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9.3 The acidic environment: 2. Acid oxides in the
Extract from Chemistry Stage 6 Syllabus (Amended October
2002). © Board of Studies, NSW.
[Edit: 11 Jun 10]
Prior learning: Preliminary modules
Background: Just as elements show a
pattern in properties in the Periodic Table so metal oxides
and non-metal oxides show a pattern in properties. Metal
oxides are usually basic and non-metal oxides usually acidic.
The extent of the acidity or basicity of an oxide can often
be predicted from the element's position in the Periodic
oxides of non-metals which act as acids and describe
the conditions under which they act as acids
- Carbon dioxide (CO2 ), sulfur dioxide
(SO2) and nitrogen dioxide (NO2) all
dissolve in water forming acid solutions. Most non-metal
oxides (except for CO, NO and N2O which are
neutral) are said to be acidic.
- To detect that a non-metal oxide gas is acidic with
indicator paper, the paper must be moist. Moisture enables
the gas to dissolve and form the acid that produces
hydrogen ions. Reaction of a hydrogen ion with an indicator
causes the colour change.
the position of these non-metals in the Periodic Table and outline
the relationship between position of elements in the Periodic
Table and acidity/basicity of oxides
When the Periodic Table outline for oxides below is
compared with the information in a full Periodic Table of
the type used for HSC exams, it can be seen that:
- metal oxides are mostly basic
- non-metal oxides are mostly acidic
oxides of the five elements close to the borderline
between metals and non-metals are amphoteric, that
is, they show both acidic and basic properties.
A substance is said to be basic if it:
dissolves in water to produce a solution that turns
litmus blue, conducts electricity and (don't try
this!) has a bitter taste
- reacts with an acid removing the acid
A substance is said to be acidic if it:
dissolves in water to produce a solution that turns
litmus red, conducts electricity and (once again,
don't try this!) has a sour taste
- reacts with a base removing the base properties.
A base can remove acid properties and an acid can remove
base properties because of a reaction called
acid + base salt + water
At the end of the 1800s, chemists pictured acid
solutions as containing hydrogen ions, H+, and
base solutions as containing hydroxide ions,
OH-. The acidic properties were due to the
hydrogen ions and the basic properties were due to the
hydroxide ions. When an acid solution and base solution
were mixed, the hydrogen ions and hydroxide ions combined
to form water molecules.
H+ + OH- H2O
The reaction between the base sodium hydroxide and
hydrochloric acid can be represented by:
- a word equation
- a full ionic equation
- an ionic equation (showing no spectator ions) or
a full formula (also known as neutral formula or
balanced formula) equation.
sodium hydroxide + hydrochloric acid sodium chloride + water
Na+ + OH- + H+ +
Cl- Na+ + Cl- +
OH- + H+ H2O
NaOH + HCl NaCl + H2O
The salt ions, Na+ and Cl-,
are called spectator ions because they
don't actually react. These ions are floating
around separately in the base and acid solutions before
mixing and are floating around in the neutralisation
mixture after mixing. If the salt solution formed is
evaporated, the salt ions will come together to form
solid salt, but this is not a chemical reaction.
A term that is sometimes used instead of basic solution
is alkaline solution. A basic solution and an
alkaline solution refer to solutions with pH > 7.
An alkali is a water soluble base; usually a group 1 or
group 2 metal hydroxide. Group 1 and group 2 refer
respectively to the elements in the first and second
columns of the Periodic Table. Group 1 elements are called
the alkali metals and group 2 elements are called
the alkaline earth metals.
Alkalis are just one type of base. The Venn diagram below illustrates that alkalis are a subset of bases.
However, in some texts, all basic solutions are called
factors which can affect the equilibrium in a reversible
The factors that can affect the equilibrium in a
reversible reaction are:
- change in concentration
- change in temperature
- change in gas pressure.
Change in concentration
The principle can be illustrated by the changes you
observe in a solution of indicator, such as litmus. An
indicator is a carbon compound that can be represented by
HIn. H is an hydrogen atom that can be released as
an hydrogen ion H+.
In represents the rest of the carbon
In a neutral purple litmus solution, there is a mixture
of red HIn molecules and blue In-
ions. There is an equilibrium between the red HIn
and the blue In-.
