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Chemistry A-Level > Equilibrium (Chapter 10.4, 10.5 + Chapter 19) > Flashcards

Equilibrium (Chapter 10.4, 10.5 + Chapter 19) Flashcards

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1
Q

What 2 things must be true in an equilibrium system?

A

1) the rate of the forward reaction is equal to the rate of the reverse reaction
2) the concentration of the products and reactants does not change

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2
Q

What is a word to describe equilibrium systems?

A

Dynamic

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3
Q

Why do the concentrations of reactants and products in an equilibrium system not change even though the forward and reverse reactions are still taking place?

A

Because as fast as reactants are becoming products, products are becoming reactants

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4
Q

What is a closed system?

A

A system isolated from its surroundings so the temperature, pressure and concentration of reactants/products are unaffected by outside influences

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5
Q

What must be true for a reaction to remain in equilibrium?

A

The system must be closed

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6
Q

What does the position of equilibrium indicate?

A

The extent of the reaction

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7
Q

What is le Chatelier’s principle?

A

When a system in equilibrium is subjected to an external change, the system readjusts itself to minimise the effect of that change

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8
Q

When will the position of equilibrium shift to the right?

A

When there is an increase in [A/B] or a decrease in [C/D]

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9
Q

What happens when the position of equilibrium shifts to the right?

A

More products are formed

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10
Q

When will the position of equilibrium shift to the left?

A

Where there is a decrease in [A/B] or an increase in [C/D]

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11
Q

What happens when the position of equilibrium shifts to the left?

A

More reactants are formed

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12
Q

What colour are aqueous chromate ions (CrO42-)?

A

Yellow

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13
Q

What colour are aqueous dichromate ions (Cr2O72-)?

A

Orange

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14
Q

What happens when you add acid to a weak acid equilibrium?

A

1) [H+] increases
2) this increases the rate of the forwards reaction
3) the position of eqm shifts right to minimise the change in [H+]
4) a new position of eqm is established towards the products, so more product is made

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15
Q

What is true about the deltaH of forwards and reverse reactions?

A

They have the same deltaH but different signs

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16
Q

What is the effect of an increase in temperature on the position of equilibrium?

A

The position of eqm shifts in the endothermic direction to counteract the increase in temperature

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17
Q

What is the effect of an decrease in temperature on the position of equilibrium?

A

The position of eqm shifts in the exothermic direction to counteract the decrease in temperature

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18
Q

What colour is [Co(H2O)6]?

A

A pink solution

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19
Q

What colour is CoCl42-?

A

Blue

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20
Q

When the forward reaction is exothermic, what is the effect of increasing the temperature?

A

The position of eqm shifts to the left, so more reactants are made

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21
Q

When the forward reaction is exothermic, what is the effect of decreasing the temperature?

A

The position of eqm shifts to the right, so more products are made

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22
Q

When the forward reaction is endothermic, what is the effect of increasing the temperature?

A

The position of eqm shifts to the right, so more products are made

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23
Q

When the forward reaction is endothermic, what is the effect of decreasing the temperature?

A

The position of eqm shifts to the left, so more reactants are made

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24
Q

When does pressure have an effect on the position of equilibrium?

A

When there are more moles/molecules on one side of the equation than the other

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25
Q

What happens when you add sodium hydroxide to a weak acid equilibrium?

A

1) the added OH- ions react with H+ ions, decreasing the [H+] (H+ + OH- = H2O)
2) the decreased concentration of reactant decreases the rate of the forwards reaction ∴ causes the position of equilibrium to shift to the left to minimise the change in concentration
3) the shift increases the concentration of the reactant that has been removed (H+)
4) a new position of equilibrium is established towards the reactants

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26
Q

What is the equation of the chromate ions and dichromate ions?

A

2CrO42- + 2H+ <=> Cr2O72- + H2O

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27
Q

What happens when you dissolve CoCl2 in water?

A

A pink solution is formed and the dissolving process produces an equilibrium:
[Co(H2O)6]2+ + 4Cl- <=> CoCl42- + 6H2O

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28
Q

What happens if you dissolve CoCl2 in water, add some HCl and out the boiling tube into iced water/boiling water bath?

