Equilibria

• Define dynamic

  Both the forwards and backwards reactions are happening at the same time
• give 2 features of a reaction at equilibrium

  — the concentrations of reactants and products stay constant

  — the rate of the forwards and backwards reactions are the same
• describe equilibrium

  — if a reversible reaction is left in a closed system, then a state of dynamic equilibrium is reached

  — a closed system is one in which no substances are either added to the system or lost from it - but energy can be transferred in or out
• what is le chatelier’s principle

  — if any factor is changed which affects an equilibrium, the position fo equilibrium will shift so as to oppose the change
• describe exothermic reactions

  — products have less energy than the reactants

  — heat energy is given off by the reactions to the surroundings
• describe endothermic reactions

  — products have more energy than the reactants

  — heat energy is absorbed by the reactions from the surroundings
• describe the relationship between the temperature and the position of equilibrium

  — increasing the temperature of a reaction favours the endothermic reaction

  — decreasing the temperature of a reaction favours the exothermic reaction
• describe relationship between pressure and position of equilibrium

  — only applies to reactions involving gases

  — if a pressure of a reaction was increased:

  —— favour the side with the least amount of moles

  — if a pressure of a reaction was decreased:

  —— favours the side with the most amount of moles
• describe the relationship between concentration and the position of equilibrium

  — if concentration of a reactant is increased, the position of equilibrium will shift to the right to oppose the increase in concentration of the reactant, so the yield increases

  — if concentration fo a reactant is decreased, the position of equilibrium will shift to the left to oppone the decrease In concentration of the reactant, so the yield decreases
• describe the relationship between catalysts and the position of equilibrium

  — adding a catalyst has no effect to the position of equilibrium

  — it just speeds up the forward and backwards reaction to the same extent

  — speeds up the rebate at which a reaction reaches dynamic equilibrium
• how does a catalyst speed up the rate of reaction?

  — lowering the activation energy

  — by providing a different pathway

  — so increasing the rate of reaction
• explain why a catalyst has no effect on the position of an equilibrium (2)

  — catalyst increases the rate of both forwards and backwards reactions

  — the increase in rate is equal
• Describe the compromise reaction: Ammonia

  — made in the Haber Process

  N2 +3H2 ————> 2NH3

  — temp: 450°C Pressure: 200-1000atm Iron Catalyst

  — low temperature gives a good yield but a slow rate

  — high pressure gives a good yield and a high rate but would lead to too high energy costs
• Describe the compromise reaction: Production of methanol from CO

  CO + 2H2 ————> CH30H

  — temperature: 400° C Pressure: 50atm catalyst: chromium and zinc oxides

  — low temperature gives a good yield but a slow rate

  — high pressure gives a good yield and a high rate, but would lead to too high energy costs
• Describe the compromise reaction: Hydration of ethene to produce ethanol

  CH2=CH2 + H20 ————> CH3CH2OH

  — temperature: 300°C Pressure: 70atm Catalyst: conc H3PO4

  — low temperature gives a good yield but a slow rate

  — high pressure gives a good yield and a high rate but would lead to too high energy costs

  — high pressure can lead to unwanted polymerisation of ethane to poly(ethene)
• how would you improve the overall yields

  — recycling unreacted reactants back into the reactor

  —if CO was extracted from the atmosphere, it could be classed as carbon neutral and only if t eh energy fro the reactor cation was not carried out by combustion

  — both methanol and ethanol are used as fuels
• describe equilibrium constant

  — at equilibrium the ratio of concentrations of reactants and products is always constant

  — the ratio is Kc, at always a fixed temperature so usually carried out in a water bath

  — Kc is unaffected by changes in pressure/catalyst
• if an equation has no units what does this mean?

  — there’s an equal number of moles on each side of the equation
• What conditions are needed for Le‘chatliers principle?

  — solution is homogenous - all reactants and products in the same state