__Collision Theory__: For a chemical reaction to occur: 1) Collisions between reactant particles must occur; 2) The collisions must be energetic enough to allow reaction; 3) The colliding particles must be oriented in a way that gives the desired reaction. The probabilities is Collision Theory are affected by changes in temperature and concentration/pressure.

Rates can be expressed in a number of ways, but for most chemical systems, the rate is equal to the change in

*concentration*over the change in time. When that change in time is (relatively) long, we have an

*average*rate. Average rates can be either in terms of

*consumption*of a reactant or

*production*of a product. The mathematical formality of "change in concentration" requires a negative sign on rates of consumption. Rates of consumption or production are related to the stoichiometric coefficients in the balances chemical equation that is being observed and can be unified as a

*rate of reaction*.

A significant disadvantage of average rates is that they change depending upon the time period being measured. To more accurately estimate the rate of a reaction, we should use smaller time periods. Taken to the extreme, we can calculate an

*instantaneous rate*at any point during a reaction. Of all the instantaneous rates for a given reaction, the only important or unique instantaneous rate is the very first one, the

*initial instantanous rate*of the reaction.

On Friday, we'll continue with kinetics. You will very likely NOT get your exams back on Friday. A number of things came up today (and continue tomorrow) that will make it difficult to get exams back on Friday. I will have them done over the weekend and will return them first thing Monday morning. Sorry for the delay.

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