Equilibrium (5.1.2 How far?)

Equilibrium (5.1.2 How far?)

1. Key definitions

   • Homogeneous equilibrium- equilibrium in which all the species making up the reactants and products are in the same physical state.
   • Heterogeneous equilibrium- equilibrium in which species making up the reactants and products are in different physical states.
   • Mole fraction- a measure of how much of a given substance is present in a reaction mixture.
   • Partial pressure- is the pressure an individual gaseous substance would exert if it occupied a whole reaction vessel on its own.

2. Kp

   • For reactions involving gases where the equilibrium expression is written using partial pressure.
   • For heterogeneous reactions- solids and pure liquids are not included in the expression.

3. Kc

   • calculating units: e.g
   • When a heterogeneous equilibrium is present, molar concentrations for solids and pure liquids do not change because their volume remains constant. this means the expression for Kc changes and the concentration of solid substances and pure liquids are not included in the expression.

4. Changing the position of equilibrium

   • changing concentrations
     • Equilibrium constants are not changed if you change the concentration of things present in the equilibrium. The only thing that changes this constant is a change of temperature.
     • The position of equilibrium is changed if you change the concentration of something present in the mixture. Equilibrium moves in a way to undo the changes you have made.
   • Changing pressure
     • Equilibrium constants are not changed if you change the pressure of the system, the only thing that can have an effect on this is the temperature.
     • The position of equilibrium however may change if you change the pressure . it moves in a way to undo the changes you have made.
   • Changing temperature
     • Equilibrium constants are changed if you change the temperature of the system. Kc and Kp are constant at constant temperatures, but they vary as the temperature changes. this is typical of what happens with any equilibrium where the forward reaction is exothermic ( increasing the temperature decreases the value of the equilibrium constant.
     • when the forward reaction is endothermic, increasing the temperature increases the value of the equilibrium constant. The position of equilibrium also changes if you change the temperature. The position of equilibrium moves in such a way as to undo the changes you have made.
     • If you increase the temperature, the position of equilibrium will move in a way as to reduce the temperature again. It will do that by favouring the reaction which absorbs heat.

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