1. Periodic Oxide Trends
YEAR 11 DP CHEMISTRY

2. Non-metal Oxides Sources
Atmospheric O2 is very reactive and reacts with many substances to form oxides
Natural formation
CO2 – from respiration (“burning” sugars for energy)
NO2 – from lightning strikes (N2 + 2O2 in the air  2NO2)
SO2 – released from volcanoes or H2S + O2 SO2 + H2O (H2S produced by bacterial decomposition of organic matter)
Human causes (bushfires and burning fossil fuels)
CO2 – fossil fuel combustion product
NO – high temperature combustion product
NO2 – NO is easily oxidised in the air (NO + O2  NO2)
SO2 – burning coal that contains S as an impurity
SO3 – SO2 is easily oxidised in the air

3. Acidic Non-metal Oxides
Many non-metal oxides react with water in the atmosphere to produce acids;
CO2 + H2O  H2CO3 (carbonic acid)
SO2 + H2O  H2SO3 (sulfurous acid)
SO3 + H2O  H2SO4 (sulfuric acid)
2NO2 + H2O  HNO3 + HNO2 (nitric and nitrous acid)
These non-metal oxides are all gases
Their acidic products all contribute to the acidity of rain

4. Oxide Trends in the Periodic Table
Oxides tend to increase in acidity from left to right
In general:
Metal oxides are basic (left side)
Non-metal oxides are acidic (right side)
Exceptions:
Amphoteric oxides (i.e. Al, Be, Ga, Sn, Pb)
Why this trend?
This is due to electronegativity increasing from left to right (see following slides for more details)

5. Oxides on the left side of the PT (basic)
Electrons are transferred to the O2-
This is due to the ionic nature of these bonds because of a large difference in electronegativities, therefore ions are formed in solution
For example: element electronegativity
Na 0.93
O 3.44
This means: Na2O(aq)  Na+ + O2- and
O2- + H+  OH- (readily)
This overall consumption of H+ ions leads to an increase of pH (i.e. Basic)

6. Oxides on the right side of the PT (acidic)
Electrons are shared with the O2-
This is due to the covalent nature of these bonds because of a small difference in electronegativities, thus no ions are formed
For example: element electronegativity
S 2.58
O 3.44
This means: Due to a partially positive S central atom,
SO3(aq) + H2O  H+ + HSO4-
This overall production of H+ ions leads to an decrease of pH (i.e. acidic). (See following slide for details of this reaction)

7. Sulfur trioxide forms an acid- O S O S OH
OH- H+ H+
Sulfur trioxide
water
Sulfuric acid solution