Chemical equilibria, Le Chatelier’s principle and Kc Flashcards
(29 cards)
Describe what is meant by dynamic equilibrium
Forward and backward reactions are occuring simultaneously at equal rates
Conc of reactants and products stay constant in a closed system
This is obviously a reversible reaction
Describe what is meant by a closed system
A system where no substances are either added to the system or lost from it, however energy is transferred in and out of this system
Describe what is meant by forward and backwards reaction
The change from left to right is known as the forwards reaction
The change from right to left is known as the backwards reaction
Describe what is meant by the term position of equilibrium
We use this to describe the composition of the equilibrium mixture
If the position of the equilibrium favours the left, or shifts towards the left, this means the equilibrium mixture will contain mostly reactants
State Le chateliers principle
If a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium moves to counteract the change.
Describe the effect on equilibrium when the temperature is increased
Equilibrium will shift to oppose this and move in the endothermic direction
This tries to reduce the temperature by absorbing the extra heat that has just been put in
If the forward reaction is exothermic and the temp has been increased in equilibrium, the position of the equilibrium will shift to the left, increasing the yield of the reactants
If the forwards reaction is endothermic and the temp has been increased the equilibrium will shift to the right increasing yield of the products
Describe the effect of decreasing the temperature on the position of equilibrium
The equilibrium will shift to oppose the decrease in temperature and move in the exothermic direction to try and increase the temperature by giving out heat
If the forwards reaction is endothermic and temp is decreased the equilibrium will move in the exothermic direction which is the left direction, to try and increase temperature giving a higher yield of the reactants
If the forwards reaction is exothermic and the temp is decreased the equilibrium will move in the exothermic direction which is the right direction to try and inc temp giving a higher yield of products
What effect would inc the temperature have on the yield of ammonia?
N2 + 3H2= 2NH3 H = -ve exo
If temperature is increased the equilibrium will shift to
oppose this and move in the endothermic, backwards
direction to try to decrease temperature. The position
of equilibrium will shift towards the left, giving a lower
yield of ammonia.
Explain why, for a reversible reaction used in an industrial process, a compromise temperature and pressure may be used.
Low temps may give a higher yield of product, however this results in slow rates of reaction, often compromise temps are used which gives a reasonable yield( economic factors) and rate( use of manpower and machinery)
Describe the effect of increasing the pressure of equilibrium
This will cause the equilibrium to shift towards the side with the fewer moles of gas to oppose this change and reduce the pressure
Describe the effect of decreasing the pressure on equilibrium
This will cause the equilibrium to shift towards the side with more moles of gas to oppose this change and increase the pressure
What effect would increasing
pressure have on the yield of methanol?
CO (g) + 2H2(g) ⇌ CH3OH (g)
If pressure is increased the equilibrium will shift to
oppose this and move towards the side with fewer
moles of gas to try to reduce the pressure . The position of equilibrium will shift towards the right because there are 3 moles of gas on the left but only 1
mole of gas on the right, giving a higher yield of methanol.
Describe why a compromise is needed when deciding a pressure to use when industrially carrying out a reaction
Increasing the pressure gives a higher yield of product however Industrially high pressures are expensive to produce( high electrical costs for pumping the gases to make a high pressure)
And the equipment is expensive to contain the high pressure in
What happens when number of moles on each side of the reaction is the same when changing the pressure
Changing pressure will have no effect on the position of the equilibrium
What would happen if you increased the concentration of A in this reaction?
A+ 2B= C+D
The position will move in the direction which counteracts this change, means that position of equilibrium will move so that A decreases again by reacting it with b and turning it into C+ D
This means position of equilibrium will move to the right
What would happen if you decreased the conc of A in this reaction
A + 2B= C+D
Position of equilibrium will move so that the conc of A increases again, means more C + D will react to replace the A that has been removed, so position of equilibrium shifts to the left
Describe the effects that cataylsts have on equilibrium
A catalyst has no effect on the position of equilibrium, but it will speed up the rate at which the
equilibrium is achieved.
It does not effect the position of equilibrium because it speeds up the rates of the forward and
backward reactions by the same amount.
Describe how the conditions selected for the haber process are compromised
N2 + 3H2 ⇌ 2NH3 H = -ve exo
T= 450oC, P= 200 – 1000 atm, catalyst = iron
Low temp gives good yield but slow rate: compromise temp used
High pressure gives good yield and high rate: too high a pressure would lead to too high energy costs
for pumps to produce the pressure
Describe how the conditions for the contact process have been compromised
Stage 1 S (s) + O2 (g) SO2 (g)
Stage 2 SO2 (g) +½O2 (g) ⇌ SO3 (g) H = -98 kJ mol-1
Low temp gives good yield but slow rate: compromise moderate temp used
High pressure only gives slightly better yield and high rate: too
high a pressure would lead to too high energy costs for pumps
to produce the pressure
Describe how the conditions for the production of ethanol by hydration of ethene are chosen as a compromised
CH2 =CH2 (g) + H2O (g) ⇌ CH3CH2OH(l) H = -ve
T= 300oC, P= 70 atm, catalyst = conc H3PO4
Low temp gives good yield but slow rate: compromise temp
used
High pressure gives good yield and high rate: too high a
pressure would lead to too high energy costs for pumps to
produce the pressure
High pressure also leads to unwanted polymerisation of
ethene to poly(ethene)
Why is a catalyst important in deciding a compromise for the conditions for an industrial process
In all cases catalysts speeds up the rate, allowing a lower temp to be used (and hence lower energy costs), but have no effect on position of equilibrium
Describe how conditions have been chosen as a compromise for production of methanol from CO
CO (g) + 2H2(g) ⇌ CH3OH (g)H = -ve exo
T= 400oC, P= 50 atm, catalyst = chromium and zinc
oxides
Low temp gives good yield but slow rate:
compromise temp used
High pressure gives good yield and high rate: too high a pressure would lead to too high energy costs for pumps to produce the pressure
What is the issue with choosing too high of a pressure
-High pressure leads to too high energy costs for pumps to produce the pressure
-Too high equipment costs to have equipment that can withstand high pressures.
What can be done to improve the yield of industrial processes ( not changing conditions)
Unreacted reactants can be recycled over the catalyst back into the reactor
