Maharashtra Board Class 11 Chemistry Chapter 12 Chemical Equilibrium PDF Download

Chemical Equilibrium

12.1 Introduction

You have learnt earlier that changes can be physical or chemical and reversible or irreversible. All the changes listed above are irreversible physical changes. You have also learnt earlier that chemical changes can be represented by chemical reactions when the exact chemical composition of reactants and products is known. In this chapter we are going to look at reversible chemical reactions. On allowing the reaction for a very long time so that the concentrations of the reactants or products do not vary, the reaction is said to have attained equilibrium.

12.1.1 Reversible Reaction

In the above activity, the change in colour of the solution is caused by the chemical reaction which reverses its direction with change of temperature.

\[\text{Co(H}_2\text{O)}_6^{2+}\text{(aq) + 4Cl}^-\text{(aq)} \xrightarrow{\text{Heat}} \text{CoCl}_4^{2-}\text{(aq) + 6H}_2\text{O(l)}\]

(Pink) (Blue)

What are the types of the following changes? Natural waterfall, spreading of smoke from burning incense stick, diffusion of fragrance of flowers. Can the above changes take place in the opposite direction?

What does violet colour of the solution in above indicate?

Dissolve 4 g cobalt chloride in 40 ml water. It forms a reddish pink solution. Add 60 ml concentrated HCl to this. It will turn violet. Take 5 ml of this solution in a test tube and place it in a beaker containing ice water mixture. The colour of solution will become pink. Place the same test tube in a beaker containing water at 90°C. The colour of the solution turns blue.

The reaction in the above activity is an example of a reversible reaction. These are many chemical reactions which appear to proceed in a single direction. For example:

\[\text{C(s) + O}_2\text{(g)} \rightarrow \text{CO}_2\text{(g)}\]

\[\text{2KClO}_3\text{(s)} \rightarrow \text{2KCl(s) + 3O}_2\text{(g)}\]

These are called irreversible reactions. They proceed only in single direction until one of the reactants is exhausted. Their direction is indicated by an arrow (\(\rightarrow\)) pointing towards the products in the chemical equation. On the contrary, reversible reactions proceed in both directions. The direction from reactants to products is the forward reaction, whereas the opposite reaction from products to reactants is called the reverse or backward reaction. A reversible reaction is denoted by drawing in between the reactants and product a double arrow, one pointing in the forward direction and other in the reverse direction (\(\rightleftharpoons\)). For example:

\[\text{H}_2\text{(g) + I}_2\text{(g)} \rightleftharpoons \text{2 HI(g)}\]

\[\text{CH}_3\text{COOH(aq) + H}_2\text{O(l)} \rightleftharpoons \text{CH}_3\text{COO}^-\text{(aq) + H}_3\text{O}^+\]

Consider an example of decomposition of calcium carbonate. Calcium carbonate when heated strongly, decomposes to form calcium oxide and carbon dioxide. If this reaction is carried out in a closed container or open container, what do we observe?

Reactions are chemically represented as follows

General representation:

(a) \[\text{CaCO}_3\text{(s)} \xrightarrow{\text{heat}} \text{CaO(s) + CO}_2\text{(g)}\]

This represents the irreversible reaction in open container

In closed container: reversible reaction

(b) \[\text{CaCO}_3\text{(s)} \rightleftharpoons \text{CaO(s) + CO}_2\text{(g)}\]

What is a closed system? In a closed system, there is no exchange of matter with the surroundings. In an open system, exchange of both matter and heat occurs with the surroundings.

If we perform the experiment at high temperature in a closed system, we find that after certain time, we have some calcium carbonate present. If we continue the experiment over a longer period of time at the same temperature, we find the concentrations of calcium carbonate, calcium oxide and carbon dioxide are unchanged. The reaction thus appears to have stopped and we say the system has attained the equilibrium. Actually, the reaction does not stop but proceeds in both the directions with equal rates. In other words calcium carbonate decomposes to give calcium oxide and carbon dioxide at a particular rate. Exactly at the same rate the calcium oxide and carbon dioxide recombine and form calcium carbonate.

Such reactions which do not go to completion and occur in both the directions simultaneously are reversible reactions. A reversible reaction may be represented in general terms as:

\[\text{A + B} \rightleftharpoons \text{C + D}\]

reactants products

The double arrow indicates that the reaction is reversible.

Consider the reaction of decomposition of calcium carbonate occurring in an open system or container. Now what will happen? We have seen that during decomposition of calcium carbonate, carbon dioxide can escape away. So, can we obtain back calcium carbonate?

No!

Such a reaction is irreversible reaction which occurs only in one direction, namely, from reactants to products.

Teacher's Note

When you heat calcium carbonate (like chalk or limestone), it breaks into calcium oxide and carbon dioxide. This is like burning paper - you cannot get the paper back. But if you do this in a closed container, the gases cannot escape, so they can react back.

