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Y12 Physical Chemistry > Equilibria > Flashcards

Equilibria Flashcards

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1
Q

Meaning of dynamic

A

Both forwards and backwards reactions occur simultaneously

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2
Q

Features of dynamic equilibrium

A
  1. Forward and backward reaction occur at equal rate.

2. Concentration of reactants and products remains constant

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3
Q

Le Chatelier’s Principle

A

Is change is imposed on a system at equilibrium, the equilibrium will shift to oppose the change.

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4
Q

Effect of increasing temperature on equililbrium

A

Equilibrium will shift to oppose the change favouring the endothermic reaction to reduce the temperature by absorbing heat. (IN u ENDO)

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5
Q

Effect of decreasing temperature on equililbrium

A

Equilibrium will shift to oppose the change favouring the exothermic reaction to increase the temperature by releasing heat.

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6
Q

If a questions asks about yield, you should talk about…

A

position of equilibrium

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7
Q

If a questions asks about rate, you should talk about…

A

frequency of collisions

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8
Q

Effect of increasing pressure on equilibrium

A

Equilibrium will shift to oppose the change favouring the reaction that decreases the number of gaseous molecules. Always state how many gaseous moles are on each side of the equation in your answer.

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9
Q

Effect of decreasing pressure on equilibrium

A

Equilibrium will shift to oppose the change favouring the reaction that increases the number of gaseous molecules. Always state how many gaseous moles are on each side of the equation in your answer.

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10
Q

Effect of increasing concentration of a substance on equilibrium

A

Equilibrium will shift to oppose the change favouring the reaction that removes that substance.

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11
Q

Effect of decreasing concentration of a substance on equilibrium

A

Equilibrium will shift to oppose the change favouring the reaction that produces that substance.

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12
Q

Effect of a catalyst on equilibrium

A

No effect on position of equilibrium as the rate of both forward and backward reactions are increased. Equilibrium will be reached faster.

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13
Q

Reaction and conditions for Haber Process

A 
N2 + 3H2 --> 2NH3
forward reaction is exothermic
T = 450 oC
P = 200-1000 atm
cat = Fe
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14
Q

Reasons for temp of Haber process

A

Forward reaction is exothermic so lower temp gives higher yield but slower rate as less frequent collisions. A compromise temperature is used

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15
Q

Reasons for pressure of Haber process

A

High pressure gives high yield and high rate but is more expensive to maintain so a lower pressure is used.

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16
Q

Contact process - reaction and conditions

A
  1. S(s) + O2(g) –> SO2(g)
  2. SO2(g) + 1/2 O2 (g) <=> SO3
    Exothermic reaction
    T = 450 oC,
    P = 10 atm
    cat = V2O5
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17
Q

Reasons for temp of Contact process

A

Forward reaction is exothermic so lower temp gives higher yield but slower rate as less frequent collisions. A compromise temperature is used

18
Q

Reasons for pressure of Contact process

A

High pressure gives high yield and high rate but is more expensive to maintain so a lower pressure is used.

19
Q

Production of methanol from CO. Reaction and conditions

A 
CO(g) + 2H2(g) <=> CH3OH
Exothermic reaction
T = 400oC
P = 50 atm
Cat = chromium and zinc oxides
20
Q

Reasons for temp of methanol production

A

Forward reaction is exothermic so lower temp gives higher yield but slower rate as less frequent collisions. A compromise temperature is used

21
Q

Reasons for pressure of methanol production

A

High pressure gives high yield and high rate but is more expensive to maintain so a lower pressure is used.

22
Q

Hydration of ethene to produce ethanol. Reaction and conditions

A 
CH2=CH2(g) + H2O(g) <=> CH3CH2OH(g)
Exothermic reaction
T = 300 oC
P = 70 atm
Cat = conc. H3PO4
23
Q

Reasons for temp of ethanol production

A

Forward reaction is exothermic so lower temp gives higher yield but slower rate as less frequent collisions. A compromise temperature is used

24
Q

Reasons for pressure of ethanol production

A

High pressure gives high yield and high rate but is more expensive to maintain so a lower pressure is used.
High pressure also leads to unwanted polymerisation to polyethene

25
Q

Why are catalysts used in industrial reversible reactions?

A

They speed up the rate of reaction allowing a lower temp to be used and so lower energy costs without having any affect on equilibrium and therefore yield.

26
Q

How can overall yields of industrial reversible reactions be increased?

A

By recycling unreacted reactants back into the reactor.

27
Q

Equation for Kc for general equation:

mA + nB <=> pC + qD

A

[A]^m x [B]^n

28
Q

How to calculate unit of Kc

A

concentration of each reactant and product is measured in mol dm^-3. Look at the Kc equations and deduce the units

29
Q

Unit of Kc in equation N2 + 3H2 <=> 2NH3

A

mol^-2 dm^6

30
Q

Unit for Kc in equation H2 + Cl2 <=> 2HCl

A

no units

31
Q

How to calculate Kc

A

Work out initial moles of each reactant and product. Work out moles of one substance at equilibrium. Work out the moles of remaining substances at eqbm by decreasing original reactant moles and increasing original product moles. Convert the moles to concentration. Calculate Kc

32
Q

The reaction H2 + Cl2 <=> HCl was done in a 600 cm^3 container. There were initially 0.5 mol of H2 and 0.6 mol of Cl2. At eqbm there were 0.2 moles of HCl. Calculate Kc

A

0.196 no unit

33
Q

In what circumstance do you not need to calculate concentration to calculate Kc

A

If Kc has no units because there are equal numbers of reactants and products.

34
Q

In the Haber process there were initially 1.5 moles of N2 and 4 moles of H2 in a 1.5 dm^3 container. At eqbm 30% of the N2 had reacted. Calculate Kc

A

0.0927 mol^-2 dm^6

35
Q

What effect does increasing pressure have on Kc

A

No effect

36
Q

What effect does adding a catalyst have on Kc

A

No effect

37
Q

What factors affect Kc?

A

ONLY changing temperature

38
Q

What does a Kc >1 mean?

A

Equilibrium favours products

39
Q

What does a Kc <1 mean

A

Eqbm favours reactants

40
Q

What effect does changing concentration have on Kc

A

no effect

Y12 Physical Chemistry (7 decks)

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