In this chapter of A2/A-Level Chemistry, we will learn how to find the rates of reaction with the Rate Equations.
Rate Equations
Rate equation = tell you how the rate is affected by the concentrations of reactants.
E.G. Rate = k [A]m [B]n
Where:
m = order of A
n = order of B
n+m = overall order
k = rate constant (always the same for a reaction at specific temp and pressure, increase temp = increase k = bigger value of k = faster reaction)
EXAMPLE
Propanone + Iodine —> Iodopropanone + H+ + I- (reaction occurs in acid)
Info: First order with respect to propanone and H+ and zero order with respect to iodine
Rate equation = k[propanone]1[H+]1[iodine]0
Simplify to;
Rate equation = k[propanone][H+] (because anything to the power of 0 is 1)
In A2/A-Level Chemistry, How to calculate rate constant from the orders and rate?
Rearrange to make k the subject and calculate.
Units of k can be found as you know concentration is moldm-3 and rate is moldm-3s-1 using a normal “cancelling” method.
Using data to deduce the order
1. The experiment: titrate sample solutions against sodium thiosulfate and starch to work out the concentration of the iodine. Repeat experiment, changing only the concentration for ONE REACTANT at a time.
Here, first we changed the concentration of propanone for experiments 1, 2 and 3.
Then, we changed the concentration of iodine in experiments 4 and 5.
Lastly, we changed concentration of H+ in experiments 6 and 7.
2. From this table we can plot 7 Concentration-Time graphs. Finding the gradient at time zero for each of these plots will give us the INITIAL rate of each.
3.Compare the results e.g.
*Reaction rates won’t be exactly double or treble due to experimental errors etc.
4. Now we can work out the rate equation:
- Rate is proportional to [propanone] so the reaction is of order 1 with respect to propanone.
- Rate does not change/is independent of [iodine] so the reaction is of order 0 with respect to iodine.
- Rate is proportional to [H+] so the reaction is of order 1 with respect to [H+].
In A2/A-Level Chemistry, Rate determining step = slowest step in a multi-step reaction
(if a reactant appears in the rate equation it MUST be a rate determining step including catalysts which may appear in a rate equation)
That's how the rate equations and the calculation works. Good job!
