Equilibrium

Equilibrium

1. Reactions

   • Closed System
   • Rate of forwards and backward reactions are equal
   • Concentrations of reactants and products are not changing
   • Dynamic Equilibrium
     • A+B⇌C+D
     • A reaction has attained dynamic equilibrium when
       • The concentrations of the reactants and the products remain the same
       • The rate of the forward reaction is the same as the reverse
   • Homogeneous system at constant temperature
     • Same state for all elements

2. Le Chatelier's Principle

   • When a factor is changed, an equilibrium will shift to oppose that change.
   • Factors
     • Temperature
       • Temperature Decrease
         • Favour exothermic direction
           • The forward reaction is exothermic, therefore equilibrium shifts to oppose the decrease in temperature
             • Resulting in a high yield in the forward reaction
           • Exothermic is ΔH = -ve
       • Temperature Increase
         • Favours endothermic direction
           • Endothermic is ΔH =+ve
     • Pressure
       • Increase Pressure
         • Favours side with less moles
       • Decrease Pressure
         • Favours side with more moles
       • All about the moles number
     • Concentration
       • Favours the other side
         • Increase concentration of Reactants
           • Produces more products
         • Decrease concentration of Reactants
           • Produces more reactants
         • Increase concentration of products
           • Produces more reactants
         • Decrease concentration of products
           • Produces more products
   • All about trying to get highest yield
   • Catalyst
     • Nothing happens to equilibrium as it speeds the forward and backwards reaction at the same rate.
     • Reach equilibrium faster but will not increase the yield
   • Compromise
     • E.G N2(g) + 3H2(g) ⇔ 2NH3(g) + Heat
       • Theoretical Conditions
         • High Pressure
           • Favours RHS so more yield
         • Decrease Tempterature
           • Favours Exothermic so more yield on RHS
       • Practical Conditions
         • Low Pressure
           • Safer and cheaper because of costly equipment
         • Increase temperature
           • Quicker rate of reaction

3. Equilibrium Constants

   • Kc
     • A + 2B ⇌ C + 3D
       • Kc = [C][D]^3/[A][B]^2
     • Kc = [C][D]/[A][B]
       • Products over reactants
       • Value only changes in temperature
         • A change in concentration will shift the equilibrium so it will all cancel out
         • Change in pressure doesn't matter
       • Units vary depending on the moles
         • Moles per dm3
       • Square brackets is concentration. Very important to include them!
     • Moles=(Volume/1000)(Molarity)
     • Analysis of Kc
       • Kc = 1
         • 1 means that equilibrium is halfway between reactants and products
       • Kc > 1
         • 1 equilibrium lies towards products
       • Kc < 1
         • 1 equilibrium lies towards reactants
   • Kp
     • aA + bB ⇌ cC + dD
     • Kp = (pC)^c x (pD)^d / (pA)^a x (pB)^b
       • Products over reactants
     • Mole Fraction = # of moles of the gas / total number moles of the gases in that equilibrium system
       • Partial Pressure of a Gas = Mole Fraction x Total Pressure
     • Conditions
       • Pressure (&concentration) change doesn't affect the constants
         • It does shift equilibrium, but the constants remain unaffected
       • Only affected by Temperature
         • Depending if the forward of backwards reaction is exothermic or endothermic
       • Catalyst also does not affect the equilibrium just the speed at which they reach equilibrium

4. Calculations

   • ICE
     • Initial moles
     • Change in moles
     • Equilibrium moles
       • Always use equilibrium moles for calculations
   • When answering start with principle then apply it to question

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