CHM 123 Chapter 10
10.1 Polar covalent Bonds and Dipole moments
Depending on the relative electronegativities of the two atoms sharing electrons, there may be partial transfer of electron density from one atom to the other. When the electronegativities are not equal, electrons are not shared equally and partial ionic charges develop.
Atoms with greater electronegativities will attract more of the shared electron density to themselves, causing a “polarity” to the bond
Nonpolar (covalent bond): a bond in which the electron density is shared “approximately” evenly between the two atoms
Polar (covalent) bond: A bond between 2 nonmetal atoms that have different electronegativities and therefore have unequal sharing of the bonding electron pair. As result, there is a “partial charge separation” occur between the two atoms
- Dipole moment: is a quantitative measure of the degree of charge separation in a molecule. It is also used to describe a polar bond.
- Partial positive charge ( δ+) and partial negative charg (δ-) indicate the direction of the polarity or dipole.
Are the following bonds polar or nonpolar? If dipole exists, show any partial charges
Polar molecules vs. Nonpolar molecules
• A molecule with separate centers of positive and negative charge is a polar molecule.
• The dipole moment (μ) of a molecule is the product of the magnitude of the charge (Q) and the distance (d) that separates the centers of positive and negative charge.
μ = Q x r
• A unit of dipole moment is the debye (D).
• Q = magnitude of the charge ( charge of e- = 1.60 x 10-19 C)
• r = distance the charge
One debye (D) is equal to 3.34 x 10–30 C· m
Example: The bond length in HI is 1.61 x 10-10 m and the measured dipole moment of Hi is 0.44D.
Calculate the expected dipole moment if the charges on the atoms were discrete instead of partial.
What is the percent ionic character of H—I bond?
Factors Affecting Dipole Moments
•Lone-pair electrons on oxygen and nitrogen project out into space away from positively charged nuclei giving rise to a considerable charge separation and contributing to the dipole moment
•Symmetrical structures of molecules cause the individual bond polarities and lone-pair contributions to exactly cancel
· Even though a molecule has polar bonds, it still maybe nonpolar. Bond dipoles are “vector” quantities. Bond dipoles can add to one another to cause molecule polarity or cancel another out to give a nonpolar molecule (depending on the shape of the molecule)
Example: Are the following molecules polar or nonpolar?
*”Hydrocarbon” – molecules containing only the elements C and H are considered to be nonpolar, no matter their shape due to the relative small difference in their electronegativies between C-C and C-H bonds.
If there is more than one type of atom (element) on the outside of the structure, one must consider the differing electronegativities involved. Depending on the structure of the molecule, bond dipoles may still cancel out to give overall nonpolar molecules.
10.2 Type of nonbonding interactions
A. Ion-ion interaction (Na+ Cl-)
B. Hydrogen bonding (OH, NH, NF) (polar interaction)
C. Dipole-dipole (polar interaction)
D. Van der waals (vdw) (nonpolar molecules)
*Ion-ion interaction between ions
- electrostatic interaction between two permanent charges
Na+ Cl-
*Dipole – dipole interaction between polar molecules
*Hydrogen bonding – strongest known dipole, due to having H on N or O or F
*van der waals interaction
Greater surface area, greater van der waals
Higher MW, higher B.P (if all polarity is equal)
Straight chain vs. branches
E.g. Rank the following molecules from highest B.P to lowest B.P then provide your reasons
water ethyl alcohol dimethyl ether propane Argon Lithium bromide
H2O CH3CH2OH CH3OCH3 CH3CH2CH3 Ar LiBr
10.3-10.5 – Phase change, evaporation, vapor pressure, and boiling point
Phase Change (State Change): A change in physical form but not the chemical identity of a substance.
Consider the boling curve below to answer the following questions:
What is the melting point of this substance? ………..
What is the boling point of this substance? ………………..
How do calculate the amount of heat required during phase change?
Heat (Enthalpy) of Fusion (ΔHfusion ): The amount of energy required to overcome enough intermolecular forces to convert a solid to a liquid.
Heat (Enthalpy) of Vaporization (ΔHvap): The amount of energy required to overcome enough intermolecular forces to convert a liquid to a gas.
· Depends on the amount of attractive (intermolecular )forces between molecules
· Stronger the force the higher ΔHvap (or boiling point)
The heating curve of water
Example: How much energy is needed to increase the temperature of 10.4 g of cadmium from 64.0°C to 325.0°C? (the specific heat of cadmium is 0.231 J/g°C)
A particular refrigerator cools by evaporating liquefied dichlorodifluoromethane, CCl2F2. How many kilograms of this liquid must be evaporated to freeze a tray of water at 0.0oC to ice at 0.0oC? The mass of water is 525 g, the heat of fusion of ice is 6.01 kJ/mol, and the heat of vaporization of dichloromethane is 17.4 kJ/mol.
For the solid à liquid phase change in water
ΔG = ΔH – TΔS = 0 at equilibrium
Or T = ΔH/ΔS
Example: The boiling of water is 100oC, and the enthalpy change for the conversion of water to steam is ΔHvap = 40.67 kJ/mol. What is the entropy change for vaporization ΔSvap, in J/(K• mol)
Vapor Pressure of pure liquid: The partial pressure of a gas in equilibrium with liquid at a constant temperature.
At equilibrium: rate (vap) = Rate (cond.)
Intermolecular forces / Enthalpy of vaporization / Boiling point / Vapor PressureLow / Low / Low / High
High / High / High / Low
Clausius-Clapeyron Equation
- this gives us a way of finding the heat of vaporization, the energy that must be supplied to vaporize a mole of molecules in the liquid state.
For water, Hvap=44.0 kJ/mol and its vapor pressure is 1.0 atm at 100.0 °C. What is vapor pressure of water at 25.0 °C and at 125.0 °C?
10.11 – Phase diagram
- summarizes the effect of temperature and pressure on a substance in a closed container. Every point in this diagram represents a possible combination of temperature and pressure for the system. The diagram is divided into three areas, which represent the solid, liquid, and gaseous states of the substance
Normal: Occurs at 1 atm.
Critical Point: A combination of temperature and pressure beyond which a gas cannot be liquefied.
• Critical Temperature: The temperature beyond which a gas cannot be liquefied regardless of the pressure.
• Critical Pressure: The pressure beyond which a liquid cannot be vaporized regardless of the temperature.
Supercritical Fluid: A state of matter beyond the critical point that is neither liquid nor gas.
Triple Point: A point at which three phases coexist in equilibrium.
Refer to the phase diagram below when answering the questions on this worksheet:
1) What is the normal freezing point of this substance? ______
2) What is the normal boiling point of this substance? ______
3) What is the normal freezing point of this substance? ______
4) If I had a quantity of this substance at a pressure of 1.25 atm and a temperature of 3000 C and lowered the pressure to 0.25 atm, what phase transition(s) would occur?
5) At what temperature do the gas and liquid phases become indistinguishable from each other? ______
6) If I had a quantity of this substance at a pressure of 0.75 atm and a temperature of -1000 C, what phase change(s) would occur if I increased the temperature to 6000 C? At what temperature(s) would they occur?
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