Fsc Part 1 Chapter 8 Chemical Equilibrium lectures 1,2,3

Reversible Chemical Reaction:

Any chemical reaction in which the reactants are converted into the products and the products are converted back into the reactants, is known as reversible chemical reaction.

For example when hydrogen gas reacts with iodine hydrochloric acid (HI ) is formed but HI immediately converts back into H2 and I2, so it is a reversible reaction , i.e
H2(g) + I2(g) 2HI(g)

In case of representing a reversible chemical reaction by means of a chemical equation, the reactants and products are separated by means two opposite half arrows (i.e. ) From the above discussion, it is clear that during a reversible chemical reaction, actually two reaction occurring. One is to the right side, converting the  reactants into the products and is known as “forward reaction”

e.g

H2 + I2 ------------------ 2HI

The other is to the left side, converting the products back into the reactants and is known as “reverse or backward reaction”

e.g

H2 + I2 ---------------------2HI

Rate of Reaction :

The change in concentration of reactants or products per unit time is called as rate of reaction.

During a chemical reaction, with the passage of time , the concentration of reactants decreases and the of the products increases.

So in terms of reactants, the rate may be defined as the decrease in concentration of reactants per unit time.

In case of reversible reactions there are two reactions i.e forward reaction and reversible reaction,

so we can discuss the rate of such reactions as :

Rate of forward reaction & Rate of reverse reaction.

Rate of Forward Reaction

The change in concentration of reactants during a reversible reaction, per unit time is known as rate of forward reaction.

Rate of Reverse Reaction.

The change in concentration of products, during a reversible reaction , reaction ,per unit time is known as rate of reverse reaction.

 

 

Chemical Equilibrium:

That state of a reversible chemical reaction, at which the rate of forward reaction becomes equal to the rate of reverse reaction, is known as chemical equilibrium.

At the state of chemical equilibrium there is no more change in the concentration of reactants and products but becomes constant although the reaction does not stop but retains dynamically.

The chemical equilibrium can be explained by considering the following

Example :

H2(G) + I2(g) 2HI

At the beginning, the concentration of reactants (H2+I2) is maximum, hence the rate o9f forward reaction will be high and only for ward reaction will occur. After sometime, enough amount of products (HI) is formed and reverse reaction starts, the rate of forward reaction will be high than rate of reverse reaction. After sometime, the concentration of reactants decreases enough and thus, rate of reverse reaction increases enough.

After sometime, the rate of forward reaction is just equal to the rate of reverse reaction and the concentration of reactants and products becomes constant and no more change occur in them, although the reaction does not stop. This is, what is known as the equilibrium state.

It is important to note that it is not necessary, that at equilibrium state concentration of reactants will be equal to the concentration of the products. May be, the concentration of reactants is more than that of the products at equilibrium and undergoes no more change or nay be the concentration of the products is more than that of the reactants and undergoes no more change or the concentration of reactants may be equal t the concentration of products at equilibrium and undergo no more change.

Types of Equilibrium:

There are two types of equilibrium.

1. Homogeneous equilibrium.
2. Heterogeneous equilibrium.

Homogeneous equilibrium.

The reversible reactions in which all the reacting substances are in same physical state i.e all gases liquids or solids. e.g consider the following equilibrium system of a reversible reaction

N2O4 ---------------> 2NO2(g)

Here all the reacting substances are in gaseous state.

Heterogeneous equilibrium.

The reversible reactions in which the reacting substances (reactants & products) are in different physical states are known as heterogeneous equilibrium. e.g

CaCo3 -------------->CaO(S) + Co2(g)

In the above equilibrium system it is clear that the reacting substances are in different physical states.

 

 

Law of  Mass Action & Equilibrium Constant Expression:

In 1864, two Norwegian chemist, Goldberg and Peter, presented a law for the rate of a chemical reaction which is known as law of Mass Action.

This law states that the rate of a chemical reaction is directly proportional to the product of the concentration (or active masses) of reading substances.

The term active mass represents the concentration (in mole* dm-3) of reactants and products. Let we have a reversible reaction, i.e

A+B----------> kr C+D

kr

The equilibrium concentration of A, B, C and D are represented in square brackets i.e (A), (B), (C) and (D) are expressed in moles xdm-3.

According to law of Mass Action, the rate of forward reaction, is proportional to the product of molar  concentration of A and B.

Rate of forward reaction (rf) * (A) (B)

Or rf  = kf (A) (B) --------I

Here Kf is the constant of proportionality and is known as “rate constant for forward reaction”

Similarly the rate of reverse reaction is,

Rate of reverse reaction (*r)----- (C) (D)

Or Rr = Kr (C) (D)-------II

Here Kr is the constant of proportionality and is known as “rate constant for reverse reaction”

As the reaction , Under consideration is a reversible reaction, therefore at equilibrium Rf = Rr.

Kc is known as equilibrium constant in terms of concentration of reactants and products.

Equilibrium Content Expression. For Heterogeneous Equilibrium:

A heterogeneous equilibrium is an equilibrium in which the reactants and products are in more than one physical state. While writing the equilibrium constant expression for a heterogeneous equilibrium, the concentration for pure solids and liquids are neglected i.e. they are not indicated, It is because of the fact, that , the concentration of a pure solid or pure liquid is constant at a constant temperature and does not  depend upon the quantity of the substance.

For example the molar concentration of copper (at 20 c) is the same, whether we have 1g or 1 ton of copper.

3Fe (s) + 4H2O(g)---------------> Fe3O4(s) + 4H2(g)

The equilibrium constant expression for the above system is ,

Kc = Concentration of Fe & Fe3 O4 are neglected because they are in their pure solid state and their concentrations do not undergo appreciable change.

Significance or Applications of Kc:

Kc ( equilibrium constant ) has the following applications,

1. Prediction of Direction of Reaction :
2. Prediction of extent of reaction.

Calculation of equilibrium concentration    

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.