A beaker contains an aqueous solution at equilibrium according to the equation below

CHEMISTRY · Multi-part · A Level · Chemistry · Chemical equilibria (Le Chatelier’s principle and Kc) · Chemical equilibria (Le Chatelier’s principle and Kc)

Problem

A beaker contains an aqueous solution at equilibrium according to the equation below. The solution is bright yellow due to the presence of chromate ions.

2\,\text{CrO}_4^{2-}(aq)+2\,\text{H}_3\text{O}^+(aq) \rightleftharpoons \text{Cr}_2\text{O}_7^{2-}(aq)+3\,\text{H}_2\text{O}(l)

A student performs three separate, independent experiments on identical samples of the equilibrium mixture at constant temperature. (a) In Experiment 1 the student adds a small volume of 6 M HCl(aq). Predict the observable color change, if any, and explain the shift in equilibrium responsible for your prediction. (b) In Experiment 2 the student adds a few pellets of solid NaOH, which dissolve completely. Predict the direction of the equilibrium shift and the resulting color change. Justify your answer in terms of concentration changes. (c) In Experiment 3 the student adds a concentrated solution of Ba(NO

_3

)

_2

, producing the insoluble salt BaCrO

_4

(s) that removes CrO

_4^{2-}

from solution. Predict both the direction of the equilibrium shift and the predominant color of the solution after equilibrium is re-established. Explain your reasoning and comment on whether the appearance of the solid affects the value of

K

for the reaction.

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