Chemistry » Acid-Base and Redox Reactions » Acid-Base Reactions Continued

# Neutralization Reactions

## Neutralization reactions

#### Fact:

A cation is an ion (charged atom or molecule) with a positive (+) charge. An anion is an ion with a negative (-) charge.

#### Fact:

Salts are not just the table salt you put on your food. A salt is any compound made up of stoichiometrically equivalent amounts of cations and anions to make a neutral, ionic compound.

Magnesium sulfate heptahydrate ($$\text{MgSO}_{4}.7\text{H}_{2}\text{O}$$), commonly known as Epsom salt, can be used as a gel to treat aches and pains, as bath salts, and has many other uses.

When an equivalent amount of acid and base react (so that neither the acid nor the base are in excess), the reaction is said to have reached the equivalence point. At this point neutralisation has been achieved.

### Definition: Equivalence point

When a stoichiometrically equivalent number of moles of both reactants has been added to the reaction vessel.

To better understand stoichiometric equivalence look at the following equations:

1. $$1\color{red}{\text{HA(aq)}} + 1\color{blue}{\text{BOH(aq)}} \to$$ $$1\text{AB}(\text{aq}) + 1\text{H}_{2}\text{O}(\text{l})$$

2. $$1\color{red}{\text{H}_{2}{\text{A(aq)}}} + 2\color{blue}{\text{BOH(aq)}} \to$$ $$1\text{AB}_{2}(\text{aq}) + 2\text{H}_{2}\text{O}(\text{l})$$

In the first example above, a stoichiometrically equivalent number of moles is one mole of $$\color{red}{\text{HA}}$$ for every one mole of $$\color{blue}{\text{BOH}}$$. In the second example, a stoichiometrically equivalent number of moles is one mole of $$\color{red}{\text{H}_{2}{\text{A}}}$$ for every two moles of $$\color{blue}{\text{BOH}}$$.

### Definition: Neutralisation

A neutralisation reaction involves an acid and a base reacting to form a salt.

Look at the following examples:

• Hydrochloric acid with sodium hydroxide

Hydrochloric acid reacts with sodium hydroxide to form sodium chloride (the salt) and water. Sodium chloride is made up of $$\text{Na}^{+}$$ cations from the base $$(\text{NaOH})$$ and $$\text{Cl}^{-}$$ anions from the acid $$(\text{HCl})$$.

$$\text{H}{\color{red}{\textbf{Cl}}}{\text{(aq)}} + \color{blue}{\textbf{Na}}{\text{OH(aq)}} \to {\color{blue}{\textbf{Na}}}{\color{red}{\textbf{Cl}}}{\text{(aq)}} + {\text{H}}_{2}{\text{O(l)}}$$

• Hydrogen bromide with postassium hydroxide

Hydrogen bromide reacts with potassium hydroxide to form potassium bromide (the salt) and water. Potassium bromide is made up of $$\text{K}^{+}$$ cations from the base $$(\text{KOH})$$ and $$\text{Br}^{-}$$ anions from the acid $$(\text{HBr})$$.

$$\text{H}{\color{red}{\textbf{Br}}}{\text{(aq)}} + \color{blue}{\textbf{K}}\text{OH(aq)} \to \color{blue}{\textbf{K}}\color{red}{\textbf{Br}}{\text{(aq)}} + {\text{H}}_{2}\text{O(l)}$$

• Hydrochloric acid with sodium hydrocarbonate

Hydrochloric acid reacts with sodium hydrocarbonate to form sodium chloride (the salt), water and carbon dioxide. Sodium chloride is made up of $$\text{Na}^{+}$$ cations from the base $$(\text{NaHCO}_{3})$$ and $$\text{Cl}^{-}$$ anions from the acid $$(\text{HCl})$$.

$$\text{H}{\color{red}{\textbf{Cl}}}{\text{(aq)}} + {\color{blue}{\textbf{Na}}}{\text{HCO}}_{3}{\text{(aq)}} \to \color{blue}{\textbf{Na}}\color{red}{\textbf{Cl}}{\text{(aq)}} + {\text{H}_{2}{\text{O(l)}}} + {\text{CO}}_{2}{\text{(g)}}$$

You should notice that in the first two examples, the base contained $$\text{OH}^{-}$$ ions, and therefore the products were a salt and water. $$\text{NaCl}$$ (table salt) and $$\text{KBr}$$ are both salts. In the third example, $$\text{NaHCO}_{3}$$ also acts as a base, despite not having $$\text{OH}^{-}$$ ions. A salt is still formed as one of the products, but carbon dioxide ($$\text{CO}_{2}$$) is produced as well as water.

