Question

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

Answer 1

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 `"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"`