Rates of reaction

Question 1

What is rate of reaction?

Answer 1

The change in concentration of a reactant or product per unit time.

Question 2

What are the units for rate of reaction?

Answer 2

mol•dm^-3•s^-1

Question 3

What is the general shape of a [reactant]-time graph and why?

Answer 3

A curve which is curving downwards but the gradient decreases until it levels off near 0 (reaction is complete). This is because as reaction progresses, [reactant] drops, the rate of reaction (rate at which reactant is depleted) also decreases; until there is no more reactant left and so rate of reaction is 0 (hence levelling off at the end).

Question 4

What is the general shape of a [product]-time graph and why?

Answer 4

A curve which is curving upwards but the gradient decreases until it levels off (reaction is complete). This is because as reaction progresses, [reactant] drops, resulting in the rate of reaction (rate at which product is formed) also decreasing; until no more reactant is left to react, hence the levelling off.

Question 5

How is an instantaneous rate of reaction obtained from a concentration-time graph?

Answer 5

Drawing and measuring gradient of tangent to curve at any instant in time along curve.

Question 6

What is the order of reaction (with respect to reactants)?

Answer 6

The power to which the concentration of reactant is raised in the rate equation.

Question 7

What is zero order (with respect to reactant)?

Answer 7

Rate ∝ [reactant]⁰

In other words, rate is independent of the concentration of the reactant.

Shape of rate-[reactant graph]:

Horizontal line

Question 8

What is first order (with respect to reactant)?

Answer 8

Rate ∝ [reactant]

In other words, rate is directly proportional to [reactant]; 2 x [reactant] → 2 x rate.

Shape of rate-[reactant] graph:

Straight line through origin.

Question 9

What is second order (with respect to reactant)?

Answer 9

Rate ∝ [reactant]²

In other words, 2 x [reactant] → 2² (4) x rate.

Question 10

What is the rate equation?

Answer 10

The rate equation for a reaction:

A + B → C is given by

rate = k[A]^m[B]ⁿ

where

k = rate constant

m,n = order of reaction with respect to A, B respectively

Question 11

What is the rate constant?

Answer 11

The constant that links the rate of reaction with the concentrations of the reactants raised to the powers of their orders in the rate equation.

Question 12

What happens to the rate constant when a physical change in the system changes rate of reaction (e.g. temperature)?

Answer 12

Increases/ decreases according to the effect of change.

Question 13

What is the overall order?

Answer 13

The sum of all individual orders in respect to every reactant in the rate equation.

Question 14

What is the intial rate of reaction?

Answer 14

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

Question 15

How can rate of reaction be monitored for acid-base reactions?

Answer 15

• Using titrations at set time intervals.
• Using a pH probe.

Question 16

How can rate of reaction be monitored for gas-producing reactions?

Answer 16

• Collect gas and measure:
  1. Change in volume.
  2. Change in pressure.
• Monitor loss in mass of reactants.

Question 17

How can rate of reaction be monitored for reactions that produce precipitates?

Answer 17

Monitor change in transparency over time with colourimeter.

Question 18

How can rate of reaction be monitored for a reaction that produces a colour change/ change in colour intensity?

Answer 18

Change in intensity of a certain colour can be monitored with colourimeter.

Question 19

What are clock reactions and how can they be used to estimate initial rates of reaction?

Answer 19

Clock reactions are reactions that produce a visible change once [reactant] decreases or [product] increases beyond certain level. The smaller the change (Δ), the closer the average rate of reaction up to that point is to the initial rate if reaction.

Question 20

How can clock reactions be used to determine order of reaction?

Answer 20

1/(time it takes for a clock reaction to reach its end-point at different [reactant]) approx. proportional to inital rate of reaction, so is also approx proportional to [reactant]^(order of reaction). Plotting 1/t against [reactant] will give good indication of order of reaction.

Question 21

What is the half life (with respect to reactant)?

Answer 21

The time taken for the [reactant] to reduce by half.

Question 22

How can half-life be used to determine order of reaction from [reactant]-time graph?

Answer 22

Order of reaction is related to half-life of reactant.

Question 23

How does the half-life of a reactant differ over time for a zero order reaction?

Answer 23

Half-life decreases rapidly as reaction progresses.

Question 24

How does the half-life of a reactant differ over time for a first order reaction?

Answer 24

Half-life stays constant as reaction progresses. In other words, half-life of a first order reaction is independent of [reactant].

Question 25

How does the half-life of a reactant differ over time for a second order reaction?

Answer 25

Half-life increases rapidly as reaction progresses.

Question 26

What is a reaction mechanism?

Answer 26

A series of stages that, together, nake up the overall reaction.

Question 27

What is the rate-determining step of a reaction?

Answer 27

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

Question 28

What is the relationship between the rate equation of reaction and the rate equation of the rate-determining step?

Answer 28

The rate equation of the reaction is usually directly related to the rate equation of the rate-determining step.

Question 29

What are the properties that a mechanism must have in order to be plausible?

Answer 29

1. Must add to overall reaction equation.
2. Rate-determining step mut predict rate equation.
3. Must obey 3-body problem; i.e. there must be no single step that involves the collision of more than 2 molecules.