In the previous section, we saw the factors which influence the rate of a chemical reaction. We also saw some basic details about reversible and irreversible reactions. In this section we will learn about the Equilibrium in Reversible Reactions.
Let us do an experiment.
1. Take the solutions of potassium nitrate (KNO3), potassium thiocyanate (KCNS) and ferric nitrate (Fe(NO3)3).
2. We are going to do some chemical reactions with these solutions. So we will want to note down the initial colors. For that, observe the color of each solution and note them down:
• KCNS solution - initial color: colourless
• Fe(NO3)3 solution - initial color: Light yellow
• KNO3 solution - initial color: colourless
3. Take a little of dilute ferric nitrate solution in a test tube and add a few drops of potassium thiocyanate to it.
• The solution becomes red in color.
• The chemical equation of the reaction is:
Fe(NO3)3 (aq) + 3KCNS (aq) ⟶ Fe(CNS)3 (aq) + 3KNO3 (aq)
• The formation of ferric thiocyanate (Fe(CNS)3) by the combination of Fe(NO3)3 and KCNS is the cause for the red color.
4. Keep the solution without disturbing. Examine after some time.
• We can see that the red color neither increases or decreases.
5. Dilute the solution and keep it without disturbing. Examine after some time.
• This time also, we can see that, the red color neither increases or decreases.
6. Transfer the dilute solution to three test tubes in equal amounts. We are going to do some tests to the tree test tubes separately
7. Test tube 1: Add Fe(NO3)3 to test tube 1.
• We can see that the red color becomes deep.
What may be the reason?
Ans: We have seen earlier that, formation of Fe(CNS)3 is responsible for the red color. Now, we have added more Fe(NO3)3 to the solution.
• This newly added Fe(NO3)3 combines with KCNS to produce more Fe(CNS)3. So naturally, the red color deepens.
8. Test tube 2: Add some KCNS to the second test tube.
• We can see that the red color becomes deep.
What may be the reason?
Ans: We have seen earlier that, formation of Fe(CNS)3 is responsible for the red color. Now, we have added more KCNS to the solution.
• This newly added KCNS combines with Fe(NO3)3 to produce more Fe(CNS)3. So naturally, the red color deepens.
9. Test tube 3: Add a drop of concentrated KNO3 to the third test tube.
• We can see that the intensity of the red color decreases significantly.
What may be the reason?
Ans: The newly added KNO3 reacts with the Fe(CNS)3 to give back the original reactants.
• So some of the Fe(CNS)3 is used up to give back the original reactants.
• As a consequence, only a less Fe(CNS)3 is available now.
• This causes the decrease in the intensity of red color.
Consider the third test tube in the above experiment:
In the next section, we will see the factors which affect equilibrium of chemical reactions.
Let us do an experiment.
1. Take the solutions of potassium nitrate (KNO3), potassium thiocyanate (KCNS) and ferric nitrate (Fe(NO3)3).
2. We are going to do some chemical reactions with these solutions. So we will want to note down the initial colors. For that, observe the color of each solution and note them down:
• KCNS solution - initial color: colourless
• Fe(NO3)3 solution - initial color: Light yellow
• KNO3 solution - initial color: colourless
3. Take a little of dilute ferric nitrate solution in a test tube and add a few drops of potassium thiocyanate to it.
• The solution becomes red in color.
• The chemical equation of the reaction is:
Fe(NO3)3 (aq) + 3KCNS (aq) ⟶ Fe(CNS)3 (aq) + 3KNO3 (aq)
• The formation of ferric thiocyanate (Fe(CNS)3) by the combination of Fe(NO3)3 and KCNS is the cause for the red color.
4. Keep the solution without disturbing. Examine after some time.
• We can see that the red color neither increases or decreases.
5. Dilute the solution and keep it without disturbing. Examine after some time.
• This time also, we can see that, the red color neither increases or decreases.
6. Transfer the dilute solution to three test tubes in equal amounts. We are going to do some tests to the tree test tubes separately
7. Test tube 1: Add Fe(NO3)3 to test tube 1.
• We can see that the red color becomes deep.
What may be the reason?
Ans: We have seen earlier that, formation of Fe(CNS)3 is responsible for the red color. Now, we have added more Fe(NO3)3 to the solution.
• This newly added Fe(NO3)3 combines with KCNS to produce more Fe(CNS)3. So naturally, the red color deepens.
8. Test tube 2: Add some KCNS to the second test tube.
• We can see that the red color becomes deep.
What may be the reason?
Ans: We have seen earlier that, formation of Fe(CNS)3 is responsible for the red color. Now, we have added more KCNS to the solution.
• This newly added KCNS combines with Fe(NO3)3 to produce more Fe(CNS)3. So naturally, the red color deepens.
9. Test tube 3: Add a drop of concentrated KNO3 to the third test tube.
• We can see that the intensity of the red color decreases significantly.
What may be the reason?
Ans: The newly added KNO3 reacts with the Fe(CNS)3 to give back the original reactants.
• So some of the Fe(CNS)3 is used up to give back the original reactants.