HIn H+ + In-
An acid is a substance that produces H+ ions
in solution. If an acid is added to a purple litmus
solution, the solution turns red. Using Le Chatelier's
principle, the higher concentration of hydrogen ions is
predicted to cause an equilibrium shift to the left,
producing the red form of litmus. This is what is actually
observed when acid is added.
Change in temperature
When an acidic oxide gas such as carbon dioxide
dissolves in water heat is released (the forward reaction as written is exothermic).
H2O(l) + CO2(g) H2CO3(aq) + heat
If the carbonic acid solution formed is heated the
equilibrium shifts to the left and carbon dioxide gas is
released. This happens whenever a solution of a gas is
heated. Raising the temperature of a solution of a gas in
water lowers the solubility of the gas.
Change in gas pressure
If the pressure of the gas above a water solution of the
gas is raised, then more gas goes into solution. If the
pressure of gas above a solution of the gas in water is
decreased, then gas comes out of solution.
You have probably noticed that when you take the lid off
a bottle of carbonated-water (water containing dissolved
carbon dioxide) soft drink that bubbles of carbon dioxide
gas form and escape the solution.
H2O(l) + CO2(g) H2CO3(aq)
When you took the lid off the bottle, the concentration
of CO2 above the solution decreased. The
equilibrium shifted to the left to produce more
data, plan and perform a
first-hand investigation to decarbonate soft drink and gather data
to measure the mass changes involved and calculate the
volume of gas released at 25oC and
- In order to carry out a valid investigation, you will
need to plan the experimental design to
remove the carbon dioxide from a quantity of soft drink in
a way that allows you to make accurate and reliable
measurements. Variations on two basic methods could be used
successfully in a school or domestic situation: a
warming method, that uses the drop in water
solubility of a gas on heating, and a salting-out method, that uses the stronger attraction soluble salt
ions have for water than dissolved gas molecules have for
water. These are described generally below.
- In planning your investigation,
consider the ideas presented below and, from them, identify
the types of data you will need to gather
during the investigation and be able to explain the
quantitative analysis that will be required for this data
to be useful.
- Perform the investigation, making
modifications as needed and analysing the effect of these
- Measure and record the quantitative
data required for the calculations to determine
the volume of gas released at 25oC and 100kPa.
Carry out repeat trials to ensure reliable data is
Methods that could be used to decarbonate soft
What you might need:
- A means of weighing to at least the nearest gram.
- 250 or 300 mL bottles or cans of soda water; buy
unopened bottles with the liquid level as low as
- For the warming method: a source of dry
heat, such as an electric hotplate or a saucepan for
gently warming the soda water, a dry towel and a
- For the salting-out method: 1g of table salt for
each 50 mL of soda water.
- It is important that heating or addition of salt is
gradual so that the soda water does not foam or spray out
of the container. Such loss of mass would require you to
start all over again.
- To achieve a more accurate result in these two methods incorporate a control that will allow you to subtract the amount of water lost due to evaporation.
Calculations required to determine the volume of
- Loss of mass due to escape of carbon dioxide gas.
- Conversion of grams of CO2 lost to moles
- Use of the knowledge that one mole of gas at
25oC and 100kPa occupies 24.8 L.
the solubility of carbon dioxide in water under various
conditions as an equilibrium process and explain
in terms of Le Chatelier's principle
- The solubility of carbon dioxide gas in water can be
fully described using four equilibrium equations:
- CO2(g) CO2(aq)
- H2O(l) + CO2(aq) H2CO3(aq)
- H2CO3(aq) H+(aq) +
- HCO3-(aq) H+(aq) +
- An equilibrium shift to the left or right in any of the equations above leads to a change in the concentration of the reactants and products in that reaction. This influences the concentrations in the other three reactions causing the equilibrium in those reactions to also shift to the left or right respectively. For example, a decrease in the pressure above the solution causes reaction 1 to shift to the left and results in a decrease in the pH of the solution as the other equlibrium reactiuons also shift to the left. Likewise, an equilibrium shift to the right in equations 2, 3 or 4 results in more carbon
dioxide gas being dissolved in equation 1.
