Chemistry 2720 Fall 2000 Assignment 3

Due: Tuesday, Oct. 3, 9:25a.m.

1.

The bombardier beetle defends itself by spraying a quinone solution at its attackers. Quinone is a noxious substance that repels most of its potential predators. The spray is forcibly expelled because the reaction that forms the quinone

$$\begin{array}{ccccccc} \mathrm{C_6H_4(OH)_{2(aq)}} & + & \ma... ...\ \mathrm{hydroquinone} &&&& \mathrm{quinone}\\ \end{array}$$

is highly exothermic and actually causes the solvent (water) to boil.

(a)

Given

$$\begin{array}{ll} \mathrm{C_6H_4(OH)_{2(aq)}} \rightarrow \m... ...\quad\Delta\bar{H}^\circ = -94.6\,\mathrm{kJ/mol} \end{array}$$

and the table of enthalpies of formation in the textbook, calculate the standard enthalpy change for the reaction of hydroquinone with hydrogen peroxide. [5 marks]

(b)

Insects are poikilothermic, i.e. they don't generate heat internally and their body temperature is thus roughly the same as the ambient temperature at any given time. Assuming that the enthalpy change is roughly constant and that the presence of solutes has little effect on the properties of water,¹ what are the minimum concentrations of hydroquinone and of hydrogen peroxide required to bring the solvent to a boil from an ambient temperature of $18^\circ\mathrm{C}$? [10 marks] Hint: How many moles of the reactants does it take to bring one liter of water to a boil?

(c)

Calculate the standard change in internal energy for the decomposition of hydroquinone to hydrogen and quinone. [5 marks]

2.

Calculate the enthalpy of combustion of propane ( C₃H_8(g)) at $500^\circ\mathrm{C}$. [10 marks]

3.

The enthalpy of vaporization of helium at its normal boiling point of 4.22K is 84J/mol. What is the entropy of vaporization of helium? [2 marks]

4.

Calculate the change in entropy of a 50g sample of potassium bromide which is heated from 12 to $53^\circ\mathrm{C}$. [5 marks]

Footnotes

... water,¹

These approximations are somewhat dubious, so the answer obtained will be a bit rough.