intermolecular forces the physical state of a substance (gas, liquid or solid) can be extremely...
TRANSCRIPT
Intermolecular Forces• The physical state of a substance (gas, liquid
or solid) can be extremely important in many systems. This includes living organisms. Water, for example, is an important solvent that is the basis for life on Earth. Individual water molecules are held together in the solid and liquid phases by very strong intermolecular forces. These forces account for the fact that water is a liquid at ambient planetary temperatures.
Classification of Substances
• In any consideration of intermolecular forces we need to be able to look at a molecular formula and determine whether a substance is made up of atoms, molecules or ions. Covalent substances and ionic substances, for example, have very different physical properties. Ionic substances have very high melting points while covalent/molecular substances have relatively low melting points.
Physical Properties – Examples:
Chemical Formula
Type of Substance
Melting Pt. (oC)
Boiling Pt. (oC)
He Atomic -272 -269Ar Atomic -189 -186Br2 Molecular -7 59
CH3OH Molecular -98 65
H2O Molecular 0 100
NaCl Ionic 801 1413MgO Ionic 2852 3600Si Network
Covalent1687 3538
Physical Properties – Trends:
• 1. Atomic substances (Noble gas elements) have very low melting and boiling points.
• 2. Covalent substances consisting of discrete molecules have “moderate” melting and boiling points.
• 3. Ionic substances and network covalent substances (no discrete molecules) have very high melting and boiling points.
Physical Properties –Trends:
• The strong coulombic forces of attraction between oppositely charged ions are easily understood. Similarly, the forces of attraction between molecules with permanent electric dipole moments are easily understood. The weak attractive forces that must exist (why?) between electrically neutral atoms (He, Ne, Ar…) and electrically neutral and nonpolar molecules are more difficult to appreciate.
Physical Properties – Weak Intermolecular Forces:
• The next two slides show melting and boiling points for the Noble gas elements and the carbon tetrahalides. The attractive forces here are due to transient atomic and molecular electrical polarity. The magnitude of the transient polarity (fluctuating dipoles) is related to atomic size. Larger atoms have electron clouds which are more readily deformed (atoms are more polarizable).
Melting and Boiling Points (Noble Gas Elements)
Element Melting Point (oC) Boiling Point (oC)
Helium -272 -269Neon -249 -246Argon -189 -186Krypton -157 -153Radon -71 -62
Aside: Liquid Ranges
• The Noble Gas elements listed on the previous slide have extremely small liquid ranges. It is fortunate that water, for example, has a much larger liquid range. Nonpolar molecules can have much larger liquid ranges than the Noble Gases – next slide.
Melting and Boiling Points of methane and Carbon Tetrahalides
Compound Melting Point (oC) Boiling Point (oC)
Methane (CH4) -187 -161
Carbon tetrafluoride (CF4)
-185 -128
Carbon tetrachloride (CCl4)
-23 76
Carbon tetrabromide (CBr4)
93 190
Carbon tetraiodide (CI4) 171 Decomposes
Slide 10 of 61
Intermolecular Forces• Van der Waals Forces
–A collection of weak attractive forces between groups of atoms or molecules.
• Instantaneous and Induced Dipoles –Displacement of electrons cause polarization giving rise to an instantaneous dipole. This dipole can affect neighbouring molecules causing induced dipoles.
• Dispersion or London forces.–Instantaneous dipole – induced dipole attraction.
–Related to polarizability.
Copyright © 2011 Pearson Canada Inc.
General Chemistry: Chapter 12
Slide 11 of 61
The phenomenon of induction
• FIGURE 12-1
Copyright © 2011 Pearson Canada Inc.
General Chemistry: Chapter 12
Slide 12 of 61
Instantaneous and induced dipoles
• FIGIURE 12-2
Copyright © 2011 Pearson Canada Inc.
General Chemistry: Chapter 12
Melting and Boiling Points (Noble Gas Elements)
Element Melting Point (oC) Boiling Point (oC)
Helium -272 -269Neon -249 -246Argon -189 -186Krypton -157 -153Radon -71 -62
Noble Gases – Physical Properties
• The melting points and boiling points of the Noble Gas elements increase as one moves to larger atoms. This is not due directly to the atoms becoming heavier. The key point is that the larger electron “clouds” in the larger/heavier Noble Gas atoms are much more readily polarized than the electron “cloud” in an atom such as He.
