[sinthreck]

A major contributor to global warming is increased emissions of CO₂. Consider the equilibria below:
CaCO₃(s) + CO₂(aq) + H₂O(l)  Ca2+(aq) + 2HCO₃ -(aq)
CaCO₃(s)  Ca²⁺(aq) + CO₃ ^2-(aq)    ΔH = -ve
Which statement is true:
(c) The increased CO2 concentration in the ocean is causing a decrease in the pH.

This is my interpretation of the problem:

If you increase CO₂(aq), the RHS of the first equation becomes favourable, hence Ca²⁺ increases.

Since Ca²⁺ increases, the LHS of the second equation becomes favourable, hence CO₃^2- which is basic is reduced.

Because you reduce the concentration of CO₃^2-, the pH decreases.

Is that correct ?

[Borek]

I have yet to see example of solution which becomes more basic (ie pH goes up) when you add acid.

[sinthreck]

lol - how silly of me, this is my second attempt at an explanation:

For every one mole of CO₂(aq) which you dissolve, you create 1 mole Ca²⁺(aq) and 2 moles of HCO₃^-(aq).
This is causing a decrease in the pH because HCO₃^-(aq) is a proton acceptor.

Looking to the second equation. Since you created the 1 mole of Ca²⁺(aq) it reacts with 1 mole of CO₃^2-(aq) to create CaCO₃(s).

The net result is that you have:
1) created 2 moles HCO₃^-(aq)
2) used 1 mole of CO₃^2-(aq)

Hence, overall this is an increase in basic anions which results in a DECREASE in pH.

[sinthreck]

ok, well I just said the exact same thing

so I've looked up on google that when you dissolve CO₂ in water, you get carbonic acid:

CO₂ + H₂O ==> H₂CO₃

I can't really see that in my equation from the first question. It produces HCO₃^- which I think is the conjugate base of H₂CO₃

Unfortunately, in the exam (which is in 2 days times) - I can't use google...

[Borek]

This about it this way. Calcium carbonate dissolves to some small extent, producing CO₃^2-:

CaCO₃ <-> Ca²⁺ + CO₃^2-

At the same time carbon dioxide dissolves in water, producing carbonic acid. So now you have carbonic acid and CO₃^2- in solution:

H₂CO₃ + CO₃^2- <-> 2HCO₃^-

You may also write carbonic dissociation first stage

H₂CO₃ <-> H⁺ + HCO₃^-

and

H⁺ + CO₃^2- <-> HCO₃^-

which yields exacltly teh same result.

But as I wrote earlier - no matter what is the system involved, when you add acid, pH goes down, when you add base, pH goes up.