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In other words, I want to know why some reactions attain equilibrium early in the reaction while some reactions obtain equilibrium at the end of the reaction.

Why is this the case?

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If you define early or end of reaction by the how much the concentration of reactants change from initial reaction to once equilibrium is reached, it is because the equilibrium constant itself is essentially a ratio of the forward and reverse rate constants (can be approximated by the Arrhenius equation posted by t.c.). A high ratio means "more" products (at least a higher concentration) are present at equilibrium than reactants (so in a sense near the "end of the reaction"), since the forward rate is much higher than the reverse, a lower concentration of reactants compared to products is needed to maintain equilibrium. A low ratio means the opposite.

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The rate of reaction is dependent on the rate constant, as well as the order of reaction.

The rate constant, which is approximated by the Arrhenius equation, is dependent on the Activation energy, Ea, and temperature, T. Activation energy can be lowered by using a catalyst (if any). $$k = A e^{-E_a/(R T)}$$

From this equation, you can see that the higher the temperature and the lower the Ea, the faster the rate.

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    $\begingroup$ I reinterpreted the question a bit, but I think your answer still fits the edit, anyway. $\endgroup$ – jonsca Oct 8 '14 at 4:08
  • $\begingroup$ I want to know why some reactions attain equilibrium early in the reaction while some reactions obtain equilibrium at the end of the reaction. $\endgroup$ – pcforgeek Oct 8 '14 at 4:11
  • $\begingroup$ @pcforgeek Okay, so you were looking for an explanation on rate and not thermodynamic equilibrium. I've edited my answer. $\endgroup$ – t.c Oct 8 '14 at 4:21
  • $\begingroup$ The way I read it the asker wants to know why some equilibrium constants are very large (essentially all reactants turn into products at equilibrium) and others are very small (almost no reactants turn into products at equilibrium). $\endgroup$ – Nicolau Saker Neto Oct 8 '14 at 12:19

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