Enthalpy of neutralization of strong acid and strong base

Aim

To determine the enthalpy of neutralization of a strong acid (HCl) and a strong base (NaOH) using a pitch calorimeter in a laboratory setting.

Apparatus Required

• Pitch calorimeter with lid and thermometer
• 100 mL measuring cylinder
• Stirrer
• 1.0 M Hydrochloric acid (HCl)
• 1.0 M Sodium hydroxide (NaOH)
• Insulating material (cotton wool or cork)
• Weighing balance
• Thermometer (0-50°C)
• Beakers (250 mL)

Theory

What is Enthalpy of Neutralization?

The enthalpy of neutralization is defined as the heat change when one gram equivalent of an acid is completely neutralized by a base in dilute solution at constant temperature.

For strong acids and strong bases like HCl and NaOH, the neutralization reaction can be represented as:

H⁺(aq) + OH⁻(aq) → H₂O(l)

The theoretical value of enthalpy of neutralization for strong acid-strong base combinations is approximately -57.1 kJ/mol.

Why is ΔH Constant for Strong Acid-Strong Base?

Strong acids and bases ionize completely in aqueous solution:

• HCl → H⁺ + Cl⁻
• NaOH → Na⁺ + OH⁻

The net ionic equation involves only H⁺ and OH⁻ ions, making the enthalpy change constant regardless of the specific strong acid or base used.

Procedure

1. Preparation: Clean and dry the pitch calorimeter thoroughly.

2. Initial Setup:

   • Note the initial temperature of the empty calorimeter
   • Take 50 mL of 1.0 M HCl in the calorimeter
   • Record the initial temperature of HCl (T₁)

3. Base Preparation:

   • Take 50 mL of 1.0 M NaOH in a separate beaker
   • Record its temperature (T₂)

4. Neutralization Process:

   • Quickly add NaOH to the calorimeter containing HCl
   • Stir the mixture continuously with the stirrer
   • Record the maximum temperature reached (T₃)

5. Final Measurements:

   • Calculate the temperature change (ΔT)
   • Note the total volume of the solution

Observation Table

Result

Calculations:

1. Total volume of solution = 50 + 50 = 100 mL
2. Mass of solution = 100 g (assuming density = 1 g/mL)
3. Specific heat capacity (s) = 4.18 J/g°C
4. Heat evolved (q) = m × s × ΔT = ___ J
5. Number of moles of HCl/NaOH = 1.0 M × 0.05 L = 0.05 mol
6. Enthalpy of neutralization (ΔH) = -q/n = ___ kJ/mol

The enthalpy of neutralization of HCl and NaOH is found to be __ kJ/mol.

Expected Result:

The experimental value should be close to the theoretical value of -57.1 kJ/mol.

Precautions

1. Temperature Measurement: Read the thermometer carefully and avoid parallax error.

2. Quick Mixing: Add NaOH to HCl quickly to minimize heat loss to surroundings.

3. Insulation: Use insulating material around the calorimeter to prevent heat exchange.

4. Stirring: Stir the solution continuously but gently to ensure uniform temperature.

5. Safety: Handle HCl and NaOH carefully as they are corrosive substances.

6. Calibration: Ensure the thermometer is properly calibrated before use.

7. Volume Measurement: Use calibrated measuring cylinders for accurate volume measurement.

Viva Questions and Answers

Q1: What is the significance of using strong acid and strong base?

A: Strong acids and bases ionize completely, making the enthalpy change constant and predictable.

Q2: Why is the enthalpy of neutralization always negative?

A: The reaction is exothermic, releasing heat, hence the enthalpy change is negative.

Q3: What would happen if weak acid/weak base is used?

A: The enthalpy of neutralization would be less than -57.1 kJ/mol due to energy required for dissociation.

Q4: Why do we take equal volumes of acid and base?

A: To ensure complete neutralization and accurate stoichiometric calculations.

Q5: What is the role of the stirrer in the experiment?

A: It ensures uniform mixing and uniform temperature distribution throughout the solution.

Q6: How does_heat loss affect the result?

A: Heat loss to surroundings results in lower temperature rise, leading to lower calculated enthalpy value.

Q7: What is calorimetry?

A: Calorimetry is the science of measuring heat changes during chemical reactions.

Q8: Why is the theoretical value -57.1 kJ/mol?

A: This represents the heat evolved when one mole of water is formed from H⁺ and OH⁻ ions.

Conclusion

The pitch calorimeter method provides an effective way to determine the enthalpy of neutralization. The experimental value should closely match the theoretical value of -57.1 kJ/mol for strong acid-strong base neutralization. This experiment validates the principle that neutralization reactions between strong acids and bases have constant enthalpy changes due to complete ionization.