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Some reactions require that temperatures of 70–100 °C to be maintained for periods of time as long as 24 hours. Why is this so?

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Quite simply, reactions happen faster at higher temperatures (because who likes waiting), and some may not proceed at all until heated. From a statistical viewpoint, you are giving more reactant molecules more energy, which thereby increases the number of collisions, which in turn increases the number of successful collisions (those which produce product). A typical way to attain such conditions is to place a reaction under reflux.

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While I think both of the other answers are good, I want to comment on two practical constraints that yield long reaction times:

  • Solvent choice - We perform a lot of chemical reactions in solvents. This allows easy mixing and usually facilitates separations and purification steps after reaction. On the other hand, if some reaction requires a high activation energy barrier, we cannot always simply raise the temperature or most practical solvents will evaporate (e.g., we can't run most reactions at 300-400 °C even if it would minimize reaction times).
  • Human timescale - Even if some reactions might complete sooner, it's practically much easier to run a reaction for ~24 hours - you set up the reaction in the morning and then it's done the next day. It's neither safe, nor practical for someone to come in at 2 AM to shut down a reaction.

From a fundamental standpoint, diffusion/transport issues limit reaction times and thermal energy increases reaction rates but practical concerns also affect reaction times.

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Long hold times are needed for one of two reasons:

(1) Mass Diffusion limitation - For example, in a drying reaction moisture needs time to diffuse through a sample to the surface before evaporating.

(2) Temperature limitation - For example, in a conventional drying reaction, the part of a sample nearest to a heating element heats first. The heat needs time to travel through a sample without getting any part of the sample too hot. If part of the sample gets too hot an undesirable reaction like combustion may occur.

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