A

Iced water: the solution turns pink

Boiling water bath: the solution turns blue

29
Q

What happens to the equilibrium when the test tube of CoCl2 is in boiling water?

A

The heat energy of system has increased

1) this causes the position of equilibrium to shift to minimise the change
2) as the forward reaction is endothermic, the position of equilibrium shifts to the right in the endothermic direction, to take heat energy in and minimise the increase in temperature
3) ∴ the solution turns blue

30
Q

If there are more molecules on the LHS, what is the effect of increasing the pressure on the equilibrium?

A

1) bc there are fewer molecules on the right hand side of the equilibrium, the position of equilibrium shifts to the right, reducing the number of gaseous moles to minimise the increase in pressure
2) this reduces the pressure of the system

31
Q

What is the effect of a catalyst on the equilibrium?

A

1) a catalyst does not change the position of equilibrium - it speeds up the rates of the forward and reverse reaction equally
2) it increases the rate at which equilibrium is established
3) catalysts affect the rate of reaction NOT the position of equilibrium

32
Q

What is the equation for the Haber process?

A

N2(g) + 3H2(g) <=> 2NH3

33
Q

Is the Haber process exothermic and endothermic and therefore what are its ideal conditions to maximise yield?

A

Exothermic - ideal conditions: high pressure and low temperature

34
Q

Why are the ideal conditions for the Haber process to maximise yield not used in reality?

A
  • The low temperature of the reaction is very slow, possibly even too slow for the equilibrium to be established
  • The high pressure requires a very strong container and lots of energy ∴ it is very expensive and there is a concern of safety as failure of the system could lead to the leak of toxic gases under pressure
35
Q

What are the compromise conditions for the Haber process and how effective are they?

A

350-500 degrees, 100-200atm and an iron catalyst - this gives a 15% yield however any unreacted nitrogen and hydrogen is recycled repeatedly so eventually nearly all of it is converted to ammonia

36
Q

What is Kc?

A

Kc = ([C]^c[D]^d)/([A]^a[B]^b)

concentration of products over concentration of reactants

37
Q

What does Kc tell us?

A
  • The magnitude of Kc indicates whether there are more reactants or more products in an equilibrium system by showing their relative proportions
  • Kc = 1 - position of equilibrium halfway between reactants and products
  • Kc > 1 - position of equilibrium towards products
  • Kc < 1 - position of equilibrium towards reactants
  • ∴ the larger the value of Kc, the further the position of equilibrium lies to the RHS and the greater the concentration of the products compared to the reactants
38
Q

How do you work out the units of Kc?

A

1) substitute units into expression for Kc

2) cancel common units (can be unitless)

39
Q

What is a homogenous equilibrium?

A

An equilibrium that contains equilibrium species that all have the same state/phase

40
Q

What is a heterogenous equilibrium?

A

An equilibrium that contains equilibrium species that have different states/phases

41
Q

What happens to the Kc expression in homogenous equilibria?

A

It contains concentrations of all species

42
Q

What happens to the Kc expression in heterogenous equilibria?

A
  • The concentrations of solids and liquids are essential constant
  • ∴ any species that are solids or liquids are omitted from the Kc expression as they are automatically incorporated within the overall equilibrium constant
  • ∴ Kc only includes species that are gases or aqueous
43
Q

Why is Kp used for gases instead of Kc?

A

1) it is easier to measure pressure than concentration for gases
2) concentration and pressure are proportional to each other

44
Q

What is the mole fraction of a gas?

A

The same as its proportion by volume to the total volume of gases in a gas mixture

45
Q

What is the mole fraction (x) of gas A?

A

number of moles of A/total number of moles in gas mixture

46
Q

What must the sum of the mole fractions in a mixture equal?

A

One

47
Q

What is the partial pressure of a gas in a gas mixture?

A

The contribution that the gas makes towards the total pressure (P)

48
Q

What does the sum of partial pressures of each gas in a mixture equal?

A

P (total pressure)

49
Q

What is the partial pressure ( p ) of gas A?

A

mole fraction of A (xA) x total pressure (P)

50
Q

What is the Kp expression?

