Practice Problems 5.5.1 — Metal and Non-metal Oxides Metal and Non-metal Oxides
1. Complete and balance the following formula equations:
(a) K20(s) + H20(/)
(b) MgO(s) + H20(/)
2. (a) Each of the following third period oxides reacts with water to produce acids. Arrange these oxides in order
from least acidic to most acidic.
S03 Cl207 Si02 P4O10 Al^
<
<
<
<
(b) Two of the above oxides react with water to produce strong acids. Write the balanced equations for those
reactions below.
3. Complete the following statements using either "ionic" or "molecular" in the blank spaces.
(a) In general, ionic oxides react with water to produce (ionic or molecular) bases.
(b) In general, molecular oxides that react with water produce acids. Acids are ; (ionic or
molecular) compounds.
The Problem of Acid
Many of the advantages we enjoy and often take for granted in our industrialized
and mobile society also come with environmental costs. One of the most serious and
Rain
widespread is the problem of acid precipitation in the form of rain, snow, fog, and even
dry deposits on particles.
As noted earlier, normal rainwater has a pH of about 5.6 because of dissolved C02.
If the pH of rainwater is below 5.3, it is referred to as acid rain.
Although natural causes also exist, virtually all of the acid precipitation that
humankind is responsible for can ultimately be traced back to the burning of fossil fuels
in our homes, vehicles, and power plants, and the processing of mineral ores in industry.
The products of such activities and the reactions they undergo are further examples of
the important impact chemistry has on our lives.
Precipitation in any form is an atmospheric event that involves water. Because our
The Human Causes
of Acid Rain .
atmosphere is a gaseous solution, it should come as no surprise that the major
substances responsible for acid precipitation are themselves gases that react with water.
Each is a non-metal oxide that reacts with water to produce an acid.
Let's discuss how our modern society produces each of the gaseous substances that
cause acids to fall from the sky.
388 Chapter S Applications of Acid-Base Reactions
© Edvantage Interactive 2012 ISBN 978-0-9864778-4-3
Sources of SuSphur
Oxides
Many coal and oil deposits contain sulfur impurities, When electrical power plants burn
these fossil fuels to drive steam turbines, the sulphur is oxidized to S02. For example, the
main sulphur impurity in coal is iron disulphide orpyrite, FeS2.The combustion of the coal
also burns the pyrite and produces S02 according to the following reaction:
4 FeS2{s) + 11 02{g) -> 2 Fe^fs) + 8 S02(g)
Although the levels of sulphur impurities are often less than 3%, the huge
quantities of fossil fuels burned worldwide result in the production of hundreds of
millions of tonnes of S02.
Another source of S02 is the process of smelting, which extracts metals from their
ores by heating. Several important metals such as copper, lead, and zinc are present in
Earth's crust, mainly as sulphides. The first step in the smelting of these minerals usually
involves roasting, in which the sulphides are heated at high temperatures in air. For
example, when ZnS is heated to about 700oC, the following reaction occurs in the roaster:
2 ZnS(s) + 3 02(g) -> 2 ZnO(s) + 2 S02{g)
Much like carbonic acid, the weak acid produced from the reaction of S02 and
water, called sulphurous acid, H2S03, is unstable and doesn't exist in its molecular form in
water. As a result, we can represent aqueous solutions of sulphur dioxide as follows:
S02(g) + 2H20(D ^ H30+(aq}-hHSOf(aq)
This means that rainwater falling through gaseous S02 will be more acidic than
normal when it reaches the ground.
The problem becomes worse in polluted air containing ozone, 03, and fine dust
particles where, especially in the presence of sunlight, oxygen and ozone will oxidize
some of the S02 to SO3:
2 S02(g) + 02(g) 2 S03(g)
S02(g) + 03(g) —> S03{g} + 02(g)
Sulphur trioxide now reacts with water to form the strong acid sulphuric acid:
S03(g) + H20(/} -> H2S04(aq)
Because 100% ionization occurs when sulphuric acid reacts with water, rainwater
in the presence of this acid is much more acidic. A gaseous mixture of S02 and S03 is
sometimes referred to as SOx.
Natural processes such as volcanic eruptions also introduce enormous amounts of
sulphur oxides into the atmosphere, but they represent only about 5% to 10% of the total
released by human activities.
