Question

Define reaction rate. Distinguish between the initial rate, average rate, and instantaneous rate of a chemical reaction. which of these rates is usually fastest? The initial rate is the rate used by convention. Give a possible explanation as to why?

(I have defined all rates. I am mainly stumped on which rate is the fastest and why initial rate is used by convention)

Answer 1

A reaction rate can be reported quite differently depending on which product or reagent selected to be monitored.

Given a reaction:   

units: g/(M s), M/s etc

The average rate of reaction, as the name suggests, is an average rate, obtained by taking the change in concentration over a time period, for example: -0.3 M / 15 minutes. This is an approximation of the reaction rate in the interval; it does not necessarily mean that the reaction has this specific rate throughout the time interval or even at any instant during that time.

aA+bB?cC+dD

The instantaneous rate of reaction is defined as the change in concentration of an infinitely small time interval, expressed as the limit or derivative expression. Instantaneous rate can be obtained from the experimental data by first graphing the concentration of a system as function of time, and then finding the slope of the tangent line at a specific point which corresponds to a time of interest

The initial rate of reaction is the rate at which the reagents are first brought together. Like the instantaneous rate mentioned above, the initial rate can be obtained either experimentally or graphically. The slope of the line tangent to the reaction curve when t=0 to concentration.

Since reactant concentrations are the largest at t = 0, the initial rate is faster than the instantaneous rate at any later time during the reaction, fastest is intial rate

We use Initial rate to avoid a complication: as a reaction proceeds in the forward direction (reactants