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AQA A Level Physical Chemistry > 5. Kinetics > Flashcards

5. Kinetics Flashcards

1
Q

What must happen for a reaction to occur between two substances?

A

Particles must collide with each other, with enough energy during the collision

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2
Q

Do all collisions result in a reaction?

A

No - most do not

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3
Q

What does the energy of a collision depend on?

A
  • the orientation at which they collide

* the speed at which they are moving

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4
Q

Which molecules have energy?

A

All molecules have some energy

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5
Q

What is activation energy?

A

The minimum energy with which particles need to collide to cause a reaction

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6
Q

What is the symbol for activation energy?

A

Eₐ

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7
Q

On a graph showing an exothermic reaction, is the energy of the reactants or the products higher?

A

Reactants

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8
Q

On a graph showing an endothermic reaction, is the energy of the reactants or the products higher?

A

Products

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9
Q

In a sample of liquid or gas where molecules are colliding with each other and the container, are the collisions elastic or inelastic?

A

Elastic because there is no loss of energy

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10
Q

In a sample at a given temperature, what will be the energy of each molecule?

A
  • small fraction will have high/low energies

* however most will be between the two extremes about the most probable energy

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11
Q

What is the graph showing the spread of energy of molecules?

A

A Maxwell-Boltzmann Distribution curve

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12
Q

What are the axes on a Maxwell-Boltzmann distribution curve?

A

Number of particles against energy

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13
Q

What is the most probable energy?

A

The energy that most particles have around, at any given time

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14
Q

For the average energy on a Maxwell-Boltzmann graph, how should the area be underneath the graph?

A

If a vertical line is drawn at the point of average energy, the energy either side of the line should be equal

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15
Q

What does the area underneath a Maxwell-Boltzmann graph distribution curve represent?

A

The total number of particles in the sample

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16
Q

How is the activation energy of a sample represented on a Maxwell-Boltzmann curve?

A

When Eₐ is marked on the x-axis of the graph, the area to the right of it represents the number of particles with at least this energy

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17
Q

Why is there an asymptote on a Maxwell-Boltzmann distribution curve?

A

Few particles have that high energy, however there is no limit on how high the energy of these particles can have - so it doesn’t touch the x-axis

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18
Q

How can the rate of reaction be calculated?

A
  • rate = product formed / time

* rate = reactant used up / time

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19
Q

How does an increase in temperature affect the rate of a reaction?

A

It increases the rate of a reaction

20
Q

In many reactions, what does a rise of 10°C mean for the rate of reaction?

A

It causes the rate of reaction to approximately double

21
Q

Why does an increase in temperature increase the rate of reaction?

A
  • at higher temperatures, particles have higher energy
  • meaning they move faster and are more likely to collide with other particles
  • when particles, collide, they do so with more energy, increasing frequency of collisions with Eₐ
22
Q

What happens to a Maxwell-Boltzmann distribution curve when temperature is increased?

A

The energy distribution moves to the right as average energy of the molecules increases

23
Q

Why does the shape of a Maxwell-Boltzmann distribution curve change when temperature is increased?

A

The spread of energies increases

24
Q

How does an increase in temperature affect the area underneath a Maxwell-Boltzmann distribution curve?

A

Doesn’t change as the number of particles stays the same

25
Q

How does an increase in temperature affect the most probable energy on Maxwell-Boltzmann distribution curve?

A

Increases due to having more energy at higher temperatures

26
Q

How does an increase in temperature affect activation energy on a Maxwell-Boltzmann distribution curve?

A

The activation energy will stay the same for both temperatures, so if a vertical line was drawn on x-axis where it is, it would stay in the same place

27
Q

How does an increase in temperature affect the number of molecules with energy greater or equal to Eₐ on a Maxwell-Boltzmann distribution curve?

A

A greater number of molecules with energy greater/equal to Eₐ for higher temps.

28
Q

How does increasing the concentration of a reagent affect the number of particles in a given volume?

A

It increases the number of particles

29
Q

How does increasing the concentration of a regent affect the rate of reaction?

A

It increases the rate of reaction

30
Q

Why does an increased concentration increase rate of reaction?

A
  • ↑ chance of successful collision
  • so ↑ frequency of collisions
  • so ↑ frequency of collisions with required Eₐ
31
Q

How does increasing the concentration of a reagent affect the shape of a Maxwell-Boltzmann distribution curve?

A

The curve follows the same shape, but the line is above the original, so the area underneath it is larger

32
Q

How does an increase in concentration affect the area underneath a Maxwell-Boltzmann distribution curve?

A

The area increases as the number of particles increases when concentration increases

33
Q

How does an increase in concentration affect the most probable energy on a Maxwell-Boltzmann distribution curve?

A

Most probable energy stays the same - as there is no increase in energy

34
Q

How does an increase in concentration affect activation energy on a Maxwell-Boltzmann distribution curve?

A

Eₐ stays the same - vertical line drawn on x-axis stays in the same place

35
Q

How does an increase in concentration affect the number of molecules with energy greater or equal to Eₐ on a Maxwell-Boltzmann distribution curve?

A

For higher concentrations, there’s a greater number of particles with Eₐ or more - due to ↑ frequency of collisions - so ↑ frequency of collisions with Eₐ or more

36
Q

How does an increase in pressure affect rate of reaction?

A

Increases rate of reaction

37
Q

What effect does an increase in pressure have on the Maxwell-Boltzmann distribution curve?

A

No change of curve - same energy and number of particles

38
Q

Why does an increase in pressure increase the rate of reaction?

A

Increasing pressure means decreasing space that gas particles are moving - so ↑ frequency of collisions

39
Q

What is a catalyst?

A

A substance that increases the rate of a chemical reaction without being changed in chemical composition or amount

40
Q

How does a catalyst work?

A
  • reactants interact to form an intermediate compound
  • then they form the product

(in order to reach this transition state they need the Eₐ)

41
Q

How does a catalyst provide an alternative reaction route?

A

By decreasing the Eₐ required and hence ↑ reaction rate

42
Q

How does use of a catalyst affect the Maxwell-Boltzmann distribution curve?

A
  • shape of the curve stays the same - same energy and number of particles
  • Eₐ lower so greater area of underneath graph with particles having required Eₐ
43
Q

Why does use of a catalyst mean that reaction rate will increase?

A
  • lower Eₐ so ↑ proportion of particles with energy equal to or greater than Eₐ
  • higher proportion of collisions are successful so rate ↑
44
Q

Do catalysts produce more product?

A

No - they produce the same amount of product more quickly

45
Q

How does adding an inert gas affect the rate of a reaction?

A

Increases rate - as this increases the pressure

AQA A Level Physical Chemistry (12 decks)

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