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Chapter 5Chemical Reactions

5.8 Energy in Chemical Reactions

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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

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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.

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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

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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)

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Summary

Reaction Energy Heat Type Change in Reaction

Endothermic Heat absorbed Reactant

Exothermic Heat released Product

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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

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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

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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.

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Reaction Rate and Catalysts

A catalyst

• increases the rate of a reaction.

• lowers the energy of activation.

• is not used up during the reaction.

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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.

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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.

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Learning Check

Indicate the effect of each factor listed on the rate of thefollowing 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

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Solution

Indicate the effect of each factor listed on the rate of thefollowing 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

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Summary of Factors That Increase Reaction Rate