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AS How Far How Fast - More equilibruim stuff

When carbon dioxide dissolves in water, the following dynamic equilibria are set up:

CO2(g) + aq <---> CO2(aq)
CO2(aq) + H2O(l) <---> H+ + HCO3-(aq)

Using these eequations and your understanding of dynamic equilibrium to explain the following observations:

i) Bubbling carbon dioxide into an aqueous solution of universal indicator turns its colour from green to orange

ii) A saturated solution of carbon dioxide effervesces when a small amount of concentrated sulphuric acid is added.


I have the mark scheme for this paper, but I don't want to just read the answer off. Can anyone explain it to me? I looked at that question and I don't even know where to begin. What information am I told/can I get from the question to help me?

Thanks.
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Avatar for oceane
oceane
10
It sounds like you haven't quite grasped the concept of equilibria so I'd ask your teacher to go over it again.

i.)U.I goes green -> orange, i.e there's the presence of acid. Look at the reactions above. Some CO2 gas has dissolved in the water - the second equilibrium eqtn shows that a proportion of this dissolved CO2 will hydrolyse into H+ ions and HCO3- ions, H+ ions causing the acidity.

ii.) You have a saturated solution of CO2 i.e, no more can dissolve into it. You add conc. acid, that is lots of H+ ions. So to restore the equilibrium the second equation shifts to the left - forming dissolved molecular CO2. But as the solution's already saturated with CO2, the first equilibrium shifts to the left too, so CO2 bubbles off as gas.

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