• As a consequence, only a less Fe(CNS)3 is available now.
• This causes the decrease in the intensity of red color.
Consider the third test tube in the above experiment:
• We added KNO3. It is one of the products. It reacted with the other product Fe(CNS)3 to give back the reactants.
■ So this is a reversible reaction.
• Once we conclude that it is a reversible reaction, we can give a detailed explanation as given below:
• Once we conclude that it is a reversible reaction, we can give a detailed explanation as given below:
1. We now know that the reaction that we saw is reversible. So we can represent it using '⇌' symbol. This is shown below:
Fe(NO3)3 (aq) + 3KCNS (aq) ⇌ Fe(CNS)3 (aq) + 3KNO3 (aq)
Fe(NO3)3 (aq) + 3KCNS (aq) ⇌ Fe(CNS)3 (aq) + 3KNO3 (aq)
2. Let us start a stop watch at the instant when KCNS is added to Fe(NO3)3.
• As the time passes by, more and more quantities of the products [Fe(CNS)3 and KNO3] will be produced
• In the first few seconds, the concentrations of the products will be very low. At the same time, the concentrations of the reactants will be high.
• As time passes by, the newly formed products will begin to react together to give back the original reactants. This is the backward reaction
♦ But as the concentrations of the products is low at the early stages, the rate of this back ward reaction will also be low in the early stages.
♦ As time passes by, the concentrations of the products increases, and the rate of backward reaction also increases.
• This 'increasing rate' is represented by the rising red curve in fig.11.7 below. We can see that, the red curve rises with the increase in time. If we mark a point 'C' on that curve, we can read off two values:
• This 'increasing rate' is represented by the rising red curve in fig.11.7 below. We can see that, the red curve rises with the increase in time. If we mark a point 'C' on that curve, we can read off two values:
♦ From the X axis, we can read off a 'particular time' corresponding to point 'C'
♦ From the Y axis, we can read off the 'rate of the backward reaction' at that 'particular time'
3. Now consider the forward reaction. As time passes by, the concentrations of the reactants decreases. This is because, more and more of the reactants are being converted into products.
• So the rate of the forward reaction decreases. This 'decreasing rate' is represented by the declining green curve in the fig.11.7.
• We can see that, the green curve declines with the increase in time. If we mark a point 'B' on that curve, we can read off two values:
♦ From the X axis, we can read off a 'particular time' corresponding to point 'B'• We can see that, the green curve declines with the increase in time. If we mark a point 'B' on that curve, we can read off two values:
♦ From the Y axis, we can read off the 'rate of the forward reaction' at that 'particular time'
4. So we have a rising curve and a declining curve. At some point in time, they will obviously meet. This meeting point is marked as 'A' in the fig.11.7
5. It is interesting to note that:
• after the 'time corresponding to A', the rate of forward reaction does not decrease
• after the 'time corresponding to A', the rate of backward reaction does not increase
6. What may be the reason? Let us analyse:
• Let R1 and R2 be the reactants. Also, let P1 and P2 be the products
♦ As time passes by, the concentrations of R1 and R2 decreases
♦ As time passes by, the concentrations of P1 and P2 increases
• At the meeting point A, let the concentrations be [R1]A, [R2]A, [P1]A and [P2]A.
• At the meting point A, rate of forward reaction = rate of backward reaction
♦ Rate of forward reaction is the 'quantity of R1 and R2 consumed in a particular interval of time'
♦ Rate of backward reaction is the 'quantity of P1 and P2 consumed in that particular interval of time'
• At the meeting point, the rates are equal. So:
Quantity of R1 and R2 consumed = Quantity of P1 and P2 consumed
• So, after the point A, there is no change in the quantities of reactants and products. That is:
After point A, [R1]A, [R2]A, [P1]A and [P2]A will not change
♦ If there is no change in the quantities of reactants, the rate of forward reaction will not change
♦ If there is no change in the quantities of products, the rate of backward reaction will not change
• That means:
♦ After point A, the rate of forward reaction will not decline
♦ After point A, the rate of backward reaction will not rise up
• So, after point A, the rate will be represented by the horizontal yellow line. We know that, all 'y values' on a horizontal graph will be the same.
At point A, the reaction is said to have attained Chemical Equilibrium
Chemical Equilibrium is a stage at which the rate of forward reaction becomes equal to the rate of backward reaction in a chemical reaction
• Even after the reaction has reached the equilibrium stage, both the forward and backward reactions will continue. But at the same rate.
♦ This continuing reactions are not visible to us because there is no color change after the equilibrium stage.
♦ There is no color change because, after equilibrium stage, the concentrations of reactants and products does not change.
• At equilibrium, both the reactants and products are present in the system
• As both the reactions are continuing even at equilibrium, it is called a dynamic equilibrium
Now we will consider the three test tubes
• Let R1 and R2 be the reactants. Also, let P1 and P2 be the products
♦ As time passes by, the concentrations of R1 and R2 decreases
♦ As time passes by, the concentrations of P1 and P2 increases
• At the meeting point A, let the concentrations be [R1]A, [R2]A, [P1]A and [P2]A.
