Chapter 9 Chemical Quantities in Reactions

1. Cold packs use an endothermic reaction. Chapter 9 Chemical Quantities in Reactions 9.5 Energy in Chemical Reactions

2. Heat of Reaction 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

3. Endothermic Reactions In an endothermicreaction, • 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)

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 C(s) + O2(g) CO2(g) + 394 kJ ΔH = –394 kJ/mol (heat released)

5. Summary Reaction Energy Change Heat Sign of ΔH Endothermic Heat absorbed Reactant + Exothermic Heat released Product ─

6. Learning Check Identify each reaction as (Ex) exothermic or (En) 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

7. Solution Identify each reaction as (Ex) exothermic or (En) endothermic. (Ex) A. N2(g)+ 3H2(g) 2NH3(g) + 92 kJ (En) B. CaCO3(s) + 556 kJ CaO(s) + CO2(g) (Ex) C. 2SO2(g) + O2(g) 2SO3(g) + heat

8. Calculations Using Heat of Reaction

9. Heat Calculations for Reactions In the reaction N2(g) + O2(g)2NO(g) ΔH = +181 kJ 181 kJ is absorbed when 1 mol of N2 and 1 mol of O2 react to produce 2 mol of NO. N2(g) + O2(g) + 181 kJ 2NO(g) This can be written as conversion factors. 181 kJ181 kJ 181 kJ 1 mol N2 1 mol O2 2 mol NO

10. Heat Calculations for Reactions (continued) N2(g) + O2(g) + 181kJ 2NO(g) If 15.0 g of NO is produced, how many kJ was absorbed? 1) 1400 kJ 2) 90 kJ 3) 45 kJ

11. Solution STEP 1 List given and needed data for the equation. Given: 15.0 g of NO produced ΔH = 180 kJ/2 mol of NO Need: kJ absorbed STEP 2Write a plan using heat of reaction and any molar mass needed. Plan: g of NO moles of NO kJ

12. Solution (continued) STEP 3Write the conversion factors including heat of reaction. 2 mol of NO = 180 kJ 180 kJ and 2 mol NO 2 mol NO 180 kJ 1 mol of NO = 30.01 g of NO 1 mol NO and 30.01 g NO 30.01 g NO 1 mol NO STEP 4Set up the problem. 15.0 g NO x 1 mol NO x 180 kJ = 45 kJ (3) 30.01 g NO 2 mol NO

13. Learning Check How many grams of O2 react if 1280 kJ is released in the following reaction? CH4(g) + 2O2(g) CO2(g) + 2H2O(l) ΔH = -890 kJ 1) 92.0 g of O2 2) 46.0 g of O2 3) 2.87 g of O2

14. Solution STEP 1 List given and needed data for the equation. Given 1280 kJ Need grams of O2 STEP 2Write a plan using heat of reaction and any molar mass needed. kilojoules moles of O2 grams of O2

15. Solution (continued) STEP 3Write the conversion factors including heat of reaction. 2 mol of O2 = 890 kJ 890 kJ and 2 mol O2 2 mol O2 890 kJ 1 mol of O2 = 30.01 g of O2 1 mol O2 and 32.00 g O2 32.00 g O2 1 mol O2

16. Solution (continued) STEP 4Set up the problem. 1280 kJ x 2 mol O2 x 32.00 g O2 = 92.0 g of O2 (1) 890 kJ 1 mol O2