For the
SAT II Chemistry test, you’ll haveto be familiar with certain aspects of chemical reactions, such asequilibrium and reaction rate. The
reaction rate is a measureof the change in the concentration of reactants or products over timein a chemical reaction. Four main external conditions affect reactionrate. The first is the concentration of reactants. Generally speaking,if we increase the concentration of one or more reactants, the reactionwill go more quickly. This is simple because the more molecules, themore collisions between molecules, and the faster the reaction will go.
The second factor that influences reactionrate is temperature. The higher the temperature of the reaction, themore quickly it will proceed. At higher temperatures, the molecules aremoving around more quickly (they have more kinetic energy); this meansthey will collide with each other with more energy, and it’s morelikely that they will overcome the activation energy needed to startthe reaction. It’s a general rule of thumb that a 10˚C increase intemperature will double the reaction rate.
The addition of a catalyst will also speed up a chemical reaction. A
catalystspeeds up the rate of reaction by lowering the activation energy.Biological catalysts are known as enzymes. The only other importantthing you need to remember about catalysts is that they are notconsumed in the course of the reaction.
The final factor that affects certainreactions is the physical state of the reactants. For example, if youmix two gases or two liquids, this represents a homogenous reaction,but if reactants are in different phases, for example, if one is a gasand one is a liquid, then the reaction area is limited to the areawhere they touch each other, and the larger this area, the faster thereaction will proceed. For example, consider a teaspoon of saltdissolving in water. If you were to dump the salt into the beaker ofwater and let it float to the bottom without stirring it, it would takemuch longer for it to dissolve than if you stirred the solution.
Now let’s quickly go through those factors that influence reaction rate again:
- Concentration of the reactants
- Temperature
- Presence of a catalyst
- Physical state of the reactants
