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Characteristics Of Chemical Equilibrium

Chemical Bonding Assignment / Homework Help
Here are the four characteristics of a state of chemical equilibrium
  • Stability of the observable properties

  • As said in the previous session, the state of chemical equilibrium is characterized by constancy of certain properties such as concentration, density, pressure or color. However, the state of equilibrium is observed only as long as the reaction conditions such as temperature and pressure are kept constant.

  • Fastened by a catalyst

  • A catalyst is a substance which fastens the chemical reactions. The state of chemical equilibrium can be attained at a faster rate in the presence of a catalyst. For example the reaction

    2SO2+O2 ↔ 2SO3

    takes place at about 500 degree and it takes very long time to attain equilibrium. However, the time required for attaining the equilibrium can be greatly reduced by adding platinum or vanadium pentoxide. The greatness of a catalyst is that the equilibrium can be attained at a faster rate without any changes in the composition of the reaction mixture i.e. the reactants SO2, O2 and the product SO3.

  • Chemical equilibrium can be attained from either direction

  • As you know, chemical equilibrium is seen in only reversible reactions in which the products can also act as reactants. Hence, it is possible to attain the state of equilibrium even in the reverse direction i.e. taking the actual products as the reactants. For example, it does not make any difference whether the reaction takes place either as

    H2 + I2 ↔ 2HI

    OR

    2HI ↔ H2 + I2

    The equilibrium state is the same in both the reactions. At the point of equilibrium, all the three gases H2, I2 , 2HI will be present in same amount whether the reaction is formation of hydrogen iodide or decomposition of hydrogen iodide.

  • Chemical equilibrium is dynamic in nature

  • As there is no change in the concentration of the reactants and the products in the state of chemical equilibrium, many of them consider chemical equilibrium is static in nature. But, chemical equilibrium is dynamic in nature. As you know both forward and backward reactions take place at the point of chemical equilibrium, the products are being formed continuously and the products are being converted back to reactants continuously. Thus, the effect of the one reaction undoes the effect of the other reaction making the state of equilibrium look like static in nature. But, the state of chemical equilibrium is highly dynamic in nature.

    For example consider the reaction

    H2 + I2 ↔ 2HI

    Let this reaction reach the state of equilibrium. Now, add radioactive iodine into the reaction mixture and let it be for some time. After some time, you can find radioactive iodine incorporated into HI. This is an indication for the conversion of reactants into products even after attaining the state of equilibrium. You can even observe that there will not be any change in the amount of HI. This means that the formation and decomposition of HI are taking place at the same rate in the state of chemical equilibrium.

  • Equilibrium is effected by change in temperature, pressure or concentration

  • As mentioned earlier the state of equilibrium is maintained only as long as the reaction conditions are maintained stable. Alteration of the conditions such as temperature, pressure or concentration will favor the state of chemical equilibrium to shift either towards the forward or backward reaction. This results in the increase of either the reactants or the products and they are not maintained at same concentration anymore.

Limitations of equation of chemical equilibrium

The equation for chemical equilibrium does not give a complete picture of how the reaction is taking place and there are certain limitations of writing the chemical equilibrium reaction. They include:

  • It is not possible to know the time taken to attain the chemical equilibrium by observing the equation of chemical equilibrium. For example in formation of water

                                H2 + ½ O2 ↔ H2O

    Water is a highly stable compound and dissociates at a very slow rate causing the equilibrium to remain towards the right side of the reaction. Thus we expect that hydrogen and oxygen combine at a faster rate to establish the equilibrium. But from experiments it is found that hydrogen and oxygen remain without reacting for a very long time.

  • The chemical equilibrium equation does not give an indication of the rate at which the state of equilibrium is attained and the conditions that favor the process of attaining chemical equilibrium. In the formation of water, the state of chemical equilibrium is obtained within seconds if the hydrogen and oxygen mixture is exploded with an electric spark.

  • The equation of chemical equilibrium does not give a clear indication of the fraction of the reactants that are being converted to products at the time of equilibrium. For example in the decomposition of N2O4


  •                             N2O4 ↔ 2NO2

    It is not possible to know the fraction of N2O4 that is dissociated into NO2. When the temperature is increased, the dissociation of N2O4 increases but the equation of the reaction remains the same.

    Despite of the above limitations, the equation of chemical equilibrium conveys certain amount of information such as
  • It states that two reactions are taking place simultaneously.

  • Equilibrium is established between two specific opposing reactions that are occurring simultaneously.

  • The equation also provides information about the number of moles of the products formed and the number of moles of reactants that gave rise to the products.
Solved problems
  • The nature of chemical equilibrium is
    • Static
    • Dynamic
    • Alters rapidly
    • Not known
    Ans: b
  • Which of the following increases the rate of attaining the chemical equilibrium
    • Increasing the concentration of reactants
    • Increasing the concentration of products
    • Use of a catalyst
    • Altering the reaction conditions
    Answer: c
  • Chemical equilibrium can be obtained from which of the following directions
    • Reactants towards products
    • Products towards reactants
    • Both the directions
    • None
    Answer: c
  • Change of which of the following conditions alters the state of equilibrium
    • Temperature
    • Pressure
    • Concentration of reactants and products
    • All
    Answer: d
  • Chemical equilibrium is characterized by
    • Stability of the observable properties like concentration and color
    • Stability of the reaction conditions
    • Stability of the rate of forward reaction
    • Stability of the rate of backward reaction
    Ans: a
  • Which of the following is true?
    • It is not possible to know the conditions favoring the state of chemical equilibrium by observing the chemical equilibrium equation.
    • Chemical equilibrium can be attained from both the directions
    • The state of chemical equilibrium is dynamic in nature
    • All
    Ans: d
  • Is it possible to know the number of moles of reactants and products that are involved in the reaction by observing the equation of chemical equilibrium?
    Ans: Yes
  • Is it possible to tell the time taken for attaining the state of chemical equilibrium by observing the equation of chemical equilibrium?
    Ans: No
  • The equation of chemical equilibrium tells about the time taken to attain the equilibrium and the conditions favoring the state of chemical equilibrium?
    Ans: False

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