Chemistry: Phase Diagrams and Heating Curves
Interpreting phase changes, temperature plateaus, and pressure-temperature graphs
Chemistry: Phase Diagrams and Heating Curves
Interpreting phase changes, temperature plateaus, and pressure-temperature graphs
Chemistry - Grade 9-12
- 1
On a heating curve for a pure substance, the temperature stays constant during melting even though heat is being added. Explain why the temperature does not increase during this part of the curve.
Temperature is related to average kinetic energy, while phase changes involve potential energy.
The temperature stays constant during melting because the added energy is used to overcome intermolecular forces and change the solid into a liquid. The average kinetic energy of the particles does not increase during the phase change. - 2
A substance is heated from 20 °C to 80 °C while remaining a liquid. Its mass is 50.0 g and its specific heat as a liquid is 2.40 J/g°C. How much heat is absorbed?
Use q = mcΔT because the substance stays in the same phase.
The substance absorbs 7,200 J of heat. Using q = mcΔT, q = (50.0 g)(2.40 J/g°C)(80 °C - 20 °C) = 7,200 J. - 3
A phase diagram shows a point where solid, liquid, and gas phases all meet. What is this point called, and what does it represent?
This point is called the triple point. It represents the temperature and pressure at which solid, liquid, and gas phases can coexist in equilibrium. - 4
On a typical phase diagram, what happens to a substance if pressure is increased at constant temperature and the starting point is in the gas region near the gas-liquid boundary?
Imagine moving straight upward on a pressure versus temperature graph.
If pressure is increased at constant temperature near the gas-liquid boundary, the gas can condense into a liquid. Increasing pressure pushes the substance into the liquid region of the phase diagram. - 5
A 25.0 g sample of ice at 0 °C melts completely at 0 °C. The heat of fusion of water is 334 J/g. How much heat is required?
During a phase change, use q = mΔH rather than q = mcΔT.
The heat required is 8,350 J. During melting, q = mΔHfus, so q = (25.0 g)(334 J/g) = 8,350 J. - 6
Identify the phase or phases present during the first flat section of a heating curve for a substance that starts as a solid and is heated until it becomes a gas.
The first flat section shows melting, so both solid and liquid are present. The temperature remains constant while the solid changes into a liquid. - 7
A liquid reaches its normal boiling point at 95 °C. What does the word normal mean in this context?
Normal boiling point refers to a standard pressure condition.
The word normal means the boiling point is measured at a pressure of 1 atmosphere. A normal boiling point is the temperature at which the vapor pressure equals 1 atm. - 8
A phase diagram has pressure on the y-axis and temperature on the x-axis. The boundary between the liquid and gas regions ends at a labeled point. What is this point called, and what happens beyond it?
This point is called the critical point. Beyond it, the substance becomes a supercritical fluid, which has properties of both liquids and gases and cannot be separated into distinct liquid and gas phases by pressure alone. - 9
A 100.0 g sample of water is heated from 25.0 °C to 100.0 °C. The specific heat of liquid water is 4.18 J/g°C. Calculate the heat absorbed before boiling begins.
This calculation only includes warming the liquid, not vaporizing it.
The water absorbs 31,350 J, or 31.35 kJ, before boiling begins. Using q = mcΔT, q = (100.0 g)(4.18 J/g°C)(100.0 °C - 25.0 °C) = 31,350 J. - 10
Explain the difference between evaporation and boiling using particle behavior and where vaporization occurs.
Evaporation occurs at the surface of a liquid and can happen below the boiling point. Boiling occurs throughout the liquid when the vapor pressure equals the external pressure, forming bubbles inside the liquid. - 11
On a phase diagram for water, the solid-liquid boundary slopes to the left. What does this unusual slope mean about the effect of pressure on the melting point of ice?
Water is unusual because its solid phase is less dense than its liquid phase.
The left-sloping solid-liquid boundary means that increasing pressure lowers the melting point of ice. This happens because liquid water is denser than ice, so pressure favors the liquid phase. - 12
A substance is at a temperature and pressure in the solid region of its phase diagram. The pressure is kept constant while temperature is increased until the substance enters the gas region without passing through the liquid region. What phase change occurred?
Look for a path that moves horizontally from the solid region to the gas region.
The phase change was sublimation. Sublimation is the change from a solid directly to a gas without becoming a liquid. - 13
A 40.0 g sample of a substance vaporizes at its boiling point. Its heat of vaporization is 850 J/g. How much heat is absorbed during vaporization?
The sample absorbs 34,000 J of heat. During vaporization, q = mΔHvap, so q = (40.0 g)(850 J/g) = 34,000 J. - 14
Describe what is happening to particle motion and particle spacing during the sloped liquid section of a heating curve.
Sloped sections show temperature changes within one phase.
During the sloped liquid section, the particles move faster as temperature increases. The substance remains a liquid, so particle spacing changes only slightly compared with the large spacing change that occurs during vaporization. - 15
A student says, "Adding heat to a substance always increases its temperature." Use heating curves to explain why this statement is not always correct.
Focus on the flat parts of a heating curve.
The statement is not always correct because during phase changes, added heat does not increase temperature. On the flat sections of a heating curve, energy is used to change phase by overcoming intermolecular forces rather than increasing particle speed.