Equilibrium

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Revision as of 17:15, 4 December 2007

Standard Level Equilibrium study guide

Basic principles

• In all reactions, there are in fact two processes occuring: A forward reaction where reactions are turned into products, and a reverse reaction where products become reactants.
  • When the reaction reaches equilibrium, the rate of the forward reaction is equal to the rate of the reverse reaction.
• Students should be familiar with Concentration/time graphs for reactants and products, as well as reaction rate/time graphs.
  • Concentration/time graphs: these show the concentration of reactants, products, or both, over time.
    • Concentration of reactants will decrease at a negative rate until dynamic equilibrium is reached.
    • Concentration of products will increase at a positive, decreasing rate until dynamic equilibrium is reached.
    • At dynamic equilibrium, concentration of reactants and products will not change.
  • Reaction rate/time graphs: these show the rate of the forward and/or reverse reaction over time.
    • forward rate will decrease until equilibrium is reached
      • This is because the forward rate decreases as the concentration of reactants decreases
    • reverse rate will increase from zero until equilibrium is reached
      • This is because the reverse rate increases as the concentration of products increases

Types of dynamic equilibrium

Dynamic equilibrium occurs in closed systems. It's called dynamic because the forward and backward processes are still occurring, but since they're happening at the same rate there is no overall change.

• physical reactions - equilibrium reached between substances of different states.
  • eg. evaporation and condensing of water
    • Because some water molecules have enough energy to melt, while others don't. (think of Maxwell-Boltzman distribution)
• chemical reactions - equilibrium reached between reactants and products
  • concentration of reactants and products reaches a constant (Kc)

Le Chatelier's Principle

Equilibrium will always 'try to compensate' for any changes to the system.

Example: A + B <-> C + D(g)

• increased concentration of A
  • leads to higher reaction rate towards the right
    • therefore more C and D created to counterbalance the A added.
      • equilibrium shifts to the right.

• decreased concentration of D
  • rate of backwards reaction decreases
    • equilibrium shifts to the right.

Factors that effect equilibrium

Concentration

See Le Chatelier's Principle example, above.

Temperature Change

If the forward reaction is exothermic (and thus the reverse reaction is endothermic),

• Increasing the temperature of the system
  • -> the endothermic reaction happens more easily
    • -> equilibrium shifts left.
• Decreasing the temperature of the system
  • -> the endothermic reaction doesn't happen as fast
    • -> equilibrium shifts right.

If the forward reaction is endothermic, the opposite happens.

If the reactions aren't exothermic or endothermic, changing temperature has no effect on equilibrium.

Pressure Change

If the reactants take up more space than the products (eg. in 2NO2(g) <-> N2O4(g) there are two moles of gas in the reactants, but only one in the products, so the reactants take up more space),

• Increasing the pressure of the system
  • -> the gas that takes up more space is more difficult to create
    • -> backwards reaction happens less
      • -> equilibrium shifts right
• Decreasing the pressure of the system
  • -> the gas that takes up more space is easier to create
    • -> equilibrium shifts left

If the reactants take up less space than the products, the opposite happens.

If the reactants and products have the same volume characteristics, changing pressure has no effect on equilibrium

Equilibrium Constant

Failed to parse (Can't write to or create math temp directory): Kc=(C^y*D^z)/(A^w*B^x)

where the reaction in question is

wA + xB <-> yC + zD

• If Kc >>(a lot more than) 1, the reaction has gone to completion.
• If Kc << 1, the reaction has hardly proceeded.
• If Failed to parse (Can't write to or create math temp directory): 10^-2 < Kc < 10^2

, the reaction has reached equilibrium.

HL has to know more about this. Somebody please add that info.