An acid was once defined as 'a substance that had a sour taste and which reacted with metals such as zinc and iron' Antoine Lavosier, 1780 proposed that 'acids were substances that contained oxygen'. This was disproved soon after as many bases contained oxygen and many acids did not. Humphry Davy, 1815 suggested that 'acids were substances that contained 'replaceable oxygen'', hydrogen that could be replaced by metals. At the time acids were chemicals that when reacting with metals produced salts and when reacted with bases formed salts and water. Svante Arrhenius, 1884 proposed that 'acids were substances that ionised in solution to produce hydrogen ions'. Acids were strong if they ionised completely and week if they only ionised slightly. Arrhenius defined a base as ' a substance that in solution produced hydroxide ions'. This definition was to specific as it excluded metallic oxides that are basic. May inaccuracies in Arrhenius' ideas. Does not recognise the solvent's role in ionisation, it depends on both the acid and solvent in determining its strength. Also Acid-base reactions often occur in solvents where the acid is not ionised at all. 1923, Bronsted and Lowry independently put forward new ideas to define acids and bases in the context of proton donors and acceptors. These definitions overcame previous problems and are currently used today. The Bronsted-Lowry definitions are that an Acid is a substance that in a solution tends to give up protons and a base is a substance that tends to accept protons.
In 1923 English chemist Thomas Lowry and Danish chemist J. N. Brønsted developed a new way to define an acid and a base that were independent of the solvent and the presence of H+ and OH- ions. They defined an Acid as substance that when in a solution would give up its protons. A base was the opposite as a substance that when in a solution would accept protons. The Bronsted - Lowry definition of Acids significantly changed the we think about acids and their definition holds true to this day. This definition was so accurate and specific it changed the way chemists thought about acids and bases. It helped increase our knowledge of acids and bases further by showing that a substance's acidity or alkalinity doesn't just rely on the structure of the acid or base. It actually relies on the acid or base's properties in relation to that of the solvent being used as well as the other reactant in the solution. This has helped scientists across the world discover why an acid or base has high or low levels of pH. The definition they gave showed that a neutralisation reaction did not need to require the involvement of ionisation to H+ and OH-ions but are able to advance straight by proton transfer. This can be shown in a NH3 + HCl reaction in a benzene solution. The Bronsted - Lowry idea also gave us an insight into the hydrolysis of salts (reaction with water). It allowed us to see that in order to change a pHs level significantly from a neutral 7, the hydrolysis of salt was simply a acid or base reaction. The concept that Bronsted and Lowry developed also provided us with knowledge for the quantitative treatment of acid to base equlibria as well as pH calculations. From this concept that these two chemists developed, chemistry was able to evolve into what it is today, having large impacts on what we currently study.
Every year, large amounts of Sulfur, Carbon and Nitrogen oxides are released into the air by power stations, metal works industries and fuel combustions. As this happens, acid rain is often formed and has many devastating effects. This happens as non metal oxides react with water vapour in the atmosphere and form dilute acids. These reactions can take from several hours to several days and can precipitate the rain hundreds of kilometres away from the pollutant source. As the water does condense, more pollutants can be pulled into the reaction, making the acidity levels of the rain higher. Acid rain is defined as any rain that has a higher hydrogen ion concentration than normal. This normal level is set as 10-5 mol/L, anything above this is considered acid rain. As well as acid rain, acid snow can be formed in a similar manner.
Acid rain has many effects on the environment both manmade and natural. During acid rains, the acidity of lakes and other water bodies increase. This has a very negative effect on the fish and other water creatures; poisoning them and sometimes killing them. The acid rain also has terrible effects on the forests of the world. As it rains, the rain damages large forests, killing many plants. Pine forests are one of the most easily damaged and sustain considerable harm parts of Europe and North America where they are abundant.
Surrounding mine and smelter sites there are generally significant amounts of vegetation. When these mine sites are located in high rainfall areas the vegetation is severely damaged as the operations there give off considerable amounts of oxides.
When an environmental area is unable to neutralise the effect of acid rain, the soil of the area is impacted significantly. The pH of the soil drops as it becomes more acidic. This impacts flowers and other plants in the soil, making it difficult to absorb both potassium and calcium. Protective waxes that are on leaves are often damaged by the acid rain causing significant damage to leave in the area.
Acid Rain also has impacts on the manmade aspects of the environment. Marble and sandstone building surfaces and decorations are greatly damaged in acid rain periods. This is due to the composition of these materials being largely carbonates, specifically calcium carbonate. As carbonates easily react with acids, when the acid rain comes in contact with the buildings they appear to melt away. Many of these effects do not always happen near the source of the pollutants. They can be experienced anywhere in the world that the pollution can travel to.
The only way to prevent acid rain is to reduce the amount of Sulfur oxides and Nitrogen oxides released into the air. Most countries now have legal limits and the amount that factories an power stations can produce.
Acid Rain can be caused by both man made pollutants and natural pollutants. Both methods of producing pollution give off oxides such as sulphur and nitrogen. Both of the pollutants react with water particles in the air whilst in a gaseous state. After the water has become acidic, it condenses and rains down to the earth as acid rain. One of the largest pollutants that form acid rain is sulphur dioxide. Naturally, Sulfur dioxide can come from releases of volcanoes, oceans, biological decay and forest fires. Man Made sources of sulfur dioxide include combustion of fossil fuels, smelting, manufacture of sulfuric acid, production of paper from wood pulp and the burning of garbage. Sulfur dioxide when reacting with water produces the sulfuric acid present in acid rain. It is given in the equation:
Situations in which Sulfur Dioxide is produced.
