YEAR 12 CHEMISTRY
ACIDS AND BASES
Acids are compounds that:









have a low pH (below 7)
taste sour
turn blue litmus paper red
react with bases to neutralize them and
produce salts
release H2 gas in reactions with active
metals
aqueous solutions conduct electricity
furnish H+
burn the skin if strong
dissolve carbonates
These are some common acids. What others can you
think of? Make a list in your notes.
ACIDS AND BASES
Bases are compounds that:









have a high pH (above 7)
taste bitter
turn red litmus paper blue
react with acids to neutralize them and
produce salts
are slippery feeling
aqueous solutions conduct electricity
burn skin if strong
react with fats to form soap
furnish OH-
What other bases/alkalis do you know?
ACIDS AND BASES
Some common laboratory acids
HCl
HBr
HI
hydrochloric acid
hydrobromic acid
hydroiodic acid
HClO4
HNO3
H2SO4
perchloric acid
nitric acid
sulfuric acid
Five of these acids are classified as monoprotic acids. They only have one
hydrogen which they are able to donate. Sulfuric acid is classified as a
diprotic acid because it has two acidic hydrogens that it can donate.
Similarly, an acid which has three donatable hydrogens would be
classified as triprotic.
ACIDS AND BASES
Some common laboratory bases
NaOH
KOH
NH3
sodium hydroxide
potassium hydroxide
ammonia
Ca(OH)2
Mg(OH)2
Na2CO3
calcium hydroxide
magnesium hydroxide
sodium carbonate
Bases are often found in everyday products such as many cleaning products
(sodium hydroxide), antacid products (magnesium hydroxide )and fertilisers
(ammonia). It is a common misconception that bases are not as dangerous as
acids. In fact, many bases can be as much or more corrosive than many acids.
pH Indicators
 A simple explanation of pH is that it is a
measure of acidity/basicity (more indepth explanation to follow)
 Many substances change colour as they
are exposed to different pH levels. These
can be used to “indicate” the pH of
substances when the colour ranges are
known
 Some natural products such as litmus,
cabbage, grapes and tea are natural
indicators while others such as
phenolphthalein and methyl orange are
synthetic
 pH indicators are themselves acids or
bases as they donate or accept protons
(more on this later)
Notice the variety of ranges where different
indicators change colours. Some have more than one
change.
pH of some common substances
pH Indicators
Some specific examples:
1.
2.
3.
4.
Litmus
Phenolphthalein
Methyl orange
Bromothymol blue
red5
8blue
colourless8.3 10red
red3.1
4.4yellow
yellow6
7.6blue
Note: colour changes within these ranges are gradual
A pH problem
Indicator
Colour
Litmus
Blue
Phenolphthalein
Colourless
Methyl orange
Yellow
Bromothymol blue
Blue
An unknown solution produces the colours above. What is the pH range of this
solution?
8-8.3
Uses of Indicators
Soil testing
 Most plants cannot survive outside pH 5.5-7.5
 Different plants require different pH level
 2 methods for testing soil pH
 Mix small sample with universal indicator and
sprinkle BaSO4 powder on top – read colour
 Mix soil with water in a test tube and add
indicator – read colour
 Changing soil pH
 Too acidic – add NH3, CaO (lime), CaCO3
 Too basic – add manure, pine bark, peat
Uses of Indicators
Pool acidity
 Pool should be pH of 7.4
 Tested with a meter or an indicator such as
phenol red
 Changing the pH
 Too acidic – add CaOCl2 (pool chlorine), Na2CO3
 Too basic – add HCl soln, NaHSO4
Non-metal Oxides
Sources
Atmospheric O2 is very reactive and reacts with many substances to
form oxides
Natural formation
1.
2.
1.
2.
3.
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)
3.
1.
2.
3.
4.
5.
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
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
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)
Oxides on the left side of the PT
 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)
Oxides on the right side of the PT
 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)
Sulfur trioxide forms an acid
O
S
O
-
O
OH-
S
O
Sulfur
trioxide
H+
water
O
O
H+
OH
Sulfuric acid solution
Industrial Pollution Research
The Industrial Revolution
brought about many
changes in the 19th
Century. One of these
changes involves an
increase in the release
of sulfur dioxide SO2
and oxides of nitrogen
such as NO2.
Summarise the
industrial origins of SO2
and NO2 and evaluate
reasons for concern
about their release into
the environment
2. Consider health issues,
visibility issues, effects
on the environment
(rivers, soil, plants), and
effects on buildings
1.