some of the gases present in our atmosphere dissolve in rain water and cause it to become acidic....

Acid Rain

Some of the gases present in our atmosphere dissolve in rain water and cause it to become acidic. Here we will examine some of these.

Carbon dioxide has been present in Earth’s atmosphere for billions of years, long before humans were around.

Thanks to Ewa Urban for sharing this image on Pixabay

CO2(g) + 2H2O(l)H3O+(aq)

Because CO2 is present in the air, it comes in contact with water droplets in clouds. When CO2 dissolves in water, it reacts with the water


To produce hydronium ions


And hydrogen carbonate ions.

CO2(g) + 2H2O(l)H3O+(aq)+ HCO3

–(aq)

H3O+ formed by dissolved CO2 has always been present


–(aq)

H3O+ formed by dissolved CO2 has always

been present and has always caused rainwater

to be slightly acidic

and has always caused rainwater to be slightly acidic


–(aq)

H3O+ formed by dissolved CO2 has always

been present and has always caused rainwater

to be slightly acidic

The presence of dissolved CO2 in natural rainwater, unaffected by human activity (click) produces enough hydronium to bring the pH as low as 5.6.


–(aq)

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 pH


–(aq)

Because natural rainwater has a pH as low as 5.6

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 pH

pH = 5.6

Natural Rainwater

Rain with any pH 5.6 or above is NOT called acid rain,

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 pH

pH = 5.6

NOT called acid rain

even though a pH of 5.6 to 7 is acidic.

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 pH

NOT called acid rain

acidic

Rain with a pH below 5.6 (click) IS called acid rain because is it more acidic than rain naturally is.

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 pH

pH = 5.6

called acid rain

Generally rainwater with a pH below 5.6 is caused by gases from human activities. Some gases produced by naturally occurring phenomena such as volcanoes or lightning can also cause acid rain in localized areas.

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 pH

Caused by gases from

human activities,

volcanoes, and lightning

called acid rain

It’s important to remember that rainwater with any pH 5.6 or above, is not technically called acid rain, while rainwater with any pH below 5.6 IS called acid rain.

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 pH

Calledacid rain

pH = 5.6

NOT calledacid rain

You may recall that Nonmetal Oxides are oxides of elements on the right side of the staircase on the periodic table.

Nonmetals

Metals

Nonmetal Oxides form

Acidic Solutions

Remember, nonmetal oxides react with water to form acidic solutions.

Nonmetals

Metals

Nonmetal Oxides form

Acidic Solutions

Nonmetal oxides produced by human activities are the major sources of acid rain.

Acid Rain Caused by

Human

Activities

The nonmetal oxides produced by human activities that contribute most to acid rain include

Nonmetal Oxides Produced by Human Activities

Nitrogen monoxide, or NO gas

NO(g),


Nitrogen dioxide, or NO2 gas

NO(g), NO2(g),


Which is sometimes referred to as N2O4 gas

NO(g), NO2(g),


Also referred to asN2O4(g)

Sulphur dioxide or SO2 gas

NO(g), NO2(g), SO2(g),


And sulphur trioxide, or SO3, which is present in the air as liquid droplets.

NO(g), NO2(g), SO2(g), and SO3(l)


Nitrogen oxides NO(g) and NO2(g) are present in exhaust gases of internal combustion engines that run on gasoline or diesel fuel.

NO NO2

Nitrogen oxides NO(g) and NO2(g) are present in exhaust gases of gasoline

and diesel engines

So the main source of nitrogen oxides, sometimes collectively referred to as NO X,is automobile exhaust.

The main source of nitrogen oxides

(sometimes referred to as NOx)

is automobile exhaust

Thanks to Pat McKane for sharing this image on Pixabay

Air is about 78% nitrogen

2(g) 2(g) (g)N O 2NOheat

Air is about:78% nitrogen (N2(g)) 21% oxygen (O2(g))

And about 21% oxygen. Air forms part of the mixture injected into the cylinders of an internal combustion engine.


