Reversible reactions and equilibrium

[emchun]

I did a prac. the other day on reversible reactions. But I don't get what happened.

We had 15mL put into two test tubes each. One was (1)control and the (2)other we added 4 drops of sulfuric acid. Told us to compare the colour, but why? The control was yellow and the other turned orange.

We then separated (2) into three test tubes.
(a) We added another 4 drops of the acid into first test tube. The colour turned darker.
(b) We added 15 drops of to second test tube but there was no colour change.
(c) We added 15 drops of NaOH to third test tube and it turned back to yellow.
Then comparing all the colours again.

By doing (a), the colour turned darker, why did it become darker and why did we have to do it?
(b), how come there was no colour change? why did we have to do this?
(c) the reaction was reversed right? why?

I really don't understand what is happening. Hope someone can help. Thanks!






 

[lzxnl]

2CrO4 2- + 2H+ <=> Cr2O72- + H2O

There's your equilibrium. For (a), if you add acid to chromate, you'll shift the equilibrium to the right and form dichromate, which is orange.
For (b), if you add lots of chromate, then you should shift the equilibrium to the right, but given that there's no colour change it may well be the case that you don't have any acid left to react.
For c, adding hydroxide ions removes H+, so the system tries to generate more H+ by forming more chromate, which is yellow.

[emchun]

thank you, I understand now.