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Chemistry IB > 16 HL Chemical Kinetics > Flashcards

16 HL Chemical Kinetics Flashcards

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1
Q

What is the rate equation when the concentration of the reactant is proportional to the rate of reaction?

A

This leads to a very common rate expression:
Rate ∝ [D] or Rate = k[D]

This rate expression means that if the concentration of D is doubled, then the rate doubles
Equally, if the concentration of D halves, then the rate halves

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2
Q

What is the rate equation for this reaction? A (aq) + B (aq) → C (aq) + D (g)

A

Rate of reaction = k [A]^m [B]^n

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3
Q

How can rate equations be determined?

A

Rate equations can only be determined experimentally and cannot be found from the stoichiometric equations

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4
Q

WHat are A and B and the exponents M and N in the rate equation k [A]^m [B]^n?

A

In the above rate equation:
[A] and [B] are the concentrations of the reactants
m and n are orders with respect to each reactant involved in the reaction

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5
Q

Can a) products b) catalysts c) intermediates be present in the rate equation?

A

Products and catalysts may feature in rate equations
Intermediates do not feature in rate equations

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6
Q

What is the order of a reactant?

A

The order of a reactant shows how the concentration of a chemical, typically a reactant, affects the rate of reaction

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7
Q

What is the order of reaction in mathematical terms?

A

It is the power to which the concentration of that reactant is raised in the rate equation

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8
Q

What numbers can the rate equation be?

A

The order can a positive, negative or fractional value
Orders that are a fraction suggest that the reaction involves multiple steps

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9
Q

Is a reactant with order of 0 included in the rate of equation?

A

When the order of reaction with respect to a chemical is 0
Changing the concentration of the chemical has no effect on the rate of the reaction
Therefore, it is not included in the rate equation

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10
Q

How does a reactant of order 1 change the rate of the equation?

A

When the order of reaction with respect to a chemical is 1
The concentration of the chemical is directly proportional to the rate of reaction, e.g. doubling the concentration of the chemical doubles the rate of reaction
The chemical is included in the rate equation

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11
Q

How does a reactant of order 2 change the rate of the equation?

A

When the order of reaction with respect to a chemical is 2
The rate is directly proportional to the square of the concentration of that chemical, e.g. doubling the concentration of the chemical increases the rate of reaction by a factor of four
The chemical is included in the rate equation (appearing as a squared term)

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12
Q

What is the overall order of the reaction?

A

The overall order of reaction is the sum of the powers of the reactants in a rate equation

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13
Q

How can you determine the order of a reactant?

A
  1. Identify two experiments where the concentration of one reactant changes, but the concentrations of all other reactants are constant
  2. Calculate what happens to the concentration
  3. Calculate what happens to the rate of reaction
  4. Deduce the order of reaction with respect to that chemical
  5. Repeat this for all of the reactants, one at a time, until you have determined the order with respect to all reactants
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14
Q

How would you construct the rate equation?

A

Once the order with respect to all of the reactants is known the rate equation can be constructed
Zero order reactants are not included in the rate equation
First order reactants are included in the rate equation - they do not require a power
Second order reactants are included in the rate equation - they are raised to the power of 2

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15
Q

What is the rate constant?

A

The rate constant, (k), of a reaction can be calculated using the initial rates and the rate equation

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16
Q

How can you calculate k from the rate equation? - equation used)

A

k = rate/concentration of reactants

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17
Q

How would you find the units of k?

A

When you are asked to calculate the rate constant, k, for a reaction you must also be able to deduce the units
This is done by replacing the values in the rearranged rate equation with the units of that value
The units can then be combined or cancelled as required

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18
Q

How does k change as rate of reaction increases?

A

As the rate of reaction increases the rate constant will increase

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19
Q

How does increasing temperature affect rate constant?

