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Chemical Equilibria Flashcards

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1
Q

What is a reversible reaction?

A

One that can proceed in both forward and backward directions

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2
Q

What is dynamic equilibrium?

A

Reversible process at equilibrium in which the rate of forward reaction is equal to the rate of backward reaction, where there is no net change in concentration of reactants and products

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3
Q

What is concentration-time graph during dynamic equilibrium?

A
  • products is upward curve with decreasing gradient
  • reactants is downward curve with decreasing gradient

(look like headless fish)

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4
Q

What is rate-time graph during dynamic equilibrium?

A
  • rate of forward reaction is downward curve
  • rate of backward reaction is upward curve
  • both curve meets at the middle to form a straight line (rate of forward = rate of backward)
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5
Q

What is a requirement for equilibrium to be achieved and why?

A
  • closed system
  • many irreversible reactions take place in an open system such that product can escape, making backward reaction impossible
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6
Q

What happens at equilibrium?

A
  • amount of each substance and hence its concentration remains constant
  • no change in properties as long as conditions remain unchanged (temperature, pressure)
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7
Q

What is Kc?

A

Equilibrium constant

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8
Q

What is Kc of aA + bB ⇌ cC + dD?

A

Kc = [C]ᶜ[D]ᵈ/[A]ᵃ[B]ᵇ

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9
Q

What is excluded from Kc?

A
  • solids
  • water when water is solvent (aq)
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10
Q

What is Kp?

A

Equilibrium constant in terms of partial pressure (which is proportional to concentration) for gas

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11
Q

What is the Kp of aA(g) + bB (g) ⇌ cC (g) + dD (g)?

A

Kp = (Pc)ᶜ(PD)ᵈ/(PA)ᵃ(PB)ᵇ

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12
Q

What is excluded from Kp?

A

solids and liquids as they add negligible pressure to the system

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13
Q

What is Dalton’s law of partial pressure?

A

Total pressure exerted by a mixture of gas which do not react is equal to the sum of partial pressures of the constituent gases
Pₜ = Pₐ + Pb + Pc

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14
Q

What is partial pressure of a gas?

A

Partial pressure of a gas in a mixture is equal to the pressure that the gas would exert if it were alone and occupying the same volume under same conditions

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15
Q

What is formula for partial pressure?

A
  • Pa = nₐ/nₜ × pₜ
  • Pa = nₐRT/V
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16
Q

What affects K?

A

Temperature

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17
Q

Is K affected by catalyst and why?

A
  • unaffected by catalyst
  • catalysts increase both the forward and backward reaction and allows equilibrium to be achieved more rapidly but does not affect yield of reaction
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18
Q

What is the power in Kc or Kp?

A

coefficient in stoichiometric equation (unlike in reaction kinetics)

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19
Q

What does Kc or Kp > 1 indicate?

A
  • high proportion of products compared to reactants
  • position of equilibrium lies far right
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20
Q

What does Kc or Kp < 1 indicate?

A
  • a high proportion of reactants compared to products
  • position of equilibrium lies far left
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21
Q

What is Qc and what is it used for?

A
  • reaction quotient
  • used to predict direction of a reaction
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22
Q

What is Qc of aA + bB ⇌ cC + dD?

A
  • Qc = [C]ᶜ[D]ᵈ/[A]ᵃ[B]ᵇ
  • where [A], [B], [C], [D] are concentration values at any given time
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23
Q

What happens to Qc over time?

A

Qc changes with time as reaction proceeds until a state of dynamic equilbirum is attained

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24
Q

What happens when Qc < Kc?

A
  • more products need to be formed
  • forward reaction is favoured
  • POE shifts to right
  • Qc will increase until equilibrium is achieved
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25
Q

What happens when Qc = Kc?

A
  • System is at dynamic equilibrium
  • no shift of POE
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26
Q

What happens when Qc > Kc?

A
  • more reactants needs to be formed
  • backward reaction will be favoured
  • POE shifts left
  • Qc will decrease until equilibrium is achieved
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27
Q

What is Le Chatelier’s Principle?

A

If a system at dynamic equilibrium is subjected to a change which disturbs the equilibrium, the system responds in a way to counteract the effect of the change

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28
Q

For A ⇌ B, what happens if more A is added?

