Melting Point and Boiling Points of Ionic and Covalent Compounds

• Melting point– According to Kinetic Molecular Theory• Particles in solids vibrate• As temperature increases, vibration increases• If the particles gain enough kinetic energy they can

break apart from neighbouring particles• When particles have enough energy to break away, the

melting point has been reached• Higher melting point – attraction between particles is

higher.

Boiling Point and Kinetic Molecular Theory

• In liquids, particles have enough energy to break away from neighbouring particles, but they are attracted to other particles

• At the boiling point – particles have enough kinetic energy to break away from all other particles and become a gas.

• High boiling point – forces b/n particles are strong

• Low boiling point – forces b/n particles are weak

Melting Points and Boiling PointsCompound Melting Point (°C) Boiling Point (°C)

ethanol , C2H5OH -114 +78.3

ammonia, NH3 -77.7 -33.3

cesium bromide, CsBr +636 +1300

hydrogen, H2 -259 -253

hydrogen chloride, HCl -114 -85

magnesium oxide, MgO +2825 +3600

methane, CH4 -182 -161

nitrogen, N2 -210 -196

sodium chloride, NaCl +801 +1465

water, H2O 0 +100Classify each into 1 of 3 categories: Low, intermediate or high boiling point.

Melting Points and Boiling PointsCompound Melting Point (°C) Boiling Point (°C)

ethanol , C2H5OH -114 +78.3

ammonia, NH3 -77.7 -33.3

cesium bromide, CsBr +636 +1300

hydrogen, H2 -259 -253

hydrogen chloride, HCl -114 -85

magnesium oxide, MgO +2825 +3600

methane, CH4 -182 -161

nitrogen, N2 -210 -196

sodium chloride, NaCl +801 +1465

water, H2O 0 +100Classify the bonds in each of the molecules as covalent, polar covalent or ionic.

How is bond type related to boiling point?

• Look at the results of your 2 lists.• How do they compare?• What does this tell you about the relative

bond strength of each type of bond?• Explain your results by referring to the Kinetic

Molecular Theory

Types of Forces Between Particles• Attractive Electrostatic Forces – occur b/n ions

of opposite charge they are strong• Dipole-dipole Forces – occur between the

positive end of one molecule and the negative end of another happens in polar molecules weak

• London Dispersion Forces – occur between any 2 molecules. Caused by the random movement of electrons...this creates temporary dipoles very weak