NEW site: Part 8 Phase equilibria - vapour pressure, boiling point and intermolecular forces AS-A2-IB-GCE Advanced Physical-Theoretical Level Chemistry Equilibria revision notes at Doc Brown's Chemistry Clinic for AQA, Edexcel, Nuffield, OCR, Salters revising for advanced level chemistry examinations docbequilibria1

KS4 SCIENCE - Additional & Applied UK Chemistry help

AQA GCSE Science - Chemistry

CCEA GCSE Science - Chemistry

Edexcel GCSE 360Science - Chemistry

OCR GCSE 21st Century Science Suite - Chemistry

OCR GCSE Gateway Science Suite - Chemistry

OCR GCSE Applied Science - Chemistry (double award)

WJEC GCSE Science - Chemistry

Doc Brown's Chemistry Clinic

GCE-AS-A2-IB Advanced Level Theoretical-Physical Chemistry revision notes

Equilibria Part 8

"Phase equilibria - vapour pressure, boiling point and intermolecular forces"

GCSE Notes on reversible reactions-equilibrium * Advanced Part 1. Equilibrium, Le Chatelier's Principle-rules * Part 2. K_c and K_p equilibrium expressions and calculations * Part 3. Equilibria and industrial processes * Part 4. Partition, solubility product and ion-exchange * Part 5. pH, weak-strong acid-base theory and calculations * Part 6. Salt hydrolysis, Acid-base titrations-indicators, pH curves and buffers * Part 7. Redox equilibria, half-cell electrode potentials, electrolysis and electrochemical series * Part 8 sub-index: 8.1 Phase equilibria-vapour pressure, boiling point and intermolecular forces (note Partition between two phases is in Equilibria Part 4.1) * The K and ΔS-ΔG connection with E^Ø_cell will be dealt with via new thermodynamics pages later, but an example of a ΔS-ΔG calculation is given at the end of the advanced kinetics pages and ΔG for cells is mentioned in Equilibria Part 7. M = old fashioned shorthand for mol dm^-3 * EMAIL query?comment

8.1 Phase equilibria - vapour pressure

e.g. liquid water water vapour or H₂O_(l) H₂O_(g)
The co-existence of water in two different phases is a clear example.
- If water (or an aqueous solution) is poured into a dry bottle and then stoppered, some of the water will then evaporate to create a vapour pressure, so water vapour and liquid water co-exist.
- However, some of the water vapour may condense if the gas molecules hit the liquid surface.
- When the rate of evaporation = rate of condensation, a dynamic physical equilibrium exists between liquid water and water vapour.
Note, that at a certain temperature and pressure (called the triple point), liquid water, water vapour and ice can all co-exist!
The maximum or saturated vapour pressure that liquid water exerts at a particular temperature can be measured and it increases exponentially with increase in temperature, i.e. the equilibrium is shifted more to the right, see graph below.
The same shaped graph line (examples below) is obtained with any liquid, though the relative position of the curve depends on the volatility of the liquid, which in turn depends on the strength of the inter-molecular attractive forces (see later paragraph).