Sources of Nitrogen
Oxides
The air we breathe is composed of about 79% nitrogen and 21% oxygen. These two gases
will not react with each other under normal conditions of temperature and pressure
because the activation energy for the reaction is far too high. However, when air is mixed
with fossil fuels and burned in the internal combustion engine of vehicles or in electric
© Edvantage Interactive 2012 ISBN 978-0-9864778-4-3
Chapter 5 Applications of Acid-Base Reactions 389
power plants, the high temperatures and pressures generated cause the two atmospheric
gases to react according to the following reaction:
N2(g) + 02(g) -» 2 HO(g)
When the NO gas in a hot exhaust stream encounters cooler outside air, it reacts
with oxygen gas to produce N02 gas. This gaseous mixture of NO and N02 is often
referred to as N0X.
Nitrogen dioxide gas reacts with water in the atmosphere to produce the strong
acid HN03 and the weak acid HN02 as shown below:
2 N02(g) + H20(/} -> HN03{aq) + HN02(ag)
As with sulphur oxides, nature also generates a significant amount of nitrogen
oxides. Lightning and decaying vegetation are both sources of NO, but we can do nothing
to affect those processes.
Figure 5.5.3 summarizes the causes and effects of acid rain.
Figure 5.5.3 Add rain kills plants, pollutes rivers, lakes, and streams and erodes stone.
Quick Check 'ÿÿ
1. Identify the three non-metal oxides that react with water in the atmosphere to produce acid precipitation.
2. How do coal-burning electrical power plants produce the sulphur oxide responsible for acid rain?
3. Why can the problem of sulphur oxide acid precipitation become worse in polluted air?
390- Chapters Applications of Acid-Base Reactions
;
© Edvantage Interactive 2012 ISBN 978-0-9864778-4-3
Acid precipitation in the form of rain, snow, fog, or dry deposits on particles has been
Consequences of
Acid Rain
recorded on every continent on Earth — even at the north and south poles! Because the
oxides of sulphur and nitrogen that cause acid precipitation are gases that are carried
into the atmosphere, prevailing winds can potentially push clouds containing these
oxides more than 1500 km from their sources. For
example, Scandinavia receives acid precipitation
from Germany, England, and countries in eastern
Europe. Southern Ontario and Quebec can receive
the acid precipitation that originated anywhere in
illl!
mtm
the U.S. industrial belt extending from Chicago to
Boston.
Figure 5.5.4 Trees, soil, water, and fish can all be damaged or destroyed by
acid rain
Measurements taken in Europe and North
America confirm how acidic some precipitation
can be. Rainwater with a pH of 2.7 (approximately
equal to vinegar) has been recorded in Sweden
fand aVirginia.
pH of 1.8 (almost equal to stomach acid) has been recorded for rain in West
jj The effects of acid rain are both serious and widespread. Both aquatic and
jj terrestrial ecosystems can be severely affected when exposed to acidic conditions.
Most species of fish die at pH levels below 5, and entire forests have been
S devastated by acidic precipitation.
jj Obviously, contaminating the water supplies that our species depends on
/jj also has serious direct and indirect consequences. The soils in many areas contain
aluminum salts that are nearly insoluble in normal groundwater, but begin to
dissolve in more acidic solutions. This dissolving soil not only releases the Al3+
ions that are very toxic to fish, but also leaches out many valuable nutrients from
the soil, which are lost in the runoff. Figure 5.5.4 shows some examples of acid rain
damage.
Many buildings, monuments, and even cemetery headstones contain CaC03
in the form of either marble or limestone. Carbonate salts dissolve in acids and
iS:-;
Figure 5.5.5 This statue shows the
damage from years of exposure to
acid rain.
Some Reasons for
Optimism
long-term exposure to acid rain significantly damages such structures. Although
not an environmental consequence, the cost of such damage to our society is
significant.
Some lakes in regions where acid precipitation occurs are naturally protected because
they are bounded by soils rich in limestone. The same reaction that dissolves the calcium
carbonate in statues and buildings allows these lakes to resist changes in their pH. As the
limestone dissolves, the hydronium ions from acid rain cause more bicarbonate ions to
form as shown below.
C032-(uq) + H30+(oq) ^ HCO-(ac/) + H20(f)
Over time, as the [HC03_] increases, the lakes become effective bicarbonate/
carbonate buffer systems and maintain a relatively constant pH.
