Equilibria Flashcards
What are the properties of a reaction at equilibrium?
- Concentrations of reactants and products remain constant.
- Forward + Reverse reactions proceed at equal rates.
Define Le Chatelier’s principle.
If a system at equilibrium is disturbed, the position of the equilibrium moves in the direction that tends to reduce the disturbance.
Use Le Chatelier’s principle to predict the effects of changes in temperature on the position of equilibrium in homogeneous reactions.
What happens when this reaction is heated?
The reaction is exothermic reaction; the system absorbs this heat when heated and moves the equilibrium in the endothermic direction; to the left, thus more reactant is formed.
Use Le Chatelier’s principle to predict the effects of changes in concentration on the position of equilibrium in homogeneous reactions.
An equilibrium is established between four substances A, B, C and D. What would happen if you increased the concentration of A?
The position of equilibrium will move in such a way as to counteract the change; the position of equilibrium will move so that the concentration of A decreases again - by reacting it with B and turning it into C + D. Thus the position of equilibrium moves to the right.
More product is made.
(Decreasing the concentration of A would mean the equilibrium would move to the left)
Use Le Chatelier’s principle to predict the effects of changes in pressure on the position of equilibrium in homogeneous reactions.
What would happen if you changed the conditions by increasing the pressure?
Increasing the pressure on a gas reaction shifts the position of equilibrium towards the side with fewer molecules; equilibrium position moves to the right (there are 3 molecules of gas on the left but only 2 on the right).
Use Le Chatelier’s principle to predict the effects of changes in pressure on the position of equilibrium in homogeneous reactions.
What would happen if you changed the conditions by DECREASING the pressure?
The equilibrium will move in such a way that the pressure increases again. It can do that by producing more molecules. In this case, the position of equilibrium will move towards the left-hand side of the reaction.
(Equilibrium position moves to the side with more gas molecules)
What effect does a catalyst have on the position of equilibrium?
There is no effect on the position of equilibrium; it is just reached faster. (a* catalyst speeds up the forward and back reaction to the same extent*)
Be able to predict qualitatively the effect of temperature on the position of equilibrium from the sign of ΔH for the forward reaction.
Increasing the temperature of a system in dynamic equilibrium favours the endothermic reaction. The system counteracts the change you have made by absorbing the extra heat.
e.g. ΔH = -250 kJ mol -1 ;exothermic, increase in temp = favour the reverse (endothermic) reaction to absorb heat.
Decreasing the temperature of a system in dynamic equilibrium favours the exothermic reaction. The system counteracts the change you have made by producing more heat.
The Haber Process combines nitrogen from the air with hydrogen derived mainly from natural gas (methane) into ammonia. The reaction is reversible and the production of ammonia is exothermic.
It takes place at 400 - 450oC, 200atm, and features an iron catalyst.
Why is there a comprimise in temperature and pressure?
Hot enough to be fast.
Not too hot to move equilibrium to left.
High pressure to make reaction fast
High pressure to make equilibrium go the right.
Not too high pressure so its expensive to maintain or dangerous.
Iron catalyst to make reaction faster.
Remove ammonia to move equilibrium to right.
Recycle hydrogen and nitrogen.
Write an equation for the hydration of ethene to form ethanol, and state the conditions.
Ethanol is manufactured by reacting ethene with steam. The reaction is reversible, and the formation of the ethanol is exothermic.
Conditions:
- 300oC/570K
- 60 - 70atm/6500kPa
- Phosphoric(V) acid catalyst
Ethene is made from cracking hydrocarbons from crude oil.
Explain the comprimise in temperature in the production of ethanol via hydration with steam.
- The forward reaction is exothermic (-45 kJ mol -1); a low temperature favours maximum ethanol production as the system will respond by moving the position of equilibrium to counteract this; producing more heat.
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However, rate is drastically reduced, though a better yield is gained; 300oC is the comprimise between yield and a fast reaction.
(Under these conditions, about 5% of the ethene reacts to give ethanol at each pass over the catalyst; but b**y removing the ethanol from the equilibrium mixture and recycling the ethene, it is possible to achieve an overall 95% conversion.)
Explain the comprimise in pressure in the production of ethanol via hydration with steam.
- There are 2 molecules on the left-hand side of the equation, but only 1 on the right; according to Le Chatelier’s Principle, if you increase the pressure the system will respond by favouring the reaction which produces fewer molecules. That will cause the pressure to fall again.
- High pressure favours the forward reaction; resulting in higher yield and rate.
- HOWEVER, high pressures are expensive. It costs more to build the original plant because you need extremely strong pipes and containment vessels. It also needs a lot of energy to produce the high pressures. That can make the ethanol uneconomic to produce.
- At high pressures, the ethene polyerises to make poly(ethene). Apart from wasting ethene, this could also clog up the plant.
Explain the comprimise in steam in the production of ethanol via hydration with steam.
- Excess steam (water is cheap) will force the equilibrium to move to the right to reduce steam concentration; resulting in more ethanol production.
- HOWEVER, too much steam dilutes the phosphoric(V) acid catalyst, which is coated onto a solid silicon dioxide support; too much steam could even wash it off the support, making it useless.
Write an equation to show the reaction of carbon monoxide with hydrogen to form methanol, and state the conditions.
Methanol is produced industrially in a reversible reactionl principally as a starting material to make other chemicals.
Conditions:
- 50 - 100atm/10,000kPa
- 500K/250oC
- Catalyst: mixture of copper, zinc and aluminium oxides.
Explain the importance of ethanol and methanol as liquid fuels.
- Both methanol and ethanol can be used as fuels for cars - either on their own, or added to petrol.
- They are ‘greener’ than petrol, considered carbon neutral, made from renewable resources and producing less pollutants (such as NOx and CO).
