Determining The Rate Law Flashcards
Which methods can be used to determine the rate equation?
Ionisation/ excess method
Initial rates method
Solving the differential equation (integrated rates method)
What is the isolation/ excess method?
Make concentration of one reactant in excess so it is effectively constant over the whole reaction
This means a 2nd order reaction can become a pseudo 1st order reaction by having one of its reactants in excess
When is isolation method used?
If we have a reaction with more than one reactant is can be difficult to interpret information as conc of both reactants are changing at the same time
In a pseudo 1st order what is the rate constant you calculate?
You calculate the Keff which is the effective rate constant
Keff= k [C]excess
Outline the basic isolation method experiment
Rate=k [A][B]^2 1) Have B in excess so B is constant Rate= Keff[A] where Keff= k[B]^2 2) Have A in excess, A is constant Rate= Keff[B]^2 where keff= k[A]
How can you calculate K from the isolation method?
Find Keff from experiment
Keff =k [C]excess
Then rearrange
K= Keff/ [C]excess
How can you calculate keff from the integrated rates method?
Keff= k’
Rate=k’[A] where k’=k[B]o
-d[A]/dt= k’[A]
1) integrate
2) find limits etc
ln[A]t=ln[A]o -k’t
Compared to y=mx + c
The gradient is -k’ so -k[B]o
What is the initial rates method?
Look at the change in initial rate as you change the initial concentration
This can let you work out the rate constant and the order of the reaction
What is ro
The initial rate of reaction
What graph would you plot for this experiment
Plot log(ro) vs log([C]o)
Outline the basic initial rates experiment
Conduct 5 experiments with 5 different initial concentrations of a reactant
For each experiment calculate ro
What is the initial rates approximation?
At the start of the experiment, the initial rate of reaction is determined by the initial concentration of reactant
Initial rate (ro)= k[A]o^a[B]o^B
What would the relationship between initial rate and concentration be if the reaction what first order?
If the rate is directly proportional to concentration then the reaction order is 1st order
How can we calculate k from first order initial rates method?
[C]o is proportional to ro so therefore ro=k[C]o
Sub in values for ro and [C]o to get k
What is the general initial rates equation?
Ro= k[A]o^x
If ro is proportional to [Cl]o^2 then what is the order of reaction?
The order is second order
Initial rates method: How can you find the rate order X and rate constant from measurements of ro and A using a graph
Ro= k[A]o^X
1) take log of both sides
2) separate using log rules
3) rearrange according to log rules
log(ro)= logk + Xlog[A]o
Compare this to the equation of a straight line
This means X is gradient, logk is y intercept
What is the rate of reaction for this reaction?
aA + bB -> cC
Rate = 1/a x d[A]/dt
