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Chemistry OCR A-Level > Chapter 18 > Flashcards

Chapter 18 Flashcards

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1
Q

How are reaction rates measured?

A

Observing the changes in the quantities of reactants/products over time

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2
Q

rate =

A

quantity reacted or produced/time

change in concentration/change in time

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3
Q

What units is concentration measured in?

A

mol dm-3

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4
Q

What do square brackets represent?

A

Concentration

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5
Q

What does [A] mean?

A

Concentration of A

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6
Q

What units does [A] have?

A

mol dm⁻³

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7
Q

What does changing concentration change?

A

Rate of a reaction

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8
Q

What is rate of reaction proportional to?

A

The concentration of a particular reactant raised to a power

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9
Q

rate ∝

A

[A]ⁿ

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10
Q

What is n in rate ∝ [A]ⁿ?

A

Order of reaction

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11
Q

What are some of the common orders?

A

1) zero order (0)
2) first order (1)
3) second order (2)

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12
Q

What is zero order?

A

1) concentration of a reactant has no effect on the rate

2) rate ∝ [A]⁰

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13
Q

What is first order?

A

1) rate depends on concentration raised to the power of 1

2) rate ∝ [A]¹

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14
Q

What is second order?

A

1) rate depends on concentration raised to the power of 2

2) rate ∝ [A]²

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15
Q

What is the rate equation?

A

The mathematical relationship between the concentrations of the reactants and the reaction rate.

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16
Q

What is the rate constant, k?

A

1) proportionality constant

2) converts between the rate of reaction and concentration and orders

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17
Q

overall order =

A

sum of orders with respect to each reactant

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18
Q

What is the overall order for the rate equation rate = k[A]ᵐ[B]ⁿ

A

m+n

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19
Q

What does ∝ mean?

A

proportional to

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20
Q

Simplify the rate equation rate = k[A]⁰[B]¹[C]²

A

rate = k[B][C]²

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21
Q

What do the units of the rate constant depend on?

A

The number of concentration terms in the rate equation

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22
Q

How can you determine the units of the rate constant, k?

A

1) rearrange equation to make k the subject
2) substitute units into the expression for k
3) cancel common units and show the final units on a single line

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23
Q

How are orders of reaction determined?

A

Experimentally by monitoring how a physical quantity changes over time

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24
Q

What is continuous monitoring?

A

Plotting graphs from continuous measurements taken during the course of a reaction

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25
Q

Give some methods of continuous monitoring that produce a gas as one of the products?

A

1) monitoring by gas collection

2) monitoring by mass loss

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26
Q

What can be used to estimate a colour change?

A

A colorimeter

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27
Q

What is the gradient of a concentration-time graph?

A

Rate of reaction

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28
Q

What is the shape of a zero order concentration-time graph?

A

1) straight line

2) negative gradient

29
Q

Does the reaction rate of a zero order reaction ever change?

A

No

30
Q

What is the shape of a first order concentration-time graph?

A

1) downward curve

2) decreasing gradient

31
Q

What is constant in a first order reaction?

A

1) half-life

3) the time for the concentration to halve

32
Q

What is the shape of a second order concentration-time graph?

A

1) downward curve

2) decreasing gradient

33
Q

What is half life?

A

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

34
Q

What is exponential decay?

A

The concentration of a first order reaction halving every half life

35
Q

How can you determine k for a first order reaction?

A

1) from the rate

2) from the half-life

36
Q

What is the formula to calculate the rate constant from the half-life?

A

k = ln2 / t₁/₂

37
Q

Why is it more accurate to use the formula for calculating the rate constant than drawing a tangent?

A

It is always difficult to judge how steep the tangent should be drawn.

38
Q

What is the shape of a zero order rate-concentration graph?

A

1) horizontal straight-line

2) zero gradient

39
Q

What is the rate equation for a zero order reaction?

A 
rate = k[A]⁰
rate = k
40
Q

What is the shape of a first order rate-concentration graph?

A

1) straight line graph through the origin

2) positive gradient

41
Q

What is the rate equation for a first order reaction?

A 
rate = k[A]¹
rate = k[A]
42
Q

What is the shape of a second order rate-concentration graph?

A

1) upward curve

2) increasing gradient

43
Q

What is the rate equation for a second order reaction?

A

rate = k[A]²

44
Q

Why can you not directly obtain the rate constant of a second order rate-concentration graph?

A

The rate-concentration graph is a curve

45
Q

How do you calculate the rate constant of a second order reaction?

A

By calculating the gradient of a graph of the rate against the concentration squared

46
Q

What is the initial rate?

A

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

47
Q

How can initial rate be found?

A

By measuring the gradient of a tangent drawn at t = 0 on a concentration-time graph

48
Q

What is a clock reaction?

A

1) takes a single measurement

2) time t from the star of an experiment is measured for a visual change to be observed, often a colour or precipitate

49
Q

What is the initial rate of a clock reaction?

A

initial rate ∝ 1/t

50
Q

How is a clock reaction repeated?

A

With different concentrations

51
Q

What colour is aqueous iodine?

A

Orange-brown

52
Q

What happens when starch is added to iodine

A

It forms a complex with iodine which is an intense dark blue-black colour

53
Q

Describe how the order of a reaction can be determined from the shape of a rate-concentration graph.

A

1) 0 order is a horizontal line
2) 1st order is a straight line. starting at the origin
3) 2nd order is an upward curve, starting at the origin

54
Q

What does a colorimeter measure?

A

1) the intensity of light passing through a sample

2) absorbance (directly linked to the concentration of the solution)

55
Q

How is the filter for a colorimeter chosen?

A

So that it is the complimentary colour to the colour being absorbed in the reaction

56
Q

When can a reaction only take place?

A

When particles collide

57
Q

What is a reaction mechanism?

A

The series of steps that make up an overall reaction

58
Q

What is the rate-determining step?

A

The slowest step in the sequence

59
Q

What does the rate equation only include?

A

Reacting species involved in the rate determining step

60
Q

What do the orders in the rate equation match?

A

The number of species involved in the rate-determining step

61
Q

What are haloalkanes hydrolysed by?

A

Hot aqueous alkali

62
Q

What happens as temperature increases?

A

1) rate increases

2) value of the rate constant k increases

63
Q

Increasing temperature shifts the Boltzmann distribution to what side?

A

Right, increasing the proportion of particles that exceed the activation energy Eₐ

64
Q

What is the Arrhenius equation?

A

1) an exponential relationship between the rate constant k and temperature T
2) k - Ae⁻ᴱᵃ/ᴿᵀ

65
Q

What is A in the Arrhenius equation?

A

Pre-exponential factor (frequency factor)

66
Q

What is -Eₐ/RT in the Arrhenius equation?

A

Exponential factor (linked to activation energy and temperature)

67
Q

What is R in the Arrhenius equation?

A

Gas constant - 8.314Jmol⁻¹K⁻¹

68
Q

What is T in the Arrhenius equation?

A

Temperature in Kelvin

Chemistry OCR A-Level (28 decks)

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