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SAT化学练习:Equilibrium and Reaction Rates

2011-11-23 17:05

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  Factors That Affect Reaction Rates  For the SAT II Chemistry test, you’ll have to be familiar with certain aspects of chemical reactions, such as equilibrium and reaction rate. The reaction rate is a measure of the change in the concentration of reactants or products over time in a chemical reaction. Four main external conditions affect reaction rate. The first is the concentration of reactants. Generally speaking, if we increase the concentration of one or more reactants, the reaction will go more quickly. This is simple because the more molecules, the more collisions between molecules, and the faster the reaction will go.   The second factor that influences reaction rate is temperature. The higher the temperature of the reaction, the more quickly it will proceed. At higher temperatures, the molecules are moving around more quickly (they have more kinetic energy); this means they will collide with each other with more energy, and it’s more likely that they will overcome the activation energy needed to start the reaction. It’s a general rule of thumb that a 10 C increase in temperature will double the reaction rate.   The addition of a catalyst will also speed up a chemical reaction. A catalyst speeds up the rate of reaction by lowering the activation energy. Biological catalysts are known as enzymes. The only other important thing you need to remember about catalysts is that they are not consumed in the course of the reaction.   The final factor that affects certain reactions is the physical state of the reactants. For example, if you mix two gases or two liquids, this represents a homogenous reaction, but if reactants are in different phases, for example, if one is a gas and one is a liquid, then the reaction area is limited to the area where they touch each other, and the larger this area, the faster the reaction will proceed. For example, consider a teaspoon of salt dissolving in water. If you were to dump the salt into the beaker of water and let it float to the bottom without stirring it, it would take much 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

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