CHM1046: Chapter 10.4 - Phase diagrams

Determine the phase changes that carbon dioxide undergoes as pressure is increased at a constant temperature of 50 °C?

Since 50 °C is above the critical temperature for carbon dioxide (31 °C ), increasing the pressure cannot cause condensation, but instead a transition to the supercritical fluid state occurs at ~7000 kPa.

If a severe storm results in the loss of electricity, it may be necessary to use a clothesline to dry laundry. In many parts of the country in the dead of winter, the clothes will quickly freeze when they are hung on the line. If it does not snow, will they dry anyway? Explain your answer.

Yes, ice will sublime, although it may take it several days. Ice has a small vapor pressure, and some ice molecules form gas and escape from the ice crystals. As time passes, more and more solid converts to gas until eventually the clothes are dry.

Elemental carbon has one gas phase, one liquid phase, and three different solid phases, as shown in the phase diagram: (camera roll pic)

camera roll

From the phase diagram for carbon dioxide in Figure 10.34, determine the state of CO2 at: (a) 20 °C and 1000 kPa

gas

From the phase diagram for carbon dioxide in Figure 10.34, determine the state of CO2 at: -40 °C and 500 kPa

gas

From the phase diagram for carbon dioxide in Figure 10.34, determine the state of CO2 at: 10 °C and 100 kPa

gas

From the phase diagram for carbon dioxide in Figure 10.34, determine the state of CO2 at: 10 °C and 2000 kPa

gas

From the phase diagram for carbon dioxide in Figure 10.34, determine the state of CO2 at: 80 °C and 10 kPa

gas

From the phase diagram for water (Figure 10.31), determine the state of water at:

gas

From the phase diagram for water (Figure 10.31), determine the state of water at:

liquid

From the phase diagram for water (Figure 10.31), determine the state of water at:(a) 35 °C and 85 kPa

liquid

From the phase diagram for carbon dioxide in Figure 10.34, determine the state of CO2 at: -80 °C and 1500 kPa

solid

From the phase diagram for water (Figure 10.31), determine the state of water at:

solid

Determine the phase changes that carbon dioxide undergoes as pressure is increased at a constant temperature of (a) -50 °C

At -50 °C and low pressures, carbon dioxide is gaseous. Increasing the pressure at this temperature will result in condensation (gas-to-liquid phase transition) at ~700 kPa and freezing (liquid-to-solid phase transition) at ~10,000 kPa.

What phase changes will take place when water is subjected to varying pressure at a constant temperature of 0.005 °C? At 40 °C? At -40 °C?

At low pressures and 0.005 °C, the water is a gas. As the pressure increases to 4.6 torr, the water becomes a solid; as the pressure increases still more, it becomes a liquid. At 40 °C, water at low pressure is a vapor; at pressures higher than about 75 torr, it converts into a liquid. At -40 °C, water goes from a gas to a solid as the pressure increases above very low values.

Is it possible to liquefy nitrogen at room temperature (about 25 °C)? Is it possible to liquefy sulfur dioxide at room temperature? Explain your answers.

From Table 10.3, the critical temperature of nitrogen is 126.0 K. Since this value is below room temperature, nitrogen cannot be liquefied at room temperature. The critical temperature is 430.5 K for sulfur dioxide, which is above room temperature. It can be liquefied at room temperature.

Pressure cookers allow food to cook faster because the higher pressure inside the pressure cooker increases the boiling temperature of water. A particular pressure cooker has a safety valve that is set to vent steam if the pressure exceeds 3.4 atm. What is the approximate maximum temperature that can be reached inside this pressure cooker? Explain your reasoning.

From looking at a phase diagram for water, the maximum temperature would be around 150 °C. Increased pressure from the pressure cooker allows water to boil at higher temperatures.

Consider a cylinder containing a mixture of liquid carbon dioxide in equilibrium with gaseous carbon dioxide at an initial pressure of 65 atm and a temperature of 20 °C. Sketch a plot depicting the change in the cylinder pressure with time as gaseous carbon dioxide is released at constant temperature.

The carbon dioxide pressure will remain roughly constant at 65 atm (the equilibrium vapor pressure of CO2 at 20 °C) as long as liquid CO2 remains in the cylinder. The gas released from the cylinder will be replaced by vaporization of the liquid. When all the liquid has vaporized, the tank pressure will drop as the cylinder continues to release gas:

Dry ice, CO2(s), does not melt at atmospheric pressure. It sublimes at a temperature of -78 °C. What is the lowest pressure at which CO2(s) will melt to give CO2(l)? At approximately what temperature will this occur? (See Figure 10.34 for the phase diagram.)

The lowest pressure at which a solid may melt is that corresponding to its triple point. Dry ice, CO2(s), will melt to give CO2(l) at ~500 kPa and -60°C.