Methods of Measurement
Rates can be measured by the following methods:
For reactions involving acids and bases
- pH change by titrations
- pH change via pH meter
For reactions involving gases
- Change in volume or pressure
- loss of mass of reactants
For reactions that produce visual changes
- The formation of a precipitate
- A colour change
For reactant A
- rate ∝ [A]^x
- where x is an order with respect to A
Orders are always defined by the reactant.
If X=0, rate is unaffected by change in [A]
If X=1, rate ∝ [A]^1 (if [A] doubles, as will rate.)
If x-2, rate ∝ [A]^2 (if [A] doubles, rate will quadruple.)
- Sum of individual orders.
- eg, rate=k[B]^1[C]^2
- overall rate = 1+2 = 3
Units of rate constants
K = rate/[A]
rate units = moldm^-3 S^-1
[A] units = moldm^-3
Rate Constant, K and Half Lives
A larger value for K means a faster reaction
- Raising the temperature speeds up the rate of most reactions by increasing K
- For many reactions, the rate doubles for each 10 Degrees Celcius increase in temperature.
The time taken for [A] to reduce by half.
First Order - [A] decreases at a constant rate. Half life decreases with time.
Second Order - [A] halves in equal time. Half life is constant.
Third Order - [A] decreases rapidly but then rate of decrease slows down. Half life increases with time.
Rate Determining Step
Multistep reactions often have one step that is slower than others.
- Reactions can only become products as fast as they can get through this slow step.
- Overall rate can be no faster than the slowest step.
The rate of reaction is dominated by this slow step, called the rate-determining step.
RDS of a reaction between X and Y is
2X -> Z
Predict the rate equation for this reaction.
rate = K[X]^2
Order with respect to the reaction tells you how many particles of the reactant are involved in RDS therefore 2X gives X 2nd Order.