Typical saturated vapour pressure curves (max. p_vap versus temperature).
The vapour pressure curves for tetrachloromethane CCl₄, ethanol C₂H₅OH, benzene C₆H₆, water H₂O and ethanoic acid CH₃COOH.
The increase in vapour pressure of any liquid with rise in temperature is due to the increasing average kinetic energy of the liquid molecules, so more molecules with sufficient KE can 'escape' the intermolecular attractive forces of the liquid molecules, increasing the rate of evaporation.
When the vapour pressure of a liquid matches the ambient pressure, the liquid boils as bubbles of vapour can now form in the bulk liquid.
- At 100^oC, water vapour pressure = normal atmospheric pressure, and so water boils at normal atmospheric pressure (1 atm/760 mmHg, see dotted lines on diagram above).
- This has consequences for brewing a cup of tea as you climb a high mountain. It gets more and more difficult to brew a good pot of tea because the boiling temperature gets lower and lower as the pressure falls with increasing height.
The greater the intermolecular forces between molecules, the lower the saturated (maximum p_vap) vapour pressure at a given temperature, here given in Pa/mm Hg 20^oC.
You also see, not surprisingly, a corresponding higher enthalpy of vaporisation (ΔH_vap/kJ mol^-1) and higher boiling point (bpt at 101325Pa/1 atm/760 mmHg) as more kinetic energy is needed by the molecules to overcome the intermolecular attractive forces.
e.g. compare five liquids of similar molecular mass and total number of electrons whose physical properties illustrate these trends.
1. ALKANE butane, CH₃CH₂CH₂CH₃, M_r = 58, 34 electrons,
  - bpt -0.5^oC, ΔH_vap = 22 kJ mol^-1, sat'd p_vap = 211316 Pa/1585 mmHg at 20^oC when liquified under pressure.
  - A non-polar molecule where the only intermolecular force operating is the weakest possible, that is the instantaneous dipole - induced dipole intermolecular forces (Van der Waals forces).
  - Van der Waals electrical attractive forces act between ANY molecules and is primarily a function of the number of electrons in the molecule. The force arises from the instantaneous and random asymmetry of the electron fields in the atomic orbitals. A transient d+ in one molecule induces a transient d- in a neighbouring molecule, so causing a very weak and transient attraction.
  - This is why, in doing comparisons, you should choose molecules of similar molecular mass and total electron number to give a 'base-line' of similar effects of Van der Waals forces, from which you can then judge the effects of changing molecular structure e.g. polar bonds increasing the inter-molecular attractive forces between polar molecules.
  - In examples 1 to 5, the molecules exhibit extra intermolecular forces due to polar bonds, and in the description it is assumed you are aware that instantaneous dipole - induced dipole are ever present.
2. HALOGENOALKANE chloroethane, CH₃CH₂Cl, M_r = 64.5, 34 electrons,
  - bpt 12.5^oC, ΔH_vap = 25 kJ mol^-1, sat'd p_vap = 133322 Pa/1000 mmHg at 20^oC when liquified under pressure,
  - Halogenoalkanes have a weakly polar C^d⁺-X^d^- bond (X = halogen) due to the difference in electronegativities (Pauling values) of carbon and halogens, e.g. Cl(3.0) > C(2.5) giving C^d⁺-Cl^d^-.
  - This gives rise to a weak, but permanent dipole, hence the extra permanent dipole - permanent dipole intermolecular attractive forces raising the bpt and ΔH_vap and lowering the vapour pressure compared to butane.
3. KETONE propanone, CH₃COCH₃, M_r = 58, 32 electrons,
  - bpt 56^oC, ΔH_vap = 29 kJ mol^-1, sat'd p_vap = 24131 Pa/181 mmHg at 20^oC,
  - Ketones are permanently polarised molecule due to the highly polar bond ^d⁺C=O^d^- caused by the difference in electronegativities of carbon and oxygen i.e. O(3.5) > C(2.5).
  - This causes the permanent dipole - permanent dipole interaction between neighbouring polar molecules via ....^d⁺C=O^d^-....^d⁺C=O^d-.... The vapour pressure is reduced, and the bpt. and ΔH_vap increased compared to butane because there is a 2nd intermolecular force operating.
  - The effect is also greater than for chloroethane because the electronegativity difference between C and O is greater than C and Cl, giving a more polar molecule, so stronger permanent dipole - permanent dipole intermolecular attractive forces.
4. ALCOHOL propan-1-ol, CH₃CH₂CH₂OH, M_r = 60, 34 electrons,
  - bpt 97^oC, ΔH_vap = 45 kJ mol^-1, sat'd p_vap = 1933 Pa/14.5 mmHg at 20^oC,
  - Alcohols are permanently polarised molecule due to the highly polar bond ^d^-O-H^d⁺ caused by the difference in electronegativities between oxygen and hydrogen i.e. O(3.5) > H(2.1). This causes the extra permanent dipole - permanent dipole interaction between neighbouring polar molecules via hydrogen bonding
    - ....^d^-O-H^d⁺....^d^-O-H^d+....^d^-O-H^d⁺....
  - So called hydrogen bonding is the strongest of the permanent dipole - permanent dipole intermolecular forces, but it is NOT a true ionic or covalent chemical bond.
  - The vapour pressure is reduced, and the bpt. and ΔH_vap considerably increased compared to butane because of this 2nd intermolecular force operating. The effect is also greater than for chloroethane/propanone for two reasons, (i) the electronegativity difference between O and H is greater than for C and Cl or C and O giving a more polar bond, and (ii) the small size of the hydrogen atom of one molecule allows it to get nearer to the oxygen of a neighbouring molecule increasing the strength of this particular intermolecular force.
5. CARBOXYLIC ACID ethanoic acid, CH₃COOH, M_r = 60, 32 electrons,
  - bpt 118^oC, ΔH_vap = 58 kJ mol^-1, sat'd p_vap = 1520 Pa/11.4 mmHg at 20^oC,
  - Of the five compared organic molecules, ethanoic acid has the highest bpt and ΔH_vap and lowest vapour pressure at 20^oC.
  - This is because as well as the Van der Waals forces (instantaneous dipole - induced dipole attraction), there are two sources of permanent dipole - permanent dipole interactions.
    - Permanent dipole - permanent dipole interaction between neighbouring polar molecules via the carbonyl group ....^d⁺C=O^d^-....^d⁺C=O^d-.... (see propanone above for more details),
    - and the even stronger hydrogen bonding via the hydroxy group ....^d^-O-H^d⁺....^d^-O-H^d+.... (see propan-1-ol above for more details).
  - This makes ethanoic acid the most polar of the molecules and exhibiting the strongest intermolecular forces.
6. for extra comparison:
  - water, H₂O, Mr = 18, 10 electrons, bpt 100^oC, ΔH_vap = 41 kJ mol^-1, sat'd p_vap = 2340 Pa/17.5 mmHg at 20^oC,
  - On the basis of molecular mass and electron number (< 1/3rd of examples 1-5), the data shows that the bpt and ΔH_vap of water are much higher, and the vapour pressure much lower than expected, even compared to the polar molecules described above.
  - This is due to water exhibiting the strongest intermolecular force hydrogen-bonding ....^d^-O-H^d⁺....^d^-O-H^d+.... as in alcohols. BUT, the two lone pairs on the oxygen and the two available hydrogens attached to the oxygen mean that water can form twice as many hydrogen bonds per molecule than alcohols. (relative diagram to do)

GENERAL REVISION NOTES

This page is My FREE UNOFFICIAL links section of revision notes for studying revising tutoring teaching Advanced Level GCE AS A2 IB CHEMISTRY courses in unofficial support the Chemistry in any advanced-subsidiary AQA, EDEXCEL, OCR, CIE, WJEC, SQA and CCEA (NI) UK or Cambridge/London/Edexcel International and OCR/CIE International examinations.