Unless otherwise stated, all images in this file have been reproduced from: Blackman, Bottle, Schmid, Mocerino and Wille, Chemistry, 3 rd Edition 2016 (John Wiley & Sons) The University of Sydney Page 1
b.socrative.com room CHEM1002 b.socrative.com CHEM1002 The University of Sydney Page 2
Phase changes and phase diagrams After lecture 26, you should be able to: Complete its worksheet and practice problems Understand how and why atomic properties such as radii and electronegativity vary across the Periodic Table Understand how and why the acid and base character of oxides varies across the Periodic Table Answer Review Problems 4.54 4.57 and 4.75 4.78 in Blackman Lecture 26: Physical States: Gases, Liquids and Solids Phase Changes and Phase Diagrams Supercritical Fluids Blackman, Chapter 7, Sections 7.1 and 7.3 The University of Sydney Page 3
Phase changes: Δ phase-change H As heat is added to a solid, the molecules vibrate more and more, until... The solid melts - an endothermic change as bonds are broken As heat is added to the liquid, the molecules move more and more, until... The liquid boils - an endothermic change as bonds are broken As heat is added to the gas, the molecules move faster and faster The University of Sydney Page 4
Normal melting and boiling points Vapour pressure: pressure exerted by vapour in equilibrium with its solid or liquid at stated temperature. Normal melting point: melting point when pressure equals 1 atm (101.3 kpa). Normal boiling point: temperature where vapour pressure of liquid equals 1 atm. The University of Sydney Page 5
Phase diagrams Give temperatures and pressures where the different phases are stable. In a one-component system, there 3 phases (solid, liquid, gas): 3 two-phase equilibria liquid-gas, solid-liquid, solid-gas 1 three-phase equilibrium all three phases co-exist: triple point The University of Sydney Page 6
Construction of a phase diagram (I) Plot vapour pressure of liquid as function of T unique pressure where both phases exist at given T co-existence curve of liquid and gas): vapour pressure = external pressure Pressure liquid gas Temperature The University of Sydney Page 7
Construction of a phase diagram (II) Plot melting point of solid as function of pressure co-existence curve of liquid & solid): Pressure solid liquid Temperature The University of Sydney Page 8
Construction of a phase diagram (III) Plot vapour pressure between solid and gas unique pressure where both phases exist at given T co-existence curve of solid & gas): Pressure solid gas Temperature Putting these all together gives phases at different P, T The University of Sydney Page 9
Construction of a phase diagram: putting it all together The University of Sydney Page 10
Phase diagram for CO 2 solid liquid At 298 K and 1 atm: CO 2 is a gas decreasing the temperature, leads to deposition to solid: increasing the pressure leads to condensation to liquid atm sp = 195 K gas 298 K The University of Sydney Page 11
Phase diagram for CO 2 solid liquid On solid /liquid boundary, increasing pressure leads to formation of more solid Solid is more dense than the liquid. gas The University of Sydney Page 12
Phase diagram for water solid liquid 1 atm 273.15 K 373.15 K gas Starting at 1 atm and warming from T < 273 K, ice melts at 273.15 K (normal melting point) water boils at 373.15 K (normal boiling point) The University of Sydney Page 13
Phase diagram for water solid liquid gas Triple point: three phases co-exist only at a single pressure and temperature - the Kelvin scale defined so that 273.16 K is the temperature at which water is at triple point Note: T triple point is slightly higher than T normal melting The University of Sydney Page 14
The unusual properties of water The solid-liquid slope for H 2 O is negative: ice melts if the pressure is increased at fixed temperature The University of Sydney Page 15
Critical temperature and pressure T & P at which boundary between liquid and vapour disappears. If sealed tube of CO 2 liquid and vapour heated, liquid-vapour phase boundary disappears at 31 o C and 73 atm: vapour liquid T > 31 C supercritial fluid no phase boundary: cannot say that it is either liquid or vapour The University of Sydney Page 16
Supercritical fluids Supercritical fluids: substance above critical temperature (T c ). Can behave as solvents dissolving a wide range of substances. New industrial reaction medium. Used in dry-cleaning and food production (e.g. caffeine extraction). The University of Sydney Page 17
Learning outcomes: phase changes and phase diagrams Learning Outcomes - you should now be able to: Complete the worksheet Relate phase diagrams to changes in state Explain the anomalous behaviour of water using its phase diagram Explain behaviour above the critical point Answer the Practice Examples (next slide) and Review Problems 6.28-6.32 and 7.5-7.7 in Blackman The University of Sydney Page 18
Practice examples 1. Which of the following statements concerning the phase diagram below is/are correct? (i) Moving from point A to B results in a phase transition from solid to liquid. (ii) Point D lies at the critical point. (iii) At point C, liquid and gas phases coexist in equilibrium. a) (i) only b) (ii) only c) (iii) only d) (i) and (ii) e) (i) and (iii) Pressure A D B C Temperature The University of Sydney Page 19
Practice examples 2. The gas methane, CH 4, has a critical point at 82 C and 46 atm. Can methane be liquefied at 25 C? Explain your answer. (2008-J-3) 3. Which of the following gases can be liquefied at 25 C? Gas CH 3 Cl SO 2 CH 4 Critical point 144 C, 66 atm 158 C, 78 atm 82 C, 46 atm a) SO 2 only b) CH 4 only c) CH 3 Cl and SO 2 d) all of them e) none of them The University of Sydney Page 20
Practice examples 4. You may recall from a lecture demonstration or your laboratory work that solid CO 2 sublimes under ambient conditions while ice melts. Define the terms sublimation and melting. 5. What is a triple point (e.g. in the phase diagram of CO 2 or H 2 O)? 6. What does the different behaviour of ice and solid CO 2 indicate about the relative positions of their respective triple points? 2007-N-6 The University of Sydney Page 21