Equilibrium Flashcards
Chemical equilibrium
Many reactions convert 100% of reactants into products - such reactions are said to go to COMPLETION and other reactions are reversible
Equilibrium reactions are reactions where both forward and backwards reactions occur at the same time - this means that the reaction mixture contains both “REACTANTS” and “PRODUCTS”
For an equilibrium reaction to be established the reaction must take place in a CLOSED SYSTEM - in this, substance cannot be lost or gained from the surroundings.
Characteristics of chemical equilibria
concentration of reactants and products change until equilibrium is established - reactants start at high conc. and decreases. Conc. of product starts at 0 and increases. When both curves are horizontal reactions has reached equilibrium where concentrations remain equal.
Chemical equilibria are dynamic - the forward & reverse reactions are continually taking place - concentrations at equilibrium don’t change as the forward and reverse rate of reaction are equal.
Effect of catalyst on an equilibrium reaction
Catalysts have no effect on the position of equilibrium as they speed up the forward and reverse reaction by the same amount.This means that reactions reach equilibrium more quickly with a catalyst.
Effect of position of equilibrium on concentration of reactants and products
If position of equilibrium lies to LEFT - reactants dominate
If position of equilibrium lies to RIGHT - products dominate
Homogeneous equilibria
All substances involved in the equilibrium are in the same phase e.g.
Heterogenous
Equilibrium mixture contains substances involved different phases
Changing equilibrium position - le chatelier’s principle
For a reaction in equilibrium if one of the conditions is changed, the position of equilibrium will shift to oppose the change
Changing concentration
Increasing the concentration of A with shift the position of equilibrium to the right - This will reduce the conc. of A and will oppose the change.
Concentration time graph example
Changing pressure
Affects position of equilibrium of reactions involving gases - changes based on no. Of moles of gas on each side of the equation.
For the reaction:
A(g) + B(g) <-> C(g)
An increase in pressure shifts the position of equilibrium to the right as there are fewer moles of gas on the RHS of the equation. This reduces the pressure and opposes the change of the increase in pressure.
Changing temperature - enthalpy change
Forward reaction is exothermic and heat is given out so this means that the backwards reaction is endothermic in which heat is taken in
When temperature is increased position of equilibrium shifts to the left since the reverse reaction is endothermic. This lowers the temperature and opposes the change of increasing the temperature.
Equilibrium constant
If you allow the reaction to reach equilibrium and measure the concentrations of each reactant and product you can combine this into the equilibrium constant - Has the same value regardless of the initial amounts of A, B, C & D.
It is affected by temp. Change but not affected by change in pressure or catalyst.
Large Kc means higher conc. of products and a low conc. of reactants - position of equilibrium lies to the right.
Kc of 1 indicates equal concentration of reactants and products.
Affect of Temp. On Kc
From graph, Kc is larger at higher temp. System would have shifted to oppose the increase in temp by favouring the endothermic reaction - forward reaction therefore must be endothermic so delta H = +ve for forward reaction.
Working out Kc units: 2A +2B <-> C + D
Calculating Kc using no.of moles
Calculate Kc using initial moles
How do gases exert pressure?
Gas particles collide with the walls of the container through random motion
Partial pressure
In a mixture of gases each gas exerts its own pressure - this is the partial pressure
Measured in Pa (Nm^-2)
Dalton’s law of partial pressures
The sum of all the partial pressures of the gases in a mixture is equal to the total pressure.
Mole fraction
Calculating partial pressure
Partial pressure = mole fraction x total pressure
Kp
Used when everything is a gas
Kp example
Compromise conditions in industry - Methanol production
Methanol is also made industrially in a reversible reaction:
2H2(g) + CO(g) = CH3OH(g)
ΔΗ = -90 kJ mol-1
Just like ethanol production, the conditions are a compromise between
keeping costs low and yield high. The conditions for this reaction are:
* a pressure of 50-100 atmospheres,
* a temperature of 250°C,
* a catalyst of a mixture of copper, zinc oxide and aluminium oxide.
Compromise conditions in industry - Ethanol production
Companies have to think about how much it costs to run a reaction and how much money they can make from it. This means they have a few factors to think about when they’re choosing the best conditions for a reaction.
Ethanol can be made via a reversible reaction between ethene and steam:
C2H4(g) + H20(g) = C2H5OH(g)
AH = -46 kJ mol-1
The industrial conditions for the reaction are:
* a pressure of 60-70 atmospheres,
* a temperature of 300 C,
* a phosphoric acid catalyst.
Because it’s an exothermic reaction, lower temperatures favour the forward reaction. This means that at lower temperatures more ethene and steam are converted to ethanol - you get a better yield. But lower temperatures mean a slower rate of reaction. You’d be daft to try to get a really high yield of ethanol if it’s going to take you 10 years. So the 300 C is a compromise between maximum yield and a faster reaction.
Higher pressure favours the forward reaction, since it moves the reaction to the side with fewer molecules of gas, so a pressure of 60-70 atmospheres is used. Increasing the pressure also increases the rate of reaction. Cranking up the pressure as high as you can sounds like a great idea, but high pressures are expensive to produce. You need stronger pipes and containers to withstand high pressure. So the 60-70 atmospheres is a compromise between maximum yield and minimum expense.
Only a small proportion of the ethene reacts each time the gases pass through the reactor. To save money and raw materials, the unreacted ethene is separated from the ethanol and recycled back into the reactor. Thanks to this, around 95% of the ethene is eventually converted to ethanol.
