Chapter 7.2 - Activity to Demonstrate Equilibrium

In the previous section, we saw solid-vapour equilibrium and equilibrium in solutions. In this section, we will see the general characteristics of equilibrium

First let us write a summary of what we have learnt so far in the previous two sections. It can be written in 4 steps:

1. Solid-liquid equilibrium
(i) For a solid-liquid reversible process, we write: Solid ⇌ Liquid
    ♦ This process is called melting
(ii) For a given solid and it’s liquid, there exists a particular temperature at which equilibrium is possible
    ♦ This temperature is called melting point
(iii) This temperature depends on the pressure experienced by the system
• So if the experiment is conducted at different pressures, we will obtain different equilibrium temperatures
• In order to avoid such a confusion, we specify that, the experiment must be conducted at 1 atm pressure
(iv) Consider the process: Ice ⇌ Water
• For equilibrium of this process,
    ♦ temperature must be 0 ^оC
    ♦ pressure must be 1 atm
        ✰ If temperature is lower than 0 ^оC, the rate of forward process will increase
        ✰ If temperature is higher than 0 ^оC, the rate of backward process will increase

2. Solid-liquid equilibrium
(i) For a liquid-vapour reversible process, we write: Liquid ⇌ Vapour
    ♦ This process is called vaporization
(ii) For a given liquid and it’s vapour, there exists a particular temperature at which equilibrium is possible
    ♦ This temperature is called boiling point
(iii) This temperature depends on the pressure experienced by the system
• So if the experiment is conducted at different pressures, we will obtain different equilibrium temperatures
• In order to avoid such a confusion, we specify that, the experiment must be conducted at 1 atm pressure
(iv) Consider the process: Water ⇌ Vapour
• For equilibrium of this process,
    ♦ temperature must be 100 ^оC
    ♦ pressure must be 1 atm
        ✰ If temperature is lower than 100 ^оC, the rate of backward process will increase
        ✰ If temperature is higher than 0 ^оC, the rate of forward process will increase

3. Solid-solution equilibrium
(i) For a solid-solution reversible process, we write: Solid ⇌ Solution
    ♦ This process is called dissolution of solid in liquid
(ii) For a given solid and it’s solution, there exists a particular temperature at which equilibrium is possible
(iv) Consider the process: Sugar ⇌ Solution
• At a particular temperature, equilibrium exists between solid sugar and the sugar solution
    ♦ If temperature is lower, then the rate of backward process will increase
        ✰ Solid sugar will precipitate out of the solution
    ♦ If temperature is higher then the rate of forward process will increase
        ✰ Solid sugar will dissolve into the solution

4. Gas-solution equilibrium
(i) For a Gas-solution reversible process, we write: Gas ⇌ Solution
    ♦ This process is called dissolution of gas in liquid
(ii) For a given gas and it’s solution, there exists a particular pressure at which equilibrium is possible
(iv) Consider the process: CO₂(g) ⇌ CO₂(sol)
• At a particular pressure, equilibrium exists between CO₂(g) and the CO₂(sol)
    ♦ If pressure is lower, then the rate of backward process will increase
        ✰ CO₂(g) will escape out of CO₂(sol)
    ♦ If pressure is higher then the rate of forward process will increase
        ✰ CO₂(g) will dissolve into the CO₂(sol)
(v) This equilibrium pressure depends on temperature also
    ♦ If the temperature is lower, lesser pressure is sufficient to cause dissolution
    ♦ If the temperature is higher, greater pressure is required to cause dissolution
• This is because, at lower temperature, the gas molecules will not have sufficient energy to escape out of the solution

