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Chemistry > Chapter 18 - Rate of reactions > Flashcards

Chapter 18 - Rate of reactions Flashcards

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1
Q

How are reaction rates measured ?

A

By observing the changes in quantities of reactants or products over time

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2
Q

What is the equation for the rate of reaction ?

A

Change in concentration / change in time

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3
Q

Why is change in concentration and time used ?

A

For consistency throughout the reaction

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4
Q

What is concentration measured in ?

A

Mol/dm^3/s

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5
Q

What is the shorthand format for concentration ?

A
  • [A] = concentration of reactant A

- It has the usual units of concentration

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6
Q

How are rate of reaction and concentration of a particular reactant raised to a power related ?

A

They are proportional

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7
Q

What is the equation for rate of reaction and concentration ?

A

rate is proportional to [A]^n

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8
Q

What is the power ?

A

The order of the reaction

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9
Q

What are the common orders of reaction ?

A
  • Zero order
  • First order
  • Second order
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10
Q

When is the order zero ?

A

When the concentration of a reactant has no effect on the rate

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11
Q

What is the equation for zero order reactions ?

A

rate is proportional to [A]^0

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12
Q

What happens in a zero order reaction ?

A
  • Any number raised to the power of zero is 1

- Concentration does not influence the rate

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13
Q

When is the order one ?

A

When the rate depends on its concentration raised to the power of one

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14
Q

What is the equation for first order reactions ?

A

rate is proportional to [A]^1

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15
Q

What happens in a first order reaction ?

A

If the concentration of A is doubled, the reaction rate increases by a factor of 2^1 = 2

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16
Q

When is the order two ?

A

When the rate depends on its concentration raised to the power of two

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17
Q

What is the equation for second order reactions ?

A

rate is proportional to [A]^2

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18
Q

What happens in a second order reaction ?

A

If the concentration of A is doubled, the reaction rate increases by a factor of 2^2 = 4

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19
Q

What does the rate equation give ?

A

It gives the mathematical relationship between the concentrations of the reactants and the reaction rate

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20
Q

What is the rate equation ?

A

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

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21
Q

What is the rate constant also known as ?

A
  • The proportionality constant

- It is the number that mathematically converts between the rate of reaction and the concentration of orders

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22
Q

What does the overall order of a reaction give ?

A

The overall effect of the concentrations of all reactants on the rate of reaction

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23
Q

How is the overall order of the reaction calculated ?

A

Overall order = sum of orders with respect to each reactant

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24
Q

What do the units of the rate constant depend upon ?

A

The number of concentration terms in the rate equation

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25
Q

How can you determine the units of k ?

A
  • Rearranging the equation to make k the subject
  • Substitute units into the expression for k
  • Cancel common units and show the final units
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26
Q

What are the units for overall order 0 ?

A

mol/dm^3/s

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27
Q

What are the units for overall order 1 ?

A

/s

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28
Q

What are the units for overall order 2 ?

A

dm^3/mol^1/s

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29
Q

What is initial rate?

A

The instantaneous rate at the beginning of an experiment when t=0.

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30
Q

In a concentration time graph, and a rate time graph, what would zero order look like?

A
  • Concentration time - linear to 0 concentration

- Rate time - Horizontal line

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31
Q

In a concentration time graph, and a rate time graph, what would first order look like?

A
  • Concentration time - curve getting closer to zero

- Rate time - Linear proportional

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32
Q

In a concentration time graph, and a rate time graph, what would second order look like?

A
  • Concentration time - steeper curve

- Rate time - proportional to x^2

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33
Q

What are the two ways of continuous monitoring when a gas is produced?

A
  • Gas collection

- Mass loss

34
Q

What is a way of utilising light to monitor rate?

A
  • Using a colorimeter.

- Measuring the amount of light absorbed by a solution.

35
Q

What is half life?

A

The time taken for half of a reactant to be used up.

36
Q

In a concentration time graph, how can you deduce whether the reactant is first or second order?

A
  • Measure multiple half lives.
  • If the half life is constant, it is first order.
  • If the half life is increasing, it is second order.
37
Q

How do you calculate the rate constant from the rate of reaction ?

A

k = rate/concentration

38
Q

How do you calculate the rate constant from the half life ?

A

k = ln|2| / t(1/2)

39
Q

How can rate concentration graphs be plotted ?

A

From measurements of the rate of reaction at different concentrations

40
Q

Why are rate concentration graphs important ?

A

They offer a route into the direct link between the rate and concentration in the rate equation

41
Q

What graph does a zero order reactant produce ?

A

Horizontal straight line with zero gradient

42
Q

What does the rate equal in a zero order reaction ?

