PROPERTIES OF SOLIDS SCH4U1

PROPERTIES OF SOLIDS SCH4U1

Intra vs. Intermolecular Bonds The properties of a substance are influenced by the force of attraction within and between the molecules.

Intra vs. Intermolecular Bonds Intramolecular Bonds: Bonds within a molecule (covalent or polar covalent) Intermolecular Bonds: Bonds between molecules.

Intermolecular Forces The physical properties of a molecule (e.g. melting point) are mainly due to the strength of intermolecular bonding: H 2 O (s)! H 2 O (l)! H 2 O (g)!!! 2H + O MP = 0 o C BP =100 0 C Decomposes: >2000 o C intermolecular bonds breaking intramolecular bonds breaking

1) Atomic Solids Noble gases form liquids and solids at very low temperature due to the very weak bonds between the atoms. Since the attraction is so weak, they are weakened and broken at very low temperature. E.g. argon (Ar): mp = -189 o C; bp = -186 o C

van der Waals (London) Forces Since they do form solids, a very weak attraction must exist between the atoms. These are explained by weak attractions between molecules called van der Waals forces. London forces are the weakest type of van der Waals attraction.

London Forces London forces form due to the attraction between instantaneous dipoles (charge imbalances) that form in the atoms. At low temperature can induce dipoles in other atoms, causing solidification of helium:

London Forces and Electrons As the number of electrons in an atomic solid increases, the mp/bp also increases. Group 18 Electrons Boiling Point ('C) He 2-268.6 Ne 10-245.9 Ar 18-185.7 Kr 36-152.3 Xe 54-107.1 Rn 86-61.8

Summary: Properties of Atomic Solids very low melting points/ boiling point do not conduct electricity mp/bp increase down the group (increasing London forces) Liquid helium is a very strange substance.

Molecular Solids Substances with covalent bonds or polar covalent bonds (N 2, CH 4, H 2 O, C 6 H 12 O 6 etc.). Can exist in all states at room temperature. Check out liquid nitrogen!

2) Non-Polar Molecular Compounds Compounds without bond dipoles only have London Forces between molecules. This results in LOW bp/mp Group 17 Boiling Point ('C) F 2-188.1 Cl 2-34.6 Br 2 58.8 I 2 184.4

BOILING POINTS OF RELATED MOLECULAR COMPOUNDS Formula Number of Electrons Boiling Point ( o C) CH 4 10-161 SiH 4 18-112 GeH 4 36-90 SnH 4 54-52

Comparing Larger Compounds When comparing non-polar compounds, the forces of attraction are greater between molecules with the greatest number of atoms. There are more locations for London (van der Waals) forces to occur between adjacent molecules.

Boiling Points of Hydrocarbons Molecular Formula Boiling Point ( o C) State at STP CH 4-161.5 gas C 2 H 6-88.6 gas C 3 H 8-42.1 gas C 4 H 10-0.5 gas C 5 H 12 36.1 liquid C 6 H 14 68.7 liquid liquid C 10 H 22 174.1 liquid liquid C 22 H 46 327 solid

3) Polar Molecular Compounds Compounds with bond dipoles AND molecular dipoles (e.g. HCl, H 2 S, CF 2 H 2 ) have higher boiling points. This is due to intermolecular forces between permanent dipoles. These are called dipole-dipole forces or bonds. Dipole-Dipole Force (Bond)

Boiling Points of Some Polar and Nonpolar Substances Substance Boiling Point ( o C) Molar Mass (g/mol) Number of Electrons HCl polar -84.9 36 18 H 2 S (molecular dipole) -60.7 34 18 F 2 nonpolar -188.1 38 18 Ar (NO molecular dipole) -185.7 40 18

4) Polar Molecules: Hydrogen Bonding If hydrogen is bonded to a VERY electronegative atom (F, O or N), a very strong dipole forms. These atoms are also very small, concentrating this positive and negative charge. Dipole-dipole bonds between molecules containing O-H, N-H or H-F bonds form hydrogen bonds.

Water bends near a charged object.

Properties of Hydrogen-bonded Molecules A hydrogen bond is about 10x weaker than a covalent bond BUT 10x stronger than a normal dipole-dipole bond Thus H-bonded molecules have the highest mp/bp of the molecular compounds: mp ( o C) bp ( o C) Propane (C 3 H 8 ) -188-42 Propanol (C 3 H 7 OH) -126 97 Glycerol (C 3 H 6 (OH) 3 ) 18 290

General Properties of Molecular Compounds Molecular compounds do not conduct electricity sine their electrons cannot move between molecules. They have relatively low bp/mp due to the existence of weaker intermolecular forces As the strength of these intermolecular forces increase, so does the mp and bp.

Unusual Properties of Water Water is called the universal solvent since it dissolves both polar molecules (e.g. sugar) and ionic compounds (e.g. NaCl). Water expands when it freezes due to the organization of the many hydrogen bonds in the solid.

