**SPOILER** AAMC Chemistry QPack Passage 3

[krazzydhoom]

Hello all. I am a little confused on this question. I know the solution will be basic, but I thought that Na will react with water to create NaOH. How am I supposed to know that the carbonate reacts with water instead? Can anyone help with writing the complete ionic equation? Thank you!

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[lockstitch]

You don't have Na (the metal) in the mixture, you have Na+ ions from the ionic solution of Na2CO3. That eliminates D. Na metal is what would react with water to form OH- ions in a (very vigorous) redox reaction. Na+ ions have already given that one valence electron away and so are very happy living as ions in the polar solvent of water.

As to why the answer is C, carbonate is a weak acid and hence a strong base (i.e. reacts with water to form OH- ions). That is probably something you can be assumed to know based on acid-base chemistry. In particular, I have a hunch they ask about carbonate here specifically because it is involved in respiration/physiological pH regulation. Look at the Khan Academy page for it, maybe it will help.

 

[krazzydhoom]

You don't have Na (the metal) in the mixture, you have Na+ ions from the ionic solution of Na2CO3. That eliminates D. Na metal is what would react with water to form OH- ions in a (very vigorous) redox reaction. Na+ ions have already given that one valence electron away and so are very happy living as ions in the polar solvent of water.

As to why the answer is C, carbonate is a weak acid and hence a strong base (i.e. reacts with water to form OH- ions). That is probably something you can be assumed to know based on acid-base chemistry. In particular, I have a hunch they ask about carbonate here specifically because it is involved in respiration/physiological pH regulation. Look at the Khan Academy page for it, maybe it will help.

Thank you! I understand it a lot better now.

 

[BerkReviewTeach]

Just an FYI here, but carbonate (CO3^2-) has no protons, so it is not an acid. I think you are confusing the conjugate pair relationship. It's conjugate (bicarbonate, HCO3^2-) is a weak acid, but carbonate is a base and ONLY a base.