Chemical Equilibrium

Question Write Kc for the following reactions.

Question Reversible reactions attain the position of equilibrium which is dynamic in nature and not static. Explain it.

Answer Let us consider the reversible reaction between hydrogen gas and iodine vapors to form hydrogen iodide at 425 OC. At equilibrium three components will be present in proportion in reaction mixture. The equilibrium is established when the rising curve of product HI and falling curve of reactants [H2] and [I2] become parallel.

Same equilibrium mixture is obtained when HI is decomposed. This situation suggest two possibilities of the state of equilibrium. (i) All reactions cease at equilibrium so that the system becomes stationary. (ii) The forward and reverse reactions are taking place simultaneously at exactly the same rate. Now second possibility is widely accepted. This is known as state of dynamic equilibrium.

Question. Why do the rates of forward reactions slow down when a reversible reaction approaches the equilibrium stage.

Answer. Consider a general reaction in which A reacts with B to produce C and D.

Question When a graph is plotted between time on x-axis and concentrations of reactants and products on y-axis for a reversible reaction, the curves become parallel to time axis at a certain stage.

a) At what stage the curves become parallel?

Answer: At equilibrium stage for the reversible reaction, the rate of forward reaction becomes equal to rate of reverse reaction. At this stage, concentrations of reactants and products become constant not equal.

b) Before the curves become parallel, the steepness of curves falls? Give the reasons.

Answer. Consider a general reaction in which A reacts with B to produce C and D.

Let the initial concentrations of A and B be equal. Initially rate of forward reaction is very fast. But as time goes on, the rate of forward reaction becomes slow. The reason lies in the fact that concentration of reactants (A and B) is decreasing as time passes and according to law of mass action rate of reaction gradually decreases. On the other hand the initial concentrations of C and D are zero. As time passes, C and D are formed. Their concentration increases rapidly at first so rate of reverse reaction is also very fast. Later concentrations of C and D is no more increasing rapidly so steepness of curves fall.

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