Question
Which of the following statements is incorrect?
A.
In equilibrium mixture of ice and water kept in perfectly insulated flask, mass of ice and water does not change with time.
B.
The intensity of red colour increases when oxalic acid is added to a solution containing iron (III) nitrate and potassium thiocyanate.
C.
On addition of catalyst, the equilibrium constant value is not affected.
D.
Equilibrium constant for a reaction with negative $$\Delta H$$ value decreases as the temperature increases.
Answer :
The intensity of red colour increases when oxalic acid is added to a solution containing iron (III) nitrate and potassium thiocyanate.
Solution :
$$\mathop {Fe_{\left( {aq} \right)}^{3 + }}\limits_{{\text{Yellow}}} + \mathop {SCN_{\left( {aq} \right)}^ - }\limits_{{\text{Colourless}}} \rightleftharpoons \mathop {\left[ {Fe\left( {SCN} \right)} \right]_{\left( {aq} \right)}^{2 + }}\limits_{{\text{Deep red}}} $$
When oxalic acid is added, it reacts with $$F{e^{3 + }}$$ ions to form stable complex ion $${\left[ {Fe{{\left( {{C_2}{O_4}} \right)}_3}} \right]^{3 - }},$$ thus decreasing the cone. of free $$Fe_{\left( {aq} \right)}^{3 + }.$$ Now, according to Le Chatelier's principle, the reaction will shift in backward direction to increase the cone. of free $$Fe_{\left( {aq} \right)}^{3 + }.$$ Thus, conc. of $${\left[ {Fe\left( {SCN} \right)} \right]^{2 + }}$$ decreases, so the intensity of red colour decreases.