Exam Trick

Remember: Reversible = two-way arrow (\(\rightleftharpoons\)). Irreversible = one-way arrow (\(\rightarrow\)). Just like a one-way street and a two-way street!

Points to Remember

Reversible reactions go forward and backward at the same time.

Irreversible reactions go only one way until one reactant is used up.

Equilibrium is reached when forward and backward reactions happen at equal rates.

In an open system, products can escape, so the reaction is irreversible.

In a closed system, products cannot escape, so the reaction can be reversible.

12.2 Equilibrium In Physical Processes

a. Liquid - Vapour Equilibrium

Let us now look at a reversible physical process of evaporation of liquid water into water vapour in a closed vessel.

Initially there is practically no vapour in the vessel. When a liquid evaporates in a closed container, the liquid molecules escape from the liquid surface into vapour phase building up vapour pressure. They also condense back into liquid state because the container is closed. In the beginning the rate of evaporation is high and the rate of condensation is low. But with time, as more and more vapour is formed, the rate of evaporation goes down and the rate of condensation increases. Eventually the two rates become equal. This gives rise to a constant vapour pressure. This state is known as an 'equilibrium state'. In this state, the number of molecules leaving the liquid surface equals the number those return to liquid from the vapour state. Across the interface, there is a lot of activity between the liquid and the vapour. This state, when the rate of evaporation is equal to the rate of condensation is called equilibrium state. It may be represented as:

\[\text{H}_2\text{O(l)} \rightleftharpoons \text{H}_2\text{O(vapour)}\]

At equilibrium, the pressure exerted by the gaseous water molecules at a given temperature remains constant, known as the equilibrium vapour pressure of water (or saturated vapour pressure of water or aqueous tension). The saturated vapour pressure increases with increase of temperature. In the case of water, the saturated vapour pressure is 1.013 bar (1 atm) at 100°C. Therefore, water boils at 100°C when exposed to 1 atm pressure. For any pure liquid at 1 atm pressure the temperature at which its saturated vapour pressure equals to atmospheric pressure is called the normal boiling point of that liquid. The boiling point of water is 100°C at 1.013 bar pressure, whereas the boiling point of another liquid ethyl alcohol is 78°C.

Teacher's Note

When you heat water in a closed bottle, the water evaporates but cannot escape. Some vapour condenses back to water. After some time, the evaporation and condensation happen at the same speed, so the amount of water stays the same.

Exam Trick

Remember: Boiling point = when vapour pressure equals atmospheric pressure. For water it is 100°C at 1 atmosphere.

Points to Remember

Evaporation means liquid becomes vapour.

Condensation means vapour becomes liquid.

At equilibrium, evaporation rate equals condensation rate.

Saturated vapour pressure depends only on temperature, not on the amount of liquid.

b. Solid - Liquid Equilibrium

Consider a mixture of ice and water in a perfectly insulated thermos flask at 273 K. It is an example of solid-liquid equilibrium. Ice and water are at constant temperature. They remain in what is called solid-liquid equilibrium.

c. Solid - Vapour Equilibrium

Place some iodine crystals in a closed vessel. Observe the change in colour intensity in it. After some time the vessel gets filled up with violet coloured vapour.

The intensity of violet colour becomes stable after certain time. What do you see in the flask?

We see both, that is, solid iodine and iodine vapour in the closed vessel. It means solid iodine sublimes to give iodine vapour and the iodine vapour condenses to form solid iodine. The stable intensity of the colour indicates a state of equilibrium between solid and vapour iodine. We can write the same as follows:

\[\text{I}_2\text{(s)} \rightleftharpoons \text{I}_2\text{(g)}\]

Other examples showing this kind of equilibrium are:

1. \[\text{Camphor(s)} \rightleftharpoons \text{Camphor(g)}\]

2. \[\text{Ammonium chloride(s)} \rightleftharpoons \text{Ammonium chloride(g)}\]

Dissolve a given amount of sugar in minimum amount of water at room temperature.

Increase the temperature and dissolve more amount of sugar in the same amount of water to make a thick sugar syrup solution.

Cool the syrup to the room temperature. Note the observation: Sugar crystals separate out.

In a saturated solution there exists dynamic equilibrium between the solute molecules in the solid state and in dissolved state.

\[\text{Sugar(aq)} \rightleftharpoons \text{Sugar(s)}\]

The rate of dissolution of sugar = The rate of crystallization of sugar

Teacher's Note

When you cool hot sugar syrup, sugar crystals appear. This shows that dissolving and crystallizing happen at equal rates in a saturated solution, just like ice melting and freezing in ice water at 0°C.

Exam Trick

Remember: Saturated solution = no more sugar can dissolve at that temperature. This is when dissolution rate equals crystallization rate.

Points to Remember

Sublimation is when solid changes directly to gas without becoming liquid.

In solid-vapour equilibrium, sublimation rate equals condensation rate.

Colour staying constant means equilibrium is reached.

A saturated solution has equilibrium between dissolved and solid solute.

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