Transcript Topic 4.5 Physical Properties of Bonds

Topic 4.5 Physical Properties of Bonds
Assessment Statements
 4.5.1 Compare and explain the following properties of
substances resulting from different types of bonding:
melting and boiling points, volatility, conductivity and
solubility.
 Look at how impurities affect these properties
 Solubilities of compounds in polar and non-polar
solvents
 Solubilities of alcohols in water being related to
chain length
General physical properties
 Depend on the forces between the particles
 The stronger the bonding between the particles, the
higher the M.P and BP
 MP tends to depend on the existence of a regular lattice
structure
Impurities and Melting points
 An impurity disrupts the regular lattice that its particle
adopts in the solid state, so it weakens the bonding.
 They always LOWER melting points
 Its often used to check purity of a known molecular
covalent compound because its MP will be off, proving
its contamination
How would this ideal heat curve look different if
the substance was contaminated?
Volatility
 A qualitative measure of how readily a liquid or
solid is vaporised upon heating or evaporation
 It is a measure of the tendency of molecules and atoms
to escape from a liquid or a solid.
 Relationship between vapour pressure and temperature
(B.P)
 Mostly dealing with liquids to gas, however can
occur from solid directly to gas (dry ice).
 The weaker the intermolecular bonds, the more
volatile
Conductivity
 Generally molecules have poor solubility in polar
solvents like water, but if they do dissolve they do not
for ions
 There are no charged particles to carry the electrical
charge across the solution.
 Example: sugar dissolves in water
 C12H22O11(s)  C12H22O11(aq)
Dissolving sugar (covalent compound)
 It takes energy to break the bonds
between the C12H22O11 molecules
in sucrose crystal structure.
 It also takes energy to break the
hydrogen bonds in water so that
one of these sucrose molecules
can fit into solution.
 In order for sugar to dissolve,
there must be a greater release of
energy when the dissolution
occurs than when the breaking of
bonds occur.
Ionic compounds
 The energy needed to break the ionic bond must be
less than the energy that is released when ions interact
with water.
 The intermolecular ion-dipole force is stronger than
the electrostatic ionic bond
 Breaks up the compound into its ions in solution.
 Soluble salt in water breaks up as
NaCl (s)  Na+ (aq) + Cl- (aq)
 http://www.mhhe.com/physsci/chemistry/essentialchemis
try/flash/molvie1.swf
Ionic compounds
 Held together by strong 3-d electrostatic forces.
 They are solid at room temperature and pressure
 If one layer moves a fraction, the ions charges are off
and now repulsion occurs. This is the reason they are
strong, yet brittle.
 Molten or dissolved ionic compounds conduct
electricity
 Insoluble in most solvents, yet H2O is polar and
attracts both the + and – ions from salts
Covalent bonding properties
Giant covalent
 Ex: diamond, silicon
dioxide
 Very hard
 Very high MP (>1000oC)
 Does not conduct
 Insoluble in all solvents
Molecular covalent
 Ex: CO2, alcohols, I2
 Usually soft, malleable
 Low MP (<200oC)
 Does not conduct
 More soluble in nonaqueous solvents, unless
they can h-bond
Solubility of methanol in water
 http://www.mhhe.com/physsci/chemistry/animations
/chang_7e_esp/clm2s3_4.swf
 Alcohols generally become less soluble, the longer the
carbon chain due to the decreasing tendency for
hydrogen bonding to occur intermolecularly.
States of matter
 Physical state depends on intermolecular forces
 The weaker the attraction, the more likely it’s a gas,
while stronger attractions indicate solid.
 http://www.chemguide.co.uk/atoms/bonding/metal
lic.html
 Metallic bonding review
 http://chemed.chem.purdue.edu/genchem/topicrev
iew/bp/ch18/soluble.php
 Solubility review
 http://wwwcsi.unian.it/educa/inglese/kevindb.html
 History involved with dissolving ionic compounds