Neutralisation reactions are foundational to the study of acid-base chemistry. These reactions exemplify the harmony of chemical interactions, providing profound insights into molecular behaviours, energy changes, and the natural quest for balance.
Formulating Equations for Neutralisation Reactions
Neutralisation reactions are characterised by the interaction of an acid with a base to produce water and salt. The quintessential formula for this kind of interaction is:
Acid + Base → Salt + Water
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Detailed Examples:
- Hydrochloric Acid and Sodium Hydroxide: This is a classic example of a strong acid reacting with a strong base. HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
- Sulphuric Acid and Potassium Hydroxide: A diprotic acid reacting with a strong base requires two moles of base for complete neutralisation. H2SO4(aq) + 2KOH(aq) → K2SO4(aq) + 2H2O(l)
- Nitric Acid and Ammonia: A strong acid reacting with a weak base results in the formation of an ammonium salt. HNO3(aq) + NH3(aq) → NH4NO3(aq)
Identifying the Parent Acid and Base from Salts
Every salt holds the secrets of its parent acid and base. Knowing how to decode these origins is crucial.
Detailed Deduction:
- The cation (positive ion) in the salt: This is usually a metal ion and is derived from the base. This is because most bases contain a metal hydroxide component.
- The anion (negative ion) in the salt: This is often a non-metal ion and is directly from the acid.
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Comprehensive Examples:
- For the salt NaCl:
- Parent Base: Sodium Hydroxide (NaOH) provides the sodium ion.
- Parent Acid: Hydrochloric Acid (HCl) donates the chloride ion.
- For the salt K2SO4:
- Parent Base: Potassium Hydroxide (KOH) contributes the potassium ions.
- Parent Acid: Sulphuric Acid (H2SO4) is the source of the sulphate ions.
- For the salt NH4NO3:
- Parent Base: Ammonia (NH3) reacts to form the ammonium ions.
- Parent Acid: Nitric Acid (HNO3) provides the nitrate ions.
Methods for Separating Salts Formed in Neutralisation Reactions
Separating the salt post-neutralisation is essential for many industrial and laboratory processes.
Comprehensive Methods:
- Evaporation: This is a straightforward technique. By gently heating the solution until all the water has evaporated, pure salt crystals are left behind. This method is suitable for salts that don't decompose at higher temperatures.
- Crystallisation: If a solution is saturated, allowing it to cool slowly can lead to the formation of large, pure salt crystals. This is often preferred when the salt can decompose upon heating.
- Filtration: In instances where insoluble impurities are present post-reaction, the solution can be filtered to obtain a cleaner salt solution. This purified solution can subsequently be evaporated or crystallised to derive solid salt.
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Exothermic Nature of Neutralisation Reactions
It's fascinating to note that neutralisation reactions are generally exothermic, which means they give off energy, usually as heat.
Understanding the Exothermic Nature in Terms of Bond Enthalpies:
- Breaking Bonds: Every bond has an associated energy. To break it, that exact amount of energy, known as bond enthalpy, must be supplied. This makes bond-breaking an endothermic process.
- Making Bonds: Conversely, when new bonds form, energy is released. The amount of energy released is equivalent to the bond enthalpy of the new bond. This is exothermic in nature.
Here's the crux: the energy released during the formation of bonds in the products (water and salt) is typically more than the energy consumed to break the bonds in the reacting acid and base. Hence, the overall reaction is exothermic.
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For instance:
In the neutralisation between hydrochloric acid and sodium hydroxide: HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
The breaking of H-Cl and Na-OH bonds requires energy. However, the formation of the O-H bonds in water and the ionic bond in NaCl releases more energy than what's absorbed, leading to a net release of heat.
With these notes, students are provided with a thorough understanding of neutralisation reactions. This intricate dance of acids and bases underlines the quest for equilibrium, highlighting the meticulous balance observed in nature.
FAQ
While the term "salt" might imply neutrality, salts formed in neutralisation reactions can be acidic, basic, or neutral, depending on the acid and base from which they derive. The pH of the resulting solution will depend on the relative strengths of the parent acid and base. If a strong acid reacts with a weak base, the resulting salt solution will be acidic. Conversely, if a weak acid reacts with a strong base, the solution will be basic. For instance, the reaction between hydrochloric acid (a strong acid) and ammonia (a weak base) produces ammonium chloride, which is slightly acidic in solution. The dissociation or hydrolysis of certain ions from the salt in water is the cause of this behaviour.
