Chapter 9 Lecture Basic Chemistry Fourth Edition 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. Chapter 9 Chemical Quantities in 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. Energy Units, Chemical Reactions

© 2014 Pearson Education, Inc. 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 = H products − H reactants Heat of Reaction, Enthalpy Change

© 2014 Pearson Education, Inc. 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 H 2 (g) + Cl 2 (g)  2HCl(g) kJ ΔH = –185 kJ/mol (heat released)

© 2014 Pearson Education, Inc. The energy of the products is less than the energy of the reactants. Exothermic Reactions

© 2014 Pearson Education, Inc. 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 N 2 (g) + O 2 (g) kJ  2NO(g) ΔH = +181 kJ (heat added) Endothermic Reactions

© 2014 Pearson Education, Inc. The energy of the products is greater than the energy of the reactants. Endothermic Reactions

© 2014 Pearson Education, Inc. Exothermic, Endothermic Reactions

© 2014 Pearson Education, Inc. Identify each reaction as exothermic or endothermic. A. N 2 (g) + 3H 2 (g)  2NH 3 (g) + 92 kJ B. CaCO 3 (s) kJ  CaO(s) + CO 2 (g) C. 2SO 2 (g) + O 2 (g)  2SO 3 (g) + heat Learning Check

© 2014 Pearson Education, Inc. Identify each reaction as exothermic or endothermic. A.N 2 (g) + 3H 2 (g)  2NH 3 (g) + 92 kJ Exothermic B.CaCO 3 (s) kJ  CaO(s) + CO 2 (g) Endothermic C. 2SO 2 (g) + O 2 (g)  2SO 3 (g) + heat Exothermic Solution

© 2014 Pearson Education, Inc. Calculations of Heat in Reactions

© 2014 Pearson Education, Inc. In the reaction N 2 (g) + O 2 (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 Calculations of Heat in Reactions

© 2014 Pearson Education, Inc. N 2 (g) + O 2 (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. Learning Check grams of NO Molar Mass moles of NO kilojoules of energy Heat of Reaction

© 2014 Pearson Education, Inc. N 2 (g) + O 2 (g)  2NO(g) ΔH = +181 kJ 1.65 g ? kJ Step 3 Write the conversion factors, including heat of reaction. 1 mole of NO = g of NO Calculations of Heat in Reactions

© 2014 Pearson Education, Inc. N 2 (g) + O 2 (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

© 2014 Pearson Education, Inc. N 2 (g) + O 2 (g)  2NO(g) ΔH = +181 kJ 1.65 g ? kJ Step 4 Set up the problem to calculate the heat. Calculations of Heat in Reactions

© 2014 Pearson Education, Inc. How many kilojoules of energy are absorbed when 23.0 g of solid ammonium nitrate is dissolved in water? NH 4 NO 3 (s) + 26 kJ  NH 4 NO 3 (aq) Learning Check H2OH2O

© 2014 Pearson Education, Inc. Solution NH 4 NO 3 (s) + 26 kJ  NH 4 NO 3 (aq) 23.0 g ? kJ Step 1 State the given and needed quantities. Given: 23.0 g of NH 4 NO 3 Need: heat absorbed H2OH2O

© 2014 Pearson Education, Inc. NH 4 NO 3 (s) + 26 kJ  NH 4 NO 3 (aq) 23.0 g ? kJ Step 2 Write a plan using the heat of reaction and any molar mass needed. Solution grams of NH 4 NO 3 Molar Mass moles of NH 4 NO 3 kilojoules Heat of Reaction

© 2014 Pearson Education, Inc. NH 4 NO 3 (s) + 26 kJ  NH 4 NO 3 (aq) 23.0 g ? kJ Step 3 Write the conversion factors, including heat of reaction. 1 mole of NH 4 NO 3 = g of NH 4 NO 3 Solution

© 2014 Pearson Education, Inc. NH 4 NO 3 (s) + 26 kJ  NH 4 NO 3 (aq) 23.0 g ? kJ Step 4 Set up the problem to calculate the heat. Solution

© 2014 Pearson Education, Inc. Concept Check