rate equations

r = k[A]m[B]n r is used as a symbol for rate The unit of r is usually mol dm-3s-1 The square brackets[A] means The concentration of A (unitmol dm-3) k is called the rate constant

m, n are called reaction orders Orders are usually integers 0,1,2 The total order for a reaction is worked out by adding all the individual orders together (m+n)

For zero order: the concentration of A has no effect on the rate of reaction r = k[A]0= k

For first order: the rate of reaction is directly proportional to the concentration of A r = k[A]1

For second order: the rate of reaction is proportional to the concentration of A squared r = k[A]2

k is the rate constant The units of k depend on the overall order of reaction. It must be worked out from the rate equation • The value of k is independent of concentration and time. It is constant at a fixed temperature. • The value of k refers to a specific temperature and it increases if we increase the temperature

When we follow one experiment over time, recording the change in concentration, we call it a continuous rate method The gradient represents the rate of reaction. The reaction is fastest at the start, where the gradient is steepest. The rate drops as the reactants start to get used up and their concentration drops. The graph will eventually become horizontal, and the gradient becomes zero, which represents the reaction having stopped.

In reactions where there are several reactants, if the The concentration of one of the reactants is kept in a large excess then that reactant will not appear to affect the rate and will be pseudo-zero order. This is because it’s concentration stays virtually constant and does not affect rate.

concentration temperature surface area

By repeating the experiment several times, varying the concentration of a reactant e.g. I–, ( keeping the other reactants at constant concentration )you can determine the order of reaction with respect to that reactant The initial rate of the reaction can be represented as (1/t

log the whole equation to make it line up with y=mx+c

look at how the rate changes when the conc of a reactant changes if when the conc doubles the rate does not change its 0 order within respects to that reagents e.g

Put the value in the rate equation that has been rearranged to give k

For zero order reactants, the ratestays constant as the reactant is used up. This means the concentration of that reactanthas no effect onrate. Rate = k [A]0 so rate = k As the rate is the gradient of the graph on the right, the gradient is also the value of the rate constant.

Increasing temperature increases the rate constant k. The relationship is given by the Arrhenius equation k = Ae-EA/RT where A is the Arrhenius constant, R is the gas constant, and EA is activation energy.

Using the rearranged version ln k = ln A– EA/(RT) k is proportional to the rate of reaction so ln k can be replaced by ln(rate) From plotting a graph of ln(rate) or ln k against 1/T the activation energy can be calculated from measuring the gradient of the line

A mechanism is a series of steps through which the reaction progresses, often forming intermediate compounds. If all the steps are added together they will add up to the overall equation for the reaction

Each step can have a different rate of reaction. The slowest step will control the overall rate of reaction.The slowest step is called the rate-determining step.