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A LEVEL - Chemistry (Inorganic) > Topic 9 - Kinetics I > Flashcards

Topic 9 - Kinetics I Flashcards

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1
Q

What are the conditions for a successful collision between particles?

A
  • Collision in the right direction

* Collision with at least a certain minimum amount of kinetic energy

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2
Q

What is the activation energy?

A

The minimum amount of energy that must be supplied to particles in order to break bonds and start a reaction.

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3
Q

Hwo does the activation energy of a reaction determine how easily it happens?

A

The lower the activation energy, the more easily the reaction happens.

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4
Q

Do all molecules in a substance have the same kinetic energy?

A

No, there is a distribution.

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5
Q

What is the name for the curve showing the distribution in energies of particles in a substance?

A

Maxwell-Boltzmann Distribution

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6
Q

On a Maxwell-Boltzmann distribution, what is on the x and y axis?

A

x-axis -> Kinetic energy

y-axis -> Number of molecules

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7
Q

Describe the shape of the Maxwell-Boltzmann distribution curve.

A
  • Starts at the origin
  • Steep gradient up to peak, then downwards gradient
  • Gradually plateauing -> Never reaches x-axis

(See diagram pg 112 of revision guide)

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8
Q

Why does the Maxwell-Boltzmann distibution curve go through the origin?

A

No molecules can have zero energy.

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9
Q

How is the activation energy for a reaction shown on a Maxwell-Boltzmann distribution curve?

A

A vertical line at a certain energy up to the curve.

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10
Q

On a Maxwell-Boltzmann distribution curve, how can you tell which particels have sufficient energy to react?

A

All the particles to the right of the vertical ‘activation energy’ line have sufficient energy to react.

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11
Q

Remember to practise drawing out the Maxwell-Boltzmann distribution curve.

A

Pg 112 of revision guide.

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12
Q

Describe how the Maxwell-Boltzmann distribution curve changes when the temperature is increased.

A
  • Shifts to the right -> More particles above activation energy line
  • Peak is more to the right, but lower than before
  • Area under curve is the same
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13
Q

What 5 factors affect the rate of a reaction?

A 
• Temperature
• Concentration
• Pressure
• Catalysts
(• Surface area)
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14
Q

Explain how increasing the temperature affects the rate of reaction.

A

• Increases it
• Because particles have on average more kinetic energy, so:
1) More collisions happen per second
2) More of these collisions happen with sufficient activation energy to react successfully

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15
Q

Explain how increasing the concentration affects the rate of reaction.

A
  • Increases it
  • Because there are more particles per unti volume, so more collisions can occur per second -> More successful collisions per second
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16
Q

Explain how increasing the pressure affects the rate of reaction.

A
  • Increases it (if any reactants are gases!)
  • Because there are more gas particles per unit volume, so more collisions can occur per second -> More successful collisions per second
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17
Q

Explain how a catalyst affects the rate of reaction.

A
  • Increases it
  • Because they lower the activation energy by providing an alternative pathway for the reaction -> So more particles have sufficient energy to react -> More successful collisions per second
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18
Q

Explain how increasing the surface area affects the rate of reaction.

A
  • Increases it
  • Because there is a larger surface area available for collisions, so more can happen per second -> More successful collisions per second
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19
Q

What is collision theory?

A

The way in which reactions of particles can be explained by their movement, etc.

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20
Q

When will increasing the pressure increase the rate of reaction?

A

When at least one of the reactants is a gas.

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21
Q

What is the reaction rate?

A

The change in amount of reactant or product per unit time.

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22
Q

Are there any defined units for rate of reaction?

A

No, it depends on what is being measured.

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23
Q

How can the rate of reaction be worked out from a graph?

A
  • Work out the gradient of the graph at the given time
  • Pick appropriate units (y units / x units)

(This is assuming the graph is of reactant or product (y) against time (x))

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24
Q

How can you work out the gradient from a curved graph?

A

Draw a tangent at the given point.

25
Q

For a graph of concentration against time, what is the unit for the rate of reaction?

A

mol / dm^3 / min

26
Q

What is the initial rate of a reaction?

A

The rate at the start of a reaction.

27
Q

How can the initial rate of reaction be worked out from a graph?

A
  • Draw a tangent at t = 0
  • Work out the gradient
  • Pick appropriate units
28
Q

In an experiment which times how long it takes for x amount of reactant to be used up or product to be formed, how can you work out the rate of reaction?

A
  • Rate of reaction = Amount of reactant used or product formed / Time taken
  • Pick appropriate units
29
Q

If a reaction takes 10 seconds to produce 20cm^3 of a gas, what is the rate of reaction?

A

2cm^3/s

30
Q

In an experiment which times how long it takes for x amount of reactant to be used up or product to be formed, what is the rate proportional to?

A

1/Time

31
Q

A student measures the time taken for a colour change to occur in a reaction as he varies the concentration of a reactant, A. His results are shown in the table. Calculate the relative rates of reaction.

