How is the rate of a chemical reaction usually expressed?

Answer 1

Let assume a simple synthesis chemical reaction in solution (the solute is inert for the considered phenomenon).

We can write

A + B -> C

and image to start with a concentration CA and CB of the components A and B and with no molecule of C.

At the beginning A and B combine to form C at high speed, since no C is yet present. While the reaction goes on, C start to be present in a certain concentration CC and also the inverse reaction starts to happen, that is C decomposes in A+B.

In an instant t, the rates of variation of the concentration of the three substances, that is the quantity of substance produced (or consumed if the rate is negative) in a very small time interval (let us call them RA, RB and RC) follows the so called chemical kinetics laws

RA = ki CC - kd CA CB

RB=RA

RC=kd CA CB - ki CC

where the parameters kd and ki are called direct and inverse reaction rates. Their values depends on the microscopic characteristics of the involved molecules, like collision section so on.

This is a very simple situation in which the synthesis happens directly by uniting an A molecule with a B molecule.

There are much more complicated reactions, where the reaction happens in a set of subsequent states and stoichiometric coefficients different from one are present.

For example oxidation of carbon oxide to carbonn dioxide

NO2 + CO -> NO + CO2

is a two step reaction, that happens as

1) NO2 + NO2 -> NO3 + NO

2) NO3 + CO -> NO2 + CO2

When a multiple step reaction is present, the rates can always be written, by their dependence from the concentration of the reaction elements is not linear, but depends on some power of the concentrations (that generally has no relation with the original reaction stoichiometry).

Also in this case however, the coefficients of such nonlinear dependence are called reaction rates.

Answer 2

Rates of reaction can be expressed depending upon their order.

For example say you have a reaction between two chemicals and the initial rate for that reaction is known :-

when:-

The concentration of one of the reactants is doubled and the other reactants concentration remains the same and the overall rate of reaction does not change - reaction is zero orderwith respect to chemical which was doubled.

The concentration of one of the reactants is doubled and other reactants concentration remains the same and the overall rate of reaction doubles - reaction is first order with respect to chemical which was doubled.

The concentration of one of the reactants is doubled and other reactants concentration remains the same and the overall rate of reaction quadruples - reaction is second order with respect to chemical which was doubled.

Zero Order

rate = k

First Order

rate = k [A] (reaction is 1st order with respect to [A] and 1st order overall)

Second Order

rate = k [A][B] (reaction is first order with respect to [A] and first order with respect to[B], reaction is second order overall)

rate = k [A]2 (reaction is second order with respect to [A] and second order overall)

Orders are simply added together in order to determine the overall order of reaction :-

rate = k [A][B][C] would be third order overall and first order with respect to each of the reactants

There are other orders of reaction, for example 2 and 3 quarter orders and third order reactions, but these are a little more complex.

Answer 3

Normally the application of heat will increase the speed/rate of a chemical reaction. As a general rule a 10 degrees centigrade increase in temperature will double the rate of a chemical reaction.

Answer 4

increases as reactant concentration increases

Answer 5

increases as reactant concentration increases