equilibria [P1] PAPER 1+2 Flashcards
equilibrium constant (Kc) equation?
ensure everything is according to CONCENTRATIONS (⚠️ volume needs to be in dm3) before calculating
i.e. [A], [B], [C] and [D] are:
• equilibrium moles of A / volume of container
• equilibrium moles of B / volume of container
etc.
concentration of water is effectively constant since it is so much larger than the other concentrations
Does the addition of a catalyst affect the equilibrium of a reaction? Explain why.
• no
• catalysts increase the rates of the forward and backwards reactions by the same amount
What affects the value of Kc?
Changing the temperature of the system
Is the value of Kc affected by changes in pressure/concentration?
No
What is meant by dynamic equilibrium?
- The forwards and backwards reaction are taking place at the same time, and at the same rates, in a closed system
- Concentrations of all reactants and products are constant
what is a closed system?
nothing can get in or out
Why are compromise temperatures and pressures used in industry?
• to maximise yield
• reduces costs
According to Le Chatelier’s Principle, what happens to the position of the equilibrium of a reaction when the temperature of the system increases?
- the forwards/backwards reaction is endothermic
- equilibrium position moves to the right/left in the endothermic direction to oppose the increase in temperature
- increases equilibrium yield of [endothermic reaction products]
[for temperature decrease - shift in the exo direction instead of endo]
According to Le Chatelier’s Principle, what happens to the position of the equilibrium of a reaction when the pressure of the system increases?
- equilibrium position moves to oppose the increase in pressure by moving to the [side with fewer moles] as the right/left side has fewer moles
- increases/decreases equilibrium yield of…
e.g.
According to Le Chatelier’s Principle, what happens to the position of the equilibrium of a reaction when the concentration of a reactant increases?
- equilibrium position moves to oppose the increase in [reactant] by moving to the right to use it up
- equilibrium yield of [product] increases
According to Le Chatelier’s Principle, what happens to the position of the equilibrium of a reaction when the concentration of a reactant decreases?
- equilibrium position moves to oppose the decrease in [reactant] by moving to the left to make more of it
- decreases equilibrium yield of [product]
working out equilibrium moles?
Initial moles
Change in moles
Equilibrium moles - ICE
See what Q has given you - calculate the change in moles and use it to calculate whatever they’re asking for
⚠️ make sure you take into account the molar ratios (from the equation)
Working out units of Kc?
• ratio of mol:dm-3 should be 1 : -3
- pretend each variable in the general Kc equation is moldm-3
- simplify the fraction - don’t be stupid with indices ⚠️
- there are no units if the moles of products are equal to the moles of reactants
e.g.
Why may a high pressure be used for a reaction in industry?
- more particles in a given volume, so more collisions in a given time
- increases the rate of reaction
- the reaction gets to equilibrium faster
For a reaction, the total number of moles on the left side is equal to the number of moles on the right side.
State the effect, if any, on the equilibrium yield of the forwards reaction products when pressure is increased.
no effect
Methanol is made by the reaction of carbon monoxide with hydrogen.
CO+2H2 ⇌CH3OH
∆H=–91kJmol–1
The reaction uses a copper-based catalyst, a pressure of 10 MPa and a temperature of 550 K.
These conditions are used to provide a balance between equilibrium yield, reaction rate and cost.
Describe how the use of a catalyst, and changes in pressure and temperature, each affect equilibrium yield, reaction rate and cost.
[6 marks]
Use of a catalyst…
• doesn’t affect equilibrium yield
• gives a faster rate
• lowers costs
Increasing pressure…
• gives a higher equilibrium yield for this reaction
• gives a faster rate
• the higher the pressure, the higher the cost
Temperature:
• higher temperature gives a faster rate
• lower temperature gives a higher equilibrium yield
• the higher the temperature, the higher the cost
The Kc of a the forwards reaction of a reversible reaction is ‘x’. What is the Kc of the backwards reaction?
1/x (1/units too if there are units)
e.g. if the Kc of the forwards reaction is 5 moldm-3, then the Kc of the backwards reaction 0.2mol-1dm3
Reaction 2 is exothermic. A typical compromise temperature of 200 °C is used industrially for this reaction.
Explain the effect of a change of temperature on both the position of equilibrium and the rate of reaction, and justify why a compromise temperature is used industrially.
[6 marks]
One reason why HIGH pressures aren’t used in industry?
expensive equipment costs
State le Chatelier’s Principle.
If any factor is changed which affects an equilibrium, the position of equilibrium will shift to oppose the change
2A + B ⇌ 3C + D (A, B, C and D are solutions)
Why would adding more water to the equilibrium mixture lower the amount of A in the mixture?
• All concentrations (of A, B, C and D) fall
• Equilibrium moves to the right, to the side with more moles, to oppose the decrease in concentration
Justify the statement that adding more water to the equilibrium mixture will lower the amount of A in the mixture.
