Chromate ions can be converted into dichromate ions with acid in an equilibrium

Question

Chromate ions can be converted into dichromate ions with acid in an equilibrium reaction. Chromate ions also react readily with barium ions to precipitate barium chromate, but do not form a precipitate with dichromate ions. 1. 3H20(1) 2 Cr042 (aq) + 2 H,O. (aq) Cr072-(aq) yellow BaCrO,(s)Ba(aq CrO (aq) + orange yellow When barium ions are added to a solution of potassium dichromate, some yellow precipitate forms and the solutions becomes slightly more yellow. What is the yellow solid? Use LeChâtelier's principle to explain the change in the system. a. b. When HCI(aq) is added a saturated solution of barium chromate, with some solid bariunm chromate at the bottom, the solid dissolves and the solution turns orange. What species gives the orange color? Use LeChâtelier's principle to explain the change in the system Since reactions are endothermic or exothermic, adding heat to or removing heat from a system at equilibrium will cause the reaction to shift because the value of K will change an equilibrium system will shift the reaction in the direction that absorbs heat. For an exothermic reaction, adding heat will shift the reaction to the left, for an endothermic reaction, it will shift to right. Removing heat will do the opposite. a. In Part IC you heated and cooled an equilibrium mixture containing CoCl and C 2. . Adding heat to and observed the changes that occurred. i. What species predominates when heat is added to the system, Coc,orCo(H,O,2 i. Is the reaction endothermic or exothermic? Explain, using your observed color b. In part 2, is the dissociation of acetic acid endothermic or exothermic? Explain, using your observed color changes.

Explanation / Answer

1a) The two equilibrium reactions are given as below.

2 CrO42- (aq) + 2 H3O+ (aq) <=======> Cr2O72- (aq) + 3 H2O (l) ……(1)

K1 = [Cr2O72-]/[CrO42-]2[H3O+]2

BaCrO4 (s) <======> Ba2+ (aq) + CrO42- (aq) ……(2)

Ksp = [Ba2+][CrO42-]

Potassium dichromate exists in equilibrium with potassium chromate as per the equilibrium reaction (1). When barium ions are added to an aqueous solution of potassium dichromate, some barium chromate is formed by combination of barium and chromate ions and hence, the solution turns slightly yellow. As the chromate ions combine, [CrO42-] is reduced. However, K1 must remain constant at a particular temperature; therefore, the reverse reaction is facilitated, producing more CrO42-. The produced CrO42- combines with Ba2+ forming BaCrO4 and thus, the solution turns more yellow.

b) Barium chromate reacts with HCl to produce barium chloride (BaCl2) and chromic acid (H2CrO4) which dissociates in aqueous solution to produce chromate ions as below.

BaCrO4 (aq) + 2 HCl (aq) --------> BaCl2 (aq) + H2CrO4 (aq)

H2CrO4 (aq) + 2 H2O (l) --------> CrO42- (aq) + 2 H3O+ (aq)

As [CrO42-] increases, the forward reaction in (1) is facilitated to keep K1 constant. Therefore, more Cr2O72- is produced and hence, the color of the solution gradually turns orange.

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