A

Kp = (pC x pD)/(pA x pB) p = equilibrium partial pressure

51
Q

What is the only thing that Kp includes?

A

Gases

52
Q

What are the units of Kp?

A

KPa/Pa/atm

53
Q

What are the three rules for controlling the position of equilibrium?

A

1) if the concentration of species is increased, the equilibrium position shifts in the direction that reduces the concentration
2) if the pressure is increased, the equilibrium position shifts towards the side with fewer gaseous moles
3) if temperature is increased, the equilibrium position shifts in the endothermic direction

54
Q

What does K indicate?

A
  • The magnitude of K indicates the extent of chemical equilibrium
  • The value of K gives the exact position of equilibrium
  • When reactants or products are mixed together, the reaction proceeds until the concentrations of the equilibrium species give the value of K when placed in the equilibrium constant expression
55
Q

When does K change?

A
  • Only if temperature changes
  • At a set temperature, K is constant and does not change despite any modifications to concentration, pressure or the presence of a catalyst
56
Q

How does temperature affect K if the forward reaction is exothermic and what does this mean?

A
  • K decreases as temperature increases

- ∴ raising the temperature decreases the equilibrium yield of the products

57
Q

What happens when Kp decreases as temperature increases?

A

1) the system is no longer in equilibrium and the ratio is now greater than Kp
2) ∴ the equilibrium partial pressures must change to give a new Kp value
3) ∴ p(products) must decrease and p(reactants) must increase
4) ∴ the position of equilibrium shifts towards the reactants (LHS) to restore the Kp value
5) a new equilibrium is reached where the ratio is equal to the new Kp value

58
Q

How does temperature affect K if the forward reaction is endothermic and what does this mean?

A
  • K increases as temperature increases

- ∴ raising the temperature increases the equilibrium yield of the products

59
Q

What happens when Kp increases as temperature increases?

A

1) the system is no longer in equilibrium and the ratio is now less than Kp
2) ∴ the equilibrium partial pressures must change to give the new Kp value
3) ∴ p(products) must increase and p(reactants) must decrease
4) ∴ the position of equilibrium shifts towards the products (RHS)
5) a new equilibrium is reached where the ratio is equal to the new Kp value

60
Q

What is the effect of concentration and pressure on K?

A
  • The value of K is unaffected by changes in concentration or pressure
  • Even though the equilibrium does shift, this shift results from the fact that K does not change
61
Q

What happens if the concentration of the more molar reactant is increased?

A

1) the ratio is now less than Kc ∴ the system is no longer in equilibrium
2) ∴ the concentrations must change to return the ratio back to the Kc value
3) [product] must increase and [reactant] must decrease
4) the position of equilibrium shifts to the RHS to restore the value of Kc
5) a new equilibrium is established where the ratio is restored to the Kc value

62
Q

Why does Le Chatelier’s principle work?

A
  • Because Kc controls the relative concentrations of reactants and products present at equilibrium
  • The shift has been directed by the value of Kp being restored
63
Q

What happens if the pressure of a system is doubled?

A

1) the ratio is now more than Kp and the system is no longer in equilibrium
2) the partial pressures must change to return the ratio back to the Kp value
3) p(product) must increase and p(reactant) must decrease
4) the position of equilibrium shifts to the left
5) a new equilibrium is established where the ratio is restored to its Kp value

64
Q

How does the presence of a catalyst affect K?

A

K is unaffected by the presence of a catalyst

65
Q

When would you include water in the Kc expression?

A

In the esterification reaction

66
Q

How do you work out the equilibrium moles?

A

1) work out the difference between the initial and equilibrium moles of the product/reactant given
2) multiply this value by the coefficients of the products/reactants
3) for products: add this value to initial moles
for reactants: subtract this value from initial moles

67
Q

What word should you use instead of just not writing units?

A

Unitless

68
Q

If changes in pressure and temperature give opposite effects on the equilibrium, why might you not know the overall effect?

A

Because you don’t know the relative effect of pressure and temperature

69
Q

What does Kp always remain?

A

Constant

Chemistry A-Level (27 decks)

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