Rate Equations Flashcards
What does the rate of reaction tell you?
Tells us how fast a reaction happens
How do you find the rate of reaction from a graph?
Gradient
Change in concentration / change in time
What is a zero order reaction?
Constant rate throughout and is not affected by concentration
[A]^0 = 1
What is a first order reaction?
Rate is directionally proportional to the concentration
If concentration halves, the rate halves
Increase conc by 3 then rate increases by 3
Rate = [A]^1 = [A]
What is a second order reaction?
Rate is proportional to the square of the concentration
Conc halves, rate will quarter
Conc increases by 3 rate increases by 9
Rate = [A]^2
How do you find the order of a reaction?
The order of reaction depends on the mechanism and must be found exponentially. It cannot be found from the chemical equation above
What is the equation for the rate equation?
Rate = K [A]^x [B]^y [C]^z
Do not include reactants of order 0
How do you work out the overall order?
Sum of the individual orders of all reactants
What factors measure the rate of reaction?
Temperature
Pressure
Concentration
What techniques can be used to measure concentration?
Titrations
0 - concentration does not affect rate
1- rate is proportion to concentration
2- rate is proportional to [conc] ^2
What is half life?
Time taken for the concentration of reagent to decrease by half
What is the purpose of a clock reaction and how do you use them?
Used to measure rates of reaction
Measure the time taken for a certain amount of product to form as you vary the concentration of the reactants.
Often an observable end point (colour change)
Used to find initial rate
What assumptions are made when doing clock reactions
Concentration does not change significantly
Constant temperature
When observing the end point, the reaction has not proceeded too far
What conclusions do you make from observing a clock reaction?
Faster a reaction reaches endpoint, the faster the initial rate
We want to measure the time taken form the begging of the reaction to the end point.
What does a large rate constant mean?
Faster rate of reaction
What effect does temperature have on rate of reaction?
Increasing the temperature means particles have more energy so more successful collisions and therefore the rate of reaction increases
What happens if the rate constant is doubled?
The rate of reaction doubles
What is the Arrhenius equation to calculate the rate constant?
K = Ae^(-Ea/RT)
K - rate constant A - pre exponential factor e - mathematical number Ea - activation energy R - gas constant T - temperature
Using the Arrhenius equation, how would you calculate the gradient?
Gradient = -Ea/RT
What is the rate determining step?
Slowest step in a reaction
What is an intermediate?
A species that is formed in one step and used up in another
That does the rate determining step control?
The rate determining step is the slowest step in a reaction and therefore will dictate how fast the whole reaction takes place.
(When measuring the rate of reaction you are actually measuring the RDS)
What does the order of reactant mean in terms of the RDS?
For any reactant in the rate equation, the order of the reactant tells us how many molecules of the reactant there are involved in the RDS
Order 0 reactants cannot be involved in the RDS
How do you propose a reaction mechanism?
The RDS is given that has the same number of molecules for the order of the reactant
All intermediates are used up in later steps
The other steps in the reaction mechanisms eventually generate the products
