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Wednesday, 23 December 2015

FSc Notes Chemistry Part 1 Chapter 8 Chemical Equilibrium Lecture 4

FSc Notes Chemistry Part 1 Chapter 8 Chemical Equilibrium Lecture 4



Prediction of Direction of Reaction:

Let we have a reversible reaction for which value of Kc is 4 the reaction is
A+B ------------------ C+D
Let at a particular time interval, their molar concentrations are (a), (b), (c) and (d) respectively. Here we are not sure whether these concentrations are the equilibrium concentration. If the provided  concentrations of reactants and products are b/f equilibrium, then the reaction may be in forward or in reverse direction.
The direction of reaction can decided easily by taking the ration of product of concentration of products and product of concentration of reactants
Now
If Qc = Kc
Then the reaction is at equilibrium

If Qc > Kc
Then the reaction is in the reverse direction.

If Q> Kc
Then the reaction is in the forward direction.

Prediction of Extent of Reaction.

If we have a reversible reaction i.e
A+B -----------------------C+D
Let their equilibrium concentrations be (a), (b), (c) and (d). Then equilibrium constant Kc will be
Kc = (c) (d)

Now there are several possibilities :
  1. If the equilibrium concentration of C& D are very large as compared to those of A & B, them the value of Kc will be large enough i.e Kc >> 1. It shows that the reactants, react very well and thus produce large quantity of produces. But on the other hand, the products do not react well. Thus at equilibrium most of the concentration of reactants is converted into the products.
  2. If the concentration of C & D is very small as compared to that of reactants A & B, at equilibrium, then the value of Kc will very very small, i.e Kc<< 1 If shows that the reactants do not react well to form the products in enough quantity but the products react well to reform the reactants.
  3. If at equilibrium, the concentration of the reactants (A,B) and products (C,D) are almost equal, then value of kc will be nearly or exactly 1. If shows that both the reactants and products react well.

Calculation of Equilibrium Concentration:

Le-chatelier's  Principle:

To the study disturbance of a system at equilibrium, by changing its conditions like temperature,
pressure , concentration or presence of catalyst, a French chemist Henry Le Chatelier (1888)
presented a principle which is known as Le-chatelier's . It states that : If a system at equilibrium is
subjected t disturbance by changing it conditions like concentration the system shifts in such a direction to re-establish a new equilibrium.

Written by: Asad Hussain

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