# chemical equilbrium

420 Points
13 years ago

Dear Nitesh

when a chemical reaction is carried out in a closed vessel, the system achieves equilibrium. Equilibrium occurs when there is a constant ratio between the concentration of the reactants and the products. Different reactions have different equilibria. Some may appear to be completely products, however, all reactions have some reactants present. A reaction may look "finished" when equilibrium is reached, but actually the forward and reverse reactions continue to happen at the same rate. A reverse reaction is when the written reaction goes from right to left instead of the forward reaction which proceeds from left to right. This is why equilibrium is also referred to as "steady state".

It is possible to write an equilibrium expression for a reaction. This can be expressed by concentrations of the products divided by the concentration of the reactants with the coefficients of each equation acting as exponents. It is important to remember that only species in either the gas or aqueous phases are included in this expression because the concentrations for liquids and solids cannot change. For the reaction:

jA + kB --> lC + mD

the equilibrium expression is:

Where:

K is the equilibrium constant
[A], [B], etc. are the molar concentrations of A, B, etc.
l, m, etc. are the coefficients of the balanced reaction

For every reaction at a specific temperature, there is only one value for K. A large value of K implies that there are more products than reactants and that the equilibrium lies to the right. A small K value implies there are more reactants than products and the reaction lies to the left. It is critical to remember that the only thing that changes K is changing temperature.

For reactions in the gas phase, equilibrium positions can also be expressed in terms of pressure. Kp, the equilibrium constant in terms of pressure, is related to K by the equation:

Kp=K(RT)Δn

Where:

Δn is the sum of the coefficients of the gaseous products minus the sum of the coefficients of the gaseous reactants.
R is the gas law constant (see the gas laws page)
T is the temperature in kelvins

The reaction quotient, Q, is an expression which deals with initial values instead of the equilibrium value that K deals with. We compare Q and K to determine which direction the reaction will proceed to obtain equilibrium. If Q is greater than K, the system will shift to the left. If Q is less than K, the system will shift to the right. If Q is equal to K than the system is already at equilibrium so it will not shift in either direction.

All the best.

AKASH GOYAL

Please feel free to post as many doubts on our discussion forum as you can. We are all IITians and here to help you in your IIT JEE preparation.

Win exciting gifts by answering the questions on Discussion Forum. So help discuss any query on askiitians forum and become an Elite Expert League askiitian.