Air is about:78% nitrogen (N2(g)) 21% oxygen (O2(g))

Inside the cylinders of this type of engine, the temperature can reach as high as 1900 degrees Celsius. At this high temperature, (click) nitrogen can react with oxygen,


To produce nitrogen monoxide gas


They can also react (click) to produce nitrogen dioxide gas, NO2.


2(g) 2(g) 2(g)N 2O 2NOheat

Some of the The NO produced (click) also reacts with oxygen

2(g) 2(g) (g)N 2NO Oheat


(g) 2(g) 2(g)2NO O 2NO

To produce NO2 gas



(g) 2(g) 2(g)2NO O 2NO

2(g) 2 ( ) 2(aq) 3(aq)2NO H O HNO HNOl

this NO2 will be present in the exhaust of vehicles using these engines, and some of it will reach the atmosphere


2(g) 2(g) 2(g)2NON 2O heat

(g) 2(g) 2(g)2O O NO2N


where it will react with water droplets



(g) 2(g) 2(g)2NO O 2NO

to produce nitrous acid, HNO2…



(g) 2(g) 2(g)2NO O 2NO



And nitric acid HNO3.



(g) 2(g) 2(g)2NO O 2NO

2(g) 2 ( 2(aq)) 3(aq)HN2NO H O HNOOl

Because it is a weak acid, HNO2



(g) 2(g) 2(g)2NO O 2NO

2(aq) 2 ( ) 3 (aq) 2(aq)HNO H O H O NOl


Undergoes partial ionization in rainwater



(g) 2(g) 2(g)2NO O 2NO






(g) 2(g) 2(g)2NO O 2NO



And nitrite ions, NO2 minus



(g) 2(g) 2(g)2NO O 2NO


2(g) 2 ( 3(a )q) q) 2(a2NO H O HN HNOOl

The strong acid HNO3, or nitric acid



(g) 2(g) 2(g)2NO O 2NO




Will completely ionize in rainwater



(g) 2(g) 2(g)2NO O 2NO







(g) 2(g) 2(g)2NO O 2NO




And nitrate ions NO3 minus



(g) 2(g) 2(g)2NO O 2NO




The presence of hydronium ions formed from both acids, causes the rain (click) to be acidic, or acid rain.



(g) 2(g) 2(g)2NO O 2NO

2(aq) 2 ( ) 2(( aq) )3 aqHHNO H O NOOl


acid rain


You may want to pause the video, take a screen shot of this page and print it. It is good to be familiar with these equations.



(g) 2(g) 2(g)2NO O 2NO



acid rain

Two other harmful substances that are produced by (click) human activities are sulphur dioxide, released from coal burning power plants and smelters

2SO

And sulphur trioxide, which forms when sulphur dioxide reacts with air.

2SO3SO

2SO

Let’s see how these two substances are formed.

3SO

2SO

Thanks to Emilian Robert Vicol for sharing this image on Pixabay.

Fossil fuels, such as coal usually contain small amounts of sulphur. When they re burned, the sulphur in them burns too.

(s) 2(g) 2(g)S + O SO

2(g) 2(g) 3(g)2SO O 2SO

(s) 2(g) (s) 2(g)2ZnS + 3O 2ZnO 2SO

Burning FuelsContaining Sulphur

Roasting Ores in Smelters

2(s) 2(g) 2 3(s) 2(g)4FeS + 11O 2Fe O 8SO

2(g) 2 ( ) 2 3(aq)SO H O H SO (sulphurous acid)l

3(g) 2 ( ) 2 4(aq)SO H O H SO (sulphuric acid)l

2 3(aq) 2 ( ) 3 (aq) 3(aq)H SO H O H O HSOl


Fossil fuels contain small amounts

of sulphur

When sulphur

(s) 2(g) 2(g)S + O SO

2(g) 2(g) 3(g)2SO O 2SO

(s) 2(g) (s) 2(g)2ZnS + 3O 2ZnO 2SO



2(s) 2(g) 2 3(s) 2(g)4FeS + 11O 2Fe O 8SO





burns in air,

(s) 2(g) 2(g)S + O SO

2(g) 2(g) 3(g)2SO O 2SO

(s) 2(g) (s) 2(g)2ZnS + 3O 2ZnO 2SO



2(s) 2(g) 2 3(s) 2(g)4FeS + 11O 2Fe O 8SO





It produces sulphur dioxide gas or SO2. But SO2 can also come from other sources…