[A] = 0.10 mol/dm3 -> 124s to colour change
[A] = 0.15 mol/dm3 -> 62s to colour change
[A] = 0.20 mol/dm3 -> 25s to colour change
A

1) Relative rate of each reaction:
When [A] = 0.10 mol/dm3 -> 1 / 124 = 0.00806 s^-1
When [A] = 0.15 mol/dm3 -> 1 / 62 = 0.0161 s^-1
When [A] = 0.20 mol/dm3 -> 1 / 25 = 0.0400 s^-1
2) Divide by the smallest relative rate to get the rates as the smallest whole number:
0.0081 : 0.016 : 0.040 = 1 : 2 : 5

32
Q

Remember to practise calculating reaction rates from graphs and experiments.

A

Pgs 114-115 of revision guide.

33
Q

Explain how a catalyst increases the rate of reaction.

A
  • A catalyst increases the rate of reaction by providing an alternative reaction pathway with a lower activation energy, so a greater proportion of collisions result in a reaction.
  • The catalyst is chemically unchanged at the end of the reaction.
34
Q

Is a catalyst used up in a reaction?

A

No, it remains chemically unchanged.

35
Q

Do catalysts take part in a reaction?

A

Yes, but they’re remade at the end of it.

36
Q

How many reactions can a catalyst catalyse?

A

Usually only 1.

37
Q

What are the two types of catalyst?

A
  • Heterogeneous

* Homogeneous

38
Q

What is a heterogeneous catalyst?

A

One that is in a different physical state to the reactants.

39
Q

Give an example of a heterogeneous catalyst.

A

In the Haber process, gases are passed over a solid iron catalyst.

40
Q

On what part of a heterogeneous catalyst does a reaction take place?

A

The surface.

41
Q

How can the effectiveness of a solid heterogeneous catalyst be increased?

A
  • Increasing its surface area

* Because this increases the number of molecules that can react at the same time -> Increasing the rate of reaction

42
Q

Describe how a solid heterogeneous catalyst works.

A

1) Reactant molecules arrive at the surface and bond with the catalyst -> Adsorption
2) Bonds been the reactant’s atoms are weakened and break up -> Lower activation energy -> This forms radicals
3) Radicals join to make products
4) Products detach from the catalyst -> Desorption

43
Q

What is the name for the reactants bonding to a heterogeneous catalyst?

A

Adsorption

NOTE: It’s adsorption, not absorption

44
Q

What is the name for the reactants detaching from a heterogeneous catalyst?

A

Desorption

45
Q

Remember to revise how a heterogeneous catalyst works.

A

Pg 116 of revision guide.

46
Q

What is a homogeneous catalyst?

A
  • One that is in the same physical state as the reactants.

* Usually, it is an aqueous catalyst for a reaction between two aqueous solutions

47
Q

What state is a homogeneous catalyst in?

A

Usually aqueous (so it catalyses reactions between aqeuous solution).

48
Q

Describe how a homogeneous catalyst works.

A

1) Reactants combine with the catalyst -> Lower activation energy E1
2) Intermediate species is formed
3) Intermediate species then reacts to form product and reform catalyst -> Lower activation energy E2

(i.e. There are two reactions, each with a lower activation energy)

49
Q

How many stages are there in a reaction catalysed by a homogeneous catalyst?

A

There are 2 separate reactions, each with a reaction energy lower than the original uncatalysed reaction.

50
Q

How many activation energies are there in a reaction catalysed by a homogeneous catalyst?

A

2

51
Q

Describe the reaction profile for a reaction catalysed by a heterogeneous catalyst.

A
  • Like the uncatalysed curve, except below it and with a lower peak
  • Reaches the products line at the same point

(See diagram pg 116 of revision guide)

52
Q

Describe the reaction profile for a reaction catalysed by a homogeneous catalyst.

A
  • Two smaller peaks below the origianl curve, each with a separate activation energy
  • After these peaks, the line is the same as the uncatalysed curve
53
Q

In the reaction profile for a reaction catalysed by a homogeneous catalyst, where is the intermediate species formed?

A

In the trough between the two small peaks.

54
Q

In the reaction profile for a catalysed reaction, does the curve reach the products line before the normal uncatalysed reaction line?

A

No, it reaches the products line at the same time.

55
Q

Remember to practise drawing out reaction profiles for an:
• Uncatalysed reaction
• Homogeneous catalyst reaction
• Heterogeneous catalyst reaction

A

Pgs 116

56
Q

How is a catalyst shown on a Maxwell-Boltzmann distribution?

A

It shifts the vertical ‘activation energy’ line to the left, so a greater proportion of the particles are to the right of it.

57
Q

What are the benefits of using catalysts in industry?

A

1) Lowers cost due to lower needed temperature and pressure
2) Faster production
3) May improve properties of product

58
Q

Give an example of a case where using a catalyst improves the properties of the product.

A 
POLYETHENE WITHOUT CATALYST:
• Less dense
• Less rigid
POLYETHENE WITH CATALYST:
• More dense
• More rigid
• Higher melting point

A LEVEL - Chemistry (Inorganic) (21 decks)

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