First slide

Enthalpy of Phase transition

Question

Δ H of combustion of yellow P and red P are –11K.J and –9.78 K.J respectively ΔH of transition of yellow P to Red P is

Moderate

Solution

$\begin{array}{l} 2{P_{(yellow)}} + \frac{5}{2}{O_{{2_{(g)}}}} \to {P_2}{O_{{5_{(s)}}}}\,\,\,\Delta {H_1} = - 11KJ\\\\ 2{P_{(red)}} + \frac{5}{2}{O_{{2_{(g)}}}} \to {P_2}{O_{{5_{(s)}}}}\,\,\,\Delta {H_2} = - 9.78KJ \end{array}$
We require $∆ H$

for
$2{P_{(yellow)}} \to 2{P_{(red)}}\,\Delta H = ?$
Substract eq.(2) from eq.(1) ; i.e eq.(1)+1/eq.(2)
$\begin{array}{l} 2{P_{(yellow)}} + \frac{5}{2}{O_{{2_{(g)}}}} \to {P_2}{O_{{5_{(s)}}}}\,\,\,\Delta {H_1} = - 11KJ\\ \underline {{P_2}{O_{{5_{(s)}}}}\, \to 2{P_{(red)}} + \frac{5}{2}{O_{{2_{(g)}}}}\,\,\Delta {H_2} = + 9.78KJ} \\ 2{P_{(yellow)}} \to 2{P_{(red)}}\,\,\Delta H = - 11 + 9.78\,\,\,\,\,\,\,\\ \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\Delta H = - 1.22KJ \end{array}$

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