Temperature Reaction - Intro Chemistry II 030.102 Chapter...

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Intro Chemistry II – 030.102 - Chapter 18 Spring, 2014 1 Effect of Temperature on Reaction Rates Little has been said thus far about the actual magnitude of the rate constants, or effect of temperature. To to do this, we will consider the connection between collision rates and rates of reactions. Gas-Phase Reaction Rate Constants. From the kinetic theory of gases (sec. 9.5), frequency of collisions calculated to be 4.1109s–1under room conditions for a small molecule such as O2. If every collision led to reaction, reaction would be complete in ~10–9s. Some reactions do occur this fast, e.g., for which observed 2nd-order rate constant is 11010L mol–1s–1. With initial concentration [CH3] = 0.04 M, in ~10 –9s concentration would drop to 0.02 M. Most reactions proceed at muchslower rates (often by orders of magnitude). 2 CH3C2H6Effect of Temperature on Reaction Rates A clue to understanding rates of reactions comes from temperature dependence. Rates of many reactions increase rapidly with T. Rule of thumb for many reactions: Increase Tby 10°C, rate doubles. First insight came from suggestion of S. Arrhenius (1889) that rate constants depend exponentially on temperature: k=AeEa/RTEais a constant with dimensions of energy, and A is a constant with same dimensions as k. Take natural logarithm of this equation: ln k=ln AEaRTEffect of Temperature on Reaction Rates ln k=ln AEaRT

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