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. (a) suppose the tank is standing in the sun and reaches a temperature of 35 C. Would you expect the pressure in the tank to be greater or less than atmospheric pressure? How does the pressure within the tank depend on how much liquid butane is in it? (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.
b pressure depends on temperature, not amount of liquid level.
c. Pressure depends on temperature, not the amount of liquid left. If the energy in the gas is released from the inner tank contents, the remaining will be cooler.
(a) When the tank is standing in the sun and reaches a temperature of 35°C, we would expect the pressure in the tank to be greater than atmospheric pressure. This is because the temperature increase causes the butane liquid inside the tank to evaporate, forming a gas. The increase in temperature leads to an increase in the average kinetic energy of the butane molecules, resulting in more frequent and energetic collisions with the container walls, hence causing an increase in pressure.
The pressure within the tank also depends on how much liquid butane is in it. As long as there is some liquid butane remaining, the pressure will remain constant as long as the temperature remains constant. This is because the vapor pressure of butane is determined by its temperature and increases as the temperature increases. However, once all the liquid butane has evaporated, the pressure in the tank will start to decrease.
(b) When the valve to the tank is opened and a few liters of butane are allowed to escape rapidly, we would expect the temperature of the remaining liquid butane in the tank to decrease. This is due to the process of evaporation, which is an endothermic process.
When the butane rapidly leaves the tank, the liquid butane molecules with the highest kinetic energy will preferentially evaporate, leaving behind molecules with lower kinetic energy. As a result, the average kinetic energy and thus the temperature of the remaining liquid butane decreases.
This phenomenon is often referred to as evaporative cooling, where the evaporation of a liquid leads to a decrease in temperature.
(a) When the tank is standing in the sun and reaching a temperature of 35°C, we can expect the pressure in the tank to be greater than atmospheric pressure. This is because the increase in temperature causes the molecules of butane to gain more kinetic energy, leading to increased collisions with the walls of the tank. These increased collisions result in an overall increase in pressure within the tank.
The pressure within the tank also depends on how much liquid butane is present. As more liquid butane is added to the tank, the number of gas molecules above the liquid increases. This increases the frequency of collisions with the walls of the tank and thus increases the pressure. Conversely, if some liquid butane evaporates or is removed from the tank, the number of gas molecules above the liquid decreases, resulting in a lower frequency of collisions and a decrease in pressure.
(b) When a few liters of butane are rapidly allowed to escape through the open valve, the temperature of the remaining liquid butane in the tank is expected to decrease. This is due to the process of evaporation, which is an endothermic process. When butane molecules escape from the liquid phase into the gas phase, they take energy from the remaining liquid butane, causing its temperature to decrease.
As the more energetic molecules escape, the average kinetic energy and temperature of the remaining liquid butane decreases. This phenomenon is often referred to as evaporative cooling. It can be observed in daily life when you feel a cooling sensation when perspiration evaporates from your skin.
Therefore, the rapid escape of a few liters of butane would lead to a decrease in temperature of the remaining liquid in the tank.