4.2 Reversible reactions and equilibria Flashcards
Reversible reaction
Some reactions go to completion, where the reactants are used up to form the product molecules and the reaction stops when the reactants have been exhausted.
In reversible reactions, the product molecules can themselves react with each other or decompose and form the reactant molecules again.
A + B ⇌ C + D
Dynamic equilibria
A reversible reaction is one that occurs in both directions.
When during the course of reaction, the rate of the forward reaction equals the rate of the reverse reaction, then the overall reaction is said to be in a state of equilibrium.
Equilibrium is dynamic i.e. the molecules on the left and right of the equation are changing into each other by chemical reactions constantly and at the same rate.
The concentration of reactants and products remains constant (given there is no other change to the system such as temperature and pressure).
It only occurs in a closed system so that none of the participating chemical species are able to leave the reaction vessel.
Ammonia as a reversible reaction
An example of a dynamic equilibrium is the reaction between H2 and N2 in the Haber process.
When only nitrogen and hydrogen are present at the beginning of the reaction, the rate of the forward reaction is at its highest, since the concentrations of hydrogen and nitrogen are at their highest
As the reaction proceeds, the concentrations of hydrogen and nitrogen gradually decrease, so the rate of the forward reaction will decrease.
However, the concentration of ammonia is gradually increasing and so the rate of the backward reaction will increase (ammonia will decompose to reform hydrogen and nitrogen).
Since the two reactions are interlinked and none of the gas can escape, the rate of the forward reaction and the rate of the backward reaction will eventually become equal and equilibrium is reached.
Haber process
Ammonia is manufactured using The Haber Process which occurs in five stages.
Stage 1: H2 and N2 are obtained from natural gas and the air respectively and are pumped into the compressor through pipe.
Stage 2: the gases are compressed to about 200 atmospheres inside the compressor.
Stage 3: the pressurised gases are pumped into a tank containing layers of catalytic iron beds at a temperature of 450°C. Some of the hydrogen and nitrogen react to form ammonia:
N2 (g) + 3H2 (g) ⇌ 2NH3 (g)
Stage 4: unreacted H2 and N2 and product ammonia pass into a cooling tank. The ammonia is liquefied and removed to pressurised storage vessels
Stage 5: the unreacted H2 and N2 gases are recycled back into the system and start over again.
Temperature for haber process
A higher temperature would favour the reverse reaction as it is endothermic (takes in heat) so a higher yield of reactants would be made.
If a lower temperature is used it favours the forward reaction as it is exothermic (releases heat) so a higher yield of products will be made
However at a lower temperature the rate of reaction is very slow.
So 450ºC is a compromise temperature between having a lower yield of products but being made more quickly.
Pressure for haber process
A lower pressure would favour the reverse reaction as the system will try to increase the pressure by creating more molecules (4 molecules of gaseous reactants) so a higher yield of reactants will be made.
A higher pressure would favour the forward reaction as it will try to decrease the pressure by creating less molecules (2 molecules of gaseous products) so a higher yield of products will be made.
However high pressures can be dangerous and very expensive equipment is needed.
So 200 atm is a compromise pressure between a lower yield of products being made safely and economically.
Position of equilibrium
The relative amounts of all the reactants and products at equilibrium depend on the conditions of the reaction.
This balance is framed in an important concept known as Le Chaterlier’s Principle, named after Henri Le Chatelier who was a French military engineer in the 19th century.
This principle states that when a change is made to the conditions of a system at equilibrium, the system automatically moves to oppose the change.
The principle is used to predict changes to the position of equilibrium when there are changes in temperature, pressure or concentration.
Knowing the energy changes, states and concentrations involved allows us to use the principle to manipulate the outcome of reversible reactions.
Temperature on equilibrium
All reversible reactions are exothermic in one direction and endothermic in the other.
If you raise the temperature of A+B ⇌ C+D where -> = exothermic, the equilibrium will shift to oppose the change. The system wats to absorb heat energy to cool it down. Therefore, the equilibrium will shift to the left (the endothermic direction).
If you reduce the temperature of A+B ⇌ C+D where -> = exothermic, the equilibrium will shift to oppose the change. The system wants to produce heat energy to heat it up. Therefore, the equilibrium will shift to the right (the exothermic direction).
Pressure on equilibrium
Changing the pressure affects reactions where the reactants and products are gases. Many of these reactions have a greater volume on one side (either of products or reactants), Greater volume means there are more gas molecules on that side of the equation and less volume means there are fewer gas molecules.
Example - The reaction below is used to make hydrogen gas. It has two gas molecules on the left and four on the right.
CH4(g) + H2O(g) ⇌ CO(g) + 3H2(g)
If you increase the pressure, the equilibrium shifts to oppose the change and moves to decrease the pressure. The position of equilibrium will shift to the left which generates less gas particles – CH4 and H₂O.
If you decrease the pressure, the equilibrium shifts to oppose the change and moves to increase the pressure. The position of equilibrium will shift to the right which generates more gas particles - CO and H₂.
Concentration on equilibrium
If you change the concentration of either the reactants or the products, the system will no longer be at equilibrium. So, the system will respond to bring itself back to equilibrium again. A+B ⇌ C+D.
If you add/increase the concentration of A or B, the position of equilibrium will shift to oppose the change and so the equilibrium will shift to the right forming more C & D and decreasing the amount of A & B.
If you add/increase the concentration of C or D, the position of equilibrium will shift to oppose the change and so the equilibrium will shift to the left forming more A & B and decreasing the amount of C & D.
Catalyst for haber process
The presence of a catalyst does not affect the position of equilibrium but it does increase the rate at which equilibrium is reached.
This is because the catalyst increases the rate of both the forward and backward reactions by the same amount (by providing an alternative pathway requiring lower activation energy).
As a result, the concentration of reactants and products is nevertheless the same at equilibrium as it would be without the catalyst.
So a catalyst is used as it helps the reaction reach equilibrium quicker
It allows for an acceptable yield to be achieved at a lower temperature by lowering the activation energy required.
Without it the process would have to be carried out at an even higher temperature, increasing costs and decreasing yield as the higher temperature decomposes more of the NH3 molecules.
