How to shift the equilibrium of a reaction
In Chemistry, the equilibrium position of a reaction can be shifted either to the right or left in order to get the maximum yield of reaction.
An equilibrium is a state in a reaction when the rate of forward reaction equals the rate of backward reaction in such a way that there is no net change in the concentrations of reacts and products with respect to time.
Equilibrium state is attained.in chemical reactions with time and there are conditions that must be obeyed in order to attain this state in chemical systems.
This is important to note that equilibrium in chemical systems can be attained when the proper conditions are set and this can be monitored and manipulated to give maximum yield of products.
Conditions for Chemical equilibrium state in Chemical reactions
- Rate of forward reaction must equal rate of backward reaction
- Concentration of reactants and products remain constant
- Chemical reactions must take place in a closed system.
| Properties of a system in equilibrium |
| 1. A chemical equilibrium can be achieved in a closed system |
| 2. A chemical equilibrium has two reactions occurring simultaneously and can be approached from the forward or backward directions. |
| 3. A chemical equilibrium is dynamic i nature |
| 4. The concentrations of all chemical species present in a chemical equilibrium remains constant |
| 5. A catalyst has no effect on equilibrium position |
Le Chateliers Principle
If an external constraint such as change in temperature, concentration or pressure is imposed on a system in equilibrium, the equilibrium position will shift so as to annul or neutralize the constraint.
The Le Chateliers Principle guides the properties of a system in equilibrium.
A + B == C
Effect of Temperature
Hint
Endothermic reaction +ve is favoured by increase in temperature
Exothermic reactions -ve is favoured by decrease in temperature
A + B===C enthalpy H =+ve
What is the effect of increase in temperature on equilibrium position?
Consider this reaction
A + B===C enthalpy H =+ve
Analysis of these conditions on equilibrium
increase in temperature.
Increase in temperature will Favour forward reaction (shifting the equilibrium to the right) producing more of C because forward is endothermic.
Reason
Because forward reactions is endothermic
Decrease in Temperature
Decrease in temperature will favour backward reaction (shifting the equilibrium position to the left producing more of A and B)
Reason
Because backward is exothermic.
Effect of concentration
Concentration is another factor that affects the chemical equilibrium of reactions.
Hint
Increase in concentration on one side favours the other side
Decrease in concentration on one side favours same side
Example
Consider the reaction
CaCO3 ===CaO +CO2
What is the effect of increasing the mass of CaCO3 on equilibrium position?
It will shift the equilibrium to the right producing more of CaO and CO2 I.e favoring forward reaction
What is the effect of decreasing the amount of CaO produced?
it will still Favour the forward reaction shifting the equilibrium position to the right
Example 2
Zn + 2HCl ===ZnCl2 + H2
Increasing concentration of HCl from 2.0mol/dm3 to 5.0mol/dm3?
It will favour the forward reaction producing more of ZnCl2 and H2
Mass = solids
Concentration = aqueous
Pressure=Gases
Effect of Pressure
Hint
When you increase in pressure, it favours the side with less no of moles or volume
When you decrease pressure, it favours side with higher no of moles
N2 +3H2===2NH3 H=-ve
Maximum yield/optimum yield
Temperature of 450C
Note
Decrease in temperature.
It will decrease rate of reaction but shift the equilibrium position to the right.
Increase in Temperature more than 450C will shift the equilibrium position to the left.
Pressure of 200 atm
Increase in pressure more than 200 atm
It will favour forward reaction.
It will destroy the equipment/ safety risks.
| Optimum yield of sulphuric acid |
| Temperature of 450C |
| Pressure of 1 atm |
| catalyst -vanadium v oxide |
| operated in a closed system. |
| Optimum yield of Haber Process |
| Temperature of 450C |
| Pressure of 200 atm |
| catalyst -finely divided iron |
| closed syatem operation. |
Decrease in pressure will cause the system to go backward.
Catalyst Finely divided iron
Why is the Catalyst made I Finely divided form
To increase surface area and then increase more active sites for the reaction
2SO2 + O2 ===2SO3 H=-ve
Conditions for optimum yield
Temp =450C
Pressure =1-2 atm
Maximum already gotten at this pressure
Catalyst =V2O5
Purify SO2 so that it will not poison the Catalyst used
Contact process Sulphuric acid
S +O2 ===SO2
2SO2 + O2 ===2SO3
SO3 +H2SO4 ===H2S2O7 oleum
H2S207 +H2O===2H2SO4
SO3 +H20===H2SO4
Why can’t we dissolve in water?
It forms misty fumes of Sulphuric acid which is difficult to condense
Effect of Pressure on chemical equilibrium
N2 +3H2===2NH3 H=-ve
increase in pressure
Increase in pressure will shift the equilibrium position to the right or favours forward reaction producing more of NH3 because it is the side with less no of moles
Decrease in pressure
Decrease in pressure favors backward reaction producing more of N2 and H2 because it is the side with higher no of moles
