Neutralisation Reactions (HSC SSCE Chemistry): Revision Notes

Neutralisation Reactions

Introduction to neutralisation reactions

When an acid reacts with a base, a neutralisation reaction takes place. This type of reaction produces salt and water as products. You can represent this using a simple net ionic equation:

$\text{H}^+_{(aq)} + \text{OH}^-_{(aq)} \rightarrow \text{H}_2\text{O}_{(l)}$

However, this equation doesn't show the full picture. There's actually another important product formed during neutralisation - heat energy. Understanding the energy changes that occur during neutralisation reactions is crucial for many applications in chemistry.

Enthalpy of neutralisation

Understanding enthalpy and energy changes

Chemical reactions always involve energy changes. When reactions occur, bonds in the reactants must be broken, and new bonds form to create the products. Breaking bonds requires energy input, while forming new bonds releases energy. The overall energy change depends on the balance between these two processes.

Enthalpy is a measure of the total energy possessed by a substance or group of substances. In chemistry, we're particularly interested in the change in enthalpy, which is commonly called the heat of reaction and given the symbol $\Delta H$. This value is measured in kilojoules per mole ($\text{kJ mol}^{-1}$).

The change in enthalpy for a chemical reaction is defined as the energy change per mole of a specified reactant or product when the reaction occurs at constant pressure. We can express this mathematically as:

$\Delta H = \text{enthalpy of products} - \text{enthalpy of reactants}$

Since we define $\Delta H$ as the energy absorbed during a reaction, a positive value indicates energy absorption, while a negative value indicates energy release.

Exothermic reactions

Exothermic reactions are chemical reactions in which the energy of the reactants is greater than the energy of the products. During these reactions, energy is released to the surroundings, usually in the form of heat. Because energy is released, exothermic reactions have negative $\Delta H$ values.

The diagram above shows an enthalpy profile for an exothermic reaction. Notice how the products have lower energy than the reactants, with the difference ($\Delta H$) being negative, representing the energy released.

Endothermic reactions

In contrast, endothermic reactions have positive $\Delta H$ values. These are reactions where the energy of the products is greater than the energy of the reactants. Energy must be absorbed from the surroundings for the reaction to proceed.

The enthalpy diagram above illustrates an endothermic reaction, where the products have higher energy than the reactants, and $\Delta H$ is positive.

Standard enthalpy of neutralisation

During a neutralisation reaction, the change in enthalpy depends on several factors, including the concentration of reactants and products in solution, and the pressure of any gases involved. To make meaningful comparisons between different reactions, chemists use standard conditions.

The standard enthalpy of neutralisation is defined as the enthalpy change when solutions of an acid and alkali react together under standard conditions ($25°\text{C}$ or $298\text{ K}$ and $100\text{ kPa}$ pressure) to produce exactly 1 mole of water.

Measuring enthalpy

Using temperature changes to measure energy

In the laboratory, one of the most practical ways to measure energy changes is by observing temperature changes in water or aqueous solutions. This method relies on the relationship between heat energy and temperature.

The specific heat capacity of water is $4.18\text{ J K}^{-1}\text{ g}^{-1}$. This means that $4.18$ joules of energy are needed to increase the temperature of $1$ gram of water by $1$ kelvin (or $1°\text{C}$).

The heat capacity equation

The total amount of energy needed to change the temperature of a substance depends on three factors:

• The mass of the substance
• The specific heat capacity of the substance
• The temperature change required

When interpreting results, remember that if the temperature change is positive, the temperature has increased and the chemical system has lost energy (exothermic). If the temperature change is negative, the temperature has decreased and the system has gained energy (endothermic).

Investigation 5.2: Measuring the enthalpy of neutralisation

Introduction

When a neutralisation reaction occurs, there is a measurable change in enthalpy. The standard enthalpy of neutralisation is the enthalpy change when solutions of an acid and alkali react together under standard conditions to produce exactly 1 mole of water.

In this investigation, you will determine the standard enthalpy of neutralisation for a reaction between aqueous sodium hydroxide solution and hydrochloric acid. You will also explore whether there is a difference in enthalpy when solid sodium hydroxide is used instead of the aqueous solution.

Aim

To determine the enthalpy of neutralisation and the effect of the state of the reactants.

Hypothesis

Students should write their own hypothesis predicting how the enthalpy change might differ between using solid sodium hydroxide versus aqueous sodium hydroxide.