A

rate = k

43
Q

How do you find the rate constant from a zero order graph ?

A

The intercept on the y axis gives the rate constant k

44
Q

What is the relationship between the rate and concentration in a zero order reaction ?

A

The reaction rate does not change with increasing concentration

45
Q

What graph does a first order reactant produce ?

A

Straight line graph through the origin

46
Q

What does the rate equal in a first order reaction ?

A

rate = k[A]

47
Q

How do you find the rate constant from a first order graph ?

A

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

48
Q

What is the relationship between the rate and concentration in a first order reaction ?

A

Rate is directly proportional to the concentration

49
Q

What graph does a second order reactant produce ?

A

Upward curve with increasing gradient

50
Q

What does the rate equal in a second order reaction ?

A

rate =k[A]^2

51
Q

How do you find the rate constant from a second order graph ?

A
  • Plot a second graph of rate against concentration squared
    • This will give u a straight line through the origin
  • The gradient of this straight line graph is equal to the rate constant
52
Q

What is the relationship between the rate and concentration in a second order reaction ?

A

Rate increases as concentration increases

53
Q

What is a clock reaction ?

A

A more convenient way of obtaining the initial rate of a reaction by taking a single measurement

54
Q

What do you look for/measure in a clock reaction ?

A

The time t from the start of an experiment is measured for a visual change to be observed

55
Q

What else are you measuring in a clock reaction ?

A
  • An average rate of a change in reactant over time
    • The shorter the period of time over which an average rate is measured, the less the rate changes over that time period
56
Q

What is the initial rate proportional to ?

A

1/t

57
Q

As temperature increases, how does the rate of the reaction change ?

A

The rate increases

58
Q

As temperature increases, how does the rate constant increase ?

A

The rate constant increases

59
Q

For many reactions, what does a 10 degree rise in temperature result in ?

A
  • Doubles the rate constant

- Doubles the rate of the reaction

60
Q

When temp increases, what two factors contribute to the increase in rate ?

A
  • Boltzmann distribution shifts right, increasing the proportion of particles that exceed the activation energy
  • As the temp increases, particles move faster and collide more frequently
61
Q

What is the change in rate mainly determined by ?

A

Activation energy

62
Q

Why is the change in rate mainly determined by the activation energy ?

A

The increased frequency of collisions is comparatively small compared with the increase in the proportion of molecules that exceed Ea from the shift in the boltzmann distribution

63
Q

What is the Arrenhius equation ?

A

An exponential relationship between the rate constant and temperature

64
Q

What is the formula for the Arrenhius equation ?

A

k = A x e^(-E(a)/RT)

65
Q

What is A known as in the Arrenhius equation ?

A

Pre exponential factor

66
Q

What is E(a)/RT known as in the Arrenhius equation ?

A

Exponential factor

67
Q

What does the exponential factor represent ?

A

The proportion of molecules that exceed the activation energy and have sufficient energy for a reaction to take place

68
Q

What is the pre exponential term also called ?

A

The frequency factor

69
Q

What does the frequency factor take into account ?

A

It takes into account the frequency of collisions with the correct orientation

70
Q

When does the frequency factor increase ?

A

Increases slightly with temperature but it is essentially constant over a small temperature range

71
Q

What does the frequency factor essentially give us ?

A

The rate if there were no activation energy

72
Q

What is the logarithmic form of the Arrhenius equation?

A

ln|k| = -E(a)/RT + ln|A|

73
Q

What can the log form of the Arrhenius equation be modelled as?

A
  • A straight line graph in y=mx + c
  • ln|k| is y
  • 1/T is x
  • E(a)/R is gradient
  • ln|A| is y intercept
74
Q

Define stoichiometry

A
  • The balancing numbers

- The relative amounts of the species in the reaction

75
Q

Is it likely for more than two particles to collide at the same time ?

A

No it is unlikely that more than two particles will collide at the same time

76
Q

Why is a single step reaction unlikely ?

A

Multiple ions colliding simultaneously is an extremely unlikely event

77
Q

How are reactions likely to take place ?

A

In a series of steps

78
Q

Define reaction mechanism

A

The series of steps that make up an overall reaction

79
Q

Define rate determining step

A

The slowest step in a multi step reaction pathway

80
Q

How can you predict reaction mechanisms (RDS) ?

A
  • The rate equation includes reacting species involved in the RDS
  • The orders in the rate equation match the number of species involved in the RDS
81
Q

Why is the RDS important ?

A

It provides important evidence in supporting or rejecting a proposed reaction mechanism

82
Q

What does investigating the hydrolysis of haloalkanes determine ?

A
  • The overall order of reaction
  • The rate equation
  • Possible mechanism for the reaction

Chemistry (29 decks)

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