1

Chapter 5 Chemical Reactions and Quantities

5.9 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 (N2 and O2)

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 H2(g) CH4(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

N2(g) + O2 (g) + 43.3 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. N2 + 3 H2 2 NH3 + 22 kcal B. CaCO3 + 133 kcal CaO + CO2

C. 2 SO2 + O2 2 SO3 + heat

8

Solution

Identify each reaction as 1) exothermic or 2) endothermic.

1 A. N2 + 3 H2 2 NH3 + 22 kcal 2 B. CaCO3 + 133 kcal CaO + CO2

1 C. 2 SO2 + O2 2 SO3 + 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) decreases 3) 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) decreases 3) 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) + O2(g) 2 CO2 (g)

A. raising the temperature B. adding O2

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) + O2(g) 2CO2 (g)

1 A. raising the temperature 1 B. adding O2

1 C. adding a catalyst 2 D. lowering the temperature

15

Summary of Factors That Increase Reaction Rate