A
  • concentration/partial pressure of A increses
  • position of equilibrium shifts to the right to consume some of A added
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29
Q

For A ⇌ B, what happens if B is removed?

A
  • concentration/partial pressure of B decreases
  • position of equilibrium shifts to the right to replenish some of the B removed
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30
Q

What happens if water is added in equilibrium?

A
  • concentration of all species decrease by the same extent
  • more species on one side (eg. left) is diluted
  • rate of backward reaction is higher than rate of forward reaction
  • POE shifts left
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31
Q

What is the effect if pressure changes for gaseous systems at equilibrium?

A
  • if total pressure is increase (by decrease volume), or decreased (by increasing volume)
  • POE will shift in the direction that decrease or increase the total number of gas particles in the system
  • to reduce or increase the pressure
32
Q

How does an increase in total pressure affect the rate of reaction in equilibrium?

A
  • Both the rates of forward and backward reactions increase as the increase in pressure increases the number of reactant particles per unit volume
  • this increases the frequency of effective collisions between particles
33
Q

How does increasing temperature affect equilibrium and Kc/Kp with ΔH < 0?

A
  • Position of equilibrium shifts left to favour the backward endothermic reaction to absorb some of heat added
  • less products formed
  • Kc/Kp decreases
34
Q

How does increasing temperature affect equilibrium and Kc/Kp with ΔH > 0?

A
  • position of equilibrium shifts right to favour the forward endothermic reaction to absorb some of the heat added
  • more products formed
  • Kc/Kp increases
35
Q

How does increasing temperature affect equilibrium and Kc/Kp with ΔH = 0?

A
  • position of equilibrium unchanged
  • change in temperature has no effect on Kc/Kp
36
Q

How does decreasing temperature affect equilibrium and Kc/Kp with ΔH < 0?

A
  • position of equilibrium shifts right to favour the forward exothermic reaction to release some heat
  • more products formed
  • Kc/Kp increases
37
Q

How does decreasing temperature affect equilibrium and Kc/Kp with ΔH > 0?

A
  • position of equilibrium shifts left to favour the backward exothermic reaction to release some heat
  • less products formed
  • Kc/Kp decreases
38
Q

How does decreasing temperature affect equilibrium and Kc/Kp with ΔH = 0?

A
  • position of equilibrium unchanged
  • change in temperature has no effect on Kc/Kp
39
Q

What happens to the rate constants of forward and backward reactions when temperature increases for endothermic reaction?

A
  • Kc = kf/kb
  • for endothermic reaction, when temperature increases
  • both kf and kb increases but kf increases more than kb, so Kc increases
40
Q

What do change in temperature affect for equilibrium?

A
  • equilibrium constants
  • positions of equilibrium
  • rate constants of forward and backward reactions
41
Q

How does catalyst affect equilibrium constant?

A
  • catalysts increase the rates of both forward and backward reactions by the same extent
  • it lowers the activation energy for both forward and backward reaction by the same extent
  • kf and kb increase by the same extent
  • thus catalysts do not affect POE and equilibrium constant
  • catalyst only affect rate at which equilibrium is established
42
Q

Is the shift in position of equilibrium a spontaneous process and why?

A
  • It is a spontaneous process as it leads to a reduction in Gibbs free energy, G
  • ΔG < 0
43
Q

What is the relationship between G and dynamic equilibrium?

A
  • G will be minimum
  • ΔG = 0
44
Q

What does the graph of Gibbs free energy and extent of reaction look like?

A
  • curve with decreasing gradient, minimum point then increasing gradient
  • when Q < K, forward reaction is spontaneous, ΔG < 0, gradient negative
  • when Q = K, ΔG = 0, dynamic equilibrium, miminum point
  • when Q > K, ΔG > 0, backward reaction is spontaneous but forward reaction is non-spontaneous, increasing gradient

(aim is to reach minimum point)

45
Q

What is the equation relating ΔG to ΔG⁰?

A

ΔG = ΔG⁰ + RT ln Q
Q is reaction quotient

46
Q

What is the equation of ΔG⁰ at equilibrium?

