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A Level Chemistry > Module 5: Chapter 18 - Rates of Reaction > Flashcards

Module 5: Chapter 18 - Rates of Reaction Flashcards

1
Q

What is rate of reaction?

A

The change of concentration of reactants/products per unit time

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2
Q

What are the units for rate of reaction?

A

mol dm⁻³ s⁻¹

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3
Q

What is the short hand for concentration of substance “A”?

A

[A]

Square brackets means “concentration of”

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4
Q

What is the order of a reactant in a reaction?

A

The power to which the concentration of a reactant is raised in the rate equation

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5
Q

How does the order of a reactant affect the rate of reaction?

A

Rate ∝ [A]ⁿ

The rate is directly proprtional to the concentration of a particular reactant raised to a power

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6
Q

What are 3 common orders of a reaction?

A
  • Zero order (0)
  • First order (1)
  • Second order (2)
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7
Q

What is a zero order of reaction?

A

A zero order of reaction means that the concentration of a reactant has no effect on the rate, the reaction is zero order with respect to the reactant

Rate ∝ [A]⁰
Rate ∝ 1 (constant)

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8
Q

What is a first order of reaction?

A

A reaction is first order with respect to a reactant when the rate depends on its concentration raised to the power of one. In a first order reaction, if the concentration of A is doubled, the rate of reaction will increase by a factor of 2, and if will increase by a factor of 3

Rate ∝ [A]¹
Rate ∝ [A]

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9
Q

What is a second of order reaction

A

A reaction is second order with respect to a reactant when the rate depends on its concentration squared. If the concentration of A is doubled, the rate of reaction will increase by a factor of 4. If the concentration of A is tripled, the rate of reaction will increase by a factor of 9

Rate ∝ [A]²

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10
Q

What is the rate equation?

A

The mathematical relationship between the concentration of reactants and the rate of reaction:

Rate = k [A]ᵐ [B]ⁿ

k = rate constant

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11
Q

What is the overall order of a reaction?

A

The overall order of a reaction gives the overall effect of the concentrations of all reactants on the rate of reaction

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12
Q

What is the equation for the overall order of reaction?

A

Overall order = sum of orders with respect to each reactant

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13
Q

What are the units of the rate constant?

A

The rate constant does not have a constant unit, it changes depending on the number of concentration terms in the rate equation. However, it can be determined by rearranging the equation to make k the subject, substitute units into the expression for k and the calculating thefinal unit

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14
Q

What would be the units for the rate constant in the rate equation rate = k[A]²[B]?

A

mol⁻² dm⁶ s⁻¹

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15
Q

What would be the units for the rate constant in the rate equation rate = [A]?

A

s⁻¹

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16
Q

What is the rate constant?

A

The proportionality constant, it is the number that mathematically converts between the rate of reaction and concentration and orders

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17
Q

How can the orders of reactants be determined?

A

Through experimental results, they cannot be determined directly from a chemical equation. When comparing the effect of different concentrations of reactants on reaction rates it is important to ensure that the rate is measured after the same amount of time, ideally as close to the beggining of the experiment so that the initial rate is used (which would give the most accurate results)

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18
Q

Determine the rate constant for the following reaction

A

k = 1.28x10⁻² dm³ mol⁻¹ s⁻¹

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19
Q

How can concentration time graphs be plotted?

A

By continuous monitoring of the concentration

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20
Q

What are 3 methods of continuous monitoring of concentration?

A
  • Monitoring by gas collection
  • Monitoring by mass loss
  • Monitoring with a colorimeter
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21
Q

When would continuous monitoring with a colorimeter be used?

A

When there is no gas product

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22
Q

What is monitoring concentration with colorimeter?

A

In a colorimeter, the wavelength of light passing through a coloured solution is controlled by a plane polarised filter. The amount of light absorbed by a solution is measured. The filter colour should be a complementary colour to the colour of the solution. You can then plot a graph of absorbance against time. By measuring the absorbances of standard solutions of the coloured compound you can plot an absorbance concentration graph. From the 2 graphs produced you can then plot a concentration time graph

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23
Q

When can continuous monitoring concentration with a colorimeter be used?

A

In a reaction with a change of colour, i.e from orange to colourless

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24
Q

How can you determine the rate of reaction from a concentration time graph?

A

The gradient

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25
Q

What would a concentration time graph of a zero order reaction look like?

