I just completed a unit test at school in which there was a simple fill in the blanks question which asked:

Would 1 mol of sulfuric acid and 1 mol calcium hydroxide in water form an acidic, basic, or neutral solution?

The answer I wrote was neutral.

My reasoning was that sulfuric acid ($\ce{H2SO4}$) has two protons it can donate, and calcium hydroxide ($\ce{Ca(OH)2}$) has two protons it can accept. However, after the test I realized that H2SO4 dissociates into HSO4- which is a weak acid and won't dissociate completely.

After some research I have consistently found that the neutralization equation is written as: $\ce{Ca(OH)2 + H2SO4 -> CaSO4 + 2H2O}$

Do sulfuric acid and calcium hydroxide completely neutralize one another? Or was I wrong to say that the resulting solution is neutral.

Could someone please explain why I am right or wrong?

  • 3
    $\begingroup$ Additionally Ca cation in aqueous solution is central atom of aquocomplex which is weakly acidic. Nothing is fully neutral, but if you use such generalisations as acidic/basic/neutral salt, CaSO4 falls under "neutral" $\endgroup$
    – Mithoron
    Dec 18 '17 at 23:49
  • $\begingroup$ @Mithoron thank you for your response, what I still do not understand is why the second hydrogen in sulfuric acid is able to to neutralize the second OH- ion in calcium hydroxide when HSO4- will not dissociate completely. $\endgroup$ Dec 19 '17 at 0:12
  • $\begingroup$ HSO4 anion can dissociate as fully as you wish depending on concentration. $\endgroup$
    – Mithoron
    Dec 19 '17 at 0:16
  • $\begingroup$ @Mithoron, does this still hold true when it is made clear in the question that both calcium hydroxide and sulfuric acid have the same concentration? $\endgroup$ Dec 19 '17 at 0:18
  • $\begingroup$ Check out chemistry.stackexchange.com/questions/36904/… $\endgroup$
    – Mithoron
    Dec 19 '17 at 0:25

The first proton transfer is simple enough and you have no doubts.

$$\ce{H2SO4 + Ca^2+ + 2 OH- -> HSO4- + Ca^2+ + OH- + H2O}\tag{1}$$

Now, you wonder whether hydrogensulphate is a strong enough acid to combine with the remaining hydroxide. Well, hydrogensulphate may be a weak acid but hydroxide is a strong base. So even the second step will go to completion:

$$\ce{HSO4- + Ca^2+ + OH- + H2O -> SO4^2- + Ca^2+ + 2 H2O}\tag{2}$$

Frankly, it does not end there. Calcium and sulphate form a salt with a very low solubility so this salt will precipitate out of solution.

$$\ce{SO4^2- + Ca^2+ + 2 H2O -> CaSO4 v + 2 H2O}\tag{3}$$

So the only thing you are left with in reasonable amounts is water and a precipitate. This, of course, is a neutral solution.

However, you don’t really always have to go that far. Check if there are strong acids and bases (don’t forget hydroxides), cancel them away and then check what remains. If you had done that, you would have noticed that there are as many hydroxide ions as acidic protons, overall giving an approximately neutral solution.


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