Equilibrium
All reactions are reversible to some extent. This is what a double arrow in a chemical equation means. When a reaction takes place, it is trying to reach equilibrium. An equilibrium is reached when:
- The concentrations of products and reactions stop changing
- The ratio of the products to reactants remains constant over time
Equilibrium Constant
Next, we can calculate the equilibrium constant (Keq). The equilibrium constant allows us to determine where the equilibrium of a reaction lies in relation to other reaction. It is the ratio of products to reactants. The larger the Keq the MORE products (higher concentration) there are in relation to the reactions, and vice verse.
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Le Chatelier's Principle
The French chemistry Henri Le Chatelier (1850-1936) studied how the equilibrium position shifts as a result of changing conditions, and he developed Le Chatelier's Principle.
- Le Chatelier's Priniciple: If stress is applied to a system in equilibrium, the system changes in a way that relieves the stress.
Concentration
- Adding more reactant produces more product, and removing the product as it forms will produce more product
- Adding more products produces more reactants, and removing the reactants as it forms will produce more reactants
Temperature
- Increasing the temperature causes the equilibrium position to shift in the direction that absorbs heat
- If heat is one of the products (just like a chemical), it is part of the equilibrium
- So cooling an exothermic reaction will produce more product, and heating it would shift the reaction to the reactant side of the equilibrium:
Pressure (Volume)
- Pressure – changes in pressure will only effect gaseous equilibria
- Increasing the pressure (decreasing the volume) will usually favor the direction that has fewer molecules (moles of gas)
- Decreasing the pressure (increasing the volume) will usually favor the direction that has more molecules (moles of gas)
- Example: N2(g) + 3H2(g) ↔ 2NH3(g)
- For every two molecules of ammonia made, four molecules of reactant are used up – this equilibrium shifts to the right with an increase in pressure (decrease in volume)
- For every two molecules of ammonia made, four molecules of reactant are used up – this equilibrium shifts to the left with an decrease in pressure (increase in volume)
Explanations
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