Chem 1102 Semester 2, 2010 PHYSICAL STATES AND PHASE DIAGRAMS

Transcription

1 Chem 1102 Semester 2, 2010 PHYSICAL STATES AND PHASE DIAGRAMS

2 Physical States and Phase Diagrams Lecture 26: Bonding forces Non-bonding forces Allotropes Lecture 27: Gases, Liquids and Solids Enthalpy changes Phase Diagrams Supercritical Fluids 2 Component Systems Alloys

3 Gases, Liquids and Solids 1. Assumes both the volume and shape of its container. 2. It is compressible 3. Flows readily 4. Diffusion within a gas occurs rapidly Assumes the shape of a portion of the container it occupies 2. Does not expand to fill its container 3. Is virtually incompressible 4. Flows readily 5. Diffusion within a liquid occurs slowly 1. Retains its own shape and volume 2. Is virtually incompressible 3. Does not flow 4. State of a substance depends on the interplay of kinetic energies of its particles versus the intermolecular forces between them.

4 Micro (μικρός, small) to macro (μaκρός, large) level NaCl Electrostatic forcesionic solid Cane Sugar (Sucrose) H-bonding-molecular solid Quartz (SiO 2 ) Covalent bondsnetwork solid (2 elements) Diamond Covalent bondsnetwork solid (1 element)

5 Water H-bonding-molecular solid Naphthalene (C10H8) London dispersion forces-molecular solid Benzoic acid Pyrite (FeS2), Beryl (Be3Al2(SiO3)6) London dispersion forces & h-bondingmolecular solid Covalent solid-network solid (2 or more elements)

6 ΔH values the heat required for changes of phase ΔH (kj/mol) H 2 O(l) H 2 O(g) ΔH 0 vap = 40.7 Heat supplied (endothermic) H 2 O(s) H 2 O(l) ΔH 0 fus = 6.02 Condensation Vaporisation Freezing Fusion (exothermic) H 2 O(g) H 2 O(l) ΔH= -ΔH0 vap = H 2 O(l) H 2 O(s) ΔH= -ΔH 0 fus = -6.02

7 ΔH Values for some Common Substances Recall Intermolecular Forces: Stronger forces higher melting points and boiling points. Similarly, the stronger the intermolecular force, the higher the ΔH values.

8 B.p. as a function of P Vapour pressure -Any collection of molecules has a distribution of kinetic energies. -At constant P, two factors affect escape of molecules from liquid to gas phase: strength of intermolecular forces and T.

9 Normal Melting & Boiling Points Normal melting point: melting point when pressure = 1 atm (101.3 kpa). Normal boiling point: temperature where vapour pressure of liquid = 1 atm.

10 Phase Diagrams In a one-component system (e.g. CO 2 ): 3 phases: solid, liquid, gas. 3 two-phase equilibria: liquid-gas, solidliquid, solid-gas (boundary lines). 1 three-phase equilibrium: all three phases co-exist: triple point

11 Construction of a Phase Diagram (Step 1) For one-component systems: Plot vapour pressure of solid as a function of temperature (T). Line represents unique pressure where both phases exist in a state of dynamic equilibrium at given T, i.e. co-existence of solid and gas phases. Pressure solid gas Temperature

12 Construction of a Phase Diagram (Step 2) Plot vapour pressure of liquid as function of T. Line represents unique pressure where both phases exist in a state of dynamic equilibrium at given T, i.e. co-existence of gas and liquid phases. Pressure liquid gas Temperature

13 Construction of a Phase Diagram (Step 3) Plot melting point of solid as function of pressure. Line represents unique pressure where both phases exist in a state of dynamic equilibrium at given T, i.e. co-existence of solid and liquid phases. Pressure solid liquid Temperature

14 Phase Diagram From the collection and collation of these data at different Ps and Ts it is possible to produce a phase diagram. Phase diagram is unique for a given substance. More complex diagram for systems made from >1 component. Pressure solid liquid gas Temperature

15 Phase Diagram Critical point. The critical T of a substance is the T above which that substance can no longer exist as a liquid, no matter how much the P is increased. Pressure liquid solid gas Temperature supercritical fluid In the same way, the critical P is the P above which the substance can no longer exist as a gas, no matter how high the T is.

