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 1

Hint: In statement (A) it is important to note the words “perfectly insulated flask”. The exactness of statement (B) is found out by writing down the chemical reactions and the products formed are noted. For, statement (C) and (D), the given conditions are to be analysed.

Complete answer:
Let us take each of these statements one by one. A brief explanation I provided as to why a statement is correct or incorrect.
In statement (A), it is said that for an equilibrium mixture ice and water, mass of ice and water in a perfectly insulated flask is unchanged with time. Since the system is in a perfectly insulated flask, it is an isolated system. In an isolated system, there is no transfer of energy or mass between the system and surroundings and hence, the given statement (A) is correct.
In statement (B), it is said that the intensity of red colour increases when oxalic acid is added to a solution containing $Fe{(N{O_3})_3}$ and $KSCN$. First, let us look at the reaction between $Fe{(N{O_3})_3}$ and $KSCN$:
\[F{e^{3 + }}\left( {aq} \right){\text{ }} + {\text{ }}SC{N^ - }\left( {aq} \right){\text{ }} \rightleftharpoons {\text{ }}FeSC{N^{2 + }}\left( {aq} \right)\]
\[FeSC{N^{2 + }}\] formed on reaction with oxalic acid forms a complex which lowers the overall concentration of free $F{e^{3 + }}$ions. This results in the decrease in the intensity of red colour. Hence, statement (B) is incorrect.
In statement (C), it is said that on adding a catalyst, the equilibrium constant value is not affected. This is correct, as catalysts only affect activation energy.
In statement (D), it is mentioned that the equilibrium constant with a negative $\Delta H$ value decreases as the temperature increases. Negative $\Delta H$value means that the reaction is exothermic in nature. For an exothermic process, when temperature is increased, the value of equilibrium constant decreases. Hence, statement (D) is correct.
Therefore, the incorrect statement is statement (B) The intensity of red colour increases when oxalic acid is added to a solution containing iron (III) nitrate and potassium thiocyanate.

Note:
While attempting such questions, it is important to understand each statement and consider it in an isolated manner, which will further help to distinguish each statement for its exactness. For statements involving chemical reactions, it is best to explain by writing down the equation for the chemical reactions, and for thermodynamic relations, it is important to remember how one term affects another term in the equation.