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

Feb 19, 2014

You predict the products of acid-base reactions by pairing each cation with the anion of the other compound.

#### Explanation:

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 ${\text{H}}^{+}$.

Let's use the neutralization of stomach acid ($\text{HCl}$) by milk of magnesia [${\text{Mg(OH)}}_{2}$] as an example.

"HCl + Mg(OH)"_2 → ?

(a.) Divide the acid into ${\text{H}}^{+}$ ions and negative ions.

$\text{HCl" → "H"^+ + "Cl"^"-}$

(b.) Divide the base into positive ions and $\text{OH"^"-}$ ions.

$\text{Mg(OH)"_2 → "Mg"^"2" + "OH"^"-}$

(c.) For products, combine the ${\text{H}}^{+}$ and $\text{OH"^"-}$ ions to make $\text{H"_2"O}$.

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, $\text{Mg"^"2+}$ and $\text{Cl"^"-}$ make ${\text{MgCl}}_{2}$.

$\left(\text{H"^+ + "Cl"^"-") + ("Mg"^"2+" + "OH"^"-") → ("Mg"^"2+" + "Cl"^"-") + ("H"^+ + "OH"^"-}\right)$

$\text{HCl + Mg(OH)"_2 → "MgCl"_2 + "H"_2"O}$

(d.) Balance the equation.

$\text{2HCl" + "Mg(OH)"_2 → "MgCl"_2 + 2"H"_2"O}$

EXAMPLE

Complete and balance an equation for the reaction between $\text{HBr}$ and ${\text{Al(OH)}}_{3}$.

Solution

$\text{HBr" + "Al(OH)"_3 → "?}$

$\left(\text{H"^+ + "Br"^"-") + ("Al"^"3+" + "OH"^"-") → ("Al"^"3+" + "Br"^"-") + ("H"^+ + "OH"^"-}\right)$

$\text{HBr + Al(OH)"_3 → "AlBr"_3 +"H"_2"O}$

$\text{3HBr + Al(OH)"_3 → "AlBr"_3 + "3H"_2"O}$