1. 7.9 Energy in Chemical Reactions
Almost every chemical reaction involves the loss or gain of energy. When cold packs are activated, an endothermic reaction
takes place.
Learning Goal Given the heat of reaction the rate of a reaction, calculate the loss or gain of heat for an exothermic or endothermic reaction.

2. Energy Units for Chemical Reactions
The SI unit for energy is the joule (J).
Often the unit of kilojoules (kJ) is used to illustrate the amount of energy lost or gained in a chemical reaction.
1 kilojoule (kJ) = 1000 joules (J)

3. Heat of Reaction
The heat of reaction is the amount of heat absorbed or released during a reaction that takes place at constant pressure. The change in energy occurs when
reactants interact.
bonds break apart.
products are formed.
The heat of reaction or enthalpy change, symbol ΔH, is the difference between the enthalpy of the products and enthalpy of the reactants.
ΔH = Hproducts − Hreactants

4. Exothermic Reactions Released
In an exothermic reaction,
heat is released.
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

5. Exothermic Reactions Released
In an endothermic reaction,
heat is absorbed.
the energy of the products is greater than the energy of the reactants.
heat is a reactant (added).
N2(g) + O2(g) kJ  2NO(g)
ΔH = +180 kJ

6. Calculating Heat Changes
The value of ΔH refers to the heat change for each substance in the balanced equation.
For the decomposition reaction,
2H2O(l)  2H2(g) + O2(g) ΔH = +572 kJ
2H2O(l) kJ  2H2(g) + O2(g)
We can write the following conversion factors:
Core Chemistry Skill Using the Heat of Reaction

7. Guide to Calculations Using Heat of Reaction

8. Study Check
How much heat in kJ is released when nitrogen and hydrogen react to form 50.0 g of ammonia?
N2(g) + 3H2(g)  2NH3(g) ΔH = −92.2 kJ

9. Solution
How much heat in kJ is released when nitrogen and hydrogen react to form 50.0 g of ammonia?
N2(g) + 3H2(g)  2NH3(g) ΔH = −92.2 kJ
STEP 1 State the given and needed quantities.
ANALYZE Given Need
THE PROBLEM g of NH heat released (kJ)
ΔH = −92.2 kJ
N2(g) + 3H2(g)  2NH3(g)

10. Solution
How much heat in kJ is released when nitrogen and hydrogen react to form 50.0 g of ammonia?
N2(g) + 3H2(g)  2NH3(g) ΔH = −92.2 kJ
STEP 2 Write a plan using the heat of reaction and any molar mass needed.
grams moles kilojoules
of NH of NH3
Molar mass
Heat of reaction

11. Solution
How much heat in kJ is released when nitrogen and hydrogen react to form 50.0 g of ammonia?
N2(g) + 3H2(g)  2NH3(g) ΔH = −92.2 kJ
STEP 3 Write the conversion factors including heat of reaction.

12. Solution
How much heat in kJ is released when nitrogen and hydrogen react to form 50.0 g of ammonia?
N2(g) + 3H2(g)  2NH3(g) ΔH = −92.2 kJ
STEP 4 Set up the problem to calculate the heat. × ×

13. Chemistry Link to Health: Cold Packs and Hot Packs
Inside the cold pack, solid ammonium nitrate (NH4NO3) is separated from the compartment containing water. When the cold pack is hit or squeezed, the compartments break and the NH4NO3 mixes with the water in an endothermic reaction. NH4NO3(s) + 26 kJ  NH4NO3(aq)
H2O

14. Chemistry Link to Health: Cold Packs and Hot Packs
Inside the hot pack, solid calcium chloride (CaCl2) is separated from the compartment containing water. When the cold pack is hit or squeezed, the compartments break and the CaCl2 mixes with the water in an exothermic reaction. CaCl2(s)  CaCl2 (aq) + 82 kJ
H2O

15. Concept Map