Rates

Question 1

Definition and units of rate of reaction

Answer 1

Change in conc. of reactants or products/time

mol dm ^-3 s^-1

Question 2

How is rate measured?

Answer 2

Plot graph of conc. against time

At any time, the rate is equal to the slope/gradient of a tangent at that point on the graph

Question 3

What is initial rate of reaction?

Answer 3

The change in concentration of a reactant or product per unit time at the start of a reaction when t = 0

Question 4

How can we obtain data for a conc. ?

a) Reactions involving acids or bases
b) Reactions that produce gases
c) Reactions that produce visual changes

Answer 4

a) Measure pH changes using titration or a pH meter
b) Change in volume or pressure, or change in mass of reactants
c) Time taken to form precipitate or colour change

Question 5

What does zero order mean?

Answer 5

The rate is unaffected by changing the conc. of this reactant

Question 6

What does first order mean?

Answer 6

If this reactant is increased by s.f A, rate also increases A times

Question 7

What does second order mean?

Answer 7

Rate increased by square of factor conc. this reactant is increased by

Question 8

What is the rate constant?

Answer 8

Rate = k x concentrations of reactants to the power of their order

Question 9

How do concentration and half life vary for zero order reactions and what is the shape of the concentration - time graph?

Answer 9

Concentration decreases at a constant rate
Half life decreases with time
Straight line with neg. gradient

Question 10

Shape of rate-conc. graph of zero order reaction

Answer 10

horizontal line

Question 11

How do concentration and half life vary for first order reactions and what is the shape of the concentration - time graph?

Answer 11

Concentration halves in equal time intervals
Half life is constant
Exponential decay curve

Question 12

Shape of rate-conc graph of first order reaction

Answer 12

Straight line

Question 13

How do concentration and half life vary for second order reactions and what is the shape of the concentration - time graph?

Answer 13

Conc. decreases rapidly but then rate of decrease slows down
Half life increases with time
Steeper exponential decay

Question 14

Shape of rate-conc graph of second order reaction

Answer 14

Exponential curve

Question 15

What effect does temp. have on rate constant?

Answer 15

Higher temp = larger value of k

Question 16

What is a reaction mechanism?

Answer 16

A series of steps that, together make up the overall reaction

Question 17

What is the rate determining step?

Answer 17

The slowest step in the reaction mechanism of a multi-step reaction

Question 18

What three things should a proposed mechanism be consistent with if it is to be regarded as plausible?

Answer 18

• Slow step should match rate equation e.g
  A + B + C -> D doesn’t match rate= k[A][B]
• No multibody collisions, max. two molecules
• Equations in mechanism must add up to overall equation