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.

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)

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

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) + 185 kJ ΔH = −185 kJ

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) + 180 kJ  2NO(g) ΔH = +180 kJ

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) + 572 kJ  2H2(g) + O2(g) We can write the following conversion factors: Core Chemistry Skill Using the Heat of Reaction

Guide to Calculations Using Heat of Reaction

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

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 50.0 g of NH3 heat released (kJ) ΔH = −92.2 kJ N2(g) + 3H2(g)  2NH3(g)

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 NH3 of NH3 Molar mass Heat of reaction

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.

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. × ×

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

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

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