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A-level Chemistry > 3.1.11 Rate Equations & 3.1.12 Kp > Flashcards

3.1.11 Rate Equations & 3.1.12 Kp Flashcards

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1
Q

Define reaction rate

A

Change in amount of reactant or product per unit time

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2
Q

State the units for reaction rate

A

Mol dm-3 s-1

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3
Q

State the generalised rate equation

A

rate = k[A]m[B]n

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4
Q

rate = k[A]m[B]n

What does m and n represent?

A

orders of the reaction

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5
Q

rate = k[A]m[B]n

What does k represent?

A

k = rate constant

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6
Q

What do orders of reactions tell you?

A

Tell you how reactant concentrations affect the rate

(e.g. m tells you how concentration of reactant A affects rate and n tells you same for reactant B)

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7
Q

Find the overall order of reaction

rate = k[A]m[B]n

A

m + n

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8
Q

How can you only find the orders of reaction?

A

From experiments

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9
Q

If [A] changes and the rate stays the same, what is the order of the reaction?

A

The order of reaction with respect to A is 0

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10
Q

If the rate is proportional to [A], what is the order of the reaction?

A

The order of reaction with respect to A is 1

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11
Q

If the rate is proportional to [A]2, what is the order of the reaction?

A

The order of reaction with respect to A is 2

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12
Q

The rate constant relates…

A

reactant concentrations to rate at a particular temperature

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13
Q

Bigger value of k = ____ __ ______

A

faster the reaction

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14
Q

k is always _____ for certain reaction at a particular temperature

A

the same

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15
Q

Why does the rate constant increase as temperature increases?

A
  • ∵ ↑ temp. = rate of reaction ↑
  • ∵ increasing no. of collisions between reactant molecules + energy of each collisions
  • But conc. of reactants and orders of reaction stay the same
  • So k must increase for rate equation to balance
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16
Q
A
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17
Q
A
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18
Q

Define the initial rate of a reaction

A

Rate right at the start of the reaction

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19
Q

Describe how you can find the initial rate of a reaction (from graph)

A

Find it from a concentration-time graph by calculating the gradient of the tangent at time = 0

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20
Q

What is the initial rates method used for?

A

Used to create rate equations

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21
Q

Describe the initial rates method

A
  1. Repeat an experiment several times using different initial concentrations of the reactants
    • Usually only change 1 conc. of at time
  2. Calculate initial rate for each experiment
  3. See how initial concentration affects initial rates and figure out the order for each reactant
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22
Q
A
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23
Q

Name 3 methods of measuring the initial reaction rate

A
  • Iodine clock reaction
  • Reactions that produce precipitates
    • Measure time it takes for mark underneath reaction vessel to disappear
  • Other reactions
    • Measure time taken for small amount of product to be formed
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24
Q

Describe the iodine clock reaction

A
  1. Add dilute sulfuric acid and starch solution to beaker
  2. Add sodium thiosulfate to reaction mixture
    1. Add potassium iodide solution
    2. Add hydrogen peroxide solution
  3. Sodium thiosulfate reacts with iodine being formed
  4. Once all sodium thiosulfate is used up = any more iodine formed remains in solution
  5. Turns starch indicator blue-black
  6. Varying conc. of iodide or hydrogen peroxide while keeping everything constant = different times for colour change
    • Used to work out reaction order
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25
Q

Iodine Clock Reaction

State the equation for how iodine is produced

A
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26
Q

Iodine Clock Reaction

State the equation for how iodine reacts with thiosulfate ions

A
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27
Q

Name 2 methods you can use to measure rate of reaction

A
  • Measuring Initial Reaction Rate
  • Continuous Monitoring
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28
Q

Describe continuous monitoring

A
  • Can follow reaction all way to its end by recording amount of product (or reactant) you have at regular time intervals
  • Use results to work out how rate changes over time
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29
Q

Name 4 examples of continuous monitoring methods

A
  • Loss of Mass
  • Colour Change
  • Gas Volume
  • Change in pH
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30
Q

What does a colorimeter do?

A

Measures absorbance

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31
Q

Colorimeter

Higher the absorbance =

A

More concentrated the colour of the solution is

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32
Q

Describe how you can measure the rate of reaction by continuous monitoring when there’s a colour change

A

Can measure change in absorbance:

  1. Plot calibration curve
    • Graph of known concentrations of coloured solution (e.g. I₂) plotted against absorbance
  2. During experiment, take small samples from your reaction solution at regular intervals and read the absorbance
  3. Use calibration curve to convert absorbance at each time point into a concentration
33
Q

Describe how you can measure the rate of reaction by continuous monitoring when there’s a loss of mass

A
  1. If gas is given off, system will lose mass
  2. Can measure this at regular intervals with a balance
  3. Use mole calculations to work out how much gas you’ve lost
  4. & thus how many moles of reactants are left
34
Q

Describe how you can measure the rate of reaction by continuous monitoring when there’s a gas volume

A
  1. If gas given off, could collect it in gas syringe & record how you’ve got at regular time intervals (e.g. every 15s)
    • e.g. acid + carbonate = CO₂
    • e.g. magnesium ribbon + HCl
  2. Find conc. of reactant at each time point
  3. Use ideal gas equation to work how many mole of gas
  4. Then use molar ratio to work out conc. of reactant
35
Q

Describe how you can measure the rate of reaction by continuous monitoring when there’s a change in pH

A

If reaction produces or uses H+ ions, can measure pH of solution at regular intervals & calculate the conc. of H+

36
Q

How can you construct a rate-concentration graph?

