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A2 Chemistry (Unit 2: F325) > Rates of reaction > Flashcards

Flashcards in Rates of reaction Deck (29)
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1

What is rate of reaction?

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

2

What are the units for rate of reaction?

mol•dm-3•s-1

3

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

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).

4

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

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. 

5

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

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

6

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

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

7

What is zero order (with respect to reactant)?

Rate ∝ [reactant]0

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

Shape of rate-[reactant graph]:

Horizontal line

8

What is first order (with respect to reactant)?

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. 

9

What is second order (with respect to reactant)?

Rate ∝ [reactant]2

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

 

10

What is the rate equation?

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

11

What is the rate constant?

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.

12

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

Increases/ decreases according to the effect of change.

13

What is the overall order?

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

14

What is the intial rate of reaction?

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

15

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

- Using titrations at set time intervals.

- Using a pH probe.

16

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

- Collect gas  and measure:
1. Change in volume.
2. Change in pressure.

- Monitor loss in mass of reactants.

17

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

Monitor change in transparency over time with colourimeter.

18

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

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

19

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

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.

20

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

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.

21

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

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

22

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

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

23

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

Half-life decreases rapidly as reaction progresses.

24

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

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

25

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

Half-life increases rapidly as reaction progresses.

26

What is a reaction mechanism?

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

27

What is the rate-determining step of a reaction?

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

28

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

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

29

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

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.