Melting and Boiling Points of Methane and Carbon Tetrahalides
Compound Melting Point (oC) Boiling Point (oC)
Methane (CH4) -187 -161
Carbon tetrafluoride (CF4)
-185 -128
Carbon tetrachloride (CCl4)
-23 76
Carbon tetrabromide (CBr4)
93 190
Carbon tetraiodide (CI4) 171 Decomposes
Slide 17 of 61
Molecular shape and polarizability
• FIGURE 12-3
Copyright © 2011 Pearson Canada Inc.
General Chemistry: Chapter 12
Molecules with Permanent Polarity
• Many covalently bonded molecules have a net non-zero molecular electric dipole moment. Clearly, a positive (coulombic) attractive force – between the positive “end” of one molecule and the negative end of another molecule will make it more difficult to, for example, vaporize electrically polar molecules.
Slide 19 of 61
Dipole-Dipole Interactions
Copyright © 2011 Pearson Canada Inc.
General Chemistry: Chapter 12
• Permanent Polarity
• Dipole-dipole interactions
Melting and Boiling Points of Fluoromethanes and Chloromethanes
• On the next two slides the melting and boiling points of several fluoromethanes and chloromethanes are presented. Some of these molecules are electrically nonpolar but all have London dispersion forces (of attraction). The dispersion forces are more important in the chloromethanes. Why?
Melting Points, Boiling Points and Dipole Moments of Fluoromethanes
Compound Melting Pt. (OC) Boiling Pt. (OC) Molecular Electric Dipole Moment (D)
Methane (CH4) -187 -161 0
Fluoromethane (CH3F) -142 -78 1.85
Difluoromethane (CH2F2) -136 -52 1.98
Trifluoromethane (CHF3) -155 -82 1.65
Tetrafluoromethane (CF4) -185 -128 0
1 D = 1 Debye = 3.3 x 10-30 coulomb.meter. Why is this unit small?
Melting Points, Boiling Points and Dipole Moments of Chloromethanes
Compound Melting Pt. (OC) Boiling Pt. (OC) Molecular Electric Dipole Moment (D)
Methane (CH4) -187 -161 0
Chloromethane (CH3Cl) -98 +24 1.86
Dichloromethane (CH2Cl2)
-97 +40 1.14
Trichloromethane (CHCl3)
-64 +61 1.15
Tetrachloromethane (CCl4)
-23 +76 0
Physical Properties of Halomethanes
• On the previous two slides how can you rationalize the boiling points of the tetrahalomethanes in comparison to the data for the trihalomethanes?
Physical Properties of N2, NO and O2
• The next slide gives the boiling points of N2, NO and O2. Two of these molecules are nonpolar while NO is weakly polar. NO has the highest boiling point. (Aside: There are a number of other complications here – including the fact that the free radical NO (unpaired electron) can dimerize (partially) to form the N2O2 molecule).
Extremely Polar Bonds – Hydrogen Bonding
• With H bonded to the very electronegative F, N and O the bonding electrons in, for example, an O-H bond are very unequally shared. The extreme electrical polarity that results leads to unusual physical properties as molecules such as these “stick together” by forming intermolecular hydrogen bonds.
Hydrogen Bonding
• Look carefully at the plots on the next slide. At what temperatures might you expect liquid water and liquid ammonia hydrogen bonding was not important for these two substances?
Slide 28 of 61
Hydrogen Bonding
Copyright © 2011 Pearson Canada Inc.General Chemistry: Chapter 12
• FIGURE 12-5• Comparison of boiling points of some hydrides of the elements of groups 14, 15, 16, and 17
Slide 29 of 61
Hydrogen bonding in gaseous hydrogen fluoride
• FIGURE 12-6
Copyright © 2011 Pearson Canada Inc.
General Chemistry: Chapter 12
Electrostatic potential map of HF
Slide 30 of 61Copyright © 2011 Pearson
Canada Inc.General Chemistry: Chapter 12
Hydrogen bonding between H2O molecules
Slide 31 of 61
Hydrogen bonding in water
• FIGURE 12-7
Copyright © 2011 Pearson Canada Inc.
General Chemistry: Chapter 12
around a molecule in the solid in the liquid
Class Examples
• We will look at a number of molecular formulas for different substances and identify the strongest type of intermolecular force present. We’ll use a knowledge of intermolecular forces to predict relative boiling points for simple molecules.