Derive relationship between ΔH and ΔU for a gaseous chemical reaction.(b) Give reason – Ozone is a good bleaching agent.(c) Write any two uses of SO2.

Answer:

(a) Enthalpy is defined as \(H = U + PV\). For a change at constant pressure,

\(\Delta H = \Delta U + P\Delta V\) ... (1)

Let \(n_1\) moles of gaseous reactants occupy volume \(V_1\) and \(n_2\) moles of gaseous products occupy \(V_2\) at the same \(T\) and \(P\). By the ideal gas equation, \(PV_1 = n_1RT\) and \(PV_2 = n_2RT\).

\(P\Delta V = P(V_2 - V_1) = (n_2 - n_1)RT = \Delta n\,RT\)

Substituting in (1): \(\Delta H = \Delta U + \Delta n\,RT\), where \(\Delta n\) = moles of gaseous products – moles of gaseous reactants. (If \(\Delta n = 0\), \(\Delta H = \Delta U\).)

(b) Ozone readily decomposes to give nascent oxygen: O₃ → O₂ + [O]. The nascent oxygen oxidises coloured organic matter to colourless products (coloured matter + [O] → colourless matter). Since it bleaches by oxidation, the bleaching is permanent; hence ozone is a good bleaching agent.

(c) Uses of SO₂ (any two): manufacture of sulphuric acid (contact process); bleaching of delicate articles like wool, silk and straw; as a preservative for fruit juices and jams (antichlor/antiseptic); refining of petroleum and sugar; as a disinfectant and fumigant.