General Characteristics of Physical Equilibrium

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From the physical equilibrium studied above, we have noticed that at equilibrium, some of the measurable properties of the system become constant. These can be summarized as follows:

Expression which acquire constant value at equilibrium

Other general characteristics of equilibrium involving physical processes are summed up as:

• The measurable properties of the system become constant at equilibrium. This is so because the concentration of the substances become constant. For example, in case of evaporation of water, vapour pressure of water becomes constant.

• Equilibrium can be established only in case of closed system. The system should neither gain matter from the surroundings nor lose matter to the surroundings. For example, evaporation of water occurs only in closed containers and not in open vessels.

• The equilibrium is always dynamic in nature. This means that the process does not stop but the changes take place in the forward and the backward directions with the same rate.

For example, we add some radioactive sugar into a saturated solution of non-radioactive sugar. When we analyze the sugar solution, we observe that the solution contains some radioactive sugar. Now, if no dissolution is occurring at equilibrium, we should not get any radioactive sugar in the solution. However, the presence of radioactive sugar in solution indicates that some radioactive sugar has passed from undissolved sugar to exchange some non-radioactive sugar in the solution. This experiment shows that even at equilibrium, the processes of dissolution and precipitation are occurring and therefore, the equilibrium is dynamic in nature.

Rate of dissolution = Rate of precipitation

Fig: 7.2 - Dynamic nature of solid in liquid equilibrium

• The measurable/visible properties of the system must remain unchanged with time. Once, an equilibrium is reached the properties of the system do not change with time.

• When equilibrium is attained, there exists an expression of concentrations of the substances involved in equilibrium, which becomes constant at a given temperature. For example, for equilibrium:

the pressure of water (related to concentration) becomes constant.

• The magnitude of the constant value of the concentration related expression (called equilibrium constant) gives an indication of the extent to which the reaction proceeds before acquiring equilibrium. For example, for the dissolution of CO₂ in water:

at a constant temperature. The greater the value of this expression, greater is the extent to which CO₂ dissolves in water.

Problem

1. The solubility of iodine in water is 1.1 x 10^-3 mol L^-1 at 288 K. 0.2 g of iodine is stirred in 100 cm³ of water till equilibrium is reached, what will be the mass of iodine found in solution and the mass that is left undissolved. After equilibrium is reached with 0.2g of iodine and 100 cm³ of water, we add 150 cm³ of water to the system. How much iodine will be dissolved and how much will be left undissolved and what will be the concentration of iodine in solution?

Solution

Solubility of iodine = 1.1 x 10^-3 mol L^-1

This means that 1.1 x 10^-3 mol of iodine is present in 1 L of water.

Molecular mass of iodine = 254

Amount of iodine dissolved per litre of water

=1.1 x 10^-3 x 254= 0.279 g

= 0.0279 g

Amount of iodine added = 0.2 g

Amount of undissolved iodine in 100 cm³of water = 0.2 - 0.0279

= 0.172 g

When 150 cm³ of water is added after the equilibrium, the total volume becomes 100 + 150 = 250 cm³

Amount of undissolved iodine = 0.2 - 0.07 = 0.130 g

Molar concentration = 0.07 g x1000 mL =0.0011 mol L^-1

254 g mol^-1x 250 mL