Study of shift in equilibrium between ferric ions and thiocynate ions by increasing/ decreasing the concentration of either of ions
Answers
Let's see how chemical equilibria behave when concentration changes.
Many chemical reactions involve a state of equilibrium. Equilibrium is said to be dynamic when the rate of forward reaction become equal to that of the backward reaction.
Consider a general reversible reaction;
«math xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mi»A«/mi»«mo»§nbsp;«/mo»«mo»+«/mo»«mo»§nbsp;«/mo»«mi»B«/mi»«mo»§nbsp;«/mo»«mo»§#8596;«/mo»«mo»§nbsp;«/mo»«mi»C«/mi»«mo»§nbsp;«/mo»«mo»+«/mo»«mo»§nbsp;«/mo»«mi»D«/mi»«/math»
With the passage of time, there is a depletion of the reactants A and B and an accumulation of the products C and D. As a result, the rate of forward reaction decreases and that of backward reaction increases. Eventually, the two reactions occur at the same rate and the system reaches a state of equilibrium.
Let us explain the 'Law of Chemical Equilibrium'
Norwegian chemists Cato Maxmillian Guldberg and Peter Waage proposed that for a chemical reaction;
The rate of reaction in either direction is proportional to the active mass (concentration) of the reactants.
By applying the Law of Mass Action;
The rate of forward reaction;
«math xmlns=¨http://www.w3.org/1998/Math/MathML¨»«msub»«mi»R«/mi»«mi»f«/mi»«/msub»«mo»=«/mo»«msub»«mi»K«/mi»«mi»f«/mi»«/msub»«msup»«mfenced close=¨]¨ open=¨[¨»«mi»A«/mi»«/mfenced»«mi»a«/mi»«/msup»«msup»«mfenced close=¨]¨ open=¨[¨»«mi»B«/mi»«/mfenced»«mi»b«/mi»«/msup»«/math»
The rate of back ward reaction;
«math xmlns=¨http://www.w3.org/1998/Math/MathML¨»«msub»«mi»R«/mi»«mi»b«/mi»«/msub»«mo»§nbsp;«/mo»«mo»=«/mo»«mo»§nbsp;«/mo»«msub»«mi»K«/mi»«mi»b«/mi»«/msub»«msup»«mfenced close=¨]¨ open=¨[¨»«mi»C«/mi»«/mfenced»«mi»c«/mi»«/msup»«msup»«mfenced close=¨]¨ open=¨[¨»«mi»D«/mi»«/mfenced»«mi»d«/mi»«/msup»«/math»
Where [A], [B], [C] and [D] are the equilibrium concentrations of A, B, C and D respectively.
a, b, c, and d represents the stoichiometric coefficients of A, B, C and D respectively.
Kf and Kb are the rate constants of forward and backward reaction.
However, at equilibrium,
Rate of forward reaction = Rate of backward reaction.
«math xmlns=¨http://www.w3.org/1998/Math/MathML¨»«msub»«mi»K«/mi»«mi»f«/mi»«/msub»«msup»«mfenced close=¨]¨ open=¨[¨»«mi»A«/mi»«/mfenced»«mi»a«/mi»«/msup»«msup»«mfenced close=¨]¨ open=¨[¨»«mi»B«/mi»«/mfenced»«mi»b«/mi»«/msup»«mo»§nbsp;«/mo»«mo»=«/mo»«mo»§nbsp;«/mo»«msub»«mi»K«/mi»«mi»b«/mi»«/msub»«msup»«mfenced close=¨]¨ open=¨[¨»«mi»C«/mi»«/mfenced»«mi»c«/mi»«/msup»«msup»«mfenced close=¨]¨ open=¨[¨»«mi»D«/mi»«/mfenced»«mi»d«/mi»«/msup»«/math»
or, «math xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mfrac»«msub»«mi»K«/mi»«mi»f«/mi»«/msub»«msub»«mi»K«/mi»«mi»b«/mi»«/msub»«/mfrac»«mo»§nbsp;«/mo»«mo»=«/mo»«mo»§nbsp;«/mo»«mfrac»«mrow»«msup»«mfenced close=¨]¨ open=¨[¨»«mi»C«/mi»«/mfenced»«mi»c«/mi»«/msup»«msup»«mfenced close=¨]¨ open=¨[¨»«mi»D«/mi»«/mfenced»«mi»d«/mi»«/msup»«/mrow»«mrow»«msup»«mfenced close=¨]¨ open=¨[¨»«mi»A«/mi»«/mfenced»«mi»a«/mi»«/msup»«msup»«mfenced close=¨]¨ open=¨[¨»«mi»B«/mi»«/mfenced»«mi»b«/mi»«/msup»«/mrow»«/mfrac»«/math»
or, «math xmlns=¨http://www.w3.org/1998/Math/MathML¨»«msub»«mi»K«/mi»«mi»c«/mi»«/msub»«mo»§nbsp;«/mo»«mo»=«/mo»«mo»§nbsp;«/mo»«mfrac»«mrow»«msup»«mfenced close=¨]¨ open=¨[¨»«mi»C«/mi»«/mfenced»«mi»c«/mi»«/msup»«msup»«mfenced close=¨]¨ open=¨[¨»«mi»D«/mi»«/mfenced»«mi»d«/mi»«/msup»«/mrow»«mrow»«msup»«mfenced close=¨]¨ open=¨[¨»«mi»A«/mi»«/mfenced»«mi»a«/mi»«/msup»«msup»«mfenced close=¨]¨ open=¨[¨»«mi»B«/mi»«/mfenced»«mi»b«/mi»«/msup»«/mrow»«/mfrac»«/math»
where, «math xmlns=¨http://www.w3.org/1998/Math/MathML¨»«msub»«mi»K«/mi»«mi»c«/mi»«/msub»«mo»§nbsp;«/mo»«mo»=«/mo»«mo»§nbsp;«/mo»«mfrac»«msub»«mi»K«/mi»«mi»f«/mi»«/msub»«msub»«mi»K«/mi»«mi»b«/mi»«/msub»«/mfrac»«/math»
Kc is called the equilibrium constant expressed in terms of molar concentrations and the above equation is known as the Law of Chemical Equilibrium.