Chemistry 2720 Fall 2000 Assignment 4

Due: Tuesday, Oct. 10, 9:25a.m.

1.

Critique the following argument: The entropy change during a phase transition is calculated by $\Delta S = \Delta H/T$. Since $\Delta H < 0$ for the freezing of a liquid, $\Delta S < 0$. According to the second law, processes for which $\Delta S < 0$ can't happen so freezing is never spontaneous. [5 marks]

2.

(a)

Use a qualitative argument to predict the sign of the entropy change associated with the combustion of ethanol at $25^\circ\mathrm{C}$. [3 marks]

(b)

Use the data below to calculate the standard entropy of combustion of ethanol. [5 marks]

Substance	$\bar{S}^\circ$ ( $\mathrm{J}\,\mathrm{K}^{-1}\mathrm{mol}^{-1}$)
C2H5OH(l)	160.67
CO2(g)	213.74
H2O(l)	69.91
O2(g)	205.138

(c)

Suppose that the reaction were carried out at $500^\circ\mathrm{C}$ instead of $25^\circ\mathrm{C}$. Outline the calculations you would have to carry out to obtain the entropy of combustion at this temperature. Clearly list all thermodynamic data you would need. [10 marks]

Note: Ethanol boils at $79^\circ\mathrm{C}$.

3.

Is the combustion of ethanol spontaneous under standard conditions? [4 marks]