SIDE NOTES CH.12 - MNHS AP Chemistry Chapter 12 Chemical...

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MNHS Chapter 12 AP Chemistry Chemical Kinetics **Notes have been derived from Zumdahl 4 th ed. - All page and table references are made to this edition. Chemical Kinetics - is the area of chemistry that concerns reaction rates. Section 12.1 - Reaction Rates Chemical kinetic deals with the speed at which these changes occur. The speed, or rate, of a process is defined as the change in a given quantity over specific period of time. Reaction Rate - change in concentration of a reactant or product per unit time. [ ] 2 1 2 1 A Concentration of A at time t Concentration of A at time t Rate = = t t t - - Note that a change can be positive (increase) or negative (decrease), for convenience, thus leading to a positive or negative reaction rate by this definition. For convenience, we will always define the rate as a positive quantity. So a negative sign will appear in front of our equation when we are focusing on the reactants. The equation will look like this: [ ] [ ] 1 A Rate = = k A t - Instantaneous rate - note that the rate is not constant, but decreases with time, so rate can measured for a given point in time. Rate = (slope of the tangent line) - When examining products of a reaction, rates are proportional to the coefficients represented in a balanced chemical equation. Section 12.2 - Rate Laws: An Introduction Chemical reactions are reversible. After a period of time, enough products accumulate so that the reverse reaction becomes important. At this point the concentration of the reactants depends on the difference in the rates of the forward and reverse reactions. This tends to complicate matters, so we will focus on the reaction soon after the reactants are mixed, before the products have had time to build up to significant levels. If we choose conditions where the reverse reaction can be neglected, the reaction rate will depend only on the concentrations of the reactants. Rate = k[ A ] n Rate law - the above expression shows how the rate depends on the concentration of the reactants. The proportionality constant " k ", called the rate constant, and " n ", called the order of the reactant, must both be determined by experiment. The order of a reactant can be an integer (including zero) or a fraction. Note two important points about the Rate law equation. 1. The concentrations of the products do not appear in the rate law because the reaction rate is being studied under conditions where the reverse reactions does not contribute to the overall rate.
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