Chapter 2 – The Acidic Environment
By: Raymond Chen
2.1 – Indicators were Identified with the Observation that Colour of Some Flowers Depends on Soil Composition
2.1.1 – Acids and Bases
- Acids are substances capable of providing hydrogen ions (H+) for chemical reactions.
- They are proton donors.
- The hydrogen ion is only present when the proton is stabilised by association with a solvent molecule.
- In aqueous solutions it exists as a hydronium ion:
Sour in taste / Acid + Metal Salt + H2
Sting and burn skin / Acid + Carbonate Salt + H2 + CO2
Conducts and burns skin / Acid + Base Salt + H2O
Turns blue litmus red
- Bases are substances that can react to form hydroxide ions, OH-, in solution.
- A soluble base is called an alkali.
Properties of Bases
Bitter taste / Soapy and slippery feel
Conducts electricity / Acid + Base Salt + H2O
Turns red litmus blue
- Neutral substances are neither acidic nor basic, some oxides such as CO and NO are neutral.
- A neutralisation reaction between strong acids and strong bases produces neutral salts.
2.1.2 – Acid-Base Indicators
- Indicators can be used to determine the acidic or basic nature of a material over a range.
- By adding a few drops to the substance, they change colour depending on how basic or acidic the solution is.
- Indicators only work for over a range and the range is identified by change in indicator colour.
Indicator / Very Acidic / Slight Acidic / Neutral / Slightly Basic / Very Basic
Litmus / Red / Red / Reddish Blue / Blue / Blue
Bromothymol Blue / Yellow / Yellow / Green / Blue / Blue
Phenolphthalein / Colourless / Colourless / Colourless / Pink / Crimson
Methyl Red / Red / Pink / Yellow / Yellow / Yellow
Methyl Orange / Red / Yellow / Yellow / Yellow / Yellow
- Universal indicator is a mixture of several indicators, thus giving a range of all colours over the entire range.
Very Acidic / Slightly Acidic / Neutral / Slightly Basic / Very Basic
Red / Orange-Yellow / Green / Blue / Deep Violet
2.1.3 – Everyday Uses of Indicators
- Phenol red is used in the testing of swimming pools.
- The right level is required, because if the water is too neutral, it will allow for the breeding of bacterium and minute organisms.
- It turns yellow if pH is below 6.8 and reddish pink if pH is above 8.2 and red when neutral.
- Bromothymol blue is used in the testing of concentration levels of CO2 in aquariums with aquatic plants.
- By adding bromothymol blue to the aquarium, the used is able to determine whether the levels are right for the fish and plants.
- Yellow is acidic at below pH of 6, whilst blue with a pH of 7.6 or higher.
- It turns bluish-green when the substance is neutral.
- Universal indicator is used to determine the acidity of soil.
- Gardeners require this as the acidity of the soil can determine what plants can be grown and whether they are able to live.
- The universal indicator has a colour chart that is followed.
- Red is the most acidic and gradually changes to green, which is neutral and then finally purple which is extremely basic.
- Upon receiving this information, gardeners can adjust the soil to suit the required pH level for their plants.
2.2 – While We Usually Think of the Air Around Us as Neutral, the Atmosphere Naturally Contains Acidic Oxides of Carbon, Nitrogen and Sulfur. The Concentrations of These Acidic Oxides have been Increasing since the Industrial Revolution
2.2.1 – Oxides of Non-Metals
- Oxides of metals are generally basic.
- Soluble metal oxides produce alkaline solutions.
- Alkali and alkali DO NOT react.
- Some metal oxides react with both acids and bases, they are known as amphoteric oxides – aluminium, tin, arsenic, lead and zinc.
- Oxides of non-metals are acidic. They react with water to form acidic solutions.
- Amphoteric oxides react with acids and bases.
- Amphiprotic oxides react like acids and bases.
2.2.2 – Non-Metal Oxides and the Periodic Table
The overall trend of acidity across the periods is:
Basic Amphoteric Acidic
The overall trend of acidity down the groups is:
Acidic Amphoteