Key Ideas

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  • Closed system: a system in which only energy is exchanged with the surroundings
  • Open system: a system that allows matter and energy to be exchanged with the surroundings

Extent of a Reaction

The extent of a reaction is the degree to which the reactants are converted into products during a chemical reaction. Different reactions proceed to different extents as they reach equilibrium. e.g. HCl virtually fully ionises in water - it describes how much product is formed with the system reaches equilibrium. The rate of reaction, however, is a measure of the change in concentration of the reactants and products with time and is not directly related to the extent of a reaction


While the reactants are forming products, the products start reacting to form reactants again. Therefore, the rate of the forward reaction is decreasing as the concentration of reactants decreases, and the rate of the reverse reaction is increasing as the concentration of products increases. Hence, this continues until the rates of the forward and reverse reactions are equal. Chemical equilibria are dynamic, as the forward and reverse reactions are occurring continuously at the same rates, so that the concentrations of all the species present remain constant

\(Q_c\) and \(K_c\)

  • The concentration fraction or the reaction quotient is denoted by \(Q_c\).
  • \(Q_c\) is the ratio of concentrations of products to reactants in a reversible reaction
  • For a reaction, the equilibrium constant, \(K_c\) (\(M^{-2}\) or \(mol^{-2} L^{-2}\)) remains constant for all equilibrium mixtures at a fixed temperature. \([A]_{eq}\) refers to the concentration of \(A\) at equilibrium
  • The species in \(Q_c\) or \(K_c\) must be aqueous or gaseous
\(Q_c\) \(>\) \(K_c\) \(\uparrow\) Reactants \(Q_c\) \(<\) \(K_c\) \(\uparrow\) Products \(Q_c\) \(=\) \(K_c\) \(=\) Equilibrium

Position of Equilibrium

The position of equilibrium refers to the relative amounts of reactants and products at equilibrium, which varies depending on the extent of the reaction
Changes to the position

  • Adding or removing a reactant or product
  • Changing the pressure
  • Changing the concentration
  • Changing the temperature
\(10^{-2} < K_c < 10^4\)
Mostly Reactants
Net Back Reaction
Reactants and Products Mostly Products
Net Forward Reaction

Le Chatelier's Principle

If an equilibrium system is subjected to a change, the system will adjust itself to partially oppose the effect of the change
The addition of a catalyst or an inert gas has no change because they affect both the forward and reverse reactions

Changing Concentration

If the concentration of one species is increased, the system changes as to partially decrease the concentration of the species. The system favours the reaction that consumes the species. This is a direct result from the fact that where would be more fruitful collisions between this species and its other reactant. The opposite occurs when decreased

\(+\) Reactants Forward Favoured \(+\) Product Reverse Favoured
\(-\) Product Net Forward \(-\) Reactants Net Reverse

Changing Pressure

In the example, \(2SO_2 (g) + O_2 (g) \rightleftharpoons 2SO_3 (g)\), the forward reaction involves an increase in the number of gas particles from 2 to 3, causing an increase in pressure, while the reverse reaction involves a decrease in the number of gas particles causing a reduction in pressure. Therefore, using this idea, we can apply the Le Châtelier Principle.

\(A + B \rightleftharpoons C + 2D\) \(A + 2B \rightleftharpoons C + D\)
\(+\) Pressure Reverse Favoured \(+\) Pressure Forward Favoured
\(-\) Pressure Net Forward \(-\) Pressure Net Reverse

Changing Temperature – Endothermic Forward Reaction

Only temperature can increase yield and rate. If the temperature is increased, the system changes as to partially decrease the temperature. Therefore, the system favours the endothermic reaction – the forward reaction. \(K_c\) increases

Changing Temperature – Exothermic Forward Reaction

If the temperature is increased, the system changes as to partially decrease the temperature. Therefore, the system favours the endothermic reaction – the reverse reaction. \(K_c\) decreases

Competing Equilibria

When equilibria have a common reactant, each equilibrium can be regarded as competing for the reactant. However, the equilibrium with the larger equilibrium constant has a greater effect on the position of the equilibrium of the other reaction. In this competing system, the production is carboxyhaemoglobin is favoured in the presence of carbon monoxide.

  • \(\text{Haemoglobin}+\text{Oxygen}\rightleftharpoons \text{Oxyhaemoglobin}\)
  • \(\text{Carbon Monoxide}+\text{Oxygen}\rightleftharpoons \text{Carboxyhaemoglobin}\)