Materials

• $4\text{ g}$ $\text{NaOH}_{(s)}$
• $100\text{ mL}$ $1.0\text{ mol L}^{-1}$ $\text{HCl}$
• $50\text{ mL}$ $2.0\text{ mol L}^{-1}$ $\text{HCl}$
• $50\text{ mL}$ $2.0\text{ mol L}^{-1}$ $\text{NaOH}$
• $100\text{ mL}$ measuring cylinder
• $-10$ to $110°\text{C}$ thermometer or temperature probe and data logger
• Spatula
• Electronic balance
• 2 polystyrene cups
• Safety glasses

Risk assessment

What are the risks in doing this investigation?	How can you manage these risks to stay safe?
NaOH is caustic.	Use a spatula to transfer the $\text{NaOH}_{(s)}$.
HCl is corrosive.	Use safety glasses and personal protective clothing. Dispose of as directed by your teacher to protect from splashing.

Method

Part A

1. Pour $100\text{ mL}$ of $1.0\text{ mol L}^{-1}$ $\text{HCl}$ into a polystyrene cup.
2. Measure the temperature of this solution using the thermometer or temperature probe.
3. Accurately weigh out approximately $4.0\text{ g}$ of $\text{NaOH}_{(s)}$ and add this to the same polystyrene cup.
4. Use the thermometer to stir, then record the highest or lowest temperature reached.

Part B

1. Pour $50\text{ mL}$ of $2.0\text{ mol L}^{-1}$ $\text{NaOH}$ into a polystyrene cup.
2. Measure the temperature of the NaOH using the thermometer or temperature probe.
3. Measure $50\text{ mL}$ of $2.0\text{ mol L}^{-1}$ $\text{HCl}$ and record its initial temperature.
4. Average the initial temperatures of the NaOH and HCl – this is the initial temperature of this experiment.
5. Pour the HCl into the polystyrene cup containing the NaOH, stir and record the highest or lowest temperature reached.

Results

Part A

Record the following data:

• Volume of HCl used
• Initial temperature of HCl
• Mass of solid NaOH used
• Final temperature of solution

Part B

Record the following data:

• Total volume of solution used
• Initial temperature of solution
• Final temperature of solution

Analysis of results

Assume the heat capacity ($C$) of each solution is $4.18\text{ J K}^{-1}\text{ g}^{-1}$.

Data for Part A will be used to determine $\Delta H$ of neutralisation when using solid sodium hydroxide, while data for Part B will be used to determine $\Delta H$ of neutralisation when using aqueous sodium hydroxide.

For each part, calculate:

1. The heat of reaction using $q = mC\Delta T$
2. The number of moles of NaOH and HCl that took part in the reaction
3. The number of moles of water produced
4. The heat of reaction per mole of water

Discussion

1. Compare the heat of reaction per mole of water for Part A and Part B and suggest reasons for any differences between the values obtained.
2. The theoretical value for enthalpy changes of neutralisation for reactions between solutions of strong acids and bases is around $-57\text{ kJ mol}^{-1}$ (of water). Explain any discrepancies in your data.

Conclusion

Students should evaluate their hypothesis based on the experimental results obtained.

Extension

Evaluate whether it would be better to clean up an acid spill with a solid or aqueous base, based on the enthalpy differences observed in this investigation.

Applications of neutralisation reactions

Neutralisation reactions play vital roles in our everyday lives, as well as in industrial processes and environmental management. Understanding these applications helps us appreciate the importance of acid-base chemistry.

Neutralisation and me

In our digestive system

In many natural systems, including our bodies, maintaining the correct acid-base balance is crucial for health. This balance is regulated by naturally occurring processes involving neutralisation reactions.

Our stomach produces hydrochloric acid, which serves two important functions: it aids in the digestion of food and kills many harmful micro-organisms that we might swallow. However, when the system is disrupted and too much acid is produced, we experience heartburn or indigestion.

The solution to excess stomach acid is commonly an antacid. Antacids contain bases such as magnesium hydroxide ($\text{Mg(OH)}_2$) or aluminium hydroxide ($\text{Al(OH)}_3$), which neutralise the excess acid through a neutralisation reaction.

Other active ingredients commonly found in antacids include:

• Magnesium oxide
• Magnesium carbonate
• Calcium carbonate
• Sodium hydrogen carbonate

Protecting our teeth

Many foods and drinks we consume are acidic. For example:

• Apples contain malic acid
• Tea contains tannic acid
• Carbonated drinks contain carbonic acid

Additionally, bacteria in your mouth produce acid when they consume sugar from food. The enamel that forms the outer layer of our teeth is composed of $95\%$ hydroxyapatite ($\text{Ca}_5(\text{PO}_4)_3\text{OH}$). This enamel is easily damaged by acids through a process called demineralisation.