A

Therefore, increasing the temperature also increases the value of the rate constant, k, assuming that the concentration of the reactants remains unchanged
An exponential relationship between the rate of reaction and temperature is observed when seen on a graph (exponential curve)

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20
Q

How much does the rate of reaction increase every 10 degrees?

A

The graph shows that the rate of reaction roughly doubles with an increase of 10 oC
This general relationship does not apply to all reactions
Also, it is not necessarily every 10 oC, the rate may double every 9 °C or 11 °C
The number of degrees needed to double the rate also changes gradually as temperature increases

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21
Q

What is the graph for the concentration of a reaction against time for a zero-order reaction?

A

In a zero-order reaction, the concentration of the reactant is inversely proportional to the time
This means that the reactant concentration decreases as time increases
The graph is a straight line going down

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22
Q

What is the value of the gradient of the zero-order reaction graph?

A

The gradient of the line is the rate of reaction
Calculating the gradient at different points on the graph, will give a constant value for the rate of reaction

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23
Q

What is the rate equation for a zero-order reaction?

A

When the order with respect to a reactant is 0, a change in the concentration of the reactant has no effect on the rate of the reaction
Therefore:
Rate = k

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24
Q

What does the gradient of a zero-order reaction graph represent?

A

This equation means that the gradient of the graph is the rate of reaction as well as the rate constant, k

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25
Q

What does the graph look like for a first-order reaction, and why?

A

In a first-order reaction, the concentration of the reactant decreases with time
The graph is a curve going downwards and eventually plateaus

(reverse exponential)

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26
Q

What does the graph look like for a second-order reaction, and why?

A

In a second-order reaction, the concentration of the reactant decreases more steeply with time
The concentration of reactant decreases more with increasing time compared to a first-order reaction
The graph is a steeper curve going downwards:

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27
Q

What can an order of reaction also be determined from?

A

The order of a reaction can also be deduced from its half-life (t1/2 )

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28
Q

How does the half-life change with time for a zero-order reaction?

A

For a zero-order reaction the successive half-lives decrease with time
This means that it would take less time for the concentration of reactant to halve as the reaction progresses
(straight line horizontally down)

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29
Q

How does the half-life change with time for a first-order reaction?

A

The half-life of a first-order reaction remains constant throughout the reaction
The amount of time required for the concentration of reactants to halve will be the same during the entire reaction
(less steep down exponential curve)

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30
Q

How does the half-life change with time for a second-order reaction?

A

For a second-order reaction, the half-life increases with time
This means that as the reaction is taking place, it takes more time for the concentration of reactants to halve

31
Q

What does the zero-order reaction, reaction rate vs concentration of reaction graph look like and why?

A

In a zero-order reaction, the rate doesn’t depend on the concentration of the reactant
The rate of the reaction therefore remains constant throughout the reaction
The graph is a horizontal line
The rate equation is rate = k

32
Q

What does the first-order reaction, reaction rate vs concentration of reaction graph look like and why?

A

In a first-order reaction, the rate is directly proportional to the concentration of a reactant
The rate of the reaction increases as the concentration of the reactant increases
This means that the rate of the reaction decreases as the concentration of the reactant decreases when it gets used up during the reaction
The graph is a straight line
The rate equation is rate = k[A]

33
Q

What does the second-order reaction, reaction rate vs concentration of reaction graph look like and why?

A

In a second-order reaction, the rate is directly proportional to the square of concentration of a reactant
The rate of the reaction increases more as the concentration of the reactant increases
This means that the rate of the reaction decreases more as the concentration of the reactant decreases when it gets used up during the reaction
The graph is a curved line
The rate equation is rate = k[A]2

34
Q

WHy is the slowest step the rate-determining step?

A

A chemical reaction can only go as fast as the slowest part of the reaction
So, the rate-determining step is the slowest step in the reaction

35
Q

Will a reactant in the rate-determining step also appear in the rate equation?

A

YES
If a reactant appears in the rate-determining step, then the concentration of that reactant will also appear in the rate equation

36
Q

What is molecularity?

