Rate Equations

Question 1

What does the rate of reaction tell you?

Answer 1

Tells us how fast a reaction happens

Question 2

How do you find the rate of reaction from a graph?

Answer 2

Gradient

Change in concentration / change in time

Question 3

What is a zero order reaction?

Answer 3

Constant rate throughout and is not affected by concentration

[A]^0 = 1

Question 4

What is a first order reaction?

Answer 4

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]

Question 5

What is a second order reaction?

Answer 5

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

Question 6

How do you find the order of a reaction?

Answer 6

The order of reaction depends on the mechanism and must be found exponentially. It cannot be found from the chemical equation above

Question 7

What is the equation for the rate equation?

Answer 7

Rate = K [A]^x [B]^y [C]^z

Do not include reactants of order 0

Question 8

How do you work out the overall order?

Answer 8

Sum of the individual orders of all reactants

Question 9

What factors measure the rate of reaction?

Answer 9

Temperature
Pressure
Concentration

Question 10

What techniques can be used to measure concentration?

Answer 10

Titrations

0 - concentration does not affect rate

1- rate is proportion to concentration

2- rate is proportional to [conc] ^2

Question 11

What is half life?

Answer 11

Time taken for the concentration of reagent to decrease by half

Question 12

What is the purpose of a clock reaction and how do you use them?

Answer 12

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

Question 13

What assumptions are made when doing clock reactions

Answer 13

Concentration does not change significantly

Constant temperature

When observing the end point, the reaction has not proceeded too far

Question 14

What conclusions do you make from observing a clock reaction?

Answer 14

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.

Question 15

What does a large rate constant mean?

Answer 15

Faster rate of reaction

Question 16

What effect does temperature have on rate of reaction?

Answer 16

Increasing the temperature means particles have more energy so more successful collisions and therefore the rate of reaction increases

Question 17

What happens if the rate constant is doubled?

Answer 17

The rate of reaction doubles

Question 18

What is the Arrhenius equation to calculate the rate constant?

Answer 18

K = Ae^(-Ea/RT)

K - rate constant
A - pre exponential factor
e - mathematical number
Ea - activation energy
R - gas constant
T - temperature

Question 19

Using the Arrhenius equation, how would you calculate the gradient?

Answer 19

Gradient = -Ea/RT

Question 20

What is the rate determining step?

Answer 20

Slowest step in a reaction

Question 21

What is an intermediate?

Answer 21

A species that is formed in one step and used up in another

Question 22

That does the rate determining step control?

Answer 22

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)

Question 23

What does the order of reactant mean in terms of the RDS?

Answer 23

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

Question 24

How do you propose a reaction mechanism?

Answer 24

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