chemical equilibrium and equilibrium constant

In a chemical reaction, chemical equilibrium is a dynamic condition when the rate of forward reaction and backward reaction are equal.

From kinetic theory, it can be said that chemical equilibrium is due to opposing forces. Considering a reversible chemical reaction, equilibrium state is reached when the rate of formation of product is equal to the rate of formation of reactant.

A + B                         C +D

At any given temperature and pressure, reactant A and B are converted to product C and D.

Considering a closed system, where the reaction between hydrogen gas and iodine reacts to give hydrogen iodide,

H2 + I2                          2HI

At constant temperature and pressure, chemical equilibrium is reached when the random collision between the hydrogen and iodine molecules leads to the formation of hydrogen iodide molecules. As the concentration of H2 and I2 is reduced the formation of HI increased. Notice that the rate of formation of HI is equal to the rate of breakdown of HI into H2 and I2.

Properties of system in equilibrium

A system in chemical equilibrium has the following characteristics:

  • The reaction type is reversible
  • The reaction is always incomplete
  • The forward and backward reaction rate are equal
  • The net free Gibb’s energy (of the system is zero
  • The disturbance of the system by any external agent can be restored by the system interaction.
  • The presence of a catalyst does not affect the equilibrium position
  • Reactants and products are always involved.

Factors affecting chemical equilibrium

In a reversible reacting system, the equilibrium either go to the right or the left if disturb by any external agent, the following agents affects chemical equilibrium: pressure, temperature and concentration.

Temperature: Equilibrium constant is a function of temperature only and temperature alters it’s value. According to van’t Hoff’s law of mobile equilibrium described temperature effect on equilibrium. The law states that an increase in temperature favors endothermic reaction process while decrease in temperature favors exothermic reaction process. (endothermic reaction is a reaction  that absorbs heat while exothermic reaction is a reaction that liberates heat out of the system as the reaction proceeds).since an increase in temperature favors endothermic reaction, this results to an increase in equilibrium constant and will shift the reaction system parameters to the direction which absorbs the input heat while decrease in temperature is the reverse case of exothermic. Consider the decomposition reaction of Nitrogen(iv)oxide to Nitrogen dioxide in a closed vessel.

N2O4                   2NO2

The reaction above is an endothermic reaction therefore increase in temperature will cause the forward reaction while a decrease in temperature will cause the reaction to the backward direction to produce N2O4

For an exothermic reaction of the form below;

N2 + 3H2                       2NH3

Industrially, this reaction is called a Haber process where vanadium(v)oxide is used as the catalyst. The process is an exothermic reaction where an increase in temperature will reduce the equilibrium constant and favor the backward reaction by decomposing Ammonia gas.

Concentration: the effect of concentration on equilibrium was explained by Le’ chaleter’s principle which states that when a system is in a state of equilibrium and it is subjected to an external constraint, the equilibrium position moves in order to annul the constraint effect. Example is given by the reaction below

2SO2 + O2                            2SO3

The reaction has an equilibrium constant as:  KC = [SO3]2/ [SO2]2[O2]

Considering the reaction and it’s equilibrium constant, an increase I the concentration of SO2 will reduce the value of equilibrium constant, therefore the concentration of SO2 and O2 must change for the value of Kc to remain constant. An increase in the concentration of SO2 and O2 will shift the equilibrium position to the right but the value of the equilibrium will be unchanged because the equilibrium position will move right to annul the effect of the increased concentration in order to the equilibrium. On the hand increasing the concentration of SO3 and decreasing the concentration of SO2 and O2 individually will move the equilibrium position left by decomposing SO3 to annul the effect.

Pressure:  the effect of pressure is more pronounced on gaseous reactions. A system at constant volume has no effect on equilibrium. According to Boyle’s law an increase in volume will have a corresponding decrease in pressure. Therefore in that effect an increase in pressure will result to reduction in pressure and the equilibrium position will move to the direction that favors decrease in volume. Consider the reaction below in a gaseous phase:

N2 +3H2                             2NH3

The number of moles of the reactant is greater than the number of moles of the product (decrease in volume). Also, increase in pressure will make the system equilibrium to shift to the direction of decreased volume (product).

Equilibrium constant

For a reversible reaction of the type shown below:

A +B                                   C +D

The rate of forward and backward reaction can be represented as:

Rate of forward reaction = Kf[A][B]

Rate of backward reaction = Kb[C][D]

Since at equilibrium;  Kf[A][B] = Kb[C][D]

Therefore,  K =   [C][D]/[A][

K is the equilibrium constant.





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