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chemistry > R2.2 - how fast? the rate of chemical change > Flashcards

R2.2 - how fast? the rate of chemical change Flashcards

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1
Q

What is a rate?

A

Reciprocal value of time and has the units of s-1

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2
Q

What is a rate of reaction?

A

Change in concentration or a product or a reactant over time. It can be measured by:
- Increase in concentration of products
- Decrease in concentration of reactants
- Measured in mol dm-3 s-1

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3
Q

What is a tangent?

A

Straight line with the same gradient as the point it touches on a gaph.

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4
Q

What are the 6 methods of measuring rates of reactions?

A

1) Change in volume
2) Change in mass
3) Change in the intensity of a specific wavelength of light being absorbed
4) Change in concentration measured using titration (quenching)
5) Change in concentration using conductivity
6) Time for cross to disappear - clock reaction

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5
Q

How do you calculate rate of reactions?

A

1) Increase in product concentration / time
2) Decrease in reactant concentration / time

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6
Q

What is the kinetic molecular theory of matter?

A

Particles in a substance move randomly as a result of kinetic energy that they possess and due to the random nature of these movements and collisions, all particles in a substance will have a range of kinetic energy values.

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7
Q

What is absolute temperature?

A

Lowest temperature when the movement of all particles stop.

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8
Q

What is the relationship with temperature and kinetic energy?

A

As temperature increases, kinetic energy increases; directly proportional relationship.

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9
Q

What happens to kinetic energy and temperature as you move from solid to gas?

A

Temperature and kinetic energy both increase.

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10
Q

What is the Maxwell-Boltzmann energy distribution curve?

A

shows the number of particles having a specific value of kinetic energy against the values of kinetic energy

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11
Q

What does the Maxwell-Boltzmann curve tell us about distribution?

A
  • large number of molecules with less energy
  • small number of molecules with more energy
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12
Q

What is required for a reaction to occur?

A
  • enough energy
    • correct orientation
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13
Q

What is activation energy?

A

minimum value of kinetic energy which particles must have so that they can react
- energy is required so that the transition state between reactants and products can occur

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14
Q

What 5 factors affect the rate of reaction?

A
  • temperature
  • surface area
  • pressure
  • volume
  • catalysts
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15
Q

How can temperature affect the rate of reaction?

A

as temperature increases, the number of molecules that have significant activation energy increases (area under the line to the right increases) and so does kinetic energy
- collision frequency also increases as the particles will be moving faster so there will be more successful collisions

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16
Q

How can temperature affect the Maxwell-Boltzmann curve?

A

The peak shifts to the right, is broader and lower.
- Same area under the curve.
- Majority of the particles have higher energy.

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17
Q

How can concentration affect the rate of reaction?

A

Increasing concentration increases the rate of reaction as the particles are closer together and the frequency of successful collisions increases.
However, as the reaction proceeds, the rate slows down as concentration drops when the reactants are being used up.

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18
Q

How can concentration change be seen in a graph?

A

Lower concentration:
- Less product made, so a lower plateau.
- Less steep gradient
Higher concentration:
- More product made, so a higher plateau.
- More steep gradient

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19
Q

How can pressure affect the rate of reaction?

A

Increasing pressure increases the rate of reaction as the particles are closer together, meaning there is less space in the container and the frequency of successful collisions increases.
- Only occurs in gases.

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20
Q

How can pressure change be seen on a graph?

A
  • same amount of product made
  • less steep for a lower pressure
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21
Q

How can surface area affect the rate of reaction?

A

smaller solids –> surface area increases
- this means that there is a larger surface area for particles to collide on, increasing the chance of successful collisions and the rate of reaction

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22
Q

How can surface area change be seen in a graph?

A
  • Same amount of product made
  • Smaller surface area - less steep gradient
  • Larger surface area - more steep gradient
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23
Q

How can catalysts affect the rate of reaction?

