Equilibrium

Equilibrium

1. Key Terms

   • Mole Fraction: A measure of how much of a given substance is present in a reaction mixture
   • Partial Pressure: The pressure that would be exerted by one of the gases in a mixture if it occupied the same volume on its own

2. Quantities present

   • Techniques and methods for finding conc
     • Colorimeter
       • Can be used to monitor the concentration of a reactant or product throughout as coloured substances alter the amount of light that gets through and is detected
     • Titration
       • Titrating either a product or reactant against a substance of known concentration will show how much of the product/reactant is present
         • Cons: The sample being tested must be removed from the mixture which will alter the position of the equilibrium and there may also be other products present that will affect the results of the titration.
     • Often the concentration of only one product is found and the others are calculated using this data and the original concentration.
   • How to work out
     • The equation first needs to be balanced as this will show the quantities made then using an ICE table any unknown quantities can be found using the data you know. The number o moles then needs to be divided by the volume of the container, in dm3, if known and can then be put into the Kc equations

3. Kc

   • • This formula is used for homogeneous equilibrium to find Kc
       • Kc is the same as with homogeneous but does not include any terms for solids, for example: C (S) + H2O (G) ⇌ CO (G) + H2 (G) - Would be: Kc = [CO][H2] / [H2O]
   • The units for the constant can be found by putting the units of each component into the original Kc formula and then simplifying it. An example is seen below:
     • Kp is done the same way but instead the units atm are used

4. Equilibrium constant

   • Not affected by concentration or pressure, any changes to the system will be countered
     • Changing concentration or pressure will change the position of equilibrium but not the constant - the equilibrium will shift to the side with less moles but the constant will remain the same unless temperature changes
     • Catalysts affect the rate of reaction but do not affect the position of equilibrium
   • Changes with temperature, the constant must be worked out for each new temperature used
     • Endothermic
       • If the forward reaction is endothermic then K increases as temperature does which leads to an increase in the yield of products and decrease in reactants
     • Exothermic
       • If the forward reaction is exothermic then K decreases as temperature decrease which leads to a decrease in products and an increase in reactant
   • If K = 1 then the equilibrium is halfway between products and reactants, if K>1 the products are favoured and if K<1 then the reactants are favoured

5. 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.

   • A heterogeneous equilibrium has things present in more than one phase. The usual examples include reactions involving solids and gases, or solids and liquids.

6. Kp

   • • This is the formula used to find Kp in a homogeneous equilibrium
       • Kp is the same as with homogeneous but does not include any terms for solids, much like Kc. The example for Kc is also used for Kp: C (S) + H2O (G) ⇌ CO (G) + H2 (G) where Kp would be Kp = PH2 x PCO / PH2O

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