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A-Level Chemistry > 18 Rates of reactions > Flashcards

18 Rates of reactions Flashcards

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1
Q

How is rate of reaction calculated?

A

Quantity reacted or produced/time

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2
Q

How is rate of reaction calculated for consistency?

A

Change in concentration/change in time

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3
Q

What is the relationship between rate of reaction and concentration of a reactant raised to a power?

A

Rate of reaction is proportional to the concentration of a reactant.

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4
Q

What does the power in the reactant mean?

A

It is the order of reaction for that reactant. This can be : zero order, first order and second order.

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5
Q

What is zero order?

A
  • Zero order is when the concentration of a reactant has no effect on the rate.
  • In a zero order reaction: any number raised to the power zero is 1 and concentration does not influence the rate.
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6
Q

What is first order?

A
  • First order is when the concentration is raised to the power of one.
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7
Q

What is second order?

A
  • Second order is when the concentration is raised to the power of two.
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8
Q

What is the rate equation?

A

rate= k[A]^m[B]^n

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9
Q

What does k stand for in the rate equation?

A

The rate constant (K) is the proportionality constant.

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10
Q

How do we calculate the overall order?

A

It is the sum of orders with respect to each reactant

- So in the rate equation the overall order is m+n

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11
Q

What is continuous monitoring?

A

When continuous measurements are taken during the course of a reaction

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12
Q

What are two methods for continuous monitoring?

A
  • Monitoring by gas collection

- Monitoring by mass loss

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13
Q

What does a colorimeter measure?

A
  • A colorimeter measures the intensity of light passing through a sample.
  • The filter is chosen so that it is the complementary colour to the colour being absorbed.
  • Absorbance is recorded, which is directly linked to the concentration of the solution.
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14
Q

Describe the process in analysing by colorimetry between propanone and iodine.

A

1) Prepare standard solutions of known concentration of the coloured chemical, iodine, I2 in this reaction.
2) Select a filter with the complementary colour of the coloured chemical. For iodine, this would be a green/blue filter but the colorimeter will usually tell you which setting to use.
3) Zero the colorimeter with water.
4) Measure the absorbance readings of the standard solutions of iodine and plot calibration curve of absorbance against iodine concentration.
5) Carry out the reaction between propanone and iodine. Take absorbance readings of the reacting mixture at measured time intervals.
6) Use the calibration curve to measure the concentration of iodine at each absorbance reading.
7) Finally plot a graph of concentration of iodine against time.

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15
Q

What shape does zero order have in concentration-time graph?

A

A zero order reaction produces a straight line with a negative gradient.

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16
Q

What is the value of the gradient in a concentration-time graph equal to?

A

The value of the gradient is equal to the rate constant,k

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17
Q

What shape does first order have in concentration-time graph?

A

A downwards curve, with decreasing gradient over time

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18
Q

What shape does second order have in concentration-time graph?

A

A downward curve, steeper at the start and tailing off more slowly

19
Q

What does half life mean?

A

The time taken for the concentration of a reactant to decrease by half.

20
Q

What is exponential decay?

A

When first order reactions have a constant half life with the concentration halving every half life.

21
Q

How can you determine k from a first order reaction?

A

1) Draw tangent at a particular concentration, calculate gradient and rearrange formula to calculate k.
2) k= In2/t 1/2
In2/half life

22
Q

What does a rate-concentration graph look like with zero order?

A

It produces a horizontal straight-line with zero gradient.

23
Q

How can we work out the rate constant, k on a zero order graph?

A

The intercept on the y axis gives the rate constant, k.

- The reaction rate does not change with increasing concentration.

24
Q

What does a rate-concentration graph look like with first order?

A
  • A first order reactant produces a straight-line graph through the origin.
  • The rate is directly proportional to concentration for a first order relationship.
25
Q

How can the rate constant be calculated from the first order rate-concentration graph?

A

The rate constant can be determined by measuring the gradient of the straight line of this graph.

26
Q

What does a rate-concentration graph look like with second order?

A

A second order reactant produces an upward curve with increasing gradient.
rate= k[A]^2

27
Q

How can the rate constant be obtained from a rate-concentration graph with second order?

A
  • As this rate-concentration graph is a curve, the rate constant cannot be obtained directly from this graph.
  • By plotting a second graph of the rate against the concentration SQUARED, the result is a straight line through the origin.
  • The gradient of this straight line graph is equal to the rate constant, k.
28
Q

What is the initial rate?

A

The instantaneous rate at the start of a reaction when the time t=0.

29
Q

How can the initial rate be found?

A

By measuring the gradient of a tangent drawn at t=0 on a concentration-time graph.

30
Q

What is a clock reaction?

A

This is when the time from the start of an experiment is measured or a visual change to be observed, e.g a colour or precipitate.
- The initial rate is proportional to 1/t

31
Q

Describe the iodine clock procedure.

A

1) Separate experiments are carried out using different concentrations of one of the reactants and all the other concentrations are kept constant.
2) The colour change is delayed by including a small amount of another chemical e.g aqueous sodium thiosulfate. (Na2SO3) which removes iodine as it forms. When the chemical is used up, the blue black colour appears.
3) The time is measured for the blue black colour of starch to appear.
4) The initial rate is proportional to 1/t .

32
Q

Why is an overall chemical equation unlikely in some events?

A
  • A reaction can only take place when particles collide.
  • For a reaction to take place, all of the reactants would have to collide together simultaneously, which is an extremely unlikely event
33
Q

What is reaction mechanism?

A
  • The series of steps that make up an overall reaction
34
Q

What is the rate determining step?

A

The slowest step in the sequence

35
Q

What does the rate equation only include?

A

It only includes reacting species involved in the rate determining step

36
Q

What is the Arrhenius equation?

A

The Arrhenius equation is an exponential relationship between the rate constant k and temperature T.

37
Q

What graph can we plot using the logarithmic form of the Arrhenius equation?

A

In k (y-axis) against 1/T (x-axis)

38
Q

What is ln K?

A

y axis

39
Q

What is -Ea/R ?

A

The gradient

40
Q

What is 1/T?

A

x axis

41
Q

What is ln A?

A

c intercept

42
Q

What is the effect on k when activation energy is increased?

A

Rate constant decreases

43
Q

What is the effect on k when temperature is increased?

A

Rate constant increases

A-Level Chemistry (27 decks)

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