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Based on the above summary, we can write 4 points:
1. For the equilibrium of Solid ⇌ Liquid, temperature is the criteria
   ♦ Also the pressure needs to be constant
• The temperature at any instant during the process will tell us:
   ♦ The direction in which, the process is going on
   ♦ The extent up to which the process has reached in that direction
• For example:
   ♦ If the temperature is lower than the specified value
         ✰ we can be sure that, the process will be taking place in the reverse direction
   ♦ If the temperature is much lower than the specified value
         ✰ we can be sure that, greater amount of solid will be present
2. For the equilibrium of Liquid ⇌ Vapour, temperature is the criteria
   ♦ Also the pressure needs to be constant
• The temperature at any instant during the process will tell us:
   ♦ The direction in which, the process is going on
   ♦ The extent up to which the process has reached in that direction
• For example:
   ♦ If the temperature is lower than the specified value
         ✰ we can be sure that, the process will be taking place in the reverse direction
   ♦ If the temperature is much lower than the specified value
         ✰ we can be sure that, greater amount of liquid will be present
3. For the equilibrium of Solid ⇌ Solution, temperature is the criteria
• The temperature at any instant during the process will tell us:
   ♦ The direction in which, the process is going on
   ♦ The extent up to which the process has reached in that direction
• For example:
   ♦ If the temperature is lower than the specified value
         ✰ we can be sure that, the process will be taking place in the reverse direction
   ♦ If the temperature is much lower than the specified value
         ✰ we can be sure that, greater amount of solid will be present
4. For the equilibrium of Gas ⇌ Solution, pressure is the criteria
   ♦ Also the temperature needs to be constant
• The pressure at any instant during the process will tell us:
   ♦ The direction in which, the process is going on
   ♦ The extent up to which the process has reached in that direction
• For example:
   ♦ If the pressure is lower than the specified value
         ✰ we can be sure that, the process will be taking place in the reverse direction
   ♦ If the pressure is much lower than the specified value
         ✰ we can be sure that, greater amount of free gas will be present

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• We know that, equilibrium can occur in both physical processes and chemical reactions
• In both cases, equilibrium is dynamic. That is., although we observe no activity at equilibrium, forward and backward reactions are going on
    ♦ We can use radioactive isotopes to prove this
    ♦ We saw it in the case of sugar solution
• But a procedure using radioactive isotopes is not suitable for school laboratories
• So instead of using isotopes, we can do a simple activity. It can be explained in 11 steps:

1. Take two measuring cylinders, each of 100 mL capacity
    ♦ Mark them as Cylinder-1 and cylinder-2
2. Take two glass tubes, each of length 30 cm
    ♦ Both must have the same diameter of 3 or 4 mm
    ♦ Mark them as tube-1’ and tube-2’
• This is shown in fig.7.2 below:

Fig.7.2

3. Fill about half of cylinder-1 with coloured water
    ♦ Potassium permanganate can be used to color the water
• Keep the measuring cylinder-2 empty
4. Put tube-1’ in cylinder-1 and tube-2’ in cylinder-2
5. Close the upper tip of tube-1’ with finger
    ♦ Transfer this tube-1’ to cylinder-2 and open the tip
    ♦ The water in the tube-1’ will fall into cylinder-2
6. Close the upper tip of tube-2’ with finger
    ♦ Transfer this tube-2’ to cylinder-1 and open the tip
    ♦ The water in the tube-2’ will fall into cylinder-1
7. The quantity transferred in (5) will be greater than quantity transferred in (6)
• This is because:
    ♦ While in cylinder-1, the tube-1’ will take in more water
    ♦ While in cylinder-2, the tube-2’ will take in only less water
8. Keep repeating the steps (5) and (6)
• We will soon see that, the level in the cylinders become equal
• This is shown in fig.7.2(b)
9. Transfer using tube-1’ is the forward process
• In the early stages, the quantity in each of this transfer is high
• But the ‘quantity continuously decreases’ due to the lowering of level in cylinder-1
• This ‘continuous decrease in quantity’ is analogous to the ‘decrease in rate of forward process’
9. Transfer using tube-2’ is the backward process
• In the early stages, the quantity in each of this transfer is low
• But the ‘quantity continuously increases’ due to the rising of level in cylinder-2
• This ‘continuous increase in quantity’ is analogous to the ‘increase in rate of backward process’
10. So the two rates are not the same
    ♦ Rate of the forward process continuously decreases
    ♦ Rate of the backward process continuously increases
• Due to this, the two rates soon become equal
◼ When this happens, we get the equilibrium
◼ The levels becoming equal in step (8) is analogous to equilibrium
11.After reaching equilibrium, we can try repeating the steps (5) and (6)
• There will be no change in the levels
◼ In a real process/reaction also, even after equilibrium, the forward and backward processes/reactions are continuing. But we observe no change

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• In the next section, we will see equilibrium in chemical reactions


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