A

A straight line with a negative gradient, the rate of reaction does not change at all during the course of the reaction

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26
Q

when is the rate constant the same as the rate of reaction?

A

When the overall order of the reaction is zero

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27
Q

What would a concentration time graph of a first order reaction look like?

A

A first order reaction produces a downard curce with a decreasing gradient over time. The time taken for the concentration to half is constant

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28
Q

What is the half-life?

A

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

29
Q

What would a concentration time graph of a second order reaction look like?

A

The same as a first order graph, but it is steeper to begin with and tails off more slowly, it does not have a constant half life

30
Q

What is exponential decay?

A

The pattern protuced in a first order reaction with the concentration halving at regular intervals

31
Q

How can you determine if a reaction is first order or second order from a concentration time graph?

A

A first order reaction would have a constant half life whereas a second order reaction would not

32
Q

How can you determine the rate constant of a first order reaction from a concentration time graph?

A

You can either calculate the rate constant from the rate of reaction at a specific point or from the half life:
* Rate of reaction - Draw a tangent to the curve and determine the gradient to find the rate of reaction. Input the rate of reaction value and the concentration at this point into the rate equation, rearrange the equation and solve for the rate constant
* Half life - Determine the half life of the reaction and plug it into the equation “ k = Ln(2)/t½”

33
Q

What is the equation for the rate constant from a first order concentration time graph?

A

k = Ln(2) / t½

Rate constant = Ln(2) / half life

34
Q

What is the shorthand writing for half life?

A

t½

35
Q

How do you plot rate-concentration graphs?

A

Using measurements of the intial rate of reactions at different concentrations

36
Q

What does the rate-concentration graph of a zero order reaction look like?

A

It produces a horizontal straight line with zero gradient

37
Q

How can you determine the rate constant from a zero order rate-concentration graph?

A

It is the y-intercept

38
Q

What does the rate-concentration graph of a first order reaction look like?

A

It produces a straight line graph through the origin

39
Q

How can you determine the rate constant from a first order rate-concentration graph?

A

The gradient of the straight line

40
Q

What does the rate-concentration graph of a second order reaction look like?

A

It produces an upward curve with an increasing gradient

41
Q

How can you determine the rate constant from a second order rate-concentration graph?

A

As it is a curve, the rate constant cannot be directly obtained from the graph. There are 2 methods to determine the rate constant:
* Plot a graph of rate against concentration squared. The gradient of this straight line graph is equal to the rate constant
* Plot a graph of log(rate) against log(concentration). As the rate equation is rate = k [A]ⁿ, this new graph will have an equation of log(rate) = n log(A) + log(k). Therefore the y intercept of this new graph is the log of the rate constant, you can then determine the rate constant from this

42
Q

What is the intial rate of a reaction?

A

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

43
Q

What is a clock reaction?

A

A type of chemical reaction that can be used to measure the intial rate of a reaction. They involve an observable end point which is used to determine when the wanted amount of product has been produced and hence when to stop the timer. Once the limited reactant is used up the product concentration will rapidly increasing causing an visual change, often a colour or precipitate.

44
Q

What assumptions are made when determining the initial rate from a clock reaction?

A

Provided there is no significant change in rate during this time, it is assumed that the average rate of reaction is the same as the initial rate

45
Q

How is the initial rate determined from a clock reaction?

A

Once the end point is reached the time is recorded. The intial rate is proportional to “1/t”. Therefore the clock reaction is repeated several times with different concentrationas and values of 1/t are calculated for each repeat run

46
Q

What is a common type of clock reaction?

A

The iodine clock reaction.

47
Q

How does the iodine clock reaction work?

A
  • A small amount of sodium thiosulfate solution and starch are added to an excess of hydrogen peroxide and iodide ions in acid solution.
  • The hydrogen peroxide reacts with the iodide and hydrogen ions to form water and iodine solution “H₂O₂ + 2I⁻ + 2H⁺ -> 2H₂O + I₂”
  • The sodium thiosulfate that is added to the reaction mixture reacts instantaneously with any iodine that forms “2S₂O₃²⁻ + I₂ -> 2I⁻ + S₄O₆²⁻”
  • To begin with all the iodine that forms is immediately used up in the reaction, however once the sodium thiosulfate runs out (it is the limiting reagent), any more iodine that forms will stay in the solution and the starch indicator will suddenly turn the solution blue-black
48
Q

How accurate are clock reaction?