A

Catalysts increase the rate of a reaction without being used up by providing an alternate reaction pathway with a lower activation energy, causing the frequency of successful collisions to increase and the rate increases as more collisions occur with greater than minimum energy needed to overcome the activation energy barrier.

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24
Q

How can catalysts be seen on the Maxwell-Boltzmann curve?

A

Shifts the activation energy line to the left, so more particles have sufficient energy to react.

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25
Q

What are systematic errors?

A

When all measurements are higher or lower than the expected value.

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26
Q

What are random errors?

A

Errors in individual measurements due to equipment, changes in surrounding, human error, misinterpreted results or insufficient number of trials.

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27
Q

What is a reaction mechanism?

A

When all reactions involve a sequence of steps.

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28
Q

What are the individual steps in a reaction mechanism known as?

A

Elementary steps

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29
Q

What is the rate-determining step?

A

The step that determines the rate of reaction is the slowest step, or the first step in a reaction.
- This is because the products can only appear as fast as the products in this elementary step, so this step determines the rate of the whole reaction.

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30
Q

What is the molecularity of an elementary step?

A

The number of reacting particles taking part in that step.

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31
Q

What are energy profiles used for?

A

Can be used to show the activation energy and transition state of the rate-determining step in a multistep reaction.

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32
Q

What is a reaction intermediate?

A

Products of elementary steps that are often used in the next step until the final product is obtained.

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33
Q

What are the 3 types of molecularity?

A

Unimolecular - 1 reactant
Bimolecular - 2 reactants
Termolecular - 3 reactants

34
Q

How can the rate-determining step be interpreted on an energy profile diagram?

A

The rate-determining step will be the step with the highest activation energy.
- This will also be the activation energy for the whole reaction.

35
Q

What do the number of peaks in an energy profile diagram show?

A

Number of peaks - number of elementary states.

36
Q

How is the activation energy shown on an energy profile diagram?

A

Difference from reactants to transition state.

37
Q

How is the overall energy change shown on an energy profile diagram?

A

Difference between the reactants and the products.

38
Q

When do transition states exist?

A

Transition states exist only as bonds are being broken or formed in the formation of products from reactants, so cannot be detected or isolated.

39
Q

What does the rate equation depend on?

A

The rate equation depends on the mechanism of the reaction and can only be determined experimentally.

40
Q

What is the order of a reaction?

A

The order of a reaction with respect to a reactant is the exponent to which the concentration of the reactant is raised in each rate equation.
- It describes the number of particles taking part in each rate-determining step.

41
Q

What is the overall reaction order?

A

Sum of orders with respect to each reactant.

42
Q

Why is rate calculated from a tangent drawn from the origin?

A

Rates are calculated from the origin line as at (0,0), there are only pure reactants and beyond this point, products are formed which can limit the reaction and slow it down.

43
Q

What are 3 ways graphs can be drawn to illustrate rates?

A
  1. Raw data - time vs volume
  2. Rate vs time
  3. Rate vs absorbance
44
Q

What is a zero-order reaction?

A

Flat horizontal line - change in concentration doesn’t affect rate.

45
Q

What is a first-order reaction?

A

Directly proportional - straight line

46
Q

What is a second-order reaction?

A

Exponential - curved line

47
Q

What happens if there is zero order in a reaction?

A

There is no rate expression, meaning that it doesn’t appear in the reaction rate.

48
Q

How do you calculate the rate for a reaction?

A

Rate = k [A]^m [B]^n
m is the order of reaction with respect to A
n is the order of reaction with respect to B
m + n = overall order of reaction

49
Q

What are 3 ways we can find the order of a reaction?

A
  1. Shape of the graph
  2. Half-life
  3. Table of results
50
Q

How can we use the shape of a graph to find the order of a reaction?

A

First order - straight line graph
Second order - curved graph
Zero order - flat horizontal line

51
Q

How can we use half-lives to prove that a reaction is first order?

A

If a reaction is first order, the half-life will be consistent throughout the whole reaction.

52
Q

How can we determine the order of a reaction using a table?