A
  • ΔG = 0, Q = K
  • ΔG⁰ = -RT ln K

standard state refers to 1 bar and specific temperature,need not be 298K

47
Q

What does ΔG⁰ < 0 indicate about K?

A
  • K&raquo_space; 1
  • position of equilibrium lies very much to right
  • products are significantly more abundant than reactants
48
Q

What does ΔG⁰ > 0 indicate about K?

A
  • K &laquo_space;1
  • position of equilibrium lies very much to the left
  • reactants are significantly more abundant than products
49
Q

What does ΔG⁰ = 0 indicate about K?

A
  • K ≈ 1
  • near equilibrium
50
Q

What happens to ΔG⁰ at a given temperature?

A

ΔG⁰ is constant for a given reaction at a given temperature

(ΔG⁰ = -RTlnK, R, T and K all constant at same temperature)

51
Q

What happens to ΔG as the reaction mixture changes?

A

ΔG changes as reaction mixture changes, reaching zero at equilibrium

52
Q

What is the equation, temperature, pressure and catalyst for the industrial preparation of ammonia?

A
  • N₂ (g) + 3H₂ (g) ⇌ 2NH₃ (g)
  • 450°C
  • 250 atm
  • finely divided iron
53
Q

Why is 450°C chosen as the temperature for haber process?

A
  • since forward reaction is exothermic, decreasing the temperature will shift POE to right, increasing yield fo NH₃
  • at low temperature, reaction would be too slow due to high activation energy to break N≡N bons, it would take too long for equilibrium to be established which is not economically viable
  • 450°C is chosen so reasonable yield can be obtained at a relatively high rate
54
Q

Why is a pressure of 250atm chosen for Haber process?

A
  • increasing pressure will shift position of equilibrium top right and favour, increasing yield fo NH₃
  • it also increases the rate at which equilibrium is established
  • however, cost of equipment to produce and maintain high pressure would be greater
  • thus pressure of 250atm used as a compromise between yield and cost
55
Q

How to increase the yield of NH₃ in the haber process?

A
  • remove NH₃ formed
  • gases are cooled while maintaining the pressure, NH₃ condenses and liquefied NH₃ is removed from system
  • shift POE to right and increasing yield
  • unreact N₂ and H₂ are recycled
56
Q

How to get H₂ for the haber process?

A
  • steam reforming
  • CH₄ (g) + H₂O (l) → CO (g) + 3H₂ (G)
57
Q

How to get N₂ for the haber process?

A

fractional distillation of liquidfied air

58
Q

What is solubility?

A

Solubility of a substance is the maximum amount of the subsatnce that will dissolve in a given volume of solvent at a specific temperature

59
Q

What does solubility of a substance indicate?

A

concentration of the substance in its saturated solution

60
Q

What is Ksp?

A

Solubility product

61
Q

What is Ksp of AₘBₙ(s) ⇌ mAⁿ⁺ (aq) + nBᵐ⁻ (aq)?

A
  • Ksp = [Aⁿ⁺]ᵐ[Bᵐ⁻]ⁿ
  • where [Aⁿ⁺] and [Bᵐ⁻] refer to concentration of ions at equilibrium in a saturated solution
62
Q

What is the relationship of Ksp and temperature?

A

Ksp usually increases as temperature increases

63
Q

What is the relationship between dynamic equilibrium and saturated solution?

A
  • When dissolving an ionic solid in water, there are two opposing processes:
    * precipitation: hydrated ions from the solution are deposited to the surfaces of the solute
    * dissolution: ions on the surface of solute are continually passing in the solution as hydrated ions
  • when rates of dissolution and precipitation are equal, dyanmic equilibrium is reached, the solution formed is called saturated solution
64
Q

Are Ksp good indicators of relative solubilities of salts?

A

Ksp is good indicator of solubility only if total number of ions produced in the solution is the same

65
Q

What is the common ion effect?

A

Reduced solubility of a sparingly soluble salt in an aqueous solution containing an ion common to that salt

66
Q

What is I.P.?

A

ionic product

67
Q

What is the IP of AₘBₙ(s) ⇌ mAⁿ⁺ (aq) + nBᵐ⁻ (aq)?

