Section 3: Controlling Chemical Reactions

• Objectives:explain that every

chemical reaction requires activation energy to get started

list factors that control the rate of chemical reactions

Where does the energy come fromto begin to light the match?

I. Getting Reactions Started

A. Activation energy1. energy needed to start a

chemical reaction.

Here the friction of striking a matchheats the substances on the match,breaking bonds in the reactant.

B. Endothermic reactions needs a supply of energy to keep it going.

1. The energy level of the product materials is higher than the starting reactant materials.

a. Endothermic reactions absorb energy.

C. Exothermic reactions do not require additional energy to complete the reaction.

1. Energy level of product materials is lower than the energy level of the reactants materials.

II. Rates of Chemical Reactions:A. Factors affecting rates of reactions:

1.Concentrationa. the amount of one material in a given amount of

another material

Bubbles of hydrogen form when magnesium reacts with an acid. The test tube on the left has a lower concentration of acid than the test tube on the right.

2. Temperaturea. increasing or decreasing temperature can control the rate of

a reaction.

The light stick on the right glows brighter than the one on the leftbecause the higher temperature causes the rate of the reaction to increase.

OR

Chemical reactions can be slowed down by decreasing the temperature.Refrigerated food does not spoil as quickly because the cooltemperatures slow down the chemical reactions of bacteria.

3. Surface Area

The concrete wall of this grain elevator in Kansas were blown apartby an explosion when grain particles and oxygen above the storedwheat exploded. Grain dust has a much greater surface area exposedto air than the top surface of the pile of grain does.

4. Catalystsa. substance that speeds up a reaction without being permanently changed.

Hydrogen peroxide slowly decomposes to form water and oxygen.A. manganese dioxide, the black powder, speeds up this reaction, causing bubbles to form faster. B. Since it is not used in the reaction, MnO2 is a catalyst.

Catalytic converter

This catalytic converter contains platinum and palladium-twocatalysts that increase the rate of reactions that make the car’sexhaust less polluting.

5. Inhibitora. substance that slows down or stops a chemical reaction.

Chemical preservatives are added to foods to inhibit or slow spoilage from bacteria and mold.