Equilibrium

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The rate of a chemical reaction is measured in terms of the amount of reactant consumed or product produced in a given time. When the rate of products formed is the same as the rate of consumption a substance is said to be in equilibrium. When alcohol is sealed in your medicine cabinet it is evaporating. Eventually, the air becomes saturated with vapor molecules and some molecules lose there kinetic energy and condense returning to liquid phase. Those molecules that condense are replaced by other molecules, with higher kinetic energy to keep the air saturated. The forward reaction is going from the original liquid alcohol to the gaseous form. The reverse reaction is the condensation of the liquid alcohol. When both the forward and reverse reactions are stabilized at the same rate they are said to be in dynamic equilibrium.
An activated complex is a transition state between the formation of two substances. In the reaction between nitrogen (I) oxide, 
                     NO + CO2
                    /
NO2 + CO --> either
                    \
                     NO2 + CO
there is an intermediate phase between reactants and products where O = N - O ··· C (triple bond) O is formed. At this point either CO2 or NO2 can be formed.
Usually, reactions don't happen spontaneously unless they have enough energy to get up the activation energy, where activated complexes can be formed.

Catalysts

Catalysts do nothing else to a reaction except speed up the rate at which it moves to a reaction. It changes the rate of the forward and backward reaction equally. This is done by changing the activated complex to a one with a lower activation energy, therefore making it easier to achieve the activation energy.

Le Chatelier's Principles

Going to completion, or going to an end is the removal of a product from a reaction, not permitting the reverse reaction to take place. The formation of a precipitate, gas or un-ionized compounds, such as water, are the requirements to go to completion. Once a reaction has gone to completion, the product can not form the reactants in a reverse reaction (there is no reverse reaction).
Le Chatelier's principle states if a chemical reaction in equilibrium is disturbed, the reaction undergoes a change to relieve the newly applied stress. After this change takes place the concentration of products and reactants is altered. The stress that can be applied are temperature change, pressure change, and concentration of products and or reactants.
Higher pressures acting on a reaction favors the formation of smaller volumes and low pressure favors the formation of high volumes. When volume is independent (i.e. products and reactants are in solid or liquid phase) pressure is irrelevant. This is because under high pressure the molecules are closer together and there is a higher chance that the reaction that favors a lesser volume will occur. Under low pressure the reaction favors a greater volume is true.
Temperature changes the rate of a reaction. The higher the temperature the more the endothermic side is favored. When the temperature is lowered the exothermic side is favored. This occurs because reactions change to undo whatever new stress is placed on them. If excess heat is present, the reaction seeks to cool off. If there is a heat deficit more heat is produced.
The concentration of reactants determines both the rate and whether or not a reaction will move to equilibrium. If one reactant far out weighs the other, the product will not.
In a redox reaction is in equilibrium once there is no potential difference, or voltage, between the reactants. When your battery is "dead" it is a state of equilibrium. Recharging the battery takes it out of equilibrium by adding electrons to one of the reactants creating a potential difference once again.

Equilibrium Calculation

  1. Make sure the equation is balanced.
  2. Place the concentration of the products of the reaction in the numerator and the concentration of the reactants in the denominator.
  3. Place square brackets around the product(s) and reactant(s) to indicate concentration.
  4. Place the coefficients in moles on each substance as you would to indicate a power on the left of the square brackets. For example, 2H2O would be [H2O]2 in the equation.
  5. Set that expression equal to KEQ. This is the equilibrium constant for that reaction at that temperature. It shows where the equilibrium will be at.
For example: 
H2 + Cl2 <==> 2HCl

        [HCl]2
KEQ = -----------
       [H2][Cl2]
K is constant, and independent of the concentration of the reactants, at any one temperature. When temperature changes the K value changes. When K is large (greater than 1) the formation of products is favored. When K is small (less than 1) the formation of reactants is favored.