5) Metallic Solids General Properties: Few valence electrons Low ionization energies Malleable, ductile and shiny Moderate mp/bp Good conductors of heat and electricity in the solid and liquid states.

Metallic Bonding Metal properties can be explained by considering them as postivie ions in an electron sea or electron cloud Delocalized or conduction electrons are shared among multiple cations are free to move throughout a crustal of positive ions. The Electron Sea Model of a Metallic Crystal Positive Metal Ion Delocalized Electron Cloud

Explaining Metallic Properties Property Conductivity (Electricity / Heat) Explanation Delocalized electrons can move between ions. Ductility and Malleability Lustre The plane of ions can move by distorting the electron cloud. Reflection is caused by loosely bonded electrons absorbing and remitting all wavelengths of light.

e.g. 1 Lithium is far more malleable than aluminum. Propose an explanation for this observation using the model of metallic bonding. Metallic bonding occurs since the loosely held (delocalized) electrons are mutually shared by a crystal of positive ions. Since Li has only 1 delocalized valence electron compared with aluminum which has 3 and aluminum has a greater nuclear charge, we can deduce that the additional protons & electrons strengthen the metallic bonding and make it more difficult to displace the network of atoms in the crystal.

e.g. 2 Which element would require the most energy to undergo vapourization, K or Sc? Explain. Scandium. The stronger the metallic bonding, the more energy required to change state. Similar explanation as above..scandium has more delocalized electrons. Kl (l) + 77 kj Sc (l) + 333 kj K (g) Sc (g)

6) Ionic Solids Solids formed by ionic bonds between metal cations (+) and non-metallic anions (-). Bonded together by a 3D array or crystal lattice without distinct molecules.

Properties of Ionic Solids High melting points and boiling points (many ionic bonds that must be broken to change states). Hard but brittle. Many are soluble in water. DO NOT conduct electricity in the solid state since ions cannot move. DO conduct electricity in the liquid or aqueous states since charged ions are mobile.

Crystal Packing Properties of ionic compounds are related to the packing of the crystals:

Factors Affecting the Strength of Ionic Bonding 1. Ionic Radius of the Cation and Anion: As the radius of the ions increases, the attraction between oppositely charged ions decreases.

Factors Affecting the Strength of Ionic Bonding 2. Ionic Charge: As the charge of the cation and anion increases, the attraction increases. Melting Point ( o C) Solubility (g/100g H2O @ 0 o C) CsCl 646 161 MgO Melting Point ( o C) Solubility (g/100g H2O @ 0 o C) 2800 0.0006 NaCl NaCl 800 35.7 800 35.7

7. Covalent Network Solids Form a lattice of continuous covalent / polar-covalent bonds. Do not contain molecules. Very hard, brittle substances. Most do not conduct since electrons are either in sigma bonds or lone pairs (filled orbitals). Some exist as different allotropes (forms with different properties)

Quartz: A Common Network Solid Quartz (SiO 2 ) and Feldspars (KAlSi 3 O 8, NaAlSi 3 O 8 & CaAl 2 Si 2 O 8 ) make up most of the Earth`s crust. Quartz is a continuous framework of tetrahedral SiO 4

Comparing CO 2 and SiO 2 Property Carbon dioxide (CO 2 ) Quartz (SiO 2 ) Type of Solid Non-polar Molecular Covalent Network Melting Point ( o C) -78(sublimates at 1 atm) 1650 Boiling Point o C) N/A 2230 Bond angle ( o ) 180 o 109 o Geometry Linear (sp) Tetrahedral (sp 3 ) Intramolecular bond Type(s) Intermolecular bond Types Polar covalent London forces Polar covalent

Allotropes of Carbon: Diamond Covalent network of sp 3 hybridized carbon (tetrahedral). Very hard; very high sublimation point (3642 o C) Does not conduct electricity.

Allotropes of Carbon: Graphite Network of sp 2 hybridized carbon (trigonal planar) Half-filled p-orbitals form pi bonds Graphite conducts electricity along the plane of the layers due to the network of delocalized p-orbital electrons/ Graphite is a good lubricant since the planes can slip over each other.

Summary: Types of Bonds Intramolecular Metallic Intermolecular 1. Ionic bonds 2. Covalent (Polar and Non-polar) Metallic bonds 1. London forces 2. Dipole-dipole forces 3. Hydrogen bonds strong bonds weak bonds increasing bond strength

Types of Solids formed by Elements Metallic Solids Network Covalent Solids Atomic Solids Non-polar Molecular Solids

Summary: Types of Solids Type Examples Intramolecular Bonds Intermolecular Bonds Relative Melting Point Atomic He, Ar van der Waals (London forces) very low Molecular Cl 2, HCl, H 2 O (non-metals) covalent bonds (polar or nonpolar) van der Waals, dipole-dipole and hydrogen bonds low Metallic Cu, Mg, Fe (metals) metallic bonds moderate-high Ionic NaCl NaNO 3 (metal + nonmetal) Network quartz (SiO 2 ) diamond (C) Silicon (Si) (non-metals) ionic bonds covalent bonds high very high