Slide 1
Heating changes the distribution of particle energies
Many chemical reactions run faster when heated. Temperature changes the distribution of particle energies, not merely their average speed.
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Particles in a sample do not all have identical energies. The energetic tail of the distribution can matter greatly when only encounters with sufficient energy can cross a reaction barrier.
Slide 2
More encounters can cross the activation barrier
Activation energy characterizes the energetic hurdle along a reaction pathway. A higher temperature often lets a larger fraction of encounters overcome it.
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The Arrhenius relation describes many rate constants over useful temperature ranges. Its exponential dependence explains why a modest temperature change can have a much larger effect than a simple proportional rule suggests.
Slide 3
Concentration also affects reaction opportunities
More reactant particles in a volume can increase opportunities to react. The precise dependence is captured by an experimentally determined rate law.
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Doubling a reactant concentration does not always double the reaction rate. The mechanism and which step limits progress determine the observed reaction order, which cannot generally be read directly from the overall equation.
Slide 4
Mixing or supply can become the limiting step
Mixing, diffusion or supply of reactants can limit an observed process. Heating the chemical step then may not proportionally speed the whole process.
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A reaction on a surface requires reactants to reach that surface. If delivery through a stagnant layer is slow, improved stirring may matter more than a further increase in intrinsic chemical rate.
Slide 5
More speed can come with unwanted reactions
Higher temperatures may also speed unwanted reactions or damage a catalyst. Process conditions balance rate with selectivity and stability.
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Biological enzymes illustrate a further limit: heating can disrupt the structure needed for activity. A rising-temperature trend observed over one range should not be extrapolated indefinitely into a different physical regime.
Sources
- Temperature and the Rate of a Chemical Reaction
American Chemical Society
- Kinetics: USNCO Coaching
American Chemical Society