Chromium(VI) forms two different oxyanions, the orange...

Chromium(VI) forms two different oxyanions, the orange dichromate ion $\left(\mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}\right)$ and the yellow chromate ion $\left(\mathrm{CrO}_{4}^{2-}\right) .$ The equilibrium reaction between the two ions is $$\mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}(a q)+\mathrm{H}_{2} \mathrm{O}(l) \rightleftharpoons 2 \mathrm{CrO}_{4}^{2-}(a q)+2 \mathrm{H}^{+}(a q)$$ The following pictures show what happens when sodium hydroxide is added to a dichromate solution. (IMAGE NOT COPY) Explain what happened.

Chromium(VI) forms two different oxyanions, the orange dichromate ion $\left(\mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}\right)$ and the yellow chromate ion $\left(\mathrm{CrO}_{4}^{2-}\right) .$ The equilibrium reaction between the two ions is
$$\mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}(a q)+\mathrm{H}_{2} \mathrm{O}(l) \rightleftharpoons 2 \mathrm{CrO}_{4}^{2-}(a q)+2 \mathrm{H}^{+}(a q)$$
The following pictures show what happens when sodium hydroxide is added to a dichromate solution.
(IMAGE NOT COPY)
Explain what happened.

Key Concepts

Chemical Equilibrium

Chemical equilibrium refers to the condition in a reversible reaction where the forward and reverse reactions occur at the same rate, resulting in constant concentrations of the reactants and products. This concept is fundamental in understanding how systems respond to disturbances like changes in concentration, temperature, or pH.

Le Chatelier's Principle

Le Chatelier's Principle states that if a dynamic equilibrium is disturbed by changing the conditions, the system will adjust itself to partially counteract the effect of the disturbance. In the context of the reaction between the dichromate and chromate ions, adding a reagent that affects the pH will result in a shift in the equilibrium position.

Acid-Base Chemistry

Acid-base chemistry involves reactions where proton donors (acids) and proton acceptors (bases) interact. In aqueous solutions, the addition of a base like sodium hydroxide neutralizes acid by removing hydrogen ions, leading to changes in the pH of the solution, which in turn can affect other chemical equilibria.

Chromate-Dichromate Equilibrium

The chromate-dichromate equilibrium is a specific example of a chemical equilibrium that is pH-dependent. In low pH (acidic) environments, the equilibrium favors the formation of dichromate ions, while in high pH (basic) environments, it shifts towards the formation of chromate ions. This shift is observable through the change in color of the solution.

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