- In accordance with Le Chatelier's principle:
- addition of acid (increased concentration of
H+) shifts equilibrium to the left
in equations 3 and 4 above and therefore results in a decrease in dissolved carbon dioxide.
- addition of base (reacts with and reduces the concentration
of H+) shifts equilibrium to the right
in equations 3 and 4 above and therefore leads to an increase in the concentraction of dissolved carbon dioxide.
- addition of a soluble carbonate (increased
concentration of CO32-) shifts
equilibrium in equation 4 above to the left and therefore results in a decrease in dissolved carbon dioxide.
volumes of gases given masses of some substances in
reactions, and calculate
masses of substances given gaseous volumes, in reactions
involving gases at 0oC and 100kPa or
25oC and 100kPa
When acid is added to certain anions, gas is formed. For
- acid + carbonate 2HCl + CaCO3 CaCl2 + CO2 +
- Net ionic equation: 2H+ +
CO32- CO2 + H2O
- acid + sulfide 2HBr + FeS FeBr2 + H2S
- Net ionic equation: 2H+ + S2-
1 mole of gas at 100kPa has a volume of 22.71 L at 273.15 K
(0oC) or 24.79 L at 298.15 K (25oC).
Calculations require the writing of balanced equations
and use of mole relationships:
Example 1: Calculate the volume of
carbon dioxide released at 100kPa and 25oC by
the reaction of 10.0 g of calcium carbonate with excess
From the equation above, it can be seen that one mole
of CaCO3 releases one mole of
10.0 g CaCO3
= 10.0 g/100 g
= 0.100 mol
Thus 0.100 mol of CO2 gas is released =
0.100 x 24.8 L = 2.48 L
Example 2: If 1.00 L of hydrogen
sulfide gas was collected at 101.3 kPa and 0oC
from the reaction of excess acid with iron(II) sulfide,
calculate the mass of FeS.
From the equation, it can be seen that one mole of FeS
produces one mole of H2S.
1.0 L gas at 100kPa and 0oC is 1.00/22.7
mol = 0.0441 mol
Thus, 0.0441 mol of FeS reacted = 0.0441 mol x 87.9 g
mol-1 = 3.87 g
natural and industrial sources of sulfur dioxide and oxides
- Some coal or oil reserves contain considerable
quantities of sulfur compounds. Most sulfur dioxide
(SO2) released into the atmosphere comes from
the burning of such coal or oil in electric power stations.
SO2 is the major contributor to acid rain that
can affect places thousands of kilometres from the source.
The low sulfur content of Australian coal is one reason why
Australia is the world's largest exporter of coal.
- Smelters where metal sulfides are heated in air to
remove the sulfur as SO2 are becoming a smaller
source of atmospheric SO2. Most of the
SO2 produced is now used to make sulfuric acid.
The huge amount of SO2 that used to be released
from Mt Isa mine's smelter in Queensland is now changed
to sulfuric acid and transported by rail 150 km away to
make ammonium sulfate and superphosphate fertilisers at
- Volcanoes are an unpredictable source of
SO2. After Mt Pinatubo erupted in the
Philippines in 1991, oxidation of the emitted
SO2 and reaction with water formed an aerosol of
sulfuric acid droplets. This aerosol reduced global
temperatures by about 0.5oC.
National Pollutant Inventory Database for
2008/2009 data within Australia - Sulfur dioxide from All Sources,
Department of Environment, Water, Heritage and the Arts, Australia.
The figures for Oxides of Nitrogen Department of Environment, Water, Heritage and the Arts, Australia.
using equations, examples of chemical reactions which release
sulfur dioxide and chemical reactions which release oxides of
information from secondary sources to summarise
the industrial origins of sulfur dioxide and oxides of
nitrogen and evaluate reasons for concern about their release
into the environment
- Information about sources of acid rain, mostly produced
by SO2 emissions and to a lesser extent by NOx
emissions, can be found by using Internet search engines or
by going to sites for scientific magazines and journals and
using site search engines. In choosing
equipment, determine the suitability and
effectiveness of your browser search facilities to locate
the relevant information.
- Some suitable starting points from which to
gather information include:
- search engines, such as Google
- popular scientific journals, such as Nature New Scientist
- science specific Internet sites, such as Nova and Science Daily
Specific information on industrial origins of pollutants
in Australia can be found in the National Pollutant Inventory Database.
