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AQA A Level Physical Chemistry > 6. Chemical Equilibria > Flashcards

6. Chemical Equilibria Flashcards

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1
Q

What is a reversible reaction?

A

A reaction that can take place in both directions and is therefore incomplete

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2
Q

Why is the reaction rate fast at the beginning of a reversible reaction?

A

There are only reactants

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3
Q

How is a dynamic equilibrium reached in a reversible reaction?

A
  • beginning - only reactants so fast rate
  • reactants produce products so forward reaction slows
  • products then react to form reactants
  • continues until equilibrium reached
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4
Q

What happens at dynamic equilibrium?

A
  • rates of forward and reverse reactions are the same

* net concentrations of the components of the reaction mixture remain constant

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5
Q

What condition must be true for dynamic equilibrium?

A

Reaction takes place in a closed system

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6
Q

If the concentration of products at equilibrium is high compared to that of the reactants, where does equilibrium lie?

A

To the right

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7
Q

If the concentration of reactants at equilibrium is high compared to that of the products, where does equilibrium lie?

A

To the left

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8
Q

What can change the position of equilibrium?

A
  • temperature
  • concentration
  • pressure
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9
Q

What principle can be used to explain which way equilibrium will shift?

A

Le Chatelier’s

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10
Q

What does Le Chatelier’s principle state?

A

That if a factor affecting the position of equilibrium is altered, the position of equilibrium shifts to oppose the effect of the change

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11
Q

Which way does equilibrium shift when the temperature of a reaction is increased?

A

In the endothermic direction (in order to ↓ the temperature)

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12
Q

Which way does equilibrium shift when the temperature of a reaction is decreased?

A

In the exothermic direction (in order to ↑ the temperature)

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13
Q

The reaction N₂ + 3H₂ ⇌ 2NH₃ has a -ve enthalpy change. What happens when the temperature is increased?

A
  • equilibrium shifts left
  • in endothermic direction
  • to take in heat energy to oppose the change and ↓ temperature
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14
Q

Would a high or low temperature increase the yield of ammonia? [N₂ + 3H₂ ⇌ 2NH₃]

A

Lower - will shift equilibrium to produce more products - but rate will be slower

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15
Q

When will changing pressure have an effect on equilibrium?

A

When reactions involve gases

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16
Q

Which way does equilibrium shift when the pressure of a reaction is increased?

A

In the direction with fewer moles of gas to oppose the change (i.e. ↓ pressure)

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17
Q

Which way does equilibrium shift when the pressure of a reaction is decreased?

A

In the direction with the most moles of gas to oppose the change (i.e. ↑ pressure)

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18
Q

What happens if pressure is increased but the number of moles is the same on both sides of the reaction?

A

The position of equilibrium remains unaffected, but equilibrium will be reached faster

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19
Q

What is the position of equilibrium for reactions in the liquid/aqueous phase affected by?

A

Concentration, not pressure

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20
Q

What happens to equilibrium when the concentration of a reactant is increased?

A

Shift position to the right

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21
Q

What happens to equilibrium when the concentration of a product is increased?

A

Shift position to the left

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22
Q

In the equation BiOCl + 2HCl ⇌ BiCl₃ + H₂O, what happens when more acid is added?

A

Equilibrium will shift right to counteract the increase in HCl

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23
Q

In the equation BiOCl + 2HCl ⇌ BiCl₃ + H₂O, what happens when more water is added?

A

Equilibrium will shift left to counteract the increase in H₂O

24
Q

How does a catalyst increase the rate of reaction?

A
  • lowering activation energy

* provide alternate route for the reaction

25
Q

What happens to equilibrium when a catalyst is added?

A

Position not affected, but rate increases so time to reach equilibrium decreases

26
Q

What happens if pressure is changed in a reaction with gases and solids as reactants, and only gases as products?

A

Only count the moles of gas when deciding which way equilibrium shifts

27
Q

How can ethanol be produced industrially?

A

Ethene (produced from cracking of crude oil) can be hydrated

28
Q

In the equation to produce ethanol using hydration of ethene [C₂H₄ + H₂O ⇌ C₂H₅OH], what is the theoretical yield?

A

100%

29
Q

Will the reaction C₂H₄ + H₂O ⇌ C₂H₅OH have an 100% yield in reality?

A

No

30
Q

Why may some reversible reactions with a theoretical yield of 100% not actually have 100% yield?

A

May produce some side reactions

31
Q

What is the initial yield of ethanol?

A

5%

32
Q

What could the yield of the production of ethanol by hydration of ethene be increased by?

A
  • ↑ pressure
  • ↑ concentration of steam as its cheaper than ethene
  • ↓ temperature
33
Q

What temperatures are used in the hydration of ethene to ethanol? Why?

A
  • 573K and 6500kPa

* compromised between best yield and fastest rate

34
Q

What is the equilibrium constant?

A

The ratio of concentrations of the reactants and products in a reaction mixture

35
Q

What can the equilibrium constant be denoted by?

A

Kc

36
Q

What does the c in Kc represent?

A

Molar concentration

37
Q

When does the value of Kc remain constant?

A

Only if the temperature remains constant

38
Q

For any reaction that reaches equilibria, how can the equation be written (with regard to Kc)?

A

ᵃA + Bᵇ ⇌ ᶜC + ᵈD

39
Q

How can the equilibrium constant be written using the equation ᵃA + Bᵇ ⇌ ᶜC + ᵈD (provided temperature is constant)?

A

Kc = [C]ᶜ [D]ᵈ / [B]ᵇ [A]ᵃ

40
Q

How can Kc be shown in an expression?

A

Kc = products/ reactants

41
Q

What is the units for Kc?

A

Vary but worked out by cancelling units for each term in the expression

42
Q

What does homogenous mean?

A

All reactants and products are in the same phase

43
Q

What happens to the equilibrium position if Kc is relatively large?

A

It lies well over to the product (right) side

44
Q

What happens to the equilibrium position if Kc is relatively small?

A

It lies well over to the reactant (left) side

45
Q

How will change in temperature affect Kc?

A

Will ↑ or ↓ depending on whether forward reaction is exothermic or endothermic

46
Q

Where will equilibrium lie if Kc > 1?

A

Concentration of products > reactants so equilibrium lies to the right

47
Q

Where will equilibrium lie if Kc < 1?

A

Concentration of reactants > products so equilibrium lies to left

48
Q

If temperature increases for an exothermic reaction, what happens to Kc?

A

Decreases

49
Q

If temperature increases for an endothermic reaction, what happens to Kc?

A

Increases

50
Q

If temperature decreases for an exothermic reaction, what happens to Kc?

A

Increases

51
Q

If temperature decreases for an endothermic reaction, what happens to Kc?

A

Decreases

52
Q

Will increasing or decreasing concentration have an effect on the value of Kc?

A

No

53
Q

Why will a change in concentration not affect Kc?

A

Kc is dependent on concentrations of reactants and products changing

e.g. if concentration of reactants is increased, then equilibrium will shift until equilibrium is restored - so no overall change in concentration of reactants or products

54
Q

Will increasing or decreasing pressure have an effect on the value of Kc?

A

No

55
Q

Will adding a catalyst have an effect on the value of Kc?

A

No - increases rate of forward and reverse reactions by the same amount

56
Q

Which reactions are regarded as going to completion?

A

Those with Kc having a value greater than 10¹⁰

57
Q

What reactions are regarded as not taking place at all?

A

Those with Kc having a value less than 10⁻¹⁰

AQA A Level Physical Chemistry (12 decks)

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