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.

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.

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

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) + 185 kJ ΔH = –185 kJ/mol (heat released)

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

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)

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

Exothermic, Endothermic Reactions

Learning Check Identify each reaction as exothermic or 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

Identify each reaction as exothermic or endothermic. Solution Identify each reaction as exothermic or endothermic. N2(g) + 3H2(g)  2NH3(g) + 92 kJ Exothermic CaCO3(s) + 556 kJ  CaO(s) + CO2(g) Endothermic C. 2SO2(g) + O2(g)  2SO3(g) + heat

Calculations of Heat in Reactions

Calculations of Heat in Reactions In the reaction N2(g) + O2(g)  2NO(g) ΔH = +181 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: 1.65 grams of NO, ΔH = +181 kJ Need: heat absorbed in kJ

N2(g) + O2(g)  2NO(g) ΔH = +181 kJ 1.65 g ? kJ Learning Check N2(g) + O2(g)  2NO(g) ΔH = +181 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

Calculations of Heat in Reactions N2(g) + O2(g)  2NO(g) ΔH = +181 kJ 1.65 g ? kJ Step 3 Write the conversion factors, including heat of reaction. 1 mole of NO = 30.01 g of NO

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

Calculations of Heat in Reactions N2(g) + O2(g)  2NO(g) ΔH = +181 kJ 1.65 g ? kJ Step 4 Set up the problem to calculate the heat.

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

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: 23.0 g of NH4NO3 Need: heat absorbed H2O

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

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 = 80.06 g of NH4NO3

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

Concept Check