1. Section 4: Chemical Reactions and Energy
Chapter 24: Chemical Reactions
Section 4: Chemical Reactions and Energy

2. Section 4: Chemical Changes
Chemical reactions involve energy exchange
Breaking chemical bonds requires energy
Forming chemical bonds releases energy
More energy out (ex-)
Exergonic reactions: energy required to break bonds is less than the energy released from new bonds that are formed; energy given off is usually light.

3. More energy out (ex-) More energy in (end-)
Exothermic reactions: energy given off in the form of heat.
More energy in (end-)
Endergonic reactions: more energy is required to break bonds than to form new ones; need energy for the reaction to occur.
If the energy needed is heat, the reaction is endothermic.

4. Changing the Speed of Reactions
Classify the following as endergonic, exergonic, endothermic or exothermic:
Sunlight decomposes hydrogen peroxide into water and oxygen
Endergonic
Sodium hydroxide is added to hydrochloric acid and gets warm
Exothermic
A campfire
Exergonic and exothermic

5. Changing the Speed of Reactions
A catalyst speeds up a chemical reaction without being permanently changed itself.
An inhibitor prevents or slows a chemical reaction or interferes with a catalyst’s reaction. (lemon juice on apples to prevent browning; food preservatives such as BHT)
Ever wonder why the shelf life of a Twinkie is years long?

6. Review Questions, p
Answer these questions (p.754) for tomorrow:
Discuss the differences between exergonic and endergonic reactions.
What happens to a catalyst in a reaction?
Crackers containing BHT stay fresh longer than those without it. Explain why.
Classify the reaction that makes a firefly glow in terms of energy input or output.
To develop a product that warms people’s hands, would you choose an exothermic or endothermic reaction to use? Why?