Non-ideal Gases Non-ideality naturally follows a consideration of intermolecular forces since these, in part, account for gas non-ideality. The next slide.

Non-ideal Gases

• Non-ideality naturally follows a consideration of intermolecular forces since these, in part, account for gas non-ideality. The next slide reviews the kinetic theory assumptions related to the ideal gas law. Let’s consider where these assumptions might break down.

Visualizing Molecular MotionCopyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 6 Slide 2 of 41

Kinetic Molecular Theory of Gases

• Particles are point masses in constant,

random, straight line motion.

• Particles are separated by great

distances.

• Collisions are rapid and elastic.

• No force between particles.

• Total energy remains constant.

Aside: rms velocities

• Ex. Calculate the root mean square velocity for gaseous CH2F2 molecules at -15 oC. Would the rms velocity of CO2 molecules be larger or smaller at the same temperature?

Non-ideal Gases:

• Treating gaseous molecules as point masses (zero molecular volume) is reasonable for small molecules and dilute gases. As molecular size increases the gas molecules less resemble point masses. As molar volume decreases (high P and low T) the volume occupied by molecules is no longer negligible compared to the overall gas volume.

Non-ideal Gases – cont’d:

• Intermolecular forces vary significantly from molecule to molecule – ranging from the very weak London dispersion forces (in He and CO2) to the very strong hydrogen bonding interactions in molecules such as H2O, NH3 and CH3OH. In some cases, hydrogen bonding can cause molecules to dimerize in the gas phase resulting in highly non-ideal behaviour.

Slide 6 of 61

An acetic acid dimer

• FIGURE 12-9

Copyright © 2011 Pearson Canada Inc.

General Chemistry: Chapter 12

CH3-COOH

Non-ideality and the Physical World• As mentioned (repeatedly?), intermolecular

forces are responsible for molecules forming condensed phases – without which the universe would be a boring place! What intermolecular forces are important in the next couple of slides?

A Beautiful Picture – Thanks to Gas Non-ideality?

“Gotta love that hydrogen bonding!”

Class examples:

• 1. Which of the following gases would you expect to behave more ideally? Why? (a) Ne(g) or CO2(g) (b) CH4(g) or CH2Cl2(g) (c) CH3CH2F(g) or CH3CH2OH(g)

• Note that the molecules which behave least ideally also have the highest melting points and the highest boiling points.

Class examples:

• 2. Which of the following gas mixtures would you expect to behave more ideally? Why? (a) Ar(g) and CH4(g) or (b) CH3OH(g) and CH3NH2(g). Indicate what types of interactions are possible for both mixtures.

Manifestation of Nonideality

• Gas nonideality manifests itself as molar volumes decrease due to external P increase or a T decrease. Initially the ratio PV/nRT is smaller than the ideal value of 1 due to intermolecular forces. At very high pressures (and/or low T’s) the volume occupied by the gas molecules collectively is not negligible compared to the “empty space” in the gas and PV/nRT gets bigger than 1.

Intermolecular forces of attractionFigure 6-22

Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 6 Slide 13 of 41

6-9 Nonideal (Real) Gases

Compressibility factor

PV/nRT = 1 for ideal gas.

Deviations for real gases.PV/nRT > 1 - molecular

volume is significant.

PV/nRT < 1 – intermolecular forces of attraction.

van der Waals Equation


P + n2a V2

V – nb = nRT

The van der Waals equation reproduces the observed behavior of gases with moderate accuracy. It is mostaccurate for gases comprising approximately spherical molecules that have small dipole moments.


Solutes, Solvents and Solutions:

• In Newfoundland and Labrador one can imagine that the following process might have been important at some time.

• NaCl(s) + Water →NaCl(aq) (Na+(aq) + Cl-(aq))

• For this process to occur the ionic bonds in the NaCl(s) lattice must be broken, the solute particles (Na+ and Cl- ions) must be separated, solvent molecules (water) must be separated and, finally, a solution is formed.

