2012 topic 4.3 intermolecular forces and physical properties
IB Chemistry Power Points Topic 4 Bondingwww.pedagogics.ca LECTURE Intermolecular Forces and Physical Properties
Much taken from AN INTRODUCTION TO BONDING andSHAPES OF MOLECULES Great thanks to JONATHAN HOPTON & KNOCKHARDY PUBLISHING www.knockhardy.org.uk/sci.htm
Intermolecular ForcesIntermolecular forces collectively describe theattractions BETWEEN the unit particles that makeup an element or compound.The nature of the intermolecular forces dependson the structure of the substance in question.WARNING: be very specific in your language usagewhen answering “explain” type questions.The stronger the intermolecular forces, the greaterthe forces of attraction. This affects properties ofsubstances such as melting and boiling point.
Complex Structures and Intermolecular ForcesIn general - intermolecular forces in complex structures arestrong, chemical bonds that involve valence electrons.• metallic bonds (in metallic structures)• ionic bonds (in ionic compounds)• covalent bonds (in giant covalent network structures)Simple Structures and Intermolecular ForcesIn general - intermolecular forces in simple molecules areweak, electrostatic attractions between particles.• Van Der Waals forces• Dipole – Dipole interactions• hydrogen “bonds”
Intermolecular Forces – Ionic BondsIonic compounds are generally visualized as solids consistingof anions and cations held together by electrostaticattractions in a crystal lattice structure. In molten NaCl, the ions have sufficient energy to overcome (“break”) the ionic bonds such that the ions are no longer held in fixed positions (note: they are still attracted to each other)
Intermolecular Forces – Metallic BondsMetallic structures are generally visualized assolids consisting of fixed cations held in place bymutual attractions for a “sea” of valence electrons.
Intermolecular Forces – Metallic BondsMetallic bonds are ELECTROSTATIC attractions betweenpositive metal ions and negative valence electrons.Heating a metal leads to an increase in the space betweenthe metal ions (thermal expansion). Increased energy ofions, increases vibration, overcomes intermolecular forces,and allows them to move apart. When melting occurs, theions are no longer “fixed” in position.
Intermolecular Forces – Giant Covalent Structures The intermolecular forces in covalent networks (giant molecules, macromolecules) are covalent bonds. In diamond, each carbon atom is covalently bonded to 4 other carbon atoms. Collectively, these 4 bonds create extremely strong intermolecular forces. It is difficult to imagine a molten diamond – where the bonds have been broken.
more on macromolecules - allotropesHow the atoms are bonded together in macromolecules canaffect the properties of the substance. Different bondedforms are called allotropes. For example, three allotropes ofpure carbon are shown below.Diamond Graphite C60 Buckminsterfullerene
more on macromoleculesPure silicon and silicon dioxide (quartz) have similar structuresto diamond.Silicon Silicon Dioxide
Simple Molecular Structures andIntermolecular Forces The intermolecular forces between simple molecules are much weaker than the covalent bonds that bind the atoms together to make the molecule itself. Be very careful with language use here. The strong intermolecular forces in ionic, metallic, and giant covalent structures are chemical bonds. The weak intermolecular forces between simple molecules are NOT chemical bonds but are sometimes referred to as “physical bonds”.
Simple Molecular Structures and Intermolecular Forces Intermolecular forces between simple covalent molecules are collectively called Van der Waals forces.Some texts, and the IBO often refer to only theweakest type of these forces as VDW forces (be aware) VDW forces - an electrostatic attraction between opposite dipoles in two different molecules.
Non-Polar Molecules – weak VDW forces attractions from temporary separations of charge force of attraction increases with molecular weight Mr
Polar Molecules – stronger attractions Dipole-Dipole attraction between oppositely charged regions of neighboring POLAR molecules. For example HCl
Hydrogen “bonding” – strongest attractions Hydrogen bonding occurs between positive hydrogen dipoles and the lone pairs of oxygen and nitrogen atoms. Look for O-H and N-H bonds in molecules! Remember - Not a chemical “bond”Hydrogen bonding in Kevlar
Melting point, boiling point, volatility The stronger the intermolecular forces, the greater the forces of attraction. Results in increased melting and boiling points, and decreased volatility (ease of evaporation). In general, metallic, ionic, and giant covalent structures have very high m.p., b.p. and low volatility. Trends in these properties in simple covalent molecules are important to understand.
Covalent Molecules and Boiling PointsThe greater the attraction between dipoles the more energy must be putin to separate molecules resulting in higher boiling points. Mr °C Mr °C CH4 16 -161 H2O 18 +100 Boiling points SiH4 32 -117 H2S 34 -61 of hydrides GeH4 77 -90 H2Se 81 -40 SnH4 123 -50 H2Te 130 -2 Those in red illustrate NH3 17 -33 HF 20 +20 hydrogen PH3 34 -90 HCl 36.5 -85 bonding AsH3 78 -55 HBr 81 -69 SbH3 125 -17 HI 128 -35
BOILING POINTS OF HYDRIDES 100 H2O The higher than expected boiling points of NH3, H2O and HF are due to intermolecular HYDROGEN BONDINGBOILING POINT / C° HF 0 Mr 50 100 140 NH3 -160
Electrical ConductivityConductivity means “movable charge”.Metals conduct: valence electrons are free to moveMolten ionic compounds, and aqueous solutionsconduct: ions are free to moveSimple covalent structures do not conductGiant covalent structures do not conduct (exceptionsilicon and graphite)
Summary Conductivity means “movable charge”. Metals conduct: valence electrons are free to move Molten ionic compounds conduct: ions are free to move Simple covalent structures do not conduct Giant covalent structures do not conduct (exception silicon and graphite)