Chemical Kinetics Flashcards
How do we measure the rate of reaction in general?
We need to work out how much of a reactant has been used up, or how much product has been produced in a set period of time.
Describe a more labour-intensive method to measuring rate of reaction
Sampling and quenching, where a small amount of the reaction mixture is removed at regular time intervals and immediately placed into ice water, cooling and diluting the mixture which effectively stops the reaction. Then each sample is individually analysed.
What is quenching?
The sudden stopping or significant slowing of a chemical reaction to allow for analysis to occur without the reaction proceeding further. Usually undertaken by cooling and diluting.
What is the advantage of sampling and quenching?
It can be used for a large range of reactions.
What are the disadvantages to sampling and quenching?
Labour and time intensive as each sample must be analysed individually, so the time intervals between measurements are longer than colorimetric methods
Sampling is only appropriate with homogenous mixtures, otherwise the sample taken may not be representative of the overall mixture
How does measuring rate of reaction change with a heterogenous catalyst?
Sampling can be taken without quenching.
How does measuring rate of reaction change with a homogenous catalyst?
Quenching is needed using cooling and dilution or by destroying the catalyst, as well as sampling.
What is the usual calculation for rate?
Change in concentration of reactants/products / Time taken
When the concentration of one reactant is doubles, how can this affect the rate of reaction?
The rate of reaction can stay the same if rate is not proportional to concentration ([A]0)
The rate of reaction can double if rate is proportional to concentration ([A]1)
The rate of reaction can increases four times if rate is proportional to concentration squared ([A]2)
What is the rate equation?
Rate = k[A]^m[B]^n
Where m is the order of the reaction with respect to A
Where n is the order of the reaction with respect to B
What is rate?
The rate of change of the concentration, or of the amount, of a particular reactant or product.
What is the rate constant?
The constant in the rate equation. It is constant for a given reaction at a particular temperature, and is not affected by changing the concentration of the reactants.
What is the order of reaction?
The order of a reaction with respect to a particular reactant is the power to which the concentration is raised in the rate equation.
How to calculate the overall order of a reaction?
The sum of all of the orders of the reaction (m+n).
Zeroth order- the total is 0
First order- the total is 1
Second order- the total is 2
Third order- the total is 3
What are the units for each order of reaction?
Zeroth order reaction- rate= moldm-3s-1 and k=moldm-3s-1
First order reaction- rate= moldm-3s-1, concentration= moldm-3 and k=s-1
Second order reaction- rate=moldm-3s-1, concentration=moldm-3 and k=mol-1dm3s-1
How do we derive a rate equation from experimental data?
- Look at information to find two experiments that differ only in the concentration of one reactant
- If doubling the reactant concentration does not affect the rate, the order in respect to that reactant is zero
- If doubling the reactant concentration doubles the rate, the order in respect to that reactant is one
- If doubling the reactant concentration increases the rate by a factor of four, the order in respect to that reactant is two
- Repeat this process for each reactant, to find the order in respect to each one
- The order of the reaction is the sum of each of these orders
What is a mechanism?
A description of the series of steps that occur during a chemical reaction. Each step in a mechanism will occur at a different rate, with its own rate equation.
What is the rate-determining step?
The slowest step in a reaction mechanism, which limits the overall rate of reaction.
What is needed for a reaction to occur in terms of collision theory?
- The reacting particles must collide
- The particles must have sufficient energy for reaction (activation energy)
How does the rate equation tell us how many particles need to collide in the rate determining step?
In a second order reaction, two particles must collide
In a third order reaction, three particles must collide
In a first order reaction, there is only one particle in the rate-determining step
How can we use chemical kinetics to prove or disprove a reaction mechanism?
We can work out what the reactants are for the rate-determining step. If the mechanism does not have a step with these reactants, it is incorrect.
What effect does increasing temperature have on the rate of reaction?
Rate of reaction increases as more collisions of particles will have the sufficient energy to react. This does not affect the concentration of each substance, so it is the rate constant that is affected by temperature.
How can we quantify the effect of temperature on the rate constant?
Use the Arrhenius equation: k=Ae^(-Ea/RT)
What does a graph of lnk over 1/T look like?
A straight line with y-intercept lnA and the gradient being -Ea/R.
How do catalysts affect rate of reaction?
Catalysts increase the rate of reaction by providing alternative routes with lower activation energies. This does not affect the concentrations in a rate equation, so it is the rate constant that is changed.
