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Chemistry > Chapter 18 > Flashcards

Chapter 18 Flashcards

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1
Q

What is the rate of reaction?

A

It’s the rate at which products are formed or reactants are used up.

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2
Q

How do you calculate rate of reaction?

A

change in concentration / change in time

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3
Q

What are the units for rate?

A

moldm-3s-1

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4
Q

What do square brackets mean?

A

‘concentration of’

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5
Q

What is zero order?

A

It is when the reactant concentration has no affect on the rate of reaction. The reaction is zero order with respect to the reactant. The concentration doesn’t influence the rate.

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6
Q

What is first order?

A

A reaction is first order with respect to reactant when the rate depends on its concentration rate the power of one (it will change by the same factor).

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7
Q

What is second order?

A

A reaction in second order with respect to a reactant when the rate depends on its concentration is raised the power of 2 (squared).

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8
Q

What is the rate of reaction proportional to?

A

The concentration of a particular reactant raised to a power.

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9
Q

What are the three common orders?

A

Zero, first, second.

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10
Q

Why is the rate equation used?

A

As it gives the mathematical relationship between the concentrations of the reactants and the reaction rate.

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11
Q

What is K?

A

It is the proportionality constant. It is temperature dependent. Its the number which mathematically converts between the rate of reaction and concentrations and orders.

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12
Q

How do you find the units of K?

A

You rearrange the equation and sub in all other units cancelling where possible.

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13
Q

What is the overall order?

A

It gives the overall effect of the concentration of a reactant in the reaction. Overall order = sum of orders with respect to each reactant.

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14
Q

How are orders of reactions determined?

A

Experimentally by monitoring how a physical quantity over a period of time changes. Orders cannot be found directly from the chemical equation.

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15
Q

What is the initial rate?

A

It is the instantaneous rate at the beginning of an experiment when t=0

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16
Q

What is continuous monitoring?

A

It is when continuous measurements are taken during a course of a reaction this means a concentration time graph can be plotted.

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17
Q

What are the chain methods for continuous monitoring to produce a gas?

A
  1. Monitoring by gas collection

2. Monitoring by mass loss

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18
Q

If the gas is not produced what are the property could be useful when measuring the rate?

A

A colour change

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19
Q

Why is a colour change a useful property?

A

As it can be estimated by eye or monitored using a colorimeter.

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20
Q

What does the filter do in a colorimeter?

A

It controls the wavelength of light which passes through the coloured solution.

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21
Q

What is measured measured using a colorimeter?

A

The amount of light absorbed by a solution.

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22
Q

What is absorbance proportional to?

A

The concentration of the coloured substance.

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23
Q

What sort of technique is colorimetry?

A

It is quantitive where the intensity of colour is followed using a colorimeter,

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24
Q

What has to maintain effectively constant in relation to the order?

A

All other reactant concentrations must remain effectively constant so that the order with respect to reactant can be obtained.

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25
Q

What does the zero order concentration time graph look like?

A

The graph has a straight line with a negative gradient. The gradient remains constant which indicates that the reaction rate doesn’t change at all during the reaction.

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26
Q

What is implied with the zero order concentration time graph in terms of the gradient value?

A

It is equal to the rate constant

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27
Q

What does the first order concentration time graph look like?

A

It produces a downward curve with a decreasing radiant overtime. As the gradient decreases the time the reaction slows down gradually. It is proportional to the decreasing concentration. The concentration of reactants to have is constant (half life).

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28
Q

What is half life?

A

It’s the time for the concentration to decrease by half

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29
Q

How can the rate constant be determined for a first-order concentration time graph?

A

Using the half life

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30
Q

What filter is used in the colorimeter?

A

A filter which has a complementary colour, the wavelength then selects the wavelengths absorbed by the solution. Coloured objects absorb the complementary colour to what they see.

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31
Q

How do use a colorimeter when monitoring a colour change?

A

At the start of the reaction a sample of the reactant mixture can be placed in the colorimeter and it’s all the absorbance measured at regular time intervals. When measuring absorbance the light absorbed is compared to that of sold by a colourist reference solution. Firstly we ‘zero’ the colorimeter using water so therefore there are no units for absorbance.

