At standard temperature and pressure, bromine (Brâ‚‚) is a red liquid. Bromine sublimes when the temperature is -25 °C and the pressure is 101.3 kPa. The phase diagram for bromine is shown below. Use this diagram for questions (1) – (9) Pressure (kPa) (1) (2) (3) 100 80 60 40 20 0 T T T T -30 -10 0 20 Temperature (°C) 40 60 Label each region of the graph as solid, liquid, or gas. Label the triple point, normal melting point, and normal boiling point on the graph and estimate their values in the spaces below. Normal Melting Point = Normal Boiling Point = Triple Point = Use the letter ‘A’ to label the line that gives the conditions for equilibrium between solid and liquid phases of bromine, the letter “B” to label the curve that gives the conditions for equilibrium between liquid and vapor phases of bromine, and the letter “C” to label the curve that gives the conditions for equilibrium between solid and vapor phases of bromine.
The Correct Answer and Explanation is :
Correct Answer
- Label Regions of the Graph
- Solid Region: Below the solid-liquid equilibrium line at low temperatures and high pressures.
- Liquid Region: Between the solid-liquid equilibrium line and the liquid-vapor equilibrium line at moderate temperatures and pressures.
- Gas Region: Above the solid-vapor or liquid-vapor equilibrium lines at higher temperatures and lower pressures.
- Key Points on the Graph
- Normal Melting Point: Estimated at -7 °C, where the solid and liquid phases are in equilibrium at 101.3 kPa.
- Normal Boiling Point: Estimated at 59 °C, where the liquid and vapor phases are in equilibrium at 101.3 kPa.
- Triple Point: Estimated at -25 °C and 5.0 kPa, where solid, liquid, and vapor phases coexist in equilibrium.
- Label Equilibrium Lines
- Line A: Solid-liquid equilibrium.
- Line B: Liquid-vapor equilibrium.
- Line C: Solid-vapor equilibrium.
Explanation
A phase diagram shows the physical states of a substance as a function of temperature and pressure. Bromine, a diatomic molecule (Br₂), exhibits three phases: solid, liquid, and gas.
- At low temperatures and high pressures, bromine is a solid, as particles have minimal kinetic energy and strong intermolecular forces dominate.
- As temperature increases at moderate pressures, bromine transitions to a liquid, where particles gain kinetic energy but remain in close contact.
- At higher temperatures and lower pressures, bromine becomes a gas as particles overcome intermolecular forces completely.
The normal melting point and boiling point are defined at standard pressure (101.3 kPa). Bromine melts at approximately -7 °C and boils at 59 °C. The triple point is a unique condition where all three phases coexist; for bromine, this occurs around -25 °C and 5.0 kPa.
The equilibrium lines represent conditions where two phases coexist:
- Line A marks solid-liquid equilibrium, indicating melting/freezing points at various pressures.
- Line B represents liquid-vapor equilibrium, describing boiling/condensation points.
- Line C shows solid-vapor equilibrium, relevant for sublimation conditions.