Cards (30)
Many chemical reactions are reversible.
In a reversible reaction at equilibrium:
forward and reverse reactions proceed at equal rates
the concentrations of reactants and products remain constant
As the reactants get used up, the forward reaction slows down - and as more product is formed, the reverse reaction speeds up. After a while the forward reaction will be going at exactly the same rate as the backward reaction so the amounts of reactants and products won't be changing anymore. This is called dynamic equilibrium.
A dynamic equilibrium can only happen in a closed system. This just means nothing can get in or out.
If you change the concentration, pressure or temperature of reversible reaction, you're going to alter the position of equilibrium. This just means you'll end up with different amounts of reactants and products at equilibrium. If the position of the equilibrium moves to the left, you'll get more reactants. If the position of the equilibrium moves to the right, you'll get more products.
Le Châtelier’s Principle states that if there is any change in conditions (concentration, pressure, temperature) when a system is at equilibrium, then the position of equilibrium shifts in such a way as to counteract the effect of the change.
You can use Le Châtelier’s Principle to work out what effect changing the concentration, pressure or temperature will have on the position of equilibrium. This only applies to hom*ogenous equilibria - that means reaction where every species is in the same physical state.
Concentration
If you increase the concentration of a reactant, the equilibrium tries to get rid of the extra reactant. It does this by making more product. So the equilibrium shifts to the right.
If you increase the concentration of the product, the equilibrium tries to remove the extra product. This makes the reverse reaction go faster. So the equilibrium shifts to the left
Decreasing the concentrations has the opposite effect.
Pressure (changing this only affects equilibria involving gases)
Increasing the pressure shifts the equilibrium to the side with fewer gas molecules. This reduces the pressure.
Decreasing the pressure shifts the equilibrium to the side with more gas molecules. This raises the pressure again
Temperature
Increasing the temperature means adding heat. The equilibrium shifts in the endothermic (positive ΔH) direction to absorb this heat
Decreasing the temperature means removing heat. The equilibrium shifts in the exothermic (negative ΔH) direction to produce more heat, in order to counteract the drop in temperature.
If the forward reaction's endothermic, the reverse direction will be exothermic, and vice versa
Catalysts have no effect on the position of equilibrium. They can't increase yield - but they do mean equilibrium is reached faster.
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 so, in industry, the chosen reaction conditions are a compromise.
Ethanol production
1. Reversible exothermic reaction between ethene and steam
2. Carried out at 60-70 atmospheres of pressure
3. Carried out at 300°C temperature
4. Catalyst of phosphoric acid used
Exothermic reaction
Lower temperatures favour the forward reaction
Lower temperatures
More ethene and steam is converted to ethanol, better yield
Lower temperatures
Slower rate of reaction
300°C is a compromise between reasonable yield and faster reaction
High pressure
Shifts equilibrium to side with fewer molecules, favours forward reaction
High pressure
Increases rate of reaction
60-70 atmospheres of pressure is used
Using even higher temperature would be really expensive and require really strong pipes and containers to withstand high pressure
60-70 atmospheres of pressure is a compromise - it gives a reasonable yield for the lowest possible cost
The equilibrium constant Kc is deduced from the equation for a reversible reaction
The concentration, in moldm^-3, of a species X involved in the expression for Kc is represented by [X].
The value of the equilibrium constant is not affected either by changes in concentration or addition of a catalyst. Catalysts don't affect Kc either - they'll speed up the reaction in both directions by the same amount, so they just help the system to reach equilibrium faster.
If you know the molar concentration of each substance at equilibrium, you can work out the equilibrium constant, Kc. The units very so you have to work them out after each calculation.
Expression for equilibrium constant
You might have to work out the equilibrium concentrations before you can find Kc.
Find out how many moles of each chemical at equilibrium
Divide each number of moles by the volume of the flask to give the molar concentrations
Put the concentrations in the expression for Kc, and calculate it
Find the units for Kc
Kc can be used to find concentrations in an equilibrium mixture.
Put all the values you know into the Kc expression
Rearrange the equation
Solve the equation
The value of Kc is only valid for one particular temperature. If you change the temperature of the system, you will also change the equilibrium concentrations of the products and reactants, so Kc will change. If the temperature means there's more product at equilibrium, Kc will rise. If it means there's less product at equilibrium, Kc will decrease.