32
Q

What does the second order graph show (NOT NEEDED)?

A

It is a steeper downward curve, and tails off more slowly. The half life is not constant.

33
Q

When do you need to produce a calibration curve?

A

When you want to find the actual concentration. This means you will need to construct and prepare a suitable range of solutions of known concentrations and measure their absorbance.

34
Q

Why are first order reactions known to have exponential decay?

A

As they have a constant half life, with the concentration halving every half-life.

35
Q

How can you confirm if a graph is first order on a conc-time graph

A

If there are successive half-lives which are constant.

36
Q

How do you work out the rate constant from a graph?

A

A tangent to the curve is drawn at a particular concentration. The gradient of the tangent is calculated giving the rate of reaction. The rate constant is calculated by rearranging the rate equation as substituting the rate (gradient) and the concentration of the position where the tangent has been drawn.

37
Q

Describe the colorimeter experiment.

A
  1. Prepare a standard solution of a known concentration of iodine.
  2. Select a filter with a complementary colour to the coloured chemical ie for iodine that is a green/blue filter, but the colorimeter will tell you which setting to use.
  3. Zero the colorimeter with water and then measure the absorbance of iodine.
  4. Plot a calibration curve of absorbance against iodine concentration. You can now convert the absorbance into conc of iodine.
  5. Carry out a reaction between propanone and iodine and take absorbance readings of reacting mixtures at measured intervals. 6. Use the calibration curve to measure iodine conc and absorbance.
  6. Plot concentration of iodine against time. from which you can now determine the order of the reaction with respect to the coloured chemical.
38
Q

How do work out rate constant by half life?

A

k= ln2/half life

39
Q

Why are rate concentration graphs useful?

A

As they offer a route into the direct link between rate and concentration in the rate equation.

40
Q

What does the zero order rate concentration graph look like?

A

It is a horizontal straight line with a zero gradient so rate= K. The y-intercept is the rate constant, the reaction rate doesn’t change with increasing concentration.

41
Q

What does the first order rate concentration graph look like?

A

The first order graph has a straight line through the origin so rate = k[A]. The rate is then directly proportional to the concentration for a first order relationship. The rate constant can be determined by working out the gradient.

42
Q

What does the second order rate concentration graph look like?

A

It is an upward curve with increasing gradient so rate = k[A]2. As the graph is a curve the rate constant can’t be obtained directly from this graph. By putting the concentrations squared graph a straight line is produced which goes through the origin, the gradient is an equal to the rate constant.

43
Q

How can you find the initial rate?

A

By measuring the gradient of a tangent at t=0 on the graph.

44
Q

How do you plot a rate- conc graph: initial rates method?

A
  1. Set up 5-6 experiments each with a different initial concentration of the reactant you wish to find the order of.
  2. Monitor continuously and plot the progress of this reaction .
  3. For each curve measure the initial rate
  4. Plot the initial rate against the initial conc of the reactant, to find the order of the reactant.
45
Q

What reaction is a more convent way of obtaining the initial rate using a single measurement?

A

A clock reaction

46
Q

What happens in a clock reaction?

A

Time from the start of the experiment is measured for a visual change to be observed often a colour or precipitate. As long as there is no significant change in rate during this time, the average rate will then be the same as the initial. This reaction is repeated several times with different concentrations and values of 1/t are calculated for each experimental run.

47
Q

What is initial rate proportional to?

A

1/t

48
Q

How do you plot a rate- conc graph using a clock reaction?

A
  1. Record the time taken for each experiment to reach a specific reactant concentration.
  2. Pick a concentration less than 15% of the extent of the reaction, at this point the curve is effectively a straight line and equivalent to the tangent.
  3. The initial rate is calculated of the concentration x and divide it by time taken X is the same for each curves so initial rate is proportional to 1/t
49
Q

What happens during an iodine clock?

A

Iodine (aq) is an orange/ brown colour. Time is measured from the start of the reaction to when the colour appears. Starch is usually added as it form a complex with iodine which forms an intense blue/black colour.

50
Q

Describe the iodine clock procedure?

