Controlling Chemical Reactions 17-3

Learning Objectives  Describe the relationship of energy to chemical reactions.  List factors that control the rate of chemical reactions.

Key Terms 1.Exothermic reaction 2.Endothermic reaction 3.Activation energy 4.Concentration 5.Catalyst 6.Enzyme 7.Inhibitor

Energy in Chemical Reactions  Light is a type of energy.  Other forms of energy include electricity and the energy of a moving object.  Every chemical reaction involves a change of energy.  Some reactions release energy and others absorb energy.

EXOTHERMIC REACTION  The reaction between gasoline and oxygen in a car engine yields carbon dioxide, water, and other products.  It also releases a lot of energy.  A reaction that releases energy in the form of heat is called an exothermic reaction.

ENDOTHERMIC REACTION  When baking soda reacts with vinegar, the reaction takes heat from the solution, making it feel cooler.  This kind of reaction, which absorbs energy, is called an endothermic reaction.

Endothermic Reaction

 In an endothermic reaction, the energy of the products is greater than the energy of the reactants, which means energy is absorbed.  That’s what happened with the baking soda and vinegar.

Endothermic Reaction

 The energy of the products in an exothermic reaction is lower than the energy of the reactants.  In an exothermic reaction, energy is released. This is the case when gasoline burns.

Getting Reactions Started  All chemical reactions need energy to get started.  The activation energy is the MINIMUM amount of energy that has to be added to start a reaction.

Example  The reaction that makes water from H 2 and O 2 is very exothermic.  It gives off tremendous amount of energy. But if you make a mixture of H 2 and O 2, it could remain for years without any noticeable change.  All it would take to ignite a tiny amount of H 2 is an electric spark.  The spark is a source of activation energy.  After the first few molecules react, the large amount of energy released provides the activation energy for more molecules to react.

Rate of Chemical Reactions  Chemical reactions don’t all occur at the same rate.  Some reactions (like explosions) are very fast.  Others, like rusting of metal, are much slower.  The rate of a reaction is affected by such factors as concentration, surface area, and temperature.

Concentration  One way to increase the rate of a reaction is to increase the concentration of the reactants.  Concentration is the amount of one material in a given volume of another material.  Increasing the concentration of reactants makes more particles available to react.

Surface Area  When a solid reacts with a liquid of a gas, only particles that are on the surface of the solid come in contact with the other reactant.  When you break the solid into smaller pieces, you have increased the surface area of the solid.  More material is exposed, so the reaction happens faster.

Temperature  A third way to increase the rate of a reaction is to add heat.  When you heat something, its particles move faster.  Faster-moving particles increase the reaction rate in two ways.  First, they come in contact more often, which means there are more chances for a reaction to happen.

 Second, faster-moving particles have more energy.  This energy helps the reactions get over the activation energy hump.

Catalysts  Another way to control the rate of a reaction is to change the activation energy.  If you decrease the activation energy, the reaction happens faster.  A catalyst is a material that increases the rate of a reaction by lowering the activation energy.  Catalysts help with the reaction, but they are not reactants themselves. Catalysts are not permanently changed in the reaction.

ENZYMES  Your body contains biological catalysts called enzymes.  Enzymes provides a surface on which reactions take place.  This helps reactions happen at lower temperatures because it lowers activation energy.  Enzymes increase the reaction rate of chemical reactions necessary for life.

Inhibitors  Sometimes a reaction is more useful when it can be slowed down rather than speeded up.  A material used to decrease the rate of a reaction is called an inhibitor.