A

Clock reactions are more accurate over a shorter period of time as you want the average rate to be as closer to the initial rate as possible. When using a clock reaction it is always an approximation but it is considered reasonably accureate provided less than 15% of the reaction has taken place

49
Q

Why are most reactions likely to take place in series of steps?

A

For a reaction to take place particles must collide, however it is unlikely that more than 2 particles will collide together at the same time. Therefore reactions involving more than 2 particles are likely to take place in a series of steps to make up an overall reaction. This is known as the reaction mechanism

50
Q

What is the rate-determining step?

A

The step in a multi-step reaction that occurs at the slowest rate.

51
Q

How can you predict the reaction mechanism?

A
  • The rate equation only includes species involved in the rate determining step
  • The orders in the rate equation match the number of species involed in the rate-determining step
  • The first step usually forms an intermediate and one of the products, the second step then involves the intermediate and forms the other products
  • The sum of the steps gives the overall equation
52
Q

The equation “(CH₃)₃CBr + OH⁻ -> (CH₃)₃COH + Br⁻” has the rate equation: “rate = k[(CH₃)₃CBr]”. Determine the reaction mechanism

A
  • Rate determining step (slow step): “(CH₃)₃CBr -> (CH₃)₃C⁺ + Br⁻”
  • Fast Step: “(CH₃)₃C⁺ + OH⁻ -> (CH₃)₃COH”
53
Q

What is the effect of temperature on the rate constant?

A

As temperature increases, the rate increases. However, the temperature cannot directly effect the concentrations of reactants, therefore, as temperature increases the value of the rate constant will increase

54
Q

What factors affect the rate constant?

A

When temperature increases, 2 factors contribute to the increased rate and rate constant:
* Increasing the temperature shifts the boltzmann distribution to the right, increasing the proportion of particles that exceed the activation energy, Eₐ.
* As the temperature increases, particles move faster and collide more frequently

55
Q

What is the Arrhenius equation (definition)?

A

An exponential relationship between the rate constant and temperature

56
Q

What is the arrhenius equation?

A
57
Q

What does the exponential factor in the arrhenius equation consider?

A

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

58
Q

What does the pre-exponential factor (frequency factor) in the arrhenius equation consider?

A

It takes into account the frequency of collisions with the correct orientation, this term does slightly increase with temperature as the overall frequency of collisions increases, but it is essentially constant over a small temperature range. This would be the rate of reaction with an activation energy of 0J

59
Q

What is the logarithmic form of the arrhenius equation?

A

Ln(K) = -Eₐ/RT + Ln(A)

60
Q

How can you determine the activation energy of a reaction and the pre exponential factor to be determined?

A

Using the logarithmic form of the arrhenius equation, plot a graph of Ln(K) against 1/T:
* The gradient is equal to -Eₐ/R
* The y intercept is equal to Ln(A)

61
Q

Calculate the rate constant:

A

27 dm⁶ mol⁻² s⁻¹

62
Q

Explain how to plot a concentration-time graph using colorimetry:

A
  1. Produce standard solutions of known concentrations
  2. Plot a calibration curve from absorbance readings of standard solutions
  3. Carry out the reaction and take absorbance readings at regular intervals
  4. Plot an absorbance-time graph
  5. Use the calibration curve to measure the concentration of the solution at each absorbance reading
  6. Plot concentration-time graph
63
Q

What are the 2 types of experiments you can do to produce rate-concentration graphs?

A
  • Initial rates method (i.e clock reaction)
  • Continuous rates method (i.e gas production or colorimetry)
64
Q

Suggest a possible two-step mechanism for the reaction “2N₂O(g) -> 2N₂(g) + O₂(g)” with the rate equation “rate = k[N₂O(g)]”

A
  • Step 1 (slow, rate determining step): N₂O(g) -> N₂(g) + O
  • Step 2 (fast step): N₂O + O -> N₂(g) + O₂(g)
65
Q

Determine the rate constant and 2-step mechanism

A

k = 0.0128 dm³ mol⁻¹ s⁻¹

Step 1 (Slow, rate determining step): O₃(g) + NO₂(g) -> O₂(g) + NO₃(g)
Step 2 (Fast Step): NO₂(g) + NO₃(g) -> N₂O₅(g)

66
Q

What are the units of gradient in the logarithmic arrhenius equation plot?

A

K

67
Q

What are the units of the pre exponential factor?

A

The same as the rate constant

68
Q
A

A Level Chemistry (31 decks)

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