I'm a student who recently learned about equilibrium and how it is not a state where nothing is made, but rather, a state where the product is decomposing at the same rate as the reactants are combining. If breaking and creating bonds takes energy, where does this energy come from? Does it have to be added to the system? Also, if a closed system, say, a glass jar, reaches equilibrium, and heat slowly leeches out of the jar, does the balance of equilibrium shift, or does the reaction slow down but stay in equilibrium?
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2 Answers
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The energy comes from the molecules themselves. See, they have certain energy of thermal motion. You might have heard of the ${3\over2}kT$ thing, but that's only an average value. Molecules have different energies, described by the so-called Maxwell distribution. Those with higher energy might take part in some reactions which are not possible for the entire bulk.
As for your second question, it's option 1: the equilibrium typically will shift. (Though the reaction might slow down a bit, too.)
answered Feb 8, 2016 at 6:27
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$\begingroup$ Thank you! I would give you an upvote thing but I don't have enough reputation for it :) $\endgroup$– N AFeb 8, 2016 at 6:51
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Recall that the molecules have a certain total energy, split into kinetic, vibrational and rotational energy. A molecule's chemical bonds can vibrate and the whole molecule can rotate as can groups within it, such as around single bonds. The total energy is determined by the temperature and energy can move from one form to another in collisions, e.g. vibration to translation. But as mentioned by @Ivan Neretin there is not a single energy but a distribution of these (with the average $3k_BT/2$). The higher the energy a molecule has, the fewer there will be with this energy.
Now to go from reactants to products the molecule has to 'wait around' until by random collisions with other molecules in the gas or in solution it acquires enough energy to surmount the activation barrier between reactants and products. [You realise that there must be an energy barrier here otherwise everything would react instantly and we could not exist ]. By a similar argument, products have to wait to get enough energy to return to reactants and so the equilibrium is established. The rate constants of forward and back reaction will be different.
In you second question, the equilibrium will shift, the lower energy species, could be reactant or product, will start to dominate. Conversely if you were to add lots of heat then the amounts of product & reactants will become more similar as there is more energy to cross the barrier both ways.
