Equilibrium(5.1.2 How Far?)

Equilibrium(5.1.2 How Far?)

1. Equilibrium constant(Kc)

   • The value of Kc determines whether the reaction favours the products or the reactants
     • If Kc equals 1 then the reaction is at dynamic equilibrium
   • If Kc is greater than 1 then the reaction favours the products
   • If Kc is less than 1 then the reaction favours the reactants
   • For the reaction aA + bB <--> cC + dD; Kc is: ([C]^c[D]^d)/([A]^a[B]^b)
     • Therefore Kc= concentrations of products/concentrations of reactants

2. Equilibrium

   • This occurs in reversible reactions
   • When the rate of the forward reaction is equal to the rate of the reverse reaction
   • Sometimes known as a dynamic equilibrium
     • To the right are some examples of dynamic equilibrium reactions
   • This does not occur immediately as the forward reaction needs to occur to produce a product
     • This then causes the reverse reaction to occur

3. Determining concentrations at equilibrium

   • Titration
     • By titrating one of the reactants/products against a substance with known concentration will allow you to find out how much of a product or reactant is present
   • Using a colorimeter
     • This works based on the fact that coloured substances alter the amount of light passing through them
       • More concentrated substances will produce more intense colours and hence absorb more light

4. Homogeneous and heterogeneous reactions

   • Homogeneous reactions are where the reactants and products are in the same phase/states
     • Method for calculating Kc and Kp DOES NOT change
   • Heterogeneous reactions are where the reactants and products are in different phases/states
     • Method for calculating Kc and Kp DOES change
       • You take into consideration the gaseous reactants and prodcuts ONLY
       • This is due to the volumes of the solids and pure liquids not changing

5. Mole fractions

   • The amount of given component within a reaction mixture; given the symbol X
     • Calculated by: number of moles of substance/total number moles of all substances

6. Partial pressures

   • The amount of pressure being exerted by an individual species within a reaction vessel; given the symbol P with subscript of the species
     • Calculated by: mole fraction x total pressure
   • The concentration of a substance is proportional to its partial pressure

7. Equilibrium constant(Kp)

   • Reactions involving gases can use partial pressures instead of concentrations
   • For the reaction aA + bB <--> cC + dD Kp = ((PC)^c(PD)^d)/((PA)^a(PB)^b)

8. Factors that affect K

   • Concentration
     • The value of K is unchanged
     • The reaction will try to self right itself upon experiencing a change
     • This can change the equilibrium position
   • Pressure
     • The value of K is unchanged
     • Same effect occurs as it does for concentration
   • Temperature
     • Increasing temperature favours the endothermic direction
     • Decreasing temperature favours the exothermic direction
     • The value of K changes with temperature
   • A catalyst only speeds up the rate of reaction(reaching equilibrium)
     • It DOES NOT affect the equilibrium position

9. Le Chatelier's principle

   • A reaction will change in order to minmise change to the equilibrium constant
     • Thus causing the value of K to remain constant
     • This subsequently prodcues more or less of the products or reactants

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