A

Molecularity is the number of reactant particles that participate in the rate-determining step

37
Q

What is a unimolecular reaction?

A

Unimolecular: one species involved in the rate-determining step

38
Q

What is a bimolecular reaction?

A

Bimolecular: two species involved in the rate-determining stepBimolecular: two species involved in the rate-determining step

39
Q

How is the intermediate derived?

A

The intermediate is derived from substances that react together to form it in the rate-determining step

40
Q

How can the rate-determining step be identified?

A

The rate-determining step can be identified from a rate equation given that the reaction mechanism is known

41
Q

From the rate equation Rate = k[CH3CH2CH3][OH-], and reaction equation, CH3CH2CH3 + Br2 + OH- → CH3CH2CH2Br + H2O + Br-
, how can it be determined what the rate-determining step is?

A

From the rate equation, it can be deduced that only CH3CH2CH3 and OH- are involved in the rate-determining step and not bromine (Br2)
CH3CH2CH3 and OH- are only involved in the first step of the reaction mechanism, therefore the rate-determining step is:
CH3CH2CH3 + OH- → CH3CH2CH2- + H2O

42
Q

What energy level will the transition state have?

A

When any reacting molecules collide with bond breaking and bond formation occurring, the interacting molecules will be in an unstable, high-energy state temporarily

This transition state will be of a higher energy than either the reactants or products and corresponds to the activation energy

43
Q

What bonds are formed between iodine and hydrogen as a reaction proceeds?

A

As the reaction proceeds, covalent bonds start to form between the hydrogen and iodine atoms from the hydrogen and iodine molecules

44
Q

Do these bonds become weaker / stronger? What does this result in? H and I

A

At the same time, the covalent bonds within the hydrogen and iodine molecules grow longer and become weaker
This results in the transition state complex shown
(square H—-H
| |
I– — I

45
Q

How is hydrogen iodide formed from the transition state?

A

From this transition state, the bonds between the hydrogen and iodine atoms can continue to grow shorter and stronger resulting in the formation of hydrogen iodide

46
Q

How are hydrogen and iodine formed back from the transition state?

A

Alternatively, the bonds within the hydrogen and iodine molecules can grow shorter and stronger which would result in the formation of the reactants

47
Q

What conditions must be met for the transition state to convert into hydrogen iodide?

A

Hydrogen iodide will only form if the hydrogen and iodine molecules collide with kinetic energy greater than or equal to the activation energy
The molecules will also need to collide in the correct orientations

48
Q

There considering activation energy, which step is the rate-determining step?

A

The reaction, or elementary, step with the greatest activation energy will be the rate-determining step and can be used to determine the rate equation

49
Q

Why is this reaction unlikely to occur in a single step?
2NO2 (g) + F2 (g) → 2NO2F (g)

A

This reaction is unlikely to occur in a single step as that would require three molecules to collide in the correct orientation and with sufficient kinetic energy
This is even less likely to occur as all three molecules are gaseous

50
Q

Which is the rate-determining step?
Step 1: NO2 + F2 → NO2F + F
Step 2: NO2 + F → NO2F
(rate equation Rate = k[NO2][F2])

A

Step 1 must be the rate-determining step as it is the only step that has reactants matching the rate equation
Therefore, on a potential energy level diagram the activation energy for step 1 will be greater than the activation energy for step 2

51
Q

What is the transition state?
Step 1: NO2 + F2 → NO2F + F
Step 2: NO2 + F → NO2F
(rate equation Rate = k[NO2][F2])

A

The transition state for this multi-step reaction must be:
NO2 + NO2F + F

This can be deduced using the the equation for elementary step 1 and the overall equation
The overall equation states that two NO2 react with one F2
Elementary step 1 uses one NO2 to form NO2F + F
This leaves one NO2 along with NO2F + F

52
Q

Therefore how many peaks are there on this potential energy level diagram for the formation of nitryl fluoride?

