[unknown_analysis]

This is fairly basic but...

I was asked to write the chemical equation (not the net-ionic equation) of the following:

a. K₂CrO₄ + HCl   ?
b. K₂CrO₄ + NaOH ?
c. K₂Cr₂O₇ + HCl   ?
d. K₂Cr₂O₇ + NaOH   ?

And can anyone explain to me why chromate turns orange in acidic medium, while dichromate turns yellow in basic medium?

Thanks for the replies!

[Borek]

There is an equilibrium between chromate & dichromate, depending on pH.

[AWK]

Both: chromate and dichromate react with an excess of HCl in redox reaction

[Borek]

In general - yes, but i doubt that's what the problem is aiming at.

Probably lousy selection of acid.

[monica]

2Cr2O7 2-  + h20    2CrO4 -  + 2 H+
orange                              yellow

Yellow chromate and orange dichromate are in equilibrium with each other in aqueous solution. The more acidic the solution, the more the equilibrium is shifted to the left towards the dichromate ion(orange colour). As hydrochloric acid is added to the chromate solution, the yellow color turns to orange. Increasing the hydrogen ion concentration is shifting the equilibrium to the left and when NaOH is added the eq. is shifted toward right according to LE Chatlier principle and the orange colour changes to yellow.

[AWK]

2Cr2O7 2-  + h20    2CrO4 -  + 2 H+
orange                              yellow

Yellow chromate and orange dichromate are in equilibrium with each other in aqueous solution. The more acidic the solution, the more the equilibrium is shifted to the left towards the dichromate ion(orange colour). As hydrochloric acid is added to the chromate solution, the yellow color turns to orange. Increasing the hydrogen ion concentration is shifting the equilibrium to the left and when NaOH is added the eq. is shifted toward right according to LE Chatlier principle and the orange colour changes to yellow.

Chromate cannot exist in the presence of H⁺

The correct reaction is:
Cr₂O₇^2- + 2OH^- = 2CrO₄^2- + H₂O

[monica]

@awk
 thanks for correction!

[unknown_analysis]

One question though, why do you say that chromate cannot exist with H⁺? 

Thanks for the replies!

[Borek]

What he means is that H⁺ shifts the equilibrium to dichromate. As presence of water implies presence of both H⁺ and OH^-, stating that chromate can't exist in the presence of H⁺ is far too fetched, and both reactions (one with H⁺/H₂O and one with OH^-/H₂O) are equivalent - although obviously equilibrium constant will be different.

[unknown_analysis]

I got it. Thank you very much!