Chemical Equilibrium



Chemical equilibrium: “A reversible reaction occur with the rate of the forward reaction equal to the backward reaction.”  Such an equilibrium is called chemical equilibrium, the two opposing processes involve a change of chemical species. Moreover the composition of the reactant and products does not change with time (if temperature, concen­tration etc. remain unaltered because the amount of products. formed at .a given time is equal to that of the reactants).

Reversible reactions: Many chemical reactions can proceed in opposite-directions i.e., the reaction products can react to reform the original reactants. For example, ammonium chloride decomposes on heating to give ammonia and hydrogen chloride. We know that ammonia and hydro­gen chloride react to give white fumes of ammonium chloride. This indicates that as soon as ammonium chloride decomposes to NH3 and HCl some of NH3 and HCl formed may react with each other to reform the original reactant : Ammonium chloride

NH4Cl (s) ⇋ NH3 (g) + HCl (g)

Reactions in which some of the products formed react to give back the reactants are called reversible-reaction.

Irreversible reaction: The reactions which proceed only in one direction ate called irreversible reactions. For example, when potassium chlorate (KClO3) is heated in the presence of MnO2 (manganese dioxide) oxygen gas is formed.

2KClO3 → 2KCl + 3O2 (g)

The reverse is not true i e., KCl and O2 gas would not combine to reproduce the reactants KClO3

State of equilibrium: Equilibrium or a state of, equili­brium is defined as a condition in which the properties of a system does not change with time.

Physical equilibrium: if we add a small quantity of sugar to a large quantity of water in certain temperature, it  soon dissolves. As we continue to add mote sugar to this solution we find that the speed with which it dissolves decreases and finally sugar starts sitting down and further addition only increases the  dissolved sugar. This is due to the fact that an equilibrium is established between the dissolved and the undissolved sugar molecules undissolved sugar (solid) ⇋ dissolved sugar (aq). Since there is only a change in the physical states, therefore it is called physical equilibrium.

Important features of chemical equilibrium:

(i) At constant temperature, the physical properties such as concen­tration, pressure, color etc. remain constant.

(ii) Chemical equilibrium can be achieved from both sides For example, hydrogen and I, react

H2(g)+I2(g) → 2HI(g)

on heating, hydrogen iodide is formed at HI decomposes to give hydrogen and iodine

2HI → H2+ I2

(iii) The presence of a catalyst does not alter the state of equilibrium.

(iv) Once attained, the equilibrium continues indefinitely provided the system is not disturbed.

Equilibrium constant (k): The ratio of the products o£ the molar concentrations of products to that of reactants raised to the powers equal to their stoichiometric coefficients in the chemical equation at a particular temperature.

The value of k is independent of the molar concentrations of the reacting substances and the products but changes only with change in temperature, k has no units.

Chemical equilibrium is referred to as dynamic equilibrium: At equilibrium molecular activity does not cease. The reactants are constantly forming the products and products are constantly changing to reactants. But the rates of two reactions; forward and the backward are equal to each other. This type of equilibrium is therefore known as dynamic


Rate of chemical reaction: The chemical reac­tions occur at different speeds. Some reactions are very fast and occur instantaneously ; some are slow and may take minutes to complete ; whereas others are very slow and take place in days, weeks or even months.

The rate of reaction of a chemical reaction is defined as the change in the concentration of reactants (or products) per unit time.

Factors affecting the Rate of reaction: The rate of reaction depends on the following five factors.

(i) Effect of temperature. (a) Increase in temperature increases the rate of reaction (b) Decrease in temperature decreases the rate of reaction.

(ii) Physical nature of reactants. (a) Use of a common solvent for the reactants increases the rate of reaction (b) On increasing the area of contact between reactants, the rate of a react ion increases.

(iii) Effect of concentration– Increasing the concentration of reactants increases the rate of a reaction.

(iv) Effect of a catalyst, A catalyst increases the rate of a reaction.

(v) Chemical nature of reactants. Some substances are more reactive so the rate of reaction is very fast and some substances arc less reactive so chemical reaction is low, for example the reaction of iron with water.

Velocity constant: In the reaction between A and B to produce C and D

A+B → C+D

The speed of reaction is proportional to the number of moles per liter of A and also proportional to the number of

moles per liter of B.

physical,Speed a [A][B]

Speed = K[A][B]

K is known as the velocity constant of reaction and its value is constant at a given temperature for a given reaction.

Law of mass reaction. It states that at constant temperature, the rate of a chemical reaction is directly proportional to the product of molar concentrations of the reactants at any given time at equilibrium position the rates of forward and backward reactions are equal.



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