# how do i write a double replacement reaction (balanced) between sulfuric acid and magnesium hydroxide

caledon | High School Teacher | (Level 3) Senior Educator

Posted on

A double replacement reaction takes place between two ionic compounds, and follows this pattern:

`(A^+ B^-) + (C^+ D^-) -> (A^+ D^-) + (C^+ B^-)`

The ionic compounds are just trading "partners", so to speak.

The Compounds:

Sulfuric acid has the formula `H_2 SO_4`

It is composed of two Hydrogen ions, with a total charge of +2, and one sulfate ion, with a charge of -2.

Magnesium hydroxide has the formula `Mg(OH)_2`

It is composed of one Magnesium ion, with a total charge of +2, and two hydroxide ions, each with a charge of -1.

The Reaction:

`(H_2^+2)(SO_4^-2) + (Mg^+2)(OH)_2^-2`

Double replacement:

`(H_2^+2)(OH)_2^-2) + (Mg^+2)(SO_4^-2)`

which gives us

`H_2(OH)_2 + MgSO_4`

The hydrogen-hydroxide compound has a formula of

`H_4O_2`

which simplifies to

`2 H_2O`

So, the complete reaction is:

`H_2SO_4 + Mg(OH)_2 -> MgSO_4 + 2H_2O`

ayl0124 | Student, Grade 12 | (Level 1) Valedictorian

Posted on

Think of a chemical reaction as a square dance. Two couples are dancing and then they swap dates. The resulting couples are a mix of the original couples.

`AB + CD -> AD + CB`

In order to write a double replacement, first find your original chemicals.

`H_2SO_4 + Mg(OH)_2 -> ?`

Note the charges on all the ions. Swap the cations...

`H_2SO_4 + Mg(OH)_2 -> MgSO_4 + H_2O`

Remember, matter cannot be created nor destroyed. There should be an equal amount of atoms in each side of the reaction. Therefore, we must balance the equation to get your final answer.

`H_2SO_4 + Mg(OH)_2 -> MgSO_4 + 2H_2O`

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