Chemistry 12

Equillibrium
Reaction Kinetics
Equilibrium
Solubility equilibrium
Acid Base Salt
Electrochemical
Word Problems
Poems
Site credit

 

Equilibrium:

A reversible reaction is said to be equilibrium when the rate of the forward reaction is equal to the rate of reverse reaction.

Example: N2O4 (g) ßà2NO2O

Note:

Inorder for the equilibrium to exist the system must be closed.

 

Closed system:

A closed system is a system in which nothing can enter or leave.

Example: Equilibrium

Notes:

  • Temperature affects the equilibrium.
  • New equilibrium is attained by new temperature.
  • The same equilibrium will exist regardless of whether the reaction started with an excess of reactant or an excess of product. (Important: therefore the Keq expression doesnt change if the equilibrium started with an excess reactant or product)
  • At equilibrium no macroscopic change occurs.

Dynamic Equilibruim:

Equilibrium situation when microscopic changes occur but macroscopic properties doesnt change.

Example:

N2O4 (g) ßà2NO2

Colorless                         Red brown  

Equilibrium exist when the color is light bron the color doesnt change overtime so no macroscopic change occuts but at microscopic level N2O4 is converting to NO2 back and forth over and over again so microscopic change occur in dynamic equilibrium.

 

Static Equilibrium:

Equilibrium doesnt move at all unless pushed is some manner.

Example: Block of wood balanced on one end in static equilibrium the equilibrium is destroyed if the blocik is pushed over.

 

Properties of Equilibrium:

  • Rate of consumption of reactants = rate of production of reactants.
  • [reactant] differs from [product] in general.
  • [reactant] is now constant in time and [product] is constant in time.
  • The forward and reverse reaction doesn't change overtime.
  • A system which is not at equilibrium will tend to move towards the position of equilibrium.

Important: Rate at which reactants are reacted equals rate at which reactants are produced and same goes for the products as well

Spontaneous:
a change which occurs by itself without outside assistance.
example: touch a match to paper and the paper burns.
Exothermic: energy barrieres are lower therfore reactions are usually spontaneous.Some exothermic reactions occur spontaneously but some don't.
Endothermic reaction: requires input of energy and such reactions don't occur spontaneously.
Note: the side of reaction having minimum energy will be favoured because molecule will tend to move towards low energy.
Entropy:
the amount of randomness in system.
example: crystal are higly ordered.
high random states are favoured over highly ordered states because there are more random states possible.
Note:
the tendency for a reaction to go to the side with max randomness
the tendency for a reaction to go to the side with the min energy.
 
Tendency to min Enthalpy:
the tendecy to minimum enthalpy favours going to the side of the reaction containing heat term.
tendency to max entropy:
gassess>>solutions>liquids>>solids
the side of equation favoured by the tendency to max randomness is the side containing. the most particles of the most random phase.
Examples:

C2H2 (g) + 2Cl2(g) ßà C2H2Cl4(l) + 38 kj

  • max entropy favours the formation of reactants.
  • Minimum Entahlpy favor the formation of products.
  • two tendencies oppose each other and the reaction forms an equilibrium.

 

CH4 (g) + 2O2(g) à CO2(g) + H2O(g)+394  kj

  • max entropy favours the formation of products.
  • min enthalpy favours the formation of products.
  • reaction will go to completion 100%
Le chatelier's principle:

if a closed system is subjected to a change processes will occur that tend to counteract that change.

incdec.gif

Examples:

Effect of temperature change:

When the amount of heat is decreased the reaction shifts so as to produce more heat.

2NO (g) +Cl2(g) ßà 2NOCl + 76Kj

The heat term is on the product side therefore it shifts to the product side.

Effect of pressure changes:

Decrease in volume increase pressure and simultaneously increase the [] of all gaseous species present.

2NO (g) +Cl2(g) ßà 2NOCl + 76Kj

If pressure increases the reaction will sift to the side having the least number of gaseous molecules. In this case the reaction will favor the product side

addition.gif

Effect of [ ]:

When the [reactant] increases, the reaction shifts to partly decrease the amount of added reactant.

2NO (g) +Cl2(g) ßà 2NOCl + 76Kj

If [Cl2] increase than the equilibrium shifts to the product side.

Effect of catalyst:

Speed up forward and reverse reaction rate by an equal amount so that a reaction at equilibrium remain constant.

 

Equilibrium Constant:
Numerical value of Keq.
Example:
Keq = [H2][CO2]/[H2O ][CO]

Note:

Keq = [products] / [reactants]

  • eliminate any concentration which have a constant value in the equilibrium expression only defined Keq in terms of concentrations which can be changed
  • solids have constqant values so the cn't be used in Keq expression.
  • Pure liquid: if there is only one liquid exist in equilibrium than it is not used in the the Keq expression but if there is more than the liquid is used in the equilirium than liquid is in Keq expression.
  • only temperature can change Keq value.
  • Large Keq value means more products pesent in equilibrium.
  • Small value of Keq means more reactants are present in equilibrium.