This experiment can be used for an informal assessment. It will help determine if the learners understand what happens in a neutralisation reaction. Learners are working with a strong acid and a strong base in this reaction. Concentrated, strong acids and bases can cause serious burns. Please remind the learners to be careful and wear the appropriate safety equipment when handling all chemicals, especially concentrated acids and bases. The safety equipment includes gloves, safety glasses, and protective clothing.

## Optional Experiment: Temperature changes in neutralisation reactions

### Aim

To investigate the temperature change associated with a neutralisation reaction.

### Apparatus

• $$\text{1}$$ $$\text{mol.dm^{-3}}$$ solution of sodium hydroxide ($$\text{NaOH}$$), $$\text{1}$$ $$\text{mol.dm^{-3}}$$ solution of hydrochloric acid ($$\text{HCl}$$)

• A thermometer, a beaker, a measuring cylinder

### Method

#### Warning:

Concentrated acids and bases can cause serious burns. We suggest using gloves and safety glasses whenever you work with an acid or a base. Remember to add the acid to the water and to avoid sniffing any laboratory chemical. Handle all chemicals with care.

1. Pour $$\text{20}$$ $$\text{cm^{3}}$$ of the sodium hydroxide solution into the beaker.

2. Measure the temperature.

3. Add $$\text{5}$$ $$\text{cm^{3}}$$ of hydrochloric acid to the beaker using the measuring cylinder.

4. Repeat steps $$\text{2}$$ and $$\text{3}$$ quite quickly until the temperature no longer changes significantly.

### Observations

You should record your temperatures and the volume in a table (remember that the volume of the sodium hydroxide is constant at $$\text{20}$$ $$\text{cm^{3}}$$):

 Volume (HCl) Temperature (℃) $$\text{0}$$ room temperature

### Discussion

You should find that the reaction releases heat and so the temperature increases. After all the base has been neutralised the temperature should no longer increase. This is because the neutralisation reaction is exothermic (it releases heat). When all the base has been neutralised there is no reaction on the addition of more acid, and no more heat is released.

After the reaction has finished (the base has been neutralised) there is no more heat being produced. As a result the temperature will not rise any more. In fact, there may even be a decrease in temperature back to room temperature as the heat from the reaction vessel (the beaker) dissipates.

#### Fact:

Salts can come in many different colours.

$$\color{purple}{\textbf{Potassium permanganate}}$$ ($$\color{purple}{\textbf{KMnO}_{4}}$$)

$$\color{blue}{\textbf{Copper sulfate}}$$ ($$\color{blue}{\textbf{CuSO}_{4}}$$)

$$\color{darkgreen}{\textbf{Nickel chloride}}$$ ($$\color{darkgreen}{\textbf{NiCl}_{2}}$$)

$$\color{gold}{\textbf{Sodium chromate}}$$ ($$\color{gold}{\textbf{Na}_{2}{\textbf{CrO}}_{4}}$$)

$$\color{red}{\textbf{Potassim dichromate}}$$ ($$\color{red}{\textbf{K}_{2}{\textbf{Cr}}_{2}{\textbf{O}}_{7}}$$)

Neutralisation reactions are very important in every day life. Below are some examples:

• Domestic uses

Calcium oxide $$(\text{CaO})$$ is used to neutralise acidic soil. Powdered limestone $$(\text{CaCO}_{3})$$ can also be used, but its action is much slower and less effective. These substances can also be used on a larger scale in farming and in rivers.

• Biological uses

Hydrochloric acid $$(\text{HCl})$$ in the stomach plays an important role in helping to digest food. It is important to note that too much acid in the stomach may lead to the formation of ulcers in cases where the stomach lining is damaged (e.g. by an infection).

Antacids (which are bases) are taken to neutralise excess stomach acid, to prevent damage to the intestines. Examples of antacids are aluminium hydroxide, magnesium hydroxide (‘milk of magnesia’) and sodium bicarbonate (‘bicarbonate of soda’).

• Industrial uses

Alkaline calcium hydroxide (limewater) is used to absorb harmful acidic $$\text{SO}_{2}$$ gas that is released from power stations and from the burning of fossil fuels.

#### Warning:

Please do not use a base to neutralise an acid if you spill some on yourself during an experiment. A strong base can burn you as much as a strong acid. Rather wash the area thoroughly with water.

#### Fact:

Bee stings are acidic and have a pH between $$\text{5}$$ and $$\text{5.5}$$. They can be soothed by using substances such as calomine lotion, which is a mild alkali based on zinc oxide. Bicarbonate of soda, or soap, can also be used. The alkalis help to neutralise the acidic bee sting and relieve some of the itchiness.