A
  • I.P. = [Aⁿ⁺]ᵐ[Bᵐ⁻]ⁿ
  • where [Aⁿ⁺] and [Bᵐ⁻] refer to instantaneous concentration of ions in any given solution
68
Q

What happens when I.P. < Ksp?

A
  • solution is not saturated
  • precipitation does not occur
69
Q

What happens when I.P. = Ksp?

A
  • Solution is saturated
  • Precipitation does not occur but is about to take place
70
Q

What happens when I.P. > Ksp?

A
  • solution is saturated
  • precipitation occurs
71
Q

What happens when precipitation takes place?

A

[Aⁿ⁺] and [Bᵐ⁻] will gradually decrease until I.P. = Ksp

72
Q

Why is white ppt. of AgCl insoluble in water but soluble in NH₃?

give equations

A
  • when AgCl is added to water, Ksp value is so small that only a very small amount of AgCl dissolves until IP = Ksp
  • AgCl (s) ⇌ Ag⁺ (aq) + Cl⁻ (aq)
    when NH₃ is added,
  • Ag⁺ (aq) + 2NH₃ (aq) ⇌ [Ag(NH₃)₂]⁺ (aq)
  • [Ag⁺] in the solution decreases and makes IP < Ksp, POE of eqn 1 shifts right to replenish some of the consumed [Ag⁺] so that IP = Ksp, causing AgCl to dissolve
73
Q

What is the order of Ksp of AgCl, AgBr, AgI after adding NH₃?

A
  • Ksp(AgCl) > Ksp(AgBr) > Ksp(AgI)
  • lowered IP of AgCl will be lower than Ksp, hence AgCl is soluble in dilute NH₃
  • lowered IP of AgBr will be still comparable to Ksp of AgBr, thus it is sparingly soluble in dilute NH₃ and soluble in concentration NH₃
  • lowered IP of AgI will not be lower than Ksp thus AgI is insoluble in NH₃
74
Q

Why when NH₃ (aq) is gradually added to Cu²⁺, a blue ppt. formed which dissolves in excess NH₃ (aq) to give a dark solution?

give equations

A
  • NH₃ (aq) + H₂O (l) ⇌ NH₄⁺ (aq) + OH⁻ (aq)
  • Cu²⁺ (aq) + 2OH⁻ (aq) → Cu(OH)₂ (s)
  • Cu(OH)₂ (s) blue ppt. forms as IP > Ksp
  • [Cu(H₂O)₆]²⁺ (aq) + 4NH₃ (aq) ⇌ [Cu(NH₃)₄(H₂O)₂]²⁺ (aq) + 4H₂O (l)
  • [Cu²⁺] decreases and IP < Ksp momentarily, POE of 2 shift to left to favour formation of Cu²⁺ for IP = Ksp, causing Cu(OH)₂ to dissolve
75
Q

Why do white ppt. of PbCl₂ dissolve in conc. HCl to give yellow soluble but remains insoluble when dilute HCl is added?

A

PbCl₂ (s) ⇌ Pb²⁺ (aq) + 2Cl⁻ (aq)
* when dilute Hcl is added, the addition of a common ion, Cl⁻ shift POE to left, making PbCl₂ even more insoluble
* but if conc. HCl is added, the high [Cl⁻] leads to formation of yellow complex ion, [PbCl₄]²⁻
* [Pb²⁺] in the solution decreases and momentarily makes IP < Ksp, POE shifts to right to favour formation of Pb²⁺ so as to let IP = Ksp, causing PbCl₂ to dissolve

76
Q

Solubility of sparingly soluble salts of weak acids in strong acids

Why is Ca₃(PO₄)₂ sparingly soluble in water but readily solute in HNO₃?

A

Ca₃(PO₄)₂ (s) ⇌ 3Ca²⁺ (aq) + PO₄³⁻ (aq)
* Ksp of Ca₃(PO₄)₂ is so small that only a very small amount dissolves until IP = Ksp
* upon addition of HNO₃, PO₄³⁻ combines with H⁺ to form H₃PO₄ (step-wise)
* [PO₄³⁻] decreases such that IP < Ksp momentarily, POE shifts right to favour formation of PO₄³⁻ to restore IP = Ksp

Chemistry (21 decks)

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