Another pollutant which reacts with water to give acid rain are the oxides of nitrogen. In contrast with Sulfur dioxide Nitrogen oxides are produced more by natural sources than manmade sources. Most of these oxides in nature are caused by volcano activity, bacterial action and lightning. Emissions from manmade sources are due to fossil fuel combustion including power generations and transport. They also come from nitric acid manufacture, the use of explosives and welding processes. Nitrogen dioxide when reacting with water produces the nitric acid present in acid rain. It is given in the equation:
Situations in which Nitrogen Dioxide is produced.
Both of these pollutants are capable of being involved in acid rain production. It does not matter whether the pollutant is produced due to man or due to the environment.
Since the industrial revolution of the late 1700s - early 1800s there has been a large increase in the emissions of toxic chemicals in the atmosphere of industrialised cities. . Since then the amount of machines within the global economy have increased and thus the amount of emissions have increased.
Air quality constantly decreased until the 1950s when it became so severe that many people died due to the pollution. This led to the introduction of laws and regulations to control emissions from factories. These laws caused a large improvement in air quality, particularly with sulfur dioxide.
Pollution from Nitrous Oxides did not emerge until the 20th century when the production of electricity began and the use of automobiles increased. Nitrous oxides first became a problem in the 1960s where photochemical smog was being produced. This led to the emission controls on cars and power plants. This one again improved the air quality situation however it only lasted for 2 decades.
Despite the invention of Chemical equipment capable of accurately measuring the level of emissions within atmosphere only becoming available in the 1970s; there are large amounts of data to suggest the increases in emissions since 1750s. This previous information was collected through the analysis of ice core samples from the Antarctic region.
The table below was released by USenergy information Administration showing the average concentration of certain pollutants prior to the start of the industrial revolution of 1750. The USenergy administration is a trusted organisation that provides data and forecasts regarding energy and the environment.
The table below released by the National Environment Protection Council in 1998 shows the average current concentrations of certain pollutants and how they are increasing. It also shows limits that were set for industries about how much of the pollutant they can release. The NEPC is a government initiative that addresses environmental protection issues.
Both NEPC and USenergy are government endorsed and release neutral information for the benefit of all industries to increase public understanding of energy use, pollution and the impact on the environment. For this reason all their information is seen as reliable and valid by the global population without any bias.
As Sulfur dioxide and Nitrogen dioxide are often washed out of the atmosphere by rain no apparent build up of these pollutants in the atmosphere is shown. There is a lack of natural evidence (such as the carbon dating and the ice cores) since nitrogen and sulfur oxides do not dissolve in water, but react with it, and that the concentrations are just too small. This is contrasted with Carbon dioxide and nitrous oxide which have increased over the past 150 years by 30% and 15% respectively. However as there are known sources of Sulfur dioxide and nitrogen oxides going into the environment they must be monitored closely over the next few years.
There is due cause for the scientific community to be concerned with the increase of both Sulfur dioxide and oxides of nitrogen in the atmosphere. The first reason for this is the large amount of health impacts that both compounds cause. Sulfur dioxide when exposed to short term can cause bronchoconstriction, emphysema and increased asthma symptoms as it is a respiratory system irritant. Long term exposure to sulfur dioxide can cause severe breathing difficulties. As sulfur dioxide can react with other compounds in the air, they are able to form small particles. These particles are breathed into the lungs and can cause respiratory disease. Nitrogen dioxide also impacts the respiratory system and if the concentration gets to high it can form nitric acid which can destroy body tissue.
The acidic pollution in the air can form photochemical smog. This smog is caused when NO2 reacts with oxygen, sunlight and ozone to from a haze in the atmosphere. This haze impairs sight of people in the region and the particles of the haze can also make their way to your lungs.
As previously said both compounds can form acid rain when they react with water vapour in the atmosphere. This acid rain is a concern as it destroys environments both manmade and natural and kills many animals. Majority of these animals are marine-types whose environment collects acid rain and becomes acidic.
Both sulfur dioxide and nitrogen oxides also have an impact on the economic side of things. As many building and vehicles are damaged by acid rain. The acidity of the rain is high enough for the rain to actually destroy limestone buildings and statues as well as corrode parts of cars. Due to this many costs are accumulated in the repairing and replacing of many, buildings, structured and motor vehicles.
As these impacts on society are great, it is obvious that society needs to reduce the amounts of pollutants that are released into the atmosphere as thus reducing the amount of acid rain. As sulfur is an impurity of coal and many power plants use coal as their primary fuel; a way to reduce sulfur dioxide would be to use coal without a lot of sulfur within in or remove the sulfur before the coal is used. A new method developed to remove sulfur is by using devices known as scrubbers. These scrubbers are able to remove the sulfur chemically without the sulfur being used.
Every person around the world is able to help reduce the amount of toxic pollutants released into our environment. By reducing the amount of electricity people use, less sulfur dioxide producing fuels need to be burnt. By rather walking, cycling or using public transport to commute people can stop producing pollutants their cars would otherwise generate. If that is not possible hydrogen or natural gas cars could be used. By society taking on some of these recommendations, it is possible that sulfur dioxide and nitrogen oxides levels can be reduced thus producing less acid rain.