Acid-base equilibria
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A tablet has got sodium hydrogencarbonate and citric acid in it, and I know that when you add water, CO2 is released. We are given the equation:
3NaHCO3 + H3A -> Na3A + 3CO2 + 3H2O, where citric acid is simplified to H3A.
We have to use ionic equations to show how the addition of water allows the release of carbon dioxide from the tablet.
I am pretty stuck - does the weak acid play a part, and how do you work out the ionic equations?
Igloo writes ...
The reaction yielding CO2 is an interaction between mobile hydrogencarbonate ions, HCO3-, and hydrogen ions, H+ (strictly speaking H3O+ ions, as you ought to know now from your A2 course, having covered the Bronsted-Lowry theory in Topic 14 if you are doing Nuffield Chemistry).
In the case of the hydrogencarbonate ions, these are obviously not mobile within the solid, but are held together with sodium ions in a crystal lattice. As regards the oxonium ions, these are only created when water is added to the solid covalent citric acid molecules.
The relevant equations are therefore:
NaHCO3(s) - with water -> Na+(aq) + HCO3-(aq)
H3A(s) + 3H2O(l) -> 3H3O+(aq) + A-(aq)
Then the mobile hydroxonium ions react with the mobile hydrogencarbonate ions, and CO2 is released:
H3O+ + HCO3-(aq) -> 2H2O(l) + CO2(g)
Incidentally, the sodium ions, Na+(aq), and citrate ions, A-(aq), are spectator ions in this reaction and therefore do not appear in the ionic equation above.
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updated: 22 August 2004
