Thermodynamics and Thermochemistry 2 Question 32
32. The enthalpy for the following reactions $\left(\Delta H^{\circ}\right)$ at $25^{\circ} \mathrm{C}$ are given below
$$ \begin{aligned} & \text { (i) } \frac{1}{2} \mathrm{H}{2}(g)+\frac{1}{2} \mathrm{O}{2}(g) \longrightarrow \mathrm{OH}(g) \Delta H^{\circ}=-10.06 \mathrm{kcal} \ & \text { (ii) } \mathrm{H}{2}(g) \longrightarrow 2 \mathrm{H}(g) \quad \Delta H^{\circ}=104.18 \mathrm{kcal} \ & \text { (iii) } \mathrm{O}{2}(g) \longrightarrow 2 \mathrm{O}(g) \quad \Delta H^{\circ}=118.32 \mathrm{kcal} \end{aligned} $$
Calculate the $\mathrm{O}-\mathrm{H}$ bond energy in the hydroxyl radical.
(1981, 2M)
Passage Based Questions
When $100 \mathrm{~mL}$ of $1.0 \mathrm{M} \mathrm{HCl}$ was mixed with $100 \mathrm{~mL}$ of $1.0 \mathrm{M}$ $\mathrm{NaOH}$ in an insulated beaker at constant pressure, a temperature increase of $5.7^{\circ} \mathrm{C}$ was measured for the beaker and its contents (Expt. 1). Because the enthalpy of neutralisation of a strong acid with a strong base is a constant $\left(-57.0 \mathrm{~kJ} \mathrm{~mol}^{-1}\right)$, this experiment could be used to measure the calorimeter constant. In a second experiment (Expt. 2), $100 \mathrm{~mL}$ of $2.0 \mathrm{M}$ acetic acid $\left(K_{a}=2.0 \times 10^{-5}\right)$ was mixed with $100 \mathrm{~mL}$ of $1.0 \mathrm{M} \mathrm{NaOH}$ (under identical conditions to Expt. 1) where a temperature rise of $5.6^{\circ} \mathrm{C}$ was measured.
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Solution:
- $\Delta H^{\circ}=\Sigma \mathrm{BE}$ (reactants) $-\Sigma \mathrm{BE}$ (products)
$\Rightarrow \quad-10.06=\frac{1}{2}(104.18)+\frac{1}{2}(118.32)-\mathrm{BE}(\mathrm{O}-\mathrm{H})$
$\mathrm{BE}(\mathrm{O}-\mathrm{H})=121.31 \mathrm{kcal}$
