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Enthalpy Of Neutralisation

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Enthalpy of Neutralisation: The Heat of Reaction in Acid-Base Chemistry



Introduction:

Neutralisation is a fundamental chemical reaction involving the reaction between an acid and a base to form salt and water. This reaction is often accompanied by a significant heat change, either released (exothermic) or absorbed (endothermic). The enthalpy of neutralisation (ΔH<sub>n</sub>) quantifies this heat change, specifically the heat released or absorbed when one mole of water is formed from the reaction of a strong acid and a strong base under standard conditions (usually 298 K and 1 atm). Understanding enthalpy of neutralisation is crucial for comprehending various chemical processes, including industrial applications and biological systems. This article delves into the concept, its measurement, factors affecting it, and its practical applications.


1. The Exothermic Nature of Neutralisation:

The most common neutralisation reactions are exothermic, meaning they release heat into their surroundings. This heat release is due to the formation of strong bonds in the water molecule. When a strong acid (like hydrochloric acid, HCl) reacts with a strong base (like sodium hydroxide, NaOH), the highly energetic H<sup>+</sup> ions from the acid and OH<sup>-</sup> ions from the base combine to form a relatively stable water molecule (H<sub>2</sub>O). This bond formation releases energy, manifesting as an increase in the temperature of the solution. The reaction can be represented as follows:

HCl(aq) + NaOH(aq) → NaCl(aq) + H<sub>2</sub>O(l) ΔH<sub>n</sub> ≈ -57 kJ/mol

The negative sign indicates an exothermic reaction, where 57 kJ of heat are released per mole of water formed.


2. Measuring the Enthalpy of Neutralisation:

The enthalpy of neutralisation is typically determined experimentally using calorimetry. A simple method involves using a polystyrene cup (to minimise heat loss) as a calorimeter. Known volumes of acid and base solutions at a specific concentration are mixed within the cup, and the temperature change (ΔT) is measured using a thermometer. Using the specific heat capacity of the solution (c), the mass of the solution (m), and the temperature change, the heat change (q) can be calculated using the formula:

q = mcΔT

The enthalpy of neutralisation (ΔH<sub>n</sub>) can then be calculated by dividing the heat change (q) by the number of moles of water formed.


3. Factors Affecting the Enthalpy of Neutralisation:

While the enthalpy of neutralisation for strong acid-strong base reactions is relatively constant (around -57 kJ/mol), it varies when weak acids or weak bases are involved. This variation is due to the energy required to ionise the weak acid or base before neutralisation can occur. For example, the neutralisation of a weak acid like ethanoic acid (CH<sub>3</sub>COOH) with a strong base involves an additional energy input to ionise the weak acid, resulting in a smaller overall enthalpy change (less negative) than that of a strong acid-strong base reaction.

The enthalpy of neutralisation also depends on the concentration of the reactants. Higher concentrations lead to a greater heat release for the same amount of reactants. However, the enthalpy change per mole of water formed remains relatively constant provided the reaction goes to completion.


4. Applications of Enthalpy of Neutralisation:

The concept of enthalpy of neutralisation has significant applications in various fields:

Industrial Processes: Many industrial processes involve neutralisation reactions, for example, wastewater treatment. Understanding the heat generated during these reactions is critical for efficient process design and control.
Chemical Engineering: Enthalpy of neutralisation data is used in the design and optimization of chemical reactors and heat exchangers.
Biological Systems: Neutralisation reactions are vital in biological systems, playing a role in maintaining pH balance and metabolic processes. The heat generated or absorbed in these biological neutralisations can impact the overall energy budget of an organism.


5. Limitations and Considerations:

The enthalpy of neutralisation value is an average and can be influenced by factors like heat loss to the surroundings during experimentation. Accurate measurement requires careful experimental design and techniques to minimize heat loss. Furthermore, the enthalpy of neutralisation value assumes complete dissociation of the strong acid and base, which might not always be the case in practice, especially at high concentrations.


Summary:

The enthalpy of neutralisation describes the heat change associated with the formation of one mole of water from the reaction of an acid and a base. While typically exothermic for strong acid-strong base reactions, it varies depending on the strength of the acid and base involved. Accurate measurement through calorimetry allows for its determination, providing essential data for various applications in industrial processes, chemical engineering, and biological systems. Understanding this concept provides crucial insight into the energy changes occurring during fundamental chemical reactions.


FAQs:

1. Why is the enthalpy of neutralisation for a strong acid-strong base reaction approximately constant? Because the limiting step is the formation of water from H<sup>+</sup> and OH<sup>-</sup> ions, which has a relatively consistent energy change. The other ions involved are spectator ions and don't significantly affect the overall enthalpy change.

2. What are the units for enthalpy of neutralisation? The standard unit is kilojoules per mole (kJ/mol).

3. How can heat loss be minimized during the experimental determination of enthalpy of neutralisation? Using an insulated calorimeter (like a polystyrene cup), ensuring rapid mixing of the reactants, and using a lid to minimize heat exchange with the atmosphere.

4. Why is the enthalpy of neutralisation different for weak acids and weak bases compared to strong acids and strong bases? Energy is needed to ionise the weak acid or base before neutralisation can occur, reducing the overall heat released or absorbed in the reaction.

5. Can the enthalpy of neutralisation be positive (endothermic)? Yes, in rare instances involving certain weak acids and bases, the enthalpy of neutralisation can be positive, indicating an endothermic reaction where heat is absorbed.

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