(s) 2(g) 2(g)S + O SO

2(g) 2(g) 3(g)2SO O 2SO

(s) 2(g) (s) 2(g)2ZnS + 3O 2ZnO 2SO



2(s) 2(g) 2 3(s) 2(g)4FeS + 11O 2Fe O 8SO





Metals occur in nature in ores, which are compounds of the metals with nonmetals. A metal combined with the nonmetal sulphur is called a sulphide ore.

(s) 2(g) 2(g)S + O SO

2(g) 2(g) 3(g)2SO O 2SO

(s) 2(g) (s) 2(g)2ZnS + 3O 2ZnO 2SO



2(s) 2(g) 2 3(s) 2(g)4FeS + 11O 2Fe O 8SO





Two examples of sulphide ores are zinc sulphide and iron(IV) sulphide.

(s) 2(g) 2(g)S + O SO

2(g) 2(g) 3(g)2SO O 2SO

(s) 2(g) (s) 2(g)2ZnS + 3O 2ZnO 2SO



2(s) 2(g) 2 3(s) 2(g)4FeS + 11O 2Fe O 8SO





Sulphur is removed from ores in a process called roasting, where the ores are heated in a high oxygen environment. This is one of the processes that occur in refining or smelting metals.

(s) 2(g) 2(g)S + O SO

2(g) 2(g) 3(g)2SO O 2SO

(s) 2(g) (s) 2(g)2ZnS + 3O 2ZnO 2SO



2(s) 2(g) 2 3(s) 2(g)4FeS + 11O 2Fe O 8SO





The equation that describes the roasting of the ore zinc sulphide is 2ZnS plus 3 O2 …

(s) 2(g) 2(g)S + O SO

2(g) 2(g) 3(g)2SO O 2SO

(s) 2(g) (s) 2(g)2ZnS + 3O 2ZnO 2SO



2(s) 2(g) 2 3(s) 2(g)4FeS + 11O 2Fe O 8SO





Gives 2ZnO or zinc oxide

(s) 2(g) 2(g)S + O SO

2(g) 2(g) 3(g)2SO O 2SO

(s) 2(g) (s) 2(g)2ZnS + 3O 2ZnO 2SO



2(s) 2(g) 2 3(s) 2(g)4FeS + 11O 2Fe O 8SO





Plus 2SO2

(s) 2(g) 2(g)S + O SO

2(g) 2(g) 3(g)2SO O 2SO

(s) 2(g) (s) 2(g)2ZnS + 3O 2ZnO 2SO



2(s) 2(g) 2 3(s) 2(g)4FeS + 11O 2Fe O 8SO





Similarly, a common iron ore, Iron(IV) sulphide, or iron pyrite, is heated in oxygen

(s) 2(g) 2(g)S + O SO

2(g) 2(g) 3(g)2SO O 2SO

(s) 2(g) (s) 2(g)2ZnS + 3O 2ZnO 2SO



2(s) 2(g) 2 3(s) 2(g)4FeS + 11O 2Fe O 8SO





To produce iron(III) oxide, or Fe2O3,

(s) 2(g) 2(g)S + O SO

2(g) 2(g) 3(g)2SO O 2SO

(s) 2(g) (s) 2(g)2ZnS + 3O 2ZnO 2SO



2(s) 2(g) 2 3(s) 2(g)4FeS + 11O 2Fe O 8SO





And SO2.

