Le Chatelier’s Principle
If a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium moves to counteract the change.
Consider the following reaction, which has an enthalpy change of -98 kJ mol-1:
The yield would decrease. This is because the equilibrium would shift in the direction of the endothermic reaction (to the left) to counteract the increase in temperature.
The yield would increase. This is because the equilibrium shifts in the direction of the side with the lowest number of gas molecules to counteract the pressure increase. The right side has the least gas molecules (2 moles vs 3 moles), so the equilibrium shifts right.
No change. A catalyst increases the rate of the forwards reaction and the rate of the backwards reactions equally.
Consider the following reaction, which is exothermic:
The yield would decrease. This is because the forward reaction is exothermic. Increasing the temperature causes the equilibrium to shift in the direction of the endothermic reaction (to the left) to absorb the heat.
The yield would decrease. This is because the equilibrium shifts in the direction of the side with the lowest number of gas molecules. The left side has 9 gas molecules and the right side has 10. The equilibrium shifts left to the side with fewer molecules.
Temperature: A compromise temperature is used. The reaction is exothermic, so a low temperature favors high yield (equilibrium shifts right). However, too low a temperature results in a slow rate of reaction.
Pressure: A high pressure should be used. There are fewer moles of gas on the right (1 mole) compared to the left (2 moles), so high pressure shifts equilibrium to the right, improving yield. High pressure also increases the rate of reaction by increasing collision frequency.
Note: Pressure is limited by safety and cost considerations in industry.
If you add NaOH, it reacts with the HCl present (neutralisation). This causes the concentration of HCl to decrease.
The system opposes this change by shifting the position of equilibrium to the left to replace the lost HCl.