16 Phase Diagram for CO 2 At 0 C, 1 atm (of CO 2 only): only gas present. from slope of liquid/solid interface: increase P, more solid formed. Also sublimes at atm P and room T I 2

17 Phase Diagram for H 2 O Triple point: three phases coexist only at single P and T. The Kelvin scale is defined by K = temperature at which water is at triple point. The solid-liquid slope for H 2 O is in the opposite direction to CO 2. Why?

18 increase P, ice liquefies solid liquid P start here (ice) T

19 Phase Diagram for N2

20 Phase Diagram for He Triple point Liquid -He II displays the highest thermal conductivity of all known substances -lowest viscosity of all known fluids being about three orders of magnitude smaller than that of air -Many technical applications: e.g. cooling of superconducting magnets of MRI scanners, space-based IR telescopes, 5.2K, 2.2bar Liquid Triple point 2.2K, 0.05bar -No triple point between S, L and G phases

21 Phase Transitions Equilibria between solid, liquid & gas: +ve -ve ΔH fusion ΔS fusion melting/freezing liquid ΔH vaporisation ΔS vaporisation -ve +ve solid vapourisation/ condensation -ve +ve ΔH sublimation ΔS sublimation sublimation/ deposition gas

22 Some Properties of SCFs T & P at which boundary between liquid & vapour disappears. vapour 31 C 73atm supercritical fluid E.g. CO 2 liquid

23 Supercritical Fluids Can behave as powerful solvents dissolving a wide range of substances: also a new industrial reaction medium. Environmentally friendly: CO 2 is a harmless solvent. Chemically inert, non-flammable, non-explosive Easily removed (open valve to release pressure). Widely used in industrial separation processes: e.g. removal of caffeine from coffee beans by supercritical CO 2 : removes caffeine from beans but leaves flavour and aroma components. Also used in dry-cleaning & as solvent for chemical industry.

24 Two Component Mixtures: Alloys Alloys are solid mixtures made up from 2 or more metals. Alloys usually have different properties from those of their component elements.

25 Two Component Mixtures: Alloys Cu 3 Au β-brass

26 Two Component Mixtures: Alloys 2 phases 1 phase Phase diagram typical for a mixture of two elements or compounds completely miscible in both the solid and liquid states. -Characteristics of components: Hume Rothery Rules (A and B need similar atomic volumes, small Δχ, similar crystal structures). 1 phase -Liquid-solid equilibria are strongly dependant on T and composition (very weakly on P). -T A = m.p. of pure A; T B = m.p. of pure B. -Solidus line: Pure A on the left to pure B on the right. -Liquidus line: -e.g. Cu-Ni mixture, NiO-MgO mixture.

27 Two Component Mixtures: Alloys 1 phase Phase diagram typical for a mixture of two elements or compounds miscible in the liquid state but completely immiscible in the solid state. 2 phases 1 phase A small amount of compound B in a sample of compound A lowers its m.p. -Liquid and two solid phases exist in equilibrium at the eutectic composition and the eutectic temperature. Also the m.p. of the eutectic alloy is lower than that of the components. e.g. if A is cinnamic acid (m.p. 137ºC) and B is benzoic acid (m.p. 122ºC), the eutectic T is 82ºC at the eutectic c. e.g. NaCl and water: eutectic point at 21.2 o C, c: 23.3% salt by mass. -ced: isothermal line. Below ced T, the mixture is entirely solid, consisting of a conglomerate of solid A and solid B. Above ced T, the mixture is either a liquid or a liquid-solid mixture, the composition of which varies.

28 Compositions of Common Alloys

29 Two Component Mixtures: Chocolate/Vanilla How different compositions and Ts can change the final product. The eutectic line indicates the temperature where the liquid transforms into two types of solids, like chocolate ripple. The solvus line indicates the limit for how much vanilla can be dissolved into the chocolate as a function of temperature.