(s) 2(g) 2(g)S + O SO

2(g) 2(g) 3(g)2SO O 2SO

(s) 2(g) (s) 2(g)2ZnS + 3O 2ZnO 2SO



2(s) 2(g) 2 3(s) 2(g)4FeS + 11O 2Fe O 8SO





Some of this SO2, in the air

(s) 2(g) 2(g)S + O SO

2(g) 2(g) 3(g)2SO O 2SO

(s) 2(g) (s )) 2(g2ZnS + 3O 2SO2ZnO



2(s) 2(g) 2 3(s 2(g))4FeS + 11O 8SO2Fe O





Will react with oxygen in the air…

(s) 2(g) 2(g)S + O SO

2(g) 2(g) 3(g)2SO O 2SO

(s) 2(g) (s) 2(g)2ZnS + 3O 2ZnO 2SO



2(s) 2(g) 2 3(s) 2(g)4FeS + 11O 2Fe O 8SO





To produce sulphur trioxide, SO3. This reaction often takes place on the surfaces of dust particles in the air.

(s) 2(g) 2(g)S + O SO

2(g) 2(g) 3( )2SO O 2SO l

(s) 2(g) (s) 2(g)2ZnS + 3O 2ZnO 2SO



2(s) 2(g) 2 3(s) 2(g)4FeS + 11O 2Fe O 8SO





SO2, being a nonmetal oxide,

(s) 2(g) 2(g)S + O SO

2(g) 2(g) 3( )2SO O 2SO l

(s) 2(g) (s) 2(g)2ZnS + 3O 2ZnO 2SO



2(s) 2(g) 2 3(s) 2(g)4FeS + 11O 2Fe O 8SO





Will react with water droplets in the air

(s) 2(g) 2(g)S + O SO

2(g) 2(g) 3( )2SO O 2SO l

(s) 2(g) (s) 2(g)2ZnS + 3O 2ZnO 2SO



2(s) 2(g) 2 3(s) 2(g)4FeS + 11O 2Fe O 8SO





2(g) 2(g) 3( )2SO O 2SO l

To produce H2SO3,

(s) 2(g) 2(g)S + O SO (s) 2(g) (s) 2(g)2ZnS + 3O 2ZnO 2SO



2(s) 2(g) 2 3(s) 2(g)4FeS + 11O 2Fe O 8SO





2(g) 2(g) 3( )2SO O 2SO l

Or sulphurous acid. Sulphurous acid is classified as a weak acid.




2(s) 2(g) 2 3(s) 2(g)4FeS + 11O 2Fe O 8SO





SO3 present in the mixture is also a nonmetal oxide,

(s) 2(g) 2(g)S + O SO

2(g) 2( 3( )g)2SO 2SOO l

(s) 2(g) (s) 2(g)2ZnS + 3O 2ZnO 2SO



2(s) 2(g) 2 3(s) 2(g)4FeS + 11O 2Fe O 8SO


3( ) 2 ( ) 2 4(aq)SO H O H SO (sulphuric acid)l l



2(g) 2 ( ) 2 3(aq)SO H O H SO (sulphurous acid)l 2(g) 2(g) 3( )2SO O 2SO l

So it will react with water




2(s) 2(g) 2 3(s) 2(g)4FeS + 11O 2Fe O 8SO




2(g) 2(g) 3( )2SO O 2SO l

Producing H2SO4,




2(s) 2(g) 2 3(s) 2(g)4FeS + 11O 2Fe O 8SO





2(g) 2(g) 3( )2SO O 2SO l

Or sulphuric acid. Sulphuric acid is classified as a strong acid.




2(s) 2(g) 2 3(s) 2(g)4FeS + 11O 2Fe O 8SO






2(g) 2(g) 3( )2SO O 2SO l

Because H2SO3 is a weak acid,




2(s) 2(g) 2 3(s) 2(g)4FeS + 11O 2Fe O 8SO




It undergoes partial ionization in water




2(g) 2(g) 3( )2SO O 2SO l (s) 2(g) 2(g)S + O SO (s) 2(g) (s) 2(g)2ZnS + 3O 2ZnO 2SO