• At the meting point A, rate of forward reaction = rate of backward reaction
♦ Rate of forward reaction is the 'quantity of R1 and R2 consumed in a particular interval of time'
♦ Rate of backward reaction is the 'quantity of P1 and P2 consumed in that particular interval of time'
• At the meeting point, the rates are equal. So:
Quantity of R1 and R2 consumed = Quantity of P1 and P2 consumed
• So, after the point A, there is no change in the quantities of reactants and products. That is:
After point A, [R1]A, [R2]A, [P1]A and [P2]A will not change
♦ If there is no change in the quantities of reactants, the rate of forward reaction will not change
♦ If there is no change in the quantities of products, the rate of backward reaction will not change
• That means:
♦ After point A, the rate of forward reaction will not decline
♦ After point A, the rate of backward reaction will not rise up
• So, after point A, the rate will be represented by the horizontal yellow line. We know that, all 'y values' on a horizontal graph will be the same.
At point A, the reaction is said to have attained Chemical Equilibrium
Chemical Equilibrium is a stage at which the rate of forward reaction becomes equal to the rate of backward reaction in a chemical reaction
♦ This continuing reactions are not visible to us because there is no color change after the equilibrium stage.
♦ There is no color change because, after equilibrium stage, the concentrations of reactants and products does not change.
• At equilibrium, both the reactants and products are present in the system
• As both the reactions are continuing even at equilibrium, it is called a dynamic equilibrium
Now we will consider the three test tubes
1. The resulting solution was transferred to three test tubes at the time when the reaction was at the equilibrium state. This is clear from the fact that, we had kept it with out disturbing for some time and no color change was observed. (see (5) at the beginning of this section)
• Further more, the transfer was made in equal amounts. (see (6) at the beginning of this section). So the concentrations in the three test tubes are the same.
2. To the test tube 1, more Fe(NO3)3 was added. This is shown in fig.11.8(b) below.
• This extra Fe(NO3)3 reacts with KCNS which is already present, to form more Fe(CNS)3. So the red colour increases.
• Thus there is an increase in rate of the forward reaction.
• But soon the rates of both forward and backward reaction will become equal, and the system will attain a new equilibrium state
3. To the test tube 2, more KCNS was added. This is shown in fig.11.8(c) above.
• Further more, the transfer was made in equal amounts. (see (6) at the beginning of this section). So the concentrations in the three test tubes are the same.
2. To the test tube 1, more Fe(NO3)3 was added. This is shown in fig.11.8(b) below.
• This extra Fe(NO3)3 reacts with KCNS which is already present, to form more Fe(CNS)3. So the red colour increases.
• Thus there is an increase in rate of the forward reaction.
• But soon the rates of both forward and backward reaction will become equal, and the system will attain a new equilibrium state
• This extra KCNS reacts with Fe(NO3)3 which is already present, to form more Fe(CNS)3. So the red colour increases
• Thus there is an increase in rate of the forward reaction.
• But soon the rates of both forward and backward reaction will become equal, and the system will attain a new equilibrium state
4. To the test tube 3, more KNO3 was added. This is shown in fig.11.8(d) above.
• This extra KCNS reacts with Fe(CNS)3 which is already present, to form more reactants. That is., some of the Fe(CNS)3 is used up. So the red colour decreases
• Thus there is an increase in rate of the backward reaction.
• But soon the rates of both forward and backward reaction will become equal, and the system will attain a new equilibrium state.
• At equilibrium, the rate of forward reaction is equal to the rate of backward reaction.
• Since reactants and products are present, both forward and backward reactions are occurring even at equilibrium.
• Since the reactions are continuing, it is called a dynamic equilibrium.
• Once equilibrium is attained, there will be no change in the concentration of the reactants or products.
• Chemical equilibrium is attained only in closed systems. That is., no substances should be added to or taken away from the system. Pressure and temperature of the system should not change.
• Thus there is an increase in rate of the forward reaction.
• But soon the rates of both forward and backward reaction will become equal, and the system will attain a new equilibrium state
4. To the test tube 3, more KNO3 was added. This is shown in fig.11.8(d) above.
• This extra KCNS reacts with Fe(CNS)3 which is already present, to form more reactants. That is., some of the Fe(CNS)3 is used up. So the red colour decreases
• Thus there is an increase in rate of the backward reaction.
• But soon the rates of both forward and backward reaction will become equal, and the system will attain a new equilibrium state.
Based on the above experiment, we can write the characteristics of Chemical equilibrium:
• At equilibrium, both reactants and products coexist. That means, at equilibrium, both reactants and products will be present in the system. • At equilibrium, the rate of forward reaction is equal to the rate of backward reaction.
• Since reactants and products are present, both forward and backward reactions are occurring even at equilibrium.
• Since the reactions are continuing, it is called a dynamic equilibrium.
• Once equilibrium is attained, there will be no change in the concentration of the reactants or products.
• Chemical equilibrium is attained only in closed systems. That is., no substances should be added to or taken away from the system. Pressure and temperature of the system should not change.
In the next section, we will see the factors which affect equilibrium of chemical reactions.
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