If a lake's soil does not contain sufficient limestone, a temporary solution is to add
CaC03 or CaO directly to the lake. Over time, however, such lakes will eventually once
again suffer the effects of acid rain unless the sources of the S02 and N02are controlled.
© Edvantage Interactive 2012 ISBN 978-0-9864778-4-3
Chapter 5 Applications of Acid-Base Reactions 391
Growing environmental awareness on a global scale over the past several decades
has focused the attention of the public, the politicians who represent them, and the
scientific community on attempting to solve the problem of acid precipitation. There is
still much work to be done, but there are few disciplines as powerful as chemistry when it
comes to solving our problems and so there are reasons for optimism.
Strict government emission standards are forcing automobile manufacturers to
produce much cleaner and more efficient vehicles. As you learned in section 1.5, modern
vehicles are equipped with a catalytic converter, which significantly reduces the levels
of carbon monoxide, unburned hydrocarbons, and nitrogen oxides released into the
atmosphere. One of the reaction chambers in the converter uses catalytic materials such
as transition metal oxides and palladium or platinum metals to convert gaseous nitrogen
oxides in the exhaust stream to nitrogen and oxygen gas before it leaves the tailpipe.The
catalyzed reaction can be represented as shown below.
2HOx(g) ^ x02{g) + U2(g)
(The value of'x" is either 1 or 2.)
Over the past several years, most major car companies have produced "hybrid"
vehicles, which normally use two distinct power sources to move the vehicle.The most
common hybrid vehicles are hybrid electric vehicles (HEVs), which combine an internal
combustion engine with one or more electric motors. In addition, several major car
companies including General Motors and Nissan have invested heavily in the production
of electric vehicles such as the Volt and the Leaf, which are available to the general public.
Industrial efforts to reduce sulphur and nitrogen oxide emissions have also
intensified over the past several decades. One method of removing sulphur dioxide gas
from the exhaust stream of a coai-fired power plant or a metal smelter is by a process
called scrubb/ng.This involves first blowing powdered limestone (CaC03) into the'
combustion chamber where heat decomposes it to CaO and C02. The calcium oxide
(lime) then combines with the sulphur dioxide gas to produce solid calcium sulphite:
CaO(s) + S02(g} -> CaS03(s)
As a second step, to remove the CaS03 and any unreacted S02, an aqueous
suspension of CaO is then sprayed into the exhaust gases before they reach the
smokestack to produce a thick suspension of CaS03 called a slurry. The process is not
without its drawbacks, however. First, it is expensive and consumes a great deal of energy.
Second, because no use has yet been found for the CaS03, the great quantities of this
compound that are produced by the process are usually buried in landfills.
A recently developed process for removing S02 involves using H2S gas to convert
the S02 into elemental sulphur according to the following reaction:
16 H2S(g) + 8 S02(g) ^ 3 S8(s) + 16 H20(/)
To reduce the nitrogen oxide emissions from power plants, gaseous ammonia is
reacted with the hot stack gases to produce nitrogen and water vapour:
4 NO(g} + 4 NH3(g) + 02(g) 4 N2(g) + 6 H20{/}
Applications of Acid-Base Reactions
© Edvantage Interactive 2012 ISBN 978-0-9864778-4-3
The problem of acid precipitation has not yet been solved. A number of major
hurdles must still be overcome, particularly as they relate to international agreements
and enforcement. Economics, employment, industrial development, political
"will, environmental protection, and scientific advancement do not always merge
harmoniously on our planet.
As you read this, consider the problem a challenge to you and your generation.
Although the stakes are very high and the problems are significant, human ingenuity and
chemistry have combined to surmount many enormous challenges in our world in the
past. Maybe it's your turn to begin such an endeavour now!
Sample Problem 5.5.2 — Acid Rain
Lead is most commonly found in Earth's crust as lead(ll) sulfide, PbS, a mineral called galena.The first step in
smelting lead converts the galena to lead(ll) oxide by the process ofroe?sf/ng.
(a) Write the formula equation for the reaction.
(b) Identify the product of the reaction that contributes to acid precipitation.
What to Think About
How to Do It
1. Roasting reacts oxygen in air with the sulphide
mineral.
2 PbS is) + 3 D2 y) -» 2 Pbo (s) + 2 (j)
2. The non-metal oxide product is precursor to
acid precipitation.
© Edvantage Interactive 2012 ISBN 978-0-9864778-4-3
Chapter 5 Applications of Acid-Base Reactions 393