2.3 Classifying Chemical Compounds: Properties of Ionic

and Covalent Compounds

Unit: Matter and Chemical Bonding

Properties of Ionic and Covalent CompoundsProperty Ionic

CompoundsCovalent

CompoundsState at Room Temperature

Crystalline solid Liquid, gas, and solid

Melting Points High Low

Electrical Conductivity as a liquid

Yes No

Solubility in water Most have high solubility

Most have low solubility

Conducts electricity when dissolved in water

yes Not usually

PROPERTY 1: Melting/Boiling PointsWHAT WE ALREADY KNOW:

• The melting point of a compound is the temperature at which it changes from a solid to a liquid at STP.

• The boiling point of a compound is the temperature at which it changes from a liquid to a gas at STP.

PROPERTY 1: Melting/Boiling Points

• A low melting or boiling point means small amounts of kinetic energy are enough for a particle to break away from its neighbours. the forces between the particles are weak.

• A very high melting or boiling point means the particles must have a very large amount of kinetic energy to break away the forces between the particles are strong

NaCl, Table Salt: An Ionic Compound• Sodium chloride=Crystalline solid at room T. • Very high melting point, at 801 ◦C• Easily dissolved in water and great electrical

conductor. (Ex. Sea water)• Uses: flavouring food (can cause blood

pressure), melts ice in the winter

High Melting and Boiling Points = IONIC COMPOUNDS

This means that the attractive electrostatic forces between oppositely charged particles create very strong bonds.

The crystal lattice arrangement of ionic compounds means that multiple ions are involved in this interaction simultaneously, which results in a great force.

The medium strength of the polar covalent bonds is a result of the dipole-dipole force.

• This is the attractive force between the positive end of one molecule and the negative end of another molecule.

Medium Melting and Boiling Points = COVALENT COMPOUNDS with POLARITY

Carbon Dioxide: A Covalent Compound

• Carbon dioxide = gas at room T but under certain T’s and P’s can be liquid.

• Very low melting point, at -79 ◦C• Easily somewhat soluble in water and a weak

electrical conductor. • Uses: cellular respiration and

photosynthesis

Low Melting and Boiling Points = NON-POLAR COVALENT COMPOUNDS• Although non-polar molecules have

no distinct separation of charge, it is still possible for the positive nuclei of atoms in one molecule to attract the electrons of a neighbouring molecule. These forces are very weak!

PROPERTY 2: Water solubility• The partially positive H in water is attracted to

negative ions of an ionic compound or the partially negative atom of a polar compound.

• The partially negative O in water is attracted to the positive ions of an ionic compound or the partially positive atom of a polar compound.

IONIC AND POLAR COVALENT COMPOUNDS ARE WATER SOLUBLE. NON-POLAR COVALENT COMPOUNDS ARE NOT WATER SOLUBLE.

PROPERTY 3: Electrical ConductivityElectrical conductivity is the ability of a

substance to allow an electric current to pass. A substance is conductive if charges can move independently of one another.

This is only possible for ionic compounds in the liquid state or dissolved in water. For this reason they are called electrolytes

Since in covalent compounds , electrons do not leave an atom completely these compounds do not conduct electricity when in liquid state. They are called non-electrolytes.

Conductivity with a Multimeter

• http://www.youtube.com/watch?v=kSPDjAhl1Qc

• Salt • Sugar

Consider the following:• Write the

formula next to each compound.

• Sort the compounds according to their boiling points.

• What do you notice?

COMPOUND BOILING POINT ( °C)ethanol +78.3ammonia +33.3cesium bromide +1300hydrogen gas -253magnesium oxide +3600methane -161nitrogen gas -196sodium chloride +1465water +100

“ThoughtLab: Ionic Or Covalent?” Imagine that you are a chemist. A colleague has just carried out a series of tests on the following compounds:Ethanol (clear, liquid)Carbon tetrachloride (clear, liquid)Glucose (white, powder, solid)Sodium chloride – table salt (white, crystalline solid)Water (clear, liquid)Potassium permanganate (bluish-purple, crystalline solid)

Unfortunately your colleague labelled the tests by sample number and not name. Use the properties of the compounds to identify them.

SAMPLE Dissolves in H2O?

Conductivity Melting point (°C)

What does it look like?

1 yes high 801 White crystalline solid

2 yes low 0.0 Clear colourless liquid

3 yes high 240 Purple crystalline solid

4 yes low 146 White powder

5 no low -23 Clear colourless liquid

6 yes low -114 Clear colourless liquid