Chemistry 2720 Fall 2000 Test 1

Formulas and data can be found at the end of this paper.

Write your answers only in the booklet provided. Use extra pages at the back of the booklet for scratch work.

1.

As the temperature of a kernel of popcorn increases, two things happen:

• The starch and protein which make up most of the kernel cook.
• The water heats up, eventually exceeding its boiling point.

When the water in the kernel vaporizes, it causes a tremendous increase in pressure, which literally causes the kernel to explode. The cooked starch and protein dehydrate rapidly, making the popped corn crispy and fluffy.¹

(a)

A typical kernel of popcorn has a moisture content of approximately 14% (by mass). How much heat is required to completely vaporize the moisture in a 15g serving of popcorn starting from $20^\circ\mathrm{C}$? Assume that the water vaporizes at its normal boiling point and that the process occurs at constant pressure.² [8 marks]

(b)

Suppose that a kernel of popcorn can hold a pressure of 1.5bar before breaking.³ What temperature would the water inside reach before boiling? Neglect the fact that the water may contain some solutes. [10 marks]

(c)

Unlike most reactions involving solids, popcorn expands spectacularly when it explodes. Suppose that you pop 20mL (initial volume) of a variety that expands by a factor of 40 when popped.⁴ How much expansion work does the popcorn do if the atmospheric pressure is 0.9bar (typical for Lethbridge)? [6 marks]

2.

The overall reaction in an alkaline battery is

$$\mathrm{Zn_{(s)}} + 2\mathrm{MnO_{2(s)}} \rightarrow \mathrm{ZnO_\mathrm{(s)}} + \mathrm{Mn_2O_{3(s)}}.$$

(a)

Calculate the maximum electrical work which an alkaline battery can perform per kilogram of zinc (molar mass 65.38g/mol) at $25^\circ\mathrm{C}$. [7 marks]

(b)

If an alkaline battery initially contains 10g of zinc and 15g of manganese (IV) oxide (molar mass 86.936g/mol), what is the maximum total electrical work which can be produced at $25^\circ\mathrm{C}$? [8 marks]

3.

The solubility of thallium (IV) fluoride ( TlF₄) in water at $25^\circ\mathrm{C}$ is $4.5\times 10^{-4}\,\mathrm{mol/L}$.

(a)

What is the standard free energy change for the dissociation reaction? [8 marks]

(b)

Suppose that you wanted to use the above result to determine the standard free energy of formation of thallium (IV) fluoride. How would you proceed? What additional data would you need? [5 marks]

Formulas and Data

To convert degrees Celsius to Kelvin, add 273.15.

The specific heat capacity of liquid water is $4.184\,\mathrm{J\,K^{-1}g^{-1}}$and its enthalpy of vaporization at $100^\circ\mathrm{C}$ is 2257J/g or 40.66kJ/mol.

For liquids and solids, a=X. For solutes, $a=c/c^\circ$, where $c^\circ = 1\,\mathrm{mol/L}$ in the usual definition of the standard state. For gases, $a=P/P^\circ$, where $P^\circ=1\,\mathrm{bar}$.

$$\begin{array}{l@{\hspace{2cm}}l} R = 8.314\,510\,\mathrm{J\,K... ...\circ}{R} \left(\frac{1}{T_2}-\frac{1}{T_1}\right) \end{array}$$

Standard thermodynamic data

in the chemists' standard state at a standard temperature of $25^\circ\mathrm{C}$ and a standard pressure of 1bar.

Species	$\Delta\bar{H}^\circ_f$	$\Delta\bar{G}^\circ_f$	$\bar{C}_p$
	(kJ/mol)	(kJ/mol)	(JK-1mol-1)
MnO2(s)	-520.0	-465.2	54.1
Mn2O3(s)	$-959\phantom{.0}$	$-881\phantom{.0}$	107.7
ZnO(s)	-350.5	-320.5	40.3

Footnotes

... fluffy.¹

H. McGee, On Food and Cooking, Collier: New York, 1984, p 242. Incidentally, this book makes for excellent pleasure reading.

... pressure.²

Most of the water probably vaporizes after the kernel breaks so that this is a sensible approximation.

... breaking.³

This is a pure guess. Relatively little is known about the chemistry and physics of popcorn.

... popped.⁴

This expansion factor is at the low end of typical values. Source: www.popcorn.org. (I'm not making this up.)