factors, including heat of reaction.
2 moles of NO = +181 kJ

16. Calculations of Heat in Reactions
N2(g) + O2(g)  2NO(g) ΔH = kJ
1.65 g ? kJ
Step 4 Set up the problem to calculate the heat.

17. Learning Check
How many kilojoules of energy are absorbed when 23.0 g of solid ammonium nitrate is dissolved in water?
NH4NO3(s) + 26 kJ  NH4NO3(aq)
H2O

18. Step 1 State the given and needed quantities. Given: 23.0 g of NH4NO3
Solution
NH4NO3(s) + 26 kJ  NH4NO3(aq)
23.0 g ? kJ
Step 1 State the given and needed quantities Given: g of NH4NO3
Need: heat absorbed
H2O

19. Solution NH4NO3(s) + 26 kJ  NH4NO3(aq) 23.0 g ? kJ
Step 2 Write a plan using the heat of reaction and any molar mass needed.
Molar Mass
Heat of Reaction
grams of
NH4NO3
moles of
NH4NO3
kilojoules

20. Solution
NH4NO3(s) + 26 kJ  NH4NO3(aq)
23.0 g ? kJ
Step 3 Write the conversion factors, including heat of reaction.
1 mole of NH4NO3 = g of NH4NO3

21. Solution
NH4NO3(s) + 26 kJ  NH4NO3(aq)
23.0 g ? kJ
Step 4 Set up the problem to calculate the heat.

22. Concept Check