Rate equations / kinetics 2

Question 1

Define the term rate of reaction

Answer 1

How fast reactants are converted into products, depending on the concentration of the reactants and the rate constant.

Question 2

How do you calculate the rate of reaction ?

Answer 2

Rate of reaction = change in concentration / change in time

Question 3

What are 2 experimental ways to measure the rate of reaction ?

Answer 3

• use a calorimeter at suitable intervals if there is a colour change
• if there is gas produced, use a gas syringe to collect the volume of gas

Question 4

How can the rate constant be determined ?

Answer 4

Only experimentally

Question 5

What affects the value of the rate constant ?

Answer 5

Only temperature

Question 6

What is a general expression for the rate of this reaction
A + B → C

Answer 6

Rate = K [A]^x [B]^y
X and y are the orders of the reaction
K is rate constant
[ ] is the concentrations of the reactants

Question 7

What is the total order of reaction ?

Answer 7

X+y = total order
Is the sum of the orders of reaction of all species in the rate expression

Question 8

What is the order of a reaction ?

Answer 8

The power to which a species’ concentration is raised in the rate equation

Question 9

How do you calculate the rate constant (K) ?

Answer 9

Rate / [A]^x [B]^y = K

Question 10

What are the units of rate ?

Answer 10

Moldm^-3s^-1

Question 11

What are the orders of reaction ?

Answer 11

Zero , 1st and 2nd

Question 12

What is zero order ?

Answer 12

• the concentration of this species has no impact on the rate because the rate is proportional
• shown as a horizontal line on a rate concentration graph

Question 13

What is first order ?

Answer 13

• the concentration of the species and the rate are directly proportional
• ie. Doubling the concentration doubles the rate
• rate = k[A]
• depicted at a straight diagonal line on a rate concentration graph

Question 14

What is second order ?

Answer 14

• the rate is proportional to the concentration squared
• doubling the concentration will increase the rate by 4
• k[A]^2
• depicted by a curved line

Question 15

How would you draw a rate concentration graph ?

Answer 15

• plot the concentration of [A] against time
• draw tangent at different values
  Or
• draw a secondary graph of rate against [A]

Question 16

How do you determine a rate equation

Answer 16

Do a series of experiments, during which you vary concentrations, so the concentration of just one reactant changes each time.
Plot a graph of concentration against time for each reactant and use a tangent at t=0 to find the initial rate of reaction.
Compare rates and concentrations between each experiment to find order of reactants and overall rate equation

Question 17

How can you calculate the rate of reaction from the thiosulfate experiment ?

Answer 17

• Record time taken for the cross to disappear
• use rate = 1/t

Question 18

What is the Arrhenius equation?

Answer 18

Ln k = Ln A - Ea/RT

Question 19

Rearrange Arrhenius to make Ln A the subject

Answer 19

Ln A = Ln k + Ea/RT

Question 20

Rearrange to make Ea the subject

Answer 20

Ea = RT(LnA - Ln K)

Question 21

Rearrange to make T the subject

Answer 21

T = Ea / R(Ln A - Ln K )

Question 22

How can you convert the Arrhenius equation into a useful form for plotting a graph?

Answer 22

In k= -Ea/RT + In A
Graph of Ink against 1/T is a straight line:
gradient = -E/R and y intercept is InA

Question 23

What is the rate determining step ?

Answer 23

The slowest step within the mechanism, which determines the overall rate of reaction.