A
  1. Take values where the concentrations of different substances is the same in one experiment, but changes for one substance in another experiment to see how it affects rate.
  2. Take values where the concentration of one substances is the same for two experiments, and changes between experiment 1 and 2 for another substance.
53
Q

What is the rate expression linked to?

A

The rate expression is not linked to the co-efficients in the balanced equation, but to the co-efficients in the rate-determining step

54
Q

What happens if a reactant is not involved in the rate-determining step?

A

It will not feature in the rate expression or will be zero order.

55
Q

What is the change in rate of a zero-order reaction if [A]:
1) doubles
2) triples
3) increases four-fold

A

No change occurs

56
Q

What is the change in rate of a first-order reaction if [A]:
1) doubles
2) triples
3) increases four-fold

A

1) rate doubles (x2)
2) rate triples (x3)
3) rate increases four-fold (x4)

57
Q

What is the change in rate of a second-order reaction if [A]:
1) doubles
2) triples
3) increases four-fold

A

1) rate x4 (2^2)
2) rate x 9 (3^2)
3) rate x16 (4^2)

58
Q

What affects the rate constant k?

A
  • k is temperature dependent
59
Q

How are the units of k determined?

A

The units of k are determined from the overall order of the reaction.

60
Q

What are the units of rate?

A

mol dm-3 s-1

61
Q

What does the power show?

A

It shows the order of the graph.
- Cannot be used to work out k on its own from the rate expression; requires extra evidence using a table or graph.

62
Q

How do you calculate k?

A

k = rate (mol dm-3 s-1) / concentration

63
Q

What are the units of k if:
rate = k?

A

mol dm-3 s-1

64
Q

What are the units of k if:
rate = k [A]

A

s-1

65
Q

What are the units of k if:
rate = k [A]^2

A

mol-1 dm3 s-1

66
Q

What are the units of k if:
rate = k [A]^3

A

mol-2 dm6 s-1

67
Q

How is the value of k calculated?

A

The value of k can be calculated from the rate equation when the concentrations of reactants and the corresponding rates are known.

68
Q

What is the Arrhenius equation?

A

k = Ae ^-Ea / RT

69
Q

How does the Arrhenius equation determine activation energy?

A

The Arrhenius equation uses the temperature dependence of the rate constant to determine activation energy.

70
Q

What does the Arrhenius factor take into account?

A

The Arrhenius factor (frequency of pre-exponential factor) takes into account the frequency of collisions with proper orientation, based on collision geometry.

71
Q

Why does increasing temperature increase the rate of reaction?

A

More particles have the required activation energy, leading to more successful collisions for the reaction to occur.

72
Q

What 2 things does the rate of reaction depend on?

A
  • The rate constant, k
  • Concentrations of reactants, raised to a power
73
Q

What is k dependent on?

A

k is dependent on temperature
- it is a general measure of the rate of reaction at a particular temperature

74
Q

What occurs to the rate of reaction if activation energy is large?

A
75
Q

What occurs to rate of reaction when activation energy is small?

A
76
Q

What is the relationship between rate constant, rate of reaction and activation energy?

A

Reaction rate, and rate constant, are proportional to the activation energy.

77
Q

How can you calculate activation energy from a graph?

A

Activation energy = gradient of line x gas constant (8.31)

78
Q

What is the method for calculating activation energy?

A
  1. Convert degrees to Kelvin.
  2. Calculate 1/T.
  3. Calculate lnk.
  4. Draw graph of ln k vs 1/T.
  5. Calculate gradient of line.
  6. Ea = gradient x R
  7. Divide by 1000 to convert J to kJ.
79
Q

How can we calculate A, Arrhenius constant, using the graph?

A

A = e^(y-intercept)
y-intercept = ln A

80
Q

How can we calculate activation energy from values of the rate constant at only 2 temperatures?

A

Solving simultaneous equations

81
Q

What is the effect of increasing temperature by 10K?

A

Doubles the value of the rate constant and the rate of reaction when activation energy range is fairly narrow.

chemistry (22 decks)

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