1 Chapter 5 Chemical Reactions 5.8 Energy in Chemical Reactions

2 Collision Theory of Reactions A chemical reaction occurs when collisions between molecules have sufficient energy to break the bonds in the reactants. bonds between atoms of the reactants (N 2 and O 2 ) are broken and new bonds (NO) can form. Copyright © 2005 by Pearson Education, Inc. Publishing as Benjamin Cummings

3 Activation Energy The activation energy is the minimum energy needed for a reaction to take place. When a collision provides energy equal to or greater than the activation energy, product can form.

4 C(s) + 2 H 2 (g) CH 4 (g) + 18 kcal In an exothermic reaction, heat is released. the energy of the products is less than the energy of the reactants. heat is a product. Exothermic Reactions Copyright © 2005 by Pearson Education, Inc. Publishing as Benjamin Cummings

5 Endothermic Reactions 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). Copyright © 2005 by Pearson Education, Inc. Publishing as Benjamin Cummings N 2 (g) + O 2 (g) kcal 2NO(g)

6 Summary Reaction Energy Heat Type Change in Reaction Endothermic Heat absorbed Reactant Exothermic Heat released Product

7 Learning Check Identify each reaction as 1) exothermic or 2) endothermic. A. N H 2 2 NH kcal B. CaCO kcalCaO + CO 2 C. 2 SO 2 + O 2 2 SO 3 + heat

8 Solution Identify each reaction as 1) exothermic or 2) endothermic. 1 A. N H 2 2 NH kcal 2 B. CaCO kcal CaO + CO 2 1 C. 2 SO 2 + O 2 2 SO 3 + heat

9 Rate of Reaction Reaction rate is the speed at which reactant is used up. is the speed at which product forms. increases when temperature rises because reacting molecules move faster providing more colliding molecules with energy of activation.

10 Reaction Rate and Catalysts A catalyst increases the rate of a reaction. lowers the energy of activation. is not used up during the reaction.

11 Learning Check State the effect of each on the rate of reaction as: 1) increases 2) decreases3) no change A. increasing the temperature. B. removing some of the reactants. C. adding a catalyst. D. placing the reaction flask in ice. E. increasing the concentration of one of the reactants.

12 Solution State the effect of each on the rate of reaction as: 1) increases 2) decreases3) no change 1 A. increasing the temperature. 2 B. removing some of the reactants. 1 C. adding a catalyst. 2 D. placing the reaction flask in ice. 1 E. increasing the concentration of one of the reactants.

13 Learning Check Indicate the effect of each factor listed on the rate of the following reaction as 1) increases 2) decreases 3) none. 2 CO(g) + O 2 (g) 2 CO 2 (g) A. raising the temperature B. adding O 2 C. adding a catalyst D. lowering the temperature

14 Solution Indicate the effect of each factor listed on the rate of the following reaction as 1) increases 2) decreases 3) none: 2CO(g) + O 2 (g) 2CO 2 (g) 1 A. raising the temperature 1 B. adding O 2 1 C. adding a catalyst 2 D. lowering the temperature

15 Summary of Factors That Increase Reaction Rate