PARAGRAPH 2
When 100 mL of 1.0 M HCl was mixed with 100 mL of 1.0 M NaOH in an insulated beaker at constant pressure, a temperature increase of 5.7 °C was measured for the beaker and its contents (Expt. 1). Because the enthalpy of neutralization of a strong acid with a strong base is a constant (-57.0 kJ mol⁻¹), this experiment could be used to measure the calorimeter constant. In a second experiment (Expt. 2), 100 mL of 2.0 M acetic acid (Kₐ = 2.0 × 10⁻⁵) was mixed with 100 mL of 1.0 M NaOH (under identical conditions to Expt. 1) where a temperature rise of 5.6 °C was measured.
(Consider heat capacity of all solutions as 4.2 J g⁻¹ K⁻¹ and density of all solutions as 1.0 g mL⁻¹)
Enthalpy of dissociation (in kJ mol⁻¹) of acetic acid obtained from the Expt. 2 is
When 100 mL of 1.0 M HCl was mixed with 100 mL of 1.0 M NaOH in an insulated beaker at constant pressure, a temperature increase of 5.7 °C was measured for the beaker and its contents (Expt. 1). Because the enthalpy of neutralization of a strong acid with a strong base is a constant (-57.0 kJ mol⁻¹), this experiment could be used to measure the calorimeter constant. In a second experiment (Expt. 2), 100 mL of 2.0 M acetic acid (Kₐ = 2.0 × 10⁻⁵) was mixed with 100 mL of 1.0 M NaOH (under identical conditions to Expt. 1) where a temperature rise of 5.6 °C was measured.
(Consider heat capacity of all solutions as 4.2 J g⁻¹ K⁻¹ and density of all solutions as 1.0 g mL⁻¹)
Enthalpy of dissociation (in kJ mol⁻¹) of acetic acid obtained from the Expt. 2 is
- (A)1.0✓ Correct
- (B)10.0
- (C)24.5
- (D)51.4
Answer: (A)
Solution
In the first experiment the strong acid-strong base neutralisation of mol releases kJ. The solution itself absorbs J, so the calorimeter takes up J over a K rise, giving a calorimeter constant of about J K. In the second experiment mol of acetic acid is neutralised, since sodium hydroxide is limiting, and the temperature rises by K, so the heat evolved is J, that is kJ per mole of acid. The measured enthalpy of neutralisation of the weak acid is therefore kJ mol. Because a fully dissociated acid would release kJ mol, the shortfall is the energy consumed in dissociating acetic acid: from we get kJ mol.
JEE Advanced · 2015 · 24 May 2015 · Paper 2 · One or more correct options