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Ch.11 - Liquids and Intermolecular Forces
Chapter 11, Problem 94

Liquid butane (C4H10) is stored in cylinders to be used as a fuel. The normal boiling point of butane is listed as -0.5 °C. (b) Suppose the valve to the tank is opened and a few liters of butane are allowed to escape rapidly. What do you expect would happen to the temperature of the remaining liquid butane in the tank? Explain.

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1
Identify the process occurring when butane escapes: This is an example of adiabatic expansion, where the butane expands rapidly and does work on the surroundings.
Understand the energy changes: In adiabatic processes, there is no heat exchange with the surroundings, so the energy for the work done comes from the internal energy of the butane.
Relate internal energy to temperature: The internal energy of a substance is related to its temperature. As the internal energy decreases due to work being done, the temperature of the remaining liquid butane will also decrease.
Consider the effect of temperature change: As the temperature decreases, the remaining liquid butane will become cooler, potentially dropping below its normal boiling point.
Conclude the expected outcome: The rapid escape of butane causes a decrease in temperature of the remaining liquid, which may lead to further condensation of butane vapor back into liquid form.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Boiling Point and Vaporization

The boiling point of a substance is the temperature at which its vapor pressure equals the external pressure, allowing it to transition from liquid to gas. For butane, the normal boiling point is -0.5 °C, meaning it will vaporize at this temperature under standard atmospheric pressure. When butane escapes from the tank, it vaporizes, absorbing heat from the remaining liquid, which can lower the temperature of the liquid butane.
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Heat of Vaporization

The heat of vaporization is the amount of energy required to convert a unit mass of a liquid into vapor without a change in temperature. For butane, this process requires significant energy, which is drawn from the remaining liquid butane in the tank. As the escaping gas absorbs heat, the temperature of the liquid butane decreases, leading to a cooling effect.
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Thermodynamics and Energy Transfer

Thermodynamics is the study of energy transfer and its effects on matter. When butane vaporizes, it undergoes an endothermic process, meaning it absorbs heat from its surroundings. This energy transfer results in a decrease in the temperature of the remaining liquid butane, illustrating the principles of energy conservation and the relationship between phase changes and temperature.
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Related Practice
Textbook Question

In Table 11.3, we saw that the viscosity of a series of hydrocarbons increased with molecular weight, doubling from the six-carbon molecule to the ten-carbon molecule.

(a) The eight-carbon hydrocarbon, octane, has an isomer, isooctane. Would you predict that isooctane would have a larger or smaller viscosity than octane? Why?

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Open Question
Acetone [(CH3)2CO] is widely used as an industrial solvent. (d) 1-Propanol (CH3CH2CH2OH) has a molecular weight that is very similar to that of acetone, yet acetone boils at 56.5 °C and 1-propanol boils at 97.2 °C. Explain the difference.
Textbook Question

The vapor pressure of ethanol (C2H5OH) at 19 °C is 40.0 torr. A 1.00-g sample of ethanol is placed in a 2.00 L container at 19 °C. If the container is closed and the ethanol is allowed to reach equilibrium with its vapor, how many grams of liquid ethanol remain?

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Textbook Question

Using information in Appendices B and C, calculate the minimum grams of propane, C3H8(g), that must be combusted to provide the energy necessary to convert 5.50 kg of ice at -20 °C to liquid water at 75 °C

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Open Question
The vapor pressure of a volatile liquid can be determined by slowly bubbling a known volume of gas through it at a known temperature and pressure. In an experiment, 5.00 L of N2 gas is passed through 7.2146 g of liquid benzene (C6H6) at 26.0 °C. The liquid remaining after the experiment weighs 5.1493 g. Assuming that the gas becomes saturated with benzene vapor and that the total gas volume and temperature remain constant, what is the vapor pressure of the benzene in torr?
Textbook Question

The relative humidity of air equals the ratio of the par- tial pressure of water in the air to the equilibrium vapor pressure of water at the same temperature times 100%. If the relative humidity of the air is 58% and its temperature is 68 °F, how many molecules of water are present in a room measuring12ft * 10ft * 8ft?

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