3. A student is studying the equilibrium represented by the equation: 2 CrO42-(a

Question

3. A student is studying the equilibrium represented by the equation: 2 CrO42-(aq, yellow) + 2 H3O+(aq) Cr2O72-(aq, orange) + 3 H2O(l) The mixture obtained by the student is yellow.

(1) Describe the color change the student should observe after adding concentrated HCl to the equilibrium mixture. The color change the student should observe after adding concentrated HCl to the equilibrium mixture when Cr2O72- is formed a orange color is observed. When HCl is added to the mixture, HCl ionizes with water to form H3O+ ion so with the addition of HCl, the concentration of reactants increases.

(2) Briefly explain your answer to (1) in terms of Le Châtelier’s principle. According to Le-Châtelier’s principle, if external stress is applied to a system at equilibrium, the system adjusts in such a way that the stress is partially offset and the system reaches a new equilibrium position so, with the increase in concentration of reactants, the equilibrium begins to shift to the right for example towards products this causes an increase in formation of products which are orange in color so on addition of HCl, the student will observe a color change from yellow to orange.

(3) Describe the color change the student should observe after adding solid sodium chromate (Na2CrO4) to the mixture prepared in (1). Recall that Na2CrO4 dissolves in water and dissociates into Na+ and CrO42- ions.

(4) Briefly explain your answer to (3) in terms of Le Châtelier’s principle.

(5) Describe the color change the student should observe after adding excess NaOH solution to the equilibrium mixture in (3).

(6) write a net ionic equation to support your answer to (5).

(7) Briefly explain your answers to (5) in terms of Le Châtelier’s principle.

Explanation / Answer

(1) and (2) According to Lechatlier's principle, as the concentration of reactants increases the equilibrium will be shifted towards right side i.e. product side. So, the should observe more and more orange colour.

(3) & (4) Again addition of Na2CrO4 increases the concentration of CrO42-, one the reactants. According to Lechatlier's principle increase in concentration of reactants shifts the equilibrium position towards right side. that is more orange colour solution is formed.

(5) addtion of excess NaOH to the equilibrium mixture, decreases the concentration of H3O+ ions. That it decreases the concentration of one of reactants.

(6) NaOH (aq.) + H3O+ (aq.) -----------> Na+ (aq.) + 2H2O (l)

(7) So, according to Lechatlier's principle decrease in concentration of reactants shifts the equilibrium position towards left side i.e towards reactants side.

So, more yellow colour solution is formed.

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