How do you predict the products in acid-base reactions?

Acid-base neutralization is a special case of double displacement reaction.

We can use the same techniques to predict the products.

In acids, the cation is always #"H"^+#.

Let's use the neutralization of stomach acid (#"HCl"#) by milk of magnesia [#"Mg(OH)"_2#] as an example.

#"HCl + Mg(OH)"_2 → ?#

(a.) Divide the acid into #"H"^+# ions and negative ions.

#"HCl" → "H"^+ + "Cl"^"-"#

(b.) Divide the base into positive ions and #"OH"^"-"# ions.

#"Mg(OH)"_2 → "Mg"^"2" + "OH"^"-"#

(c.) For products, combine the #"H"^+# and #"OH"^"-"# ions to make #"H"_2"O"#.

Then combine the positive and negative ions to make a salt.

Remember to balance the positive and negative charges in the salt.

In this case, #"Mg"^"2+"# and #"Cl"^"-"# make #"MgCl"_2#.

#("H"^+ + "Cl"^"-") + ("Mg"^"2+" + "OH"^"-") → ("Mg"^"2+" + "Cl"^"-") + ("H"^+ + "OH"^"-")#

#"HCl + Mg(OH)"_2 → "MgCl"_2 + "H"_2"O"#

(d.) Balance the equation.

#"2HCl" + "Mg(OH)"_2 → "MgCl"_2 + 2"H"_2"O"#

EXAMPLE

Complete and balance an equation for the reaction between #"HBr"# and #"Al(OH)"_3#.

Solution

#"HBr" + "Al(OH)"_3 → "?"#

#("H"^+ + "Br"^"-") + ("Al"^"3+" + "OH"^"-") → ("Al"^"3+" + "Br"^"-") + ("H"^+ + "OH"^"-")#

#"HBr + Al(OH)"_3 → "AlBr"_3 +"H"_2"O"#

#"3HBr + Al(OH)"_3 → "AlBr"_3 + "3H"_2"O"#