A

Can use data from concentration-time graph to construct it

37
Q

What can a rate-concentration graph tell you?

A

Reaction order

38
Q

Describe how you can construct a rate-concentration graph

A
  1. Find gradient at various points on graph
    1. Gives you rate at that particular concentration
    2. For curve, need to draw tangents
  2. Plot each point on new graph with axes rate and concentration
    1. Then draw smooth line or curve through points
    2. Shape of line will tell you the order of the reaction with respect to that reactant
39
Q

State order of the reaction with respect to reactant [X]

A
40
Q

State order of the reaction with respect to reactant [X]

A
41
Q

State order of the reaction with respect to reactant [X]

A
42
Q

In a multi-step reaction, each step can have a ___ ___

A

different rate

43
Q

State what is meant by the rate determining step

A

Slowest step in a multi-step reaction

44
Q

What is the overall rate decided by?

A

By the step with the slowest rate = rate determining step (aka rate-limiting step)

45
Q

Explain how you know reactants in rate equations affect the rate

A

If a reactant appears in the rate equation, it must affect the rate. ∴ this reactant, or something derived from it, must be in the rate determining step.

46
Q

Rate determining step doesn’t have to be the ___ step in a mechanism

A

first

47
Q

Reaction mechanism can’t usually be predicted from just the ____ _____

A

chemical equation

48
Q

What does the order of a reaction with respect to a reactant show?

A

Shows the number of molecules of that reactant that are involved in the rate determining step

49
Q

Determine the rate equation from these equations

A
50
Q

Rate determining step can sometimes involve an _____ that isn’t in the full equation

A

intermediate

51
Q

rate = k[(CH3)2CBr]. State which mechanism is correct.

A
52
Q

What does the Arrhenius Equation show?

A

How rate constant (k) varies with temperature (T) and activation energy (Ea)

53
Q

State what each letter represents and their units

A
  • k = rate constant
  • Ea = activation energy (J)
  • T = temperature (K)
  • R = gas constant (8.31 J K-1 mol-1)
  • A = the Arrhenius constant
54
Q

Explain why as Ea increases, the rate constant decreases?

A
  • Large Ea = slow rate
  • If reactions has high Ea, not many reactant particles have enough energy to react
  • ∴ few of collisions will result in reaction occurring & rate will be slow
55
Q

Explain why as temperature increases, the rate constant increases?

A
  • Higher temperatures mean reactant particles move around faster with more energy
  • More likely to collide and more likely to collide with E ≥ Ea
  • So reaction rate ↑
56
Q
A
57
Q

What can you use Arrhenius plot to find?

A

Ea or Arrhenius constant

58
Q

How can you use Arrhenius equation to create an Arrhenius plot?

A

By plotting lnk against 1/T

59
Q

State the gradient of an Arrhenius plot

A

-Ea/R

60
Q
A
61
Q

What is partial pressure?

A

In a mixture of gases, each gas exerts its own pressure = partial pressure

62
Q

What is the total pressure of a gas mixture? (i.e how you calculate it)

A

sum of all partial pressure of the individual gases

63
Q
A
64
Q

What is a mole fraction?

A

Proportion of a gas mixture that is made up a particular gas

65
Q

State how you calculate mole fraction of a gas in a mixture

A
66
Q

State how you calculate partial pressure of a gas

A
67
Q

PCl5(g) ⇌ PCl3(g) + Cl2(g)

A
68
Q

What is the equilibrium constant for reversible reactions where all reactants and products are gases?

A

Kp

69
Q

State the expression for Kp

A

(Put partial pressures in expressions)

70
Q

PCl5(g) ⇌ PCl3(g) + Cl2(g)

A
71
Q

Just like Kc, value of Kp is affected by _____

A

Temperature

(Kp is only valid for a given temp.)

72
Q

Explain why Kp is only valid for a given temperature

A
  • ∵ changing temp. changes how much product is formed at equilibrium
  • Changes mole fractions of gases present which changes their partial pressures
73
Q

Just as changing conc. doesn’t change Kc, changing _____ doesn’t affect Kp

A

pressure

Equilibrium will shift to keep it the same

74
Q

Adding a catalyst ______ Kp

A

won’t affect

75
Q

Explain what will happen to Kp if temperature is increased

A
76
Q

The rate equation for a reaction is rate = k[E]. Explain qualitatively why doubling the temperature has a much greater effect on the rate of the reaction than doubling the concentration of E. (3)

A
  • Reaction occurs when molecules have E ≥ Ea
  • Doubling T causes many more molecules to have this E
  • Whereas doubling [E] only doubles the number with this E
77
Q

Suggest why initial rates of reaction are used to determine orders rather than rates of reaction at other times during the experiments (1)

A

At time 0, the conc. are known

78
Q

State how the initial rate is obtained from a graph of the concentration of the product against time (1)

A
  • Calculate gradient of the tangent
  • At t = 0 OR at start of graph
79
Q

How can you work out the rate of reaction from reactions where there’s a sudden colour change when a product reaches a certain concentration

A
  • By measuring the time it takes for the colour change to happen
  • Shorter the time = faster rate

A-level Chemistry (34 decks)

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