Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 109 - General Chemistry - Spring 2015

Lecture Notes 37: 29 April

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Liquids & Solids, cont.

Liquids, cont.

• Freezing
  • heat of fusion/crystallization (note T constant until all of solid melts or all of liquid freezes, depending on direction)

• Heating/cooling curves, below.
• Viscosity
  • due to van der Waals forces and long molecules
  • due to strong H-bonding (ethylene glycol, HOCH₂CH₂OH has a viscosity much like high MW syrup)
• Surface Tension: due to attraction of molecules of liquid for each other (diagram).
  • meniscus
• detergents/surfactants and wetting (water striders, ducks etc.)

Weak bonds range from about 10% as strong as a covalent or ionic bond to <1% as strong. Note the examples in the table below:

Interaction Type	Example	Average Strength, kcal/mol (kJ/mol)	Range**
Charge-dipole	-NH3+ Cl-		1/r2
Dipole-dipole	ClCH3 ClCH3		1/r3
Dipole-induced dipole*	CH4 ClCH3	0.1-0.2 (0.4-4)	1/r6
Dispersion* (induced dipole-induced dipole)	CH4 CH4	0.1-0.2 (0.4-4)	1/r6
Hydrogen bond		3-8 (12-30)
van der Waals repulsion			1/r12

*van der Waals interactions, **from Zubay Biochemistry 3rd. Table 4.3, pg. 89.

The van der Waals bonds are strictly the dipole-induced dipole and dispersion types, but is also often used to refer to other weak bonds other than hydrogen bonds. Notice that the bonds are not only very weak (about 0.1 - 0.3% as strong as a covalent bond), they also do not act at a distance. Essentially they are contact bonds - they sort of act like a weak tape. The corollary is that they increase in importance with increases in molecular size (and thus contact surface).

Thus for hydrocarbons, which are essentially completely non-polar, we see a very low boiling point for methane (CH₄) of - 161°C and a fairly regular increase in boiling point as carbons are added (ethane, C₂H₆ - 88°C; butane, C₄H₁₀ - 0.5°C; hexane, C₆H₁₄ 69°C; octane, C₈H₁₈ 126°C; etc.) until very large molecules such as paraffin (about 100 C's) and polyethylene (>1,000 C's) are essentially non-volatile. Note also though, in these very large molecules the forces holding the substance together have become significant due to the very large contact areas.

Hydrogen bonds are a special case of weak bonds. Note that they are significantly stronger (>100 fold) than the other weak bonds at about 4-10% as strong as a covalent bond. Hydrogen bonds only occur when a hydrogen bound to a small, very electronegative atom is brought close to another small, very electronegative atom. Essentially this means that we only see hydrogen bonds between hydrogens bound to N, O, or F (second Period electronegative elements) and N, O, or F. So we can have O-H O, O-H N, O-H F, N-H O, N-H N etc. hydrogen bonds. This is because hydrogen bonds involve dipole-dipole interactions, but they also have covalent character (about 10% of the sharing we see in true covalent bonds) which requires that the participating atoms be small enough to get close enough to allow such partial sharing. (Grp IVA-VIIA bp plot, text Fig 10.4, p 456, Zumdahl 9th:

H-bonding examples/discussion, examples of F, O, N H-bonding strength, F and O H-bond numbers for individual molecules and in bulk.

Hydrogen bonding accounts for much of the special properties of water, such as its very high boiling point (261°C higher than methane with only a 10% increase in MW), high viscosity, high heat capacity etc. which in turn are due to the strong bonds between the individual molecules so they stick together.

Examples of water excluding non-polar substances to force the formation of biomembranes, separate out oils etc.

Occurs when rate of evaporation = rate of condensation. Must have some liquid (or solid for sublimation) present. (Figure 10.39, p 460)

Recall that:

• Equilibrium vapor pressure depends only on T and substance.
• Boiling occurs when vapor pressure = atmospheric pressure.
• Heat of Vaporization is the energy required to convert a mole of liquid to a gas.

Quantitative variation of vapor pressure with temperature:

Plot (P_vap vs. T; upward curve) Figure 10.40a, p 486, Zumdahl 9th:

public domain image via Wikipedia Creative Commons

 

Plot (lnP_vap vs 1/T; linear with negative slope, T = K) Figure 10.40b, p 486, Zumdahl 9th:

For the linear plot can find the equation (y = ax + b):

where a = the slope = -H_vap/R and R = 8.315 JK^-1mol^-1. So

This expression is known as the Clausius-Clapeyron Equation.We can use this equation to find useful information such as the boiling points of liquids at different elevations (and thus pressures).

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Last modified 29 April 2015