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Chapter 6: Problem 86

(a) For most efficient use, refrigerator freezer compartments should be fully packed with food. What is the thermochemical basis for this recommendation? (b) Starting at the same temperature, tea and coffee remain hot longer in a thermal flask than chicken noodle soup. Explain.

Short Answer

Expert verified
(a) Packing a freezer compartment full is more efficient because solids and liquids conduct heat less efficiently than air, thereby reducing the rate of heat transfer and energy consumption. \n(b) Tea and coffee stay hot longer than soup in a thermal flask because they lose heat primarily through conduction and radiation whereas soup, having mixed particles of different densities, experiences more significant heat loss through convection, causing it to cool faster.

Step by step solution

01

Understand the Thermochemical Basis for Packing Freezer Compartments Fully

Packing the freezer full of food is more efficient because food items, being solids and liquids, conduct heat less efficiently than air. When the compartment is fully packed, there is less air circulating, reducing the rate of heat transfer and thus, the freezer doesn't have to work as hard to remove heat. This reduces energy consumption, thus making it more efficient.
02

Understanding Why Beverages Stay Hot Longer than Soup in a Flask

The main principle here is the difference in heat transfer methods. In the case of tea and coffee, the heat loss is primarily by conduction and minorly by radiation. However, in the case of chicken noodle soup, there is an additional presence of convection currents due to mixed particles of different densities causing the soup to cool faster. In liquids, particles closer to the surface gain more energy and form convection currents. In more homogenous liquids like coffee or tea, these currents are less due to the uniform density, thus making them cool slower compared to chicken noodle soup.

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Most popular questions from this chapter

Describe the interconversions of forms of energy occurring in these processes: (a) You throw a softball up into the air and catch it. (b) You switch on a flashlight. (c) You ride the ski lift to the top of the hill and then ski down. (d) You strike a match and let it burn down.

Consider this reaction: $$ \begin{aligned} 2 \mathrm{CH}_{3} \mathrm{OH}(l)+3 \mathrm{O}_{2}(g) \longrightarrow & 4 \mathrm{H}_{2} \mathrm{O}(l)+2 \mathrm{CO}_{2}(g) \\ \Delta H &=-1452.8 \mathrm{~kJ} / \mathrm{mol} \end{aligned} $$ What is the value of \(\Delta H\) if (a) the equation is multiplied throughout by \(2,\) (b) the direction of the reaction is reversed so that the products become the reactants and vice versa, (c) water vapor instead of liquid water is formed as the product?

If energy is conserved, how can there be an energy cricis?

State Hess's law. Explain, with one example, the usefulness of Hess's law in thermochemistry.

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