Formation of Ionic Solutions


General Chemistry: Chapter 13 Slide 17 of 46

FIGURE 13-6

•An ionic crystal dissolving in water

Enthalpies involved in the solution process:



NaCl(s) → Na+(g) + Cl-(g) ΔHlattice > 0

Na+(g) + xs H2O(l) → Na+(aq) ΔHhydration < 0

Cl-(g) + xs H2O(l) → Cl-(aq) ΔHhydration < 0

ΔHsoln > 0 (~ 5 kcal/mol) but ΔGsolution < 0

Water of Hydration:

• When ions move through water they are commonly surrounded by several water molecules. This is true, in particular, of the smaller and higher charged metal ions. The combination of transition metal ions and water molecules gives us beautiful colours (blue for hydrated Cu2+). Water molecules commonly remain when ionic salts are crystallized – eg. CuSO4∙5H2O.

Solution Process – Energetics:

• The energetics of the solution process are discussed on the next slide. The overall solution process/change can be exothermic or endothermic. It’s easy to rationalize solutions forming where the intermolecular forces in the solute molecules (before mixing with solvent) are similar in type and magnitude to the intermolecular forces between solvent molecules.

Intermolecular Forces and the Solution Process



FIGURE 13-2•Enthalpy diagram for solution formation

Intermolecular Forces in Mixtures



FIGURE 13-3•Intermolecular forces in a solution

ΔHsoln = 0

Magnitude of ΔHa, ΔHb, and ΔHc depend on intermolecular forces.

Ideal solution

Forces are similar between all combinations of components.

Hydrocarbons – CXHY:

• Most hydrocarbon molecules are nonpolar or very weakly polar. Dispersion forces and very weak dipole-dipole forces hold hydrocarbons together in condensed phases. It’s not surprising that different hydrocarbon molecules easily mix to form nearly “ideal” solutions (ΔHSolution approximately zero).

Two components of a nearly ideal solutionFIGURE 13-4



“Like Dissolves Like”:

• It seems reasonable that highly polar molecules – such as CH2Cl2 and CH2Br2 – might happily mix together (form a solution). This is true.

• One might expect that methanol (CH3OH) and methyl amine (CH3NH2) might also form a solution since both molecules can hydrogen bond. This is also true.

More Complex Examples:

• Water also “happily” forms solutions with CH3OH and CH3CH2OH but, more complex alcohols, such as CH3(CH2)6OH are only sparingly soluble in water. Why?

Solubilities of Alcohols and Alkanes

Alcohol Alcohol Solubility (g/L)

Alkane (Hydrocarbon)

Alkane Solubility (g/L)

CH3OH Miscible CH3CH3

CH3CH2OH Miscible CH3CH2CH3

CH3CH2CH2OH Miscible CH3CH2CH2CH3 0.061

CH3(CH2)3OH 73 CH3(CH2)3CH3 0.040

CH3(CH2)4OH 22 CH3(CH2)4CH3 0.0095

CH3(CH2)5OH 5.9

CH3(CH2)6OH

Boiling Points of Alcohols and Alkanes

Alcohol Alcohol Boiling Point

Alkane (Hydrocarbon)

Alkane Boiling Point

Boiling Point Difference

CH3OH 66 oC CH3CH3 -88 oC 154 oC

CH3CH2OH 78 oC CH3CH2CH3 -43 oC 121 oC

CH3CH2CH2OH 97 oC CH3CH2CH2CH3 -1 oC 98 oC

CH3(CH2)3OH 118 oC CH3(CH2)3CH3 36 oC 82oC

CH3(CH2)4OH 137 oC CH3(CH2)4CH3 69 oC 68oC

Copyright 2011 Pearson Canada Inc. 13 - 29

Gas Law Review

• Ex. A sample of O2 gas initially present in a 2.00 L container at a pressure of 2.40 atm and 22 oC is moved to a 5.00 L container at 15 OC. A sample of Ne gas initially held at a pressure of 4.00 atm and -16 OC in a 4.00 L container is added to the oxygen gas in the 5.00 L container (still at 15 OC). What is the total pressure in the 5.00 L container?

Non-ideal Gases Non-ideality naturally follows a consideration of intermolecular forces since these, in part, account for gas non-ideality. The next slide.

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