Building and balancing chemical equations (1) Neutralisation

How to build neutralisation equations

If combustion equations present us with problems to solve them so do neutralisation equations

Did you know there are at least two types of neutralisation equation?

Essentially neutralisation occurs when an acid neutralises a base or an alkali (an alkali being a water soluble base)

Here are three typical neutralisation equations:

First

Hydrochloric acid  +   sodium hydroxide   =         sodium chloride  +  water

HCl     +                     NaOH                      =         NaCl               +          H₂O

This is an example of an acid neutralising a water soluble base (alkali)

Second

Nitric acid +   sodium carbonate = sodium nitrate + water + carbon dioxide

HNO₃       +   Na₂CO₃                = NaNO₃            +   H₂O  +  CO₂

This is an example of an acid plus an insoluble base i.e. a carbonate

Third

Sulfuric acid  +  magnesium oxide = magnesium sulphate + water

H₂SO₄          +       MgO                 =              MgSO₄         +   H₂O

This is an example of an acid plus an insoluble base i.e. a metal oxide

So how do we construct and balance these kinds of equations?

A) Let’s look at the first example: acid and soluble base

How can we build an equation for the reaction between say nitric acid and potassium hydroxide?

Let’s put the words down first:

This is the pattern:

Acid   +   soluble base   = water  +  soluble salt

Nitric acid  +  potassium hydroxide = ????

Next add the formulas of the two reactants so:

Nitric acid  +  potassium hydroxide = ?????

HNO₃        +  KOH   =  

Now notice that water forms so remove the elements of water from the reactants.

I have highlighted these elements so

Nitric acid  +  potassium hydroxide = ?????

HNO₃        +  KOH   =  

These will combine to form one water molecule

Nitric acid  +  potassium hydroxide = water

HNO₃        +  KOH                          =   H2O

The remaining parts of the reactants form the soluble salt potassium nitrate

Nitric acid  +  potassium hydroxide = water  +  potassium nitrate

HNO3       +   KOH                          =   H2O   +   KNO3

With a diprotic acid like sulphuric acid things gets a little trickier

B) Let’s look at a second example: acid and insoluble base a metal oxide

How can we build an equation for the reaction between sulfuric acid and copper oxide?

Let’s put the words down first:

This is the pattern:

Acid   +   insoluble metal oxide   =  water  +  soluble salt

sulfuric acid  +  copper oxide = ????

Next add the formulas of the two reactants so:

sulfuric acid  +  copper oxide = ????

H2SO4           +   CuO 

Now notice that water forms so remove the elements of water from the reactants.

I have highlighted these elements so

sulfuric acid  +  copper oxide = ????

H2SO4           +   CuO 

These will combine to form one water molecule

sulfuric acid  +  copper oxide = water

H₂SO₄             +   CuO          =    H₂O

The remaining parts of the reactants form the soluble salt copper sulfate

sulfuric acid  +  copper oxide = water   +   copper sulfate

H₂SO₄             +   CuO        =    H₂O    +     CuSO₄

With a monoprotic acid like nitric acid things gets a little trickier

C) Let’s look at a third example: acid and insoluble base a metal carbonate

How can we build an equation for the reaction between nitric acid and magnesium carbonate?

Let’s put the words down first:

This is the pattern:

Acid   +   insoluble metal carbonate   =  water  +  carbon dioxide + soluble salt

nitric acid  + magnesium carbonate  = ????

Next add the formulas of the two reactants so:

nitric acid  + magnesium carbonate  = ????

HNO₃        +    MgCO₃   =……….?

Now notice that water and carbon dioxide form so remove the elements of water and carbon dioxide from the reactants.

I have highlighted these elements so

nitric acid  + magnesium carbonate  = ????

HNO₃        +    MgCO₃   =……….?

These will combine to form one carbon dioxide molecule but there is not enough hydrogen for a water molecule, so we have to double up the nitric acid formulas.

nitric acid  + magnesium carbonate  = water   +  carbon dioxide

2HNO₃      +    MgCO₃                      =  H₂O   +       CO₂

The remaining parts of the reactants form the soluble salt magnesium nitrate and note that there are now two nitrates to one magnesium

Note too that the nitrates are placed in brackets with the 2 outside to show the magnesium to nitrate ratio is 1:2. 

nitric acid + magnesium carbonate = water + carbon dioxide + magnesium carbonate

2HNO₃     +    MgCO₃                     =  H₂O   +        CO₂    +             Mg(NO₃)₂

You need to be able to make yourself familiar with as many examples like these as you can imagine for example:

Nitric acid   +  sodium hydroxide

Sulphuric acid  + potassium hydroxide

Hydrochloric acid  +  ammonium hydroxide

Sulphuric acid   +   ammonium hydroxide

Nitric acid  +   copper oxide

Nitric acid  +   zinc oxide

Nitric acid  +   iron (III) oxide

Sulfuric acid  +   magnesium oxide

Sulphuric acid   +   sodium oxide

Sulphuric acid   +   iron (II) oxide

Hydrochloric acid   +   copper oxide

Hydrochloric acid   +   iron (II) oxide

Hydrochloric acid   +   magnesium oxide

Nitric acid  +  copper carbonate

Sulphuric acid   +  potassium carbonate

Hydrochloric acid   +  ammonium carbonate  etc……….