Unit 3.1.6 - Chemical equilibria, Le Chatelier’s principle and Kc
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This document is a summary of equilibria containing information about Le Chatelier’s principle, changes in certain factors and how to calculate the equilibrium constant. It also includes key information for methanol production, ethanol production and the haber process.
Unit 3.1.6 - Chemical equilibria, Le Chatelier’s principle and Kc
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PHYSICAL CHEMISTRY I
equilibria
Equilibrium is where a reversible reaction has the forward and backwards reaction going at the same rate. All the
products and reactants react in a closed system. The concentrations also remain constant.
The conditions for equilibrium:
- Closed system
- Final position of equilibrium will be the same
if temperature, pressure and concentration are
unchanged
- Reached when macroscopic properties of the
system do not change
Le Chatelier’s Principle
‘ when a system is subject to change, the system will alter to lessen the effect’
Changes in Concentration
- When the concentration of reactants is increased, equilibrium shifts to the right, so more products are formed. When
the concentration of products is increased, equilibrium shifts to the left to produce more reactants
- When the concentration of reactants is decreased, the equilibrium shifts to the left so more reactants are formed.
When the concentration of products is decreased, the equilibrium shifts to the right so more products are formed.
Changes in Pressure
- When pressure is increased, equilibrium shifts to the side with fewer gas molecules, increasing the yield of that side
- When pressure is decreased, equilibrium shifts to the side with more gas molecules, increasing the yield of that side
Changes in Temperature
- An increase in temperature will favour the endothermic reaction, as it will be able to absorb the heat to bring the temperature back to its
original value, which therefore increases the yield of the endothermic products.
- A decrease in the temperature will favour the exothermic reaction, because it will release more heat to bring the temperature up to it
original value, which furthermore increases the yield of the exothermic products
Catalysts
Catalysts have no effect on the position of equilibrium, it speeds up the rate of the forwards reaction, to allow equilibrium to
be reached faster
Equilibrium Constant
aA + bB ⇌ cC + dD
𝑐 𝑑
[𝐶] [𝐷]
𝐾𝐶 = 𝑎 𝑏
[𝐴] [𝐵]
Measured in concentration, concentration = moles/volume
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