Reasons for concern about release of these gases into the
environment can be found by:
- searching the Internet using a search engine
- accessing material safety data sheets (MSDS) for these
gases using CD-ROMs, such as the one supplied to schools
with the Chemical Safety In Schools (CSIS)
- analyse the information you access to
evaluate the reasons for concern about the release of
sulfur dioxide and the oxides of nitrogen into the
environment. Your evaluation might describe cause and
effect relationships and could be based on criteria such as
economic, social cost, health effects, environmental,
political and energy benefit. List the criteria or values
you use in your evaluation.
evidence which indicates increases in atmospheric
concentration of oxides of sulfur and nitrogen
- There is extensive evidence for an increase of over 25%
in atmospheric carbon dioxide levels over the last two
hundred years. The evidence comes from quantitative
analysis of trapped air bubbles in Antarctic ice and
measurement of carbon isotopes in old trees, grass seeds in
museum collections and calcium carbonate in coral.
Finding evidence for increases in atmospheric sulfur
oxides and nitrogen oxides is more difficult for the
- Whereas atmospheric CO2 concentrations
are about 360 parts per million (ppm), the levels for
SO2 and NOx are only about 0.001 ppm in
populated parts of the Earth.
- The chemical instruments able to measure very low
concentrations, like those for SO2, have
only been commercially available since the 1970s.
CO2 changes to carbonate ions when it
dissolves in water and most carbonates are insoluble.
Seashells and coral are made up of carbonates that
came from atmospheric CO2. Isotope ratio
measurements using mass spectrometers on shells and
corals of different ages give clues as to past
atmospheric CO2 concentrations.
On the other hand, SO2 eventually forms
sulfate ions and NO2 forms nitrate ions.
Most sulfates and all nitrates are water-soluble.
Soluble sulfates and nitrates circulate in the
hydrosphere and biosphere and are chemically changed
while insoluble carbonates tend to stay in inert
forms such as shells or coral.
- Some evidence that has been observed that indicate the increasing atmospheric concentrations of oxides of sulfur (SOx) and nitrogen (NOx) are:
- Higher atmospheric concentrations of SOx and NOx have been detected in industrial areas, particularly in Europe. These correspond to an increase in acidity in lakes and rivers in these areas.
- Damage to forests, buildings and aquatic life has also been observed in industrialised areas. This is thought to be due to acid rain formed by elevated levels of atmospheric SOx and NOx.
- Information can be found at http://www.apis.ac.uk/ that indicates the atmospheric concentrations of oxides and nitrogen have been decreasing in recent decades. Acces this and other souces of information to make your assessment. Make sure you assess the validity and reliability of the secondhand sources you access and make your final assessment based on the most valid and reliable information.
- Distilled water in contact with the atmosphere is not
neutral. It has a pH of 5.5 to 6, due to absorption of the
acidic gas CO2 from the atmosphere.
- In Australia, unpolluted rainwater has a pH between 5
and 6. If the pH is below 5, an acidic substance, such as
SO2 or NO2, has dissolved in the
water, which is sometimes called acid rain. In the
Northern Hemisphere, pHs as low as 2 have been recorded in
acid rain. The source of the SO2 or
NO2 could be hundreds or thousands of kilometres
from where the acid rain falls.
- SO2 sources, such as fossil fuel burning
power stations and metal sulfide smelters, are larger but
fewer in number than NO2 sources, like internal
combustion engines in vehicles.
If the quantity of acid rain is greater than the capacity
of an environment to neutralise it then the following can
- Soil pH can drop, making it difficult for plants to
absorb sufficient calcium or potassium.
- Soil chemistry can change, leading to the death of
important micro-organisms and release of normally
insoluble aluminium and mercury into soil water.
- Protective waxes can be lost from leaves, causing
- Buildings made of carbonates, such as concrete,
mortar, limestone and marble, can be gradually
- Aquatic animals can die as water acidity drops
below pH 5.
- Smog and acid rain can combine to form killer
fog, as happened after the Second World War in
London, when many homes burnt sulfur dioxide-releasing
Acid rain effects described United
States Environmental Protection Agency, US
(These web sites last checked 27 June 2008)