2(s) 2(g) 2 3(s) 2(g)4FeS + 11O 2Fe O 8SO









2(s) 2(g) 2 3(s) 2(g)4FeS + 11O 2Fe O 8SO


And hydrogen sulphite ions







2(s) 2(g) 2 3(s) 2(g)4FeS + 11O 2Fe O 8SO


And because H2SO4 is a strong acid







2(s) 2(g) 2 3(s) 2(g)4FeS + 11O 2Fe O 8SO


It ionizes completely in water







2(s) 2(g) 2 3(s) 2(g)4FeS + 11O 2Fe O 8SO









2(s) 2(g) 2 3(s) 2(g)4FeS + 11O 2Fe O 8SO


And hydrogen sulphate ions







2(s) 2(g) 2 3(s) 2(g)4FeS + 11O 2Fe O 8SO


The presence of hydronium ions in the water results in the (click) formation of acid rain

2 3(aq) 2 ( ) 3(a( q) )3 aqHH SO H O HSO Ol


acid rain





2(s) 2(g) 2 3(s) 2(g)4FeS + 11O 2Fe O 8SO


You may want to pause the video, take a screen shot of this page and print it. It is good to be familiar with these equations.



acid rain





2(s) 2(g) 2 3(s) 2(g)4FeS + 11O 2Fe O 8SO


Humans are not the only source of acid rain. It can also be caused by natural phenomena.

Acid Rain Caused by

Natural Phenomena

Volcanoes can release large amounts of SO2 into the atmosphere.

SO2 SO2

Mt St Helens volcano eruption Image Number: AC80-0513-29 Date: 1980 - NASA photograph

And the extreme localized heating caused by lightning can cause nitrogen and oxygen in the air to react


And form nitrogen monoxide, which using the sequence of reactions we discussed, can cause acid rain.


Thanks to sethink for sharing this image on Pixabay

The reason people are concerned about acid rain is it has some serious effects.

Effects of Acid Rain

We’ll outline some of the effects of acid rain here: Acid rain

Acid rain: causes soil and rock to release Al3+ ions,

which are toxic to many fish species and can affect the health of trees and crops

washes away many nutrients that are important for the growth of trees and crops

damages stone buildings and statues by dissolving limestone (CaCO3) in the rocks

dissolves substances that can contaminate water supplies

causes soil and rock to release Al3+ ions. Aluminum ions are toxic to many fish species and can seriously affect the health of trees and crops






Acid rain can also dissolve and wash away many of the good nutrients that are needed by trees and crops. When trees are weakened by acid rain they are more susceptible to disease






Acid rain dissolves certain rocks such as limestone or marble, which are made up of calcium carbonate. Many buildings and statues have been damaged by years of exposure to acid rain.






Acid rain can also dissolve toxic metals present in rocks and soils. The resulting ions of these metals can then enter water supplies and contaminate them.






Acid rain is not as big a problem as it once was in many places. This is due to increased awareness of the problem and pressure on governments to react.

Measures to Minimize Acid rain: Since the 1970’s governments have imposed strict

regulations on air quality. This includes emissions from thermal power plants, vehicles, and industrial processes.

New vehicles are all equipped with multiple devices to control emissions. These include sophisticated catalytic converters to break down exhaust gases. In addition, an increasing number of hybrid vehicles are being sold. These have lower emissions than other vehicles.

Alternate forms of energy are being employed in many countries. These include wind-powered, tidal powered, and geothermal generating plants.

SO2 from smelting is reclaimed and used to produce sulphuric acid instead of being released to the environment.

Beginning in 1970’s, many governments have imposed strict regulations concerning air quality. This includes emissions from thermal power plants, vehicles, and industrial processes






You’re probably aware that new vehicles are all equipped with numerous devices to control emissions. One of the most important are catalytic converters, which greatly minimize the release of harmful exhaust products.






You may also have noticed that there is are more and more hybrid vehicles of the roads. These have lower emissions than other vehicles. There are also some vehicles that run on fuel cells, which have zero emissions of harmful gases.






In many places in the world, thermal coal-burning power plants are being phased out and alternate sources of energy, such as wind, tidal power, and geothermal energy are being used instead.






We had mentioned that SO2 is a product of smelting sulphide ores of metals. In many places, this SO2 is reclaimed and used to produce sulphuric acid on an industrial scale, with very little of it reaching the environment.






some of the gases present in our atmosphere dissolve in rain water and cause it to become acidic....

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