A

To find the order withe respect to iodine, the concentration of S2O8 is kept the same.
A small quantity of sodium thiosulfate solution and a small quantity of starch solution is added in all experiments, the amount is kept the same for all.
The S2O8 ions react with iodine as soon as it’s produced, the solution remains colourless.
When all S2O8 ions are gone the iodine product will start to build up, as starch is present the starch iodine complex is now seen immediately. The time taken for the blue/black colour to appear is measured.For each experiment it occurs at the same % of the reaction which has occurred ie when a specific quantity of iodine has been produced.
It can be assumed that initial rate is proportional to 1/t

51
Q

How accurate are clock reactions?

A

In these reactions you are measuring the average rate during the reaction. Its assumed that the rate of reaction is constant and is the same as the initial rate. You measure the average rate of a change over time. The shorter the period of time over which an average rate is measured the less the rate changes over that time period. At a smaller time period the average rate is nearly identical to the initial which implies its more accurate than at a longer time period.

52
Q

What is the purpose of the overall chemical equation?

A

It compares the reactants and the products. The balancing numbers give the stoichiometry, the relative amount of species in the reaction.

53
Q

Why is it unlikely that a reaction will happen in one step when there is more than two species?

A

As it’s less likely that more than two particles will collide at the same time therefore more steps are needed as reactions only occur when particles collide.

54
Q

What is the reaction mechanism?

A

When a series of steps make up the overall reaction.

55
Q

What is meant by the rate determining step?

A

The steps and a multistep reaction will take place at different rates the slowest step in the sequence is called the rate determining step.

56
Q

What does the rate equation include?

A

It includes reacting species involved in the rate determining step. The orders in the rate equation match the number of species involved in the rate determining step. This allows us to use rate equations in order to protect reaction mechanisms.

57
Q

What are halo alkanes hydrolysed by?

A

Aqueous alkali

58
Q

Why are the hydrolysis reaction of halo alkanes investigated experimentally?

A

To determine the order of reaction, the rate equation and possible mechanisms for the reaction.

59
Q

What happens if the rate determining step is not the first step?

A

A reagent from the previous step may affect the rate even though it doesn’t take part in the rate determining step. This would be the case if the reagent in question was necessary for the production of a reagent in the rate determining step,

60
Q

What happens as temperature increases in terms of rate of reaction?

A

The rate rate increases and the value of the rate constant will also increase. For many reactions a 10°C in rise in temperature doubles the rate constant and doubled the rate of reaction.

61
Q

What are the two factors which contribute to the increase rate and rate constant?

A

Increasing the temperature shifts the Boltzmann distribution to the right increasing the proportion of particles which exceed activation energy.
As the temperature increases particles move faster and collide more frequently.

62
Q

What must happen in order for particles to react?

A

With sufficient energy and the correct orientation.

63
Q

How can the rate constant be determined experimentally?

A

Carry out the same experiment at different temperatures and then calculate K at different temperatures.

64
Q

What does the Arrhenius equation show?

A

The exponential relationship between the rate constant and temperature.

65
Q

What is the equation?

A

k = A e ^ -Ea/RT

66
Q

What does the pre-exponential factor take into account?

A

The frequency of collisions with the correct orientation and increases slightly with temperature it can be regarded as remaining constant over a small temperature range.

67
Q

What does the exponential factor represent?

A

The proportion of molecules that exceed the activation energy.

68
Q

What is temperature measured in?

A

Kelvin

69
Q

What is the gas constant?

A

8.314 JmolK

70
Q

What happens if there is no activation energy?

A

The frequency factor that will give the rate.

71
Q

What is the logarithmic form of the Arrhenius equation?

A

lnk= -Ea/RT + lnA

72
Q

What form does the logarithmic form take?

A 
A straight line equation 
y= lnk
m= -Ea/R
x= 1/t 
c= ln A
73
Q

What is the gradient equal to when you plot lnk against 1/t?

A

-Ea/R

74
Q

How do you work out the activation energy?

A

-(gradient x R)

75
Q

How do you work out A

A

You use your y-intercept (lnA)

A= e^ y-intercept

Chemistry (23 decks)

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