[mr0624]

I'm looking at the equilibrium system 2CrO4-2 + 2H3O+ <---> Cr2O7-2 + 3H20

When the equilibrium lies favoring the reactants, the solution is yellow, since the CrO4 gives off a yellow color. When the equilibrium favors the products, it is orange because of the C2O7-2. When HCl is added, the H3O+ concentration is greatly increased, which causes the equilibrium to shift to the right (this much information i was given by my teacher). When I did this in lab, the solution DID turn orange, as expected.

However, when asked to explain the color change in terms of both Le Chatelier's principle AND the Kc equation, there's a contradiction.

According to Le Chatelier's Principle, if there is more of the H3O, more reactant, then more product should be produced as equilibrium would lie to the right. However, since the Kc equilibrium equation is in products/reactants, when i calculate Kc I get a smaller kc than before, which suggests that more reactant is being produced (smaller kc = more reactant, larger kc = more product).

I'm not actually calculating Kc by the way, being that I don't have specific molarities, I'm simply looking at the assumed equilibrium equation which should be .

What am I doing wrong?

[Borek]

You are not "calculating Kc". Kc - as the name implies - is constant, so its value - when calculated for the system in equilibrium - is always the same. I guess you calculated just value of the reaction quotient, but not for equilibrium - you have used concentration of reactants as added, not taking into account that they have reacted. If you will try to use Kc to calculate concentrations of reactants and products after equilibrium was reached you will see it agrees with Le Chatelier's principle predictions.

[mr0624]

Ah, that makes sense. Thanks.