For the thermite reaction a measured value of has been reported

CHEMISTRY · Single correct · A Level · Chemistry · Energetics · Energetics

Problem

For the thermite reaction

\displaystyle 2\,\text{Al}(s) + \text{Fe}_2\text{O}_3(s) \rightarrow \text{Al}_2\text{O}_3(s) + 2\,\text{Fe}(s),

a measured value of

\Delta S_{\text{exp}}^\circ = +95.0\,\text{J\,mol}^{-1}\,\text{K}^{-1}

has been reported. Use the two reactions below to calculate

\Delta S_{\text{calc}}^\circ

for the thermite reaction. (i)

\displaystyle \text{Al}_2\text{O}_3(s) \rightarrow 2\,\text{Al}(s) + \tfrac32\,\text{O}_2(g) \qquad \Delta S_i^\circ = +180.5\,\text{J\,mol}^{-1}\,\text{K}^{-1}

(ii)

\displaystyle 2\,\text{Fe}(s) + \tfrac32\,\text{O}_2(g) \rightarrow \text{Fe}_2\text{O}_3(s) \qquad \Delta S_{ii}^\circ = -151.2\,\text{J\,mol}^{-1}\,\text{K}^{-1}

Which of the following is the correct value of

\Delta S_{\text{calc}}^\circ

? (a)

+29.3\,\text{J\,mol}^{-1}\,\text{K}^{-1}

(b)

+331.7\,\text{J\,mol}^{-1}\,\text{K}^{-1}

(c)

-29.3\,\text{J\,mol}^{-1}\,\text{K}^{-1}

(d)

-209.8\,\text{J\,mol}^{-1}\,\text{K}^{-1}