While neutralisation reactions generally result in the production of benign salts and water, they can also be accompanied by significant heat release, especially with strong acids and bases. This can lead to boiling or splattering of the solution, posing a burn risk. Additionally, if incorrect or excessive amounts of acids or bases are mixed, the resulting solution might remain strongly acidic or alkaline, presenting potential harm. It's essential to handle these substances with care, using appropriate safety measures, like wearing protective eyewear and gloves, and conducting reactions in a well-ventilated area or under a fume hood.
Temperature can influence the rate of a neutralisation reaction, the extent of the reaction, and the amount of heat released. As with many chemical reactions, increasing the temperature typically speeds up the rate of neutralisation because the reacting particles possess more kinetic energy and collide more frequently and with greater energy. Moreover, the heat released during an exothermic neutralisation reaction can itself raise the temperature of the reaction mixture, further increasing the rate. However, in some specific reactions, particularly those involving weak acids or bases, temperature can also influence the position of equilibrium and thus the extent of the reaction.
Neutralisation is a fundamental process in wastewater treatment. Many industrial processes produce acidic or alkaline wastewater, which can be harmful to the environment and aquatic life. Before discharging or further treating such wastewater, it's essential to bring its pH closer to neutral. This is done by adding acids to alkaline wastewaters or bases to acidic wastewaters. Lime (calcium carbonate) and caustic soda (sodium hydroxide) are common neutralising agents. Once the pH is adjusted, the treated water can be safely released into the environment or undergo further treatment processes. Neutralisation thus plays a pivotal role in ensuring that industrial effluents do not harm the ecosystem.
Neutralisation reactions occur frequently in our daily lives, even if we don't always notice them. One of the most familiar is the treatment of acid indigestion. Overproduction of stomach acid can cause heartburn and indigestion. Antacid tablets, often made of calcium carbonate or magnesium hydroxide, neutralise excess acid to relieve discomfort. Another example is the treatment of soil in gardening. If soil becomes too acidic, gardeners might add lime (calcium carbonate) to neutralise the excess acidity and make conditions more favourable for plant growth. Similarly, in sting treatments: bee stings are acidic and can be neutralised by using a basic substance like baking soda, while wasp stings are alkaline and can be treated with an acid like vinegar.
Practice Questions
a. Write down the balanced chemical equation for this reaction.
b. Identify the parent acid and base for the resulting salt.
c. Explain, in terms of bond enthalpies, why the neutralisation process is exothermic.
The balanced chemical equation for the neutralisation of sulphuric acid (H2SO4) with sodium hydroxide (NaOH) is: H2SO4(aq) + 2NaOH(aq) → Na2SO4(aq) + 2H2O(l).
The parent acid for the resulting salt, sodium sulphate (Na2SO4), is sulphuric acid (H2SO4). The parent base is sodium hydroxide (NaOH).
Regarding the exothermic nature of the neutralisation process, every bond possesses an associated bond enthalpy, which is the energy required to break it, making bond-breaking endothermic. Conversely, when new bonds are formed, energy is released, an exothermic process. For the above reaction, the energy released from the formation of O-H bonds in water and the ionic bonds in sodium sulphate outweighs the energy consumed to break the bonds in H2SO4 and NaOH. Hence, the overall reaction releases heat, making it exothermic.
a. Write the balanced equation for the neutralisation of nitric acid with ammonia.
b. Identify the name and formula of the salt produced.
c. Suggest a method the student could use to obtain solid salt from the solution and explain the rationale behind the choice.
The balanced chemical equation for the neutralisation of nitric acid (HNO3) with ammonia (NH3) is: HNO3(aq) + NH3(aq) → NH4NO3(aq).
The salt produced from the neutralisation of nitric acid and ammonia is ammonium nitrate with the formula NH4NO3.
To obtain solid ammonium nitrate from the solution, the student can use the crystallisation method. The rationale behind choosing crystallisation over evaporation is that ammonium nitrate can decompose when exposed to high temperatures. By allowing the saturated solution to cool slowly, large and pure ammonium nitrate crystals will form without the risk of decomposition. This method ensures the integrity and purity of the salt.
