Equilibrium

Equilibrium

1. Mole fraction - a measure of how much of a given substance is present in a reaction mixture.

2. Partial pressure - the pressure an individual gaseous substance would exert if it occupied a whole reaction vessel on its own.

3. Reversible reactions

   • A reversible reaction is a chemical change in which the products can be converted back to the original reactants under suitable conditions. A reversible reaction is shown by the sign:
   • Not all reactions go to completion, an equilibrium may be established. Equilibrium can only happen if the forward reaction has produced a high enough concentration of the product.
     • According to the equilibrium law, for the reaction:

4. Units for Kc

   • You must work out the units of Kc afresh for each different reaction, this is by arranging them in the equation and cancelling them out such as:

5. Le Chatelier's principle

   • Le Chateliers' Principle states that if an 'instantaneous' change is imposed on an equilibrium, the position of the equilibrium will further change to minimise the 'enforced' change. In other words, if a change is 'instantaneously' imposed, the equilibrium attempts to restore itself to its original state.
     • Raising the temperature favours the endothermic direction (ΔH +ve). The system absorbs the heat energy from the surroundings to try to minimise the temperature increase.
     • Increasing the pressure favours the side of the equilibrium with the least number of gaseous molecules as indicated by the balanced symbol equation. The system attempts to reduce the number of gas molecules present to reduce the pressure increase.
     • A catalyst speeds up both the forward and reverse reactions by providing a mechanistic pathway with a lower activation energy, but there is no way it can influence the final 'balanced' concentration ratios.

6. Expressions

   • A homogeneous equilibrium has everything present in the same phase. The usual examples include reactions where everything is a gas, or everything is present in the same solution.
     • Kc
       • It applies where everything in the equilibrium mixture is present as a gas, or everything is present in the same solution.
   • A heterogeneous equilibrium has things present in more than one phase. The usual examples include reactions involving solids and gases, or solids and liquids.
     • Kp
       • Solids will not be included in the expression and neither will pure liquids.
     • Kc
       • The concentration of solid substances are not included in the expression and neither are the concentrations of pure liquids.

7. Calculating unknown equilibrium concentrations

   • From balanced equations you can find the reacting quantities needed to prepare a required quantity of a product, also, the quantities of products formed by reacting together known quantities of reactants.

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