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Equilibrium/Le Chatelier's Principle

When there is an increased amount of reactants, the system shifts in favour of the products. When there is an increased amount of products, it shifts to the reactants. That's fine and good.

But when a different substance is added, for example,
CoCl42- (aq) + 6H2O (l) <---> Co(H2O)62+ (aq) + 4Cl- (aq)

has AgNO3(aq) added to it, the system shifts in favour of the products.

Can someone please explain to me why this happens?
Reply 1
AgNO3 reacts with Cl-

Equilibrium shifts to right to make more Cl-

So whenever a substance is added to an equilibrium, think about what it might react with. The equilibrium will then shift in the direction of that reactant/product.

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Ag(NO3) will break down into Ag+ and NO3-

The Ag+ will react with the Cl- decreasing the concentration of Cl-, so the equilibrum will shift to the right
That makes sense. Thank you.
I have one more quick question, along the same lines.
H+ (aq) + HTb- (aq) <----> H2Tb (aq)
Yellow Red
HTb- (aq) <---> H+ (aq) + Tb2- (aq)
Yellow Blue

When HCl is added to the first reaction, there is an increase in the amount of H+, so the system shifts to the left and turns red. When NaOH is added to the same solution (the red solution) it turns blue. How would I describe this and why does this happen?
Typos due to me copying and pasting. The yellow is the colour of the reactants and the red is the products for the first one.
The yellow is the reactants and the blue the products for the second one.

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