C401Ch12LN2 - Chapter 12: Chemical Kinetics, Part 2...

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Unformatted text preview: Chapter 12: Chemical Kinetics, Part 2 Zero-Order Reactions (or zeroth order) For a zero-order reaction , the rate is independent of the concentration of the reactant(s). (of course, there does have to be some reactant present in order for any rxn to occur) One example is the following rxn: As it is zero-order, the rate law is: But the rate is also equal to the change in a reactant over the change in time, so we may write: Rearranging we get: Integrating we get: Rearranging to get an equation of a line gives us this: A plot of [ A ] t versus t is a straight line with slope - k and intercept [ A ] 0 (sometimes called [A] i . Here are typical examples of zero-order graphs: Half-life , t 1/2 , is the time required for the concentration of a reactant to decrease to half its original value. That is, half life, t 1/2 , is the time required for [ A ] to reach 0.5[ A ] . Mathematically, First-Order Reactions For a first-order reaction , the rate doubles as the concentration of a reactant doubles. Therefore: Integrating: We get: Rearranging: An alternative form: A plot of ln[ A ] t versus t is a straight line with slope - k and intercept ln[ A ] . Note that in this equation we use the natural logarithm, ln (log to the base e ). Here are typical examples of graphs for 1st-order rxns: Problem (see handout): Half-Life for 1 st order rxns: Mathematically t 1/2 is: Note that the half-life of a first-order reaction is independent of the initial concentration of the reactant. Problem (see handout): Second-Order Reactions A second-order reaction is one whose rate depends on the reactant concentration to the second power or on the concentration of two reactants, each raised to the first power. power or on the concentration of two reactants, each raised to the first power....
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C401Ch12LN2 - Chapter 12: Chemical Kinetics, Part 2...

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