A

2

53
Q

What are 3 key features of the potential energy level diagram for the formation of nitryl fluoride?

A

The overall reaction is endothermic, as stated
The rate-determining step is the step that has the greatest activation energy
There is a labelled energy level for the transition state

54
Q

How can a catalyst be identified in a rate mechanism?

A

When a rate equation includes a species that is not part of the chemical reaction equation then this species is a catalyst
being used in step 1 and forming as a product in step 2

55
Q

Can a catalyst be part of the rate equation?

A

YES

56
Q

WHat can chemical kinetics be used for and what can they not prove?

A

Chemical kinetics can only suggest a reaction mechanism, they cannot prove it
However, they can be used to disprove a proposed mechanism

57
Q

What are elementary steps?

A

Elementary steps are the steps involved in a reaction mechanism

58
Q

What is it important the elementary steps agree with?

A

It is important that the elementary steps for a proposed mechanism agree with the overall stoichiometric equation

59
Q

When is k constant?

A

However, k only remains constant if the concentration of the reactants is the only factor which is changed
If the temperature is changed or a catalyst is used or changed, then the rate constant, k, changes

60
Q

What does the Arrhenius equation represent? (in data booklet)

A

The relationship between the rate constant, the temperature and also the activation energy is given by the Arrhenius equation:

61
Q

What is A in the Arrhenius equation?

A

A - arrhenius constant ( a constant related to the collision frequency and orientation of the molecules)

62
Q

What is R in the Arrhenius equation?

A

R = gas constant 8.31 J K-1 mol

63
Q

What are the units of Ea in the Arrhenius equation?

A

J mol^-1

64
Q

What are the units of T in the Arrhenius equation?

A

kelvin

65
Q

WHat can be done to the Arrhenius equation for it to be easier to use?

A

The Arrhenius equation is easier to use if you take natural logarithms of each side of the equation, which results in the following equation:

66
Q

How does the Arrhenius equation show the effect that a change in temperature has on the rate constant?

A

An increase in temperature (higher value of T) gives a greater value of ln k and therefore a higher value of k
Since the rate of the reaction depends on the rate constant, k, an increase in k also means an increased rate of reaction

67
Q

How does the Arrhenius equation show the effect that a change in activation energy has on the rate constant?

A

An increase in the activation energy, Ea, means that the proportion of molecules which possess at least the activation energy is less
This means that the rate of the reaction, and therefore the value of k, will decrease

68
Q

How are K and T determined? What can the Arrhenius equation then be used to calculate?

A

The values of k and T for a reaction can be determined experimentally
These values of k and T can then be used to calculate the activation energy for a reaction

69
Q

What is the Arrhenius equation most commonly used to calculate?

A

Very often, the Arrhenius equation is used to calculate the activation energy of a reaction

70
Q

What graph can be plotted to calculate Ea?

A

A graph of ln k against 1/T can be plotted, and then used to calculate Ea
This gives a line which follows the form y = mx + c

71
Q

How can the Arrhenius equation be rearranged to form the equation of a straight line, and define the variable y, x , m and c

A

lnk = -Ea/R 1/T + lnA
y=lnk
x=1/T
m=-Ea/R (gradient!!!!)
c = lnA (y-intercept)

72
Q

How would you find the Ea by using the Arrhenius equation?

A

find gradient and multiply by R (8.31)

73
Q

By looking at two lines, will the steeper or not steeper one have a higher activation energy?

A

The reaction with a steeper gradient has the higher activation energy, Ea
This indicates that the rate constant, and therefore rate, will change quicker with temperature changes

74
Q

Apart from using a graph, how can Ea be calculated?

A

The activation energy, Ea, can be calculated using rate constant values, k1 and k2, for two given temperatures, T1 and T2
This requires the use of the following equation that is given in the data booklet:

ln(k1/k2) = Ea/R (1/T2 - 1/T1)
K1 and K2 are two different k values, same with T1 and T2

Chemistry IB (13 decks)

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