The acid reacts with the enamel, causing it to break down according to the reaction:

$\text{Ca}_5(\text{PO}_4)_3\text{OH} + 4\text{H}^+ \rightarrow 5\text{Ca}^{2+} + 3(\text{HPO}_4)^{2-} + \text{H}_2\text{O}$

Toothpaste is alkaline and helps neutralise acids in the mouth while removing food particles that would otherwise decay and produce more acid. Typical toothpaste ingredients include:

• Calcium carbonate (mild abrasive)
• Aluminium oxides (mild abrasive)
• Magnesium carbonates (mild abrasive)

These compounds remove food particles and neutralise acids. Many toothpastes also contain fluoride, which strengthens and remineralises tooth enamel by replacing the $\text{OH}^-$ ions in hydroxyapatite to form fluorhydroxyapatite ($\text{Ca}_5(\text{PO}_4)_3\text{F}$), which is more resistant to acid attack.

In cooking

The reaction between an acid and a carbonate or hydrogen carbonate to produce salt, water and carbon dioxide is essential in baking. The carbon dioxide produced forms thousands of tiny bubbles in dough, causing bread, cakes and other baked goods to rise when heated in the oven.

Baking soda is sodium hydrogen carbonate ($\text{NaHCO}_3$), which is basic. When combined with acidic ingredients such as buttermilk, lemon juice or sour cream, it reacts to produce carbon dioxide, causing the dough to rise.

Baking powder contains both an acid and a base in dry form. The base is sodium hydrogen carbonate, and the acid is usually tartaric acid (cream of tartar). When mixed with liquids like water or milk, these components react to produce the carbon dioxide needed for rising.

Neutralisation and industry

Agriculture and soil pH

The use of neutralisation is crucial in agriculture because the ability of plants to absorb nutrients from soil is affected by soil pH. While most plants grow well in neutral soil, some prefer acidic conditions (such as azaleas, blueberries, parsley and potato), while others thrive in alkaline conditions (such as lilac, thyme and leek).

If soil is too acidic:

• Add slaked lime (calcium hydroxide) or limestone (calcium carbonate)
• Use organic sources of calcium carbonate such as eggshells or oyster shells

If soil is too alkaline:

• Add substances that act as acids, including gypsum (calcium sulfate) or powdered sulfur
• Use naturally occurring acidic materials such as pine needles, coffee grounds or fresh manure

Fertiliser production

Fertilisers containing ammonium sulfate ($(\text{NH}_4)_2\text{SO}_4$) or ammonium nitrate ($\text{NH}_4\text{NO}_3$) are manufactured using neutralisation reactions between sulfuric acid or nitric acid and ammonia gas ($\text{NH}_3$).

Textile industry

Many processes in the textile industry occur under alkaline conditions, and neutralisation is used at the end of these steps. For example, fabric made from natural fibres is difficult to dye because the fibres contain natural oils and waxes. These are removed through a process called scouring, which involves:

1. Treating fabric with strong alkali solution of sodium hydroxide (or a mixture of sodium hydroxide and sodium carbonate)
2. Boiling the fabric for 1-2 hours
3. Rinsing the fabric in cold water with acetic acid to neutralise the sodium hydroxide

Wastewater treatment

Much wastewater from industrial processes is either acidic or alkaline. If released untreated into creeks and rivers, it could cause significant environmental damage. Neutralisation reactions are used to treat this wastewater before disposal.

Methods for neutralising acidic wastewater:

• Passing acidic water through a limestone (calcium carbonate) bed
• Mixing acid waste with lime (calcium oxide) slurries
• Adding caustic soda (sodium hydroxide) or soda ash (sodium carbonate)

Common neutralising agents for acidic wastes:

• Quicklime ($\text{CaO}$)
• Slaked lime ($\text{Ca(OH)}_2$)
• Caustic soda ($\text{NaOH}$)
• Soda ash ($\text{Na}_2\text{CO}_3$)
• Calcium and magnesium oxides (used in slurries due to moderate solubility)

For alkaline wastewater:

Wastewater that is too alkaline is commonly neutralised using sulfuric acid, hydrochloric acid or nitric acid. Among these, sulfuric acid is most widely used.

Managing industrial spills

Industrial spills involving acids and bases must be dealt with quickly due to their corrosive nature. Neutralisation reactions are used for spill management:

• For acid spills: Sodium carbonate or sodium hydrogen carbonate is widely used
• For base spills: Dilute sulfuric acid or hydrochloric acid is used

Hydrangeas and soil pH

Hydrangeas are unique plants because their flower colour changes depending on soil pH. The shrub produces blue flowers when grown in acidic soil and pink or red flowers when grown in neutral to alkaline soils.

Summary