3.1.6 Chemical equilibria, Le Chatelier’s principle and Kc Flashcards
Le Chatelier’s principle
If external condition is changed, the equilibrium will shift to
oppose the change and try to reverse it.
Position of equilibrium
Composition of the equilibrium mixture
If position of equilibrium favours reactants = equilibrium
mixture contains mostly reactants.
Dynamic equilibrium state
- Forward and backward reactions occur at equal states
2. Concentrations of reactants and products stay constant
Position of equilibrium- Temperature
Temp increase:
equilibrium shifts to oppose this and moves in the
endothermic direction to try and reduce the temp by
absorbing heat
Position of equilibrium- Temperature
Temp decrease:
equilibrium shifts to oppose this and moves in the
exothermic direction to try and increase the temp by giving
out heat
Compromise temp in industry
Low temp= high yield but slow rate
Compromise temp used to give a reasonable yield and rate to
make best possible yield.
Position of equilibrium- Pressure
Pressure increase:
equilibrium shifts towards the side with fewer moles of gas
to oppose the change so reduces the pressure.
Position of equilibrium- Pressure
Pressure decrease:
equilibrium shifts towards the side with more moles of gas to
oppose the change therefore increasing the pressure.
Advantage and disadvantages of increasing pressure
industrially
Adv: higher yield of product and faster rate of reaction
Disadv: Expensive: equipment to contain high pressures and
high electricity costs for pumping gases to make high
pressures.
Position of equilibrium- Catalyst
No affect apart from speeding up rate at which equilibrium is
achieved.
Speed up rates of forward and backward reactions by the
same amounts
Haber process
N2 + 3H2 2NH3 ΔH = -ve (exo)
Temp: 450*C (low temp: good yield, slow rate- compromise
temp used)
Pressure: 200-1000 atm (high pressure = good yield and high
rate)
Catalyst: iron
Expensive: equipment required and electricity needed
Use of ammonia: fertiliser
Contact process
Stage 1:
S (s) + O2 (g) –> SO2 (g)
Stage 2:
SO2 (g) + 1/2O2(g) SO3 (g) ΔH = -98kJ mol-1
Temp: 450*C (low temp: good yield, slow rate-
compromise temp used)
Pressure: 10 atm (high pressure = slightly better yield and high rate; too high temp = high energy costs to produce the pressure)
Catalyst: V2O5 (vanadium (v) oxide)
Producing methanol from carbon monoxide:
CO (g) + 2H2(g) CH3OH (g) ΔH = -ve (exo)
CO (g) + 2H2(g) CH3OH (g) ΔH = -ve (exo)
Temp: 400*C
Pressure: 50 atm
Catalyst: Chromium and
ZnOx
Hydrating ethene to produce ethanol
CH2=CH2 (g) + H2O(g) CH3CH2 (l) ΔH = -ve (exo)
Temp: 300*C
Pressure: 70 atm - too high pressures = polymerisation of ethene to poly(ethene)
Catalyst: concentrated H3PO4- phosphoric acid
Catalysts in industry
Catalysts: speed up rate of reaction allowing lower temps to be used so lower energy costs
No affect on position of equilibrium
