Two types of reactions, based on energy.

Exothermic reaction - a reaction that releases energy. The reactants have more energy than the products. Energy must be released as the products form. The container in which an exothermic reaction is taking place will feel hot.

Endothermic reaction - a reaction that absorbs energy. The products have more energy than the reactants. Energy must be absorbed as the products form. The container in which an endothermic reaction is taking place will feel cold.

Activation energy - the energy needed to start a chemical reaction.

It is very low for some reactions and very high for others.

Reaction rate - the rate at which products form or reactants are used up.

The collision theory relates molecule collisions to reaction rate. According to this theory, reacting molecules must collide with sufficient energy if they are to form products.

Reaction rate is affected by these four things:

Concentration: an increase in concentration means there are more particles. More particles means there will be more collisions. This should increase the reaction rate.

Surface area: when one of the reactants is a solid, the reaction can only take place at the surface of the solid. Breaking the solid into smaller pieces will increase the surface area exposed to the other reactant. This should increases the reaction rate.

Temperature: since temperature is a measure of the motion of particels, increasing the temperature will cause the particles to move faster. When particles move faster, more collisions occur and the collisions are more violent. This should increase the reaction rate.

Catalysts: catalysts are substances that change the rate of a chemical reaction without being changed in the reaction. Catalysts are most often used to speed up a chemical reaction. They do this by changing the steps needed between the reactants and the formation of the products. If the number of steps can be reduced, the reaction rate should increase.

1. Work with your table to find an exothermic reaction that you can produce in class. Check the SHS chemical inventory to see if the chemicals are available.
   This reaction may not be potentially explosive
   like sodium reacting with water.

2. With your table, find an endothermic reaction that you can produce in class. Check the inventory for these chemicals.
3. Write a balanced equation for both reactions and describe what you expect to see happen.
4. Have these approved by your facilitator and produce both these reactions.

Day 2

Sodium metal reacts with water. Is the reaction endothermic or exothermic?

Biochemist

Test Your Concept Understanding: Today's automobiles have a catalytic converter in the exhaust system to reduce pollutants. Use the Internet to find out what chemical reaction takes place in a catalytic converter. The compound H2PtCl6 may be involved. Write the equation for the chemical reaction that takes place in a catalytic converter. How does a catalytic converter reduce air pollution?

8th Grade Science

Activation energy is the energy needed to start a chemical reaction.
All chemical reactions require energy to begin. Enough energy is available from the environment to start some reactions. For others, additional energy must be added before the reaction will begin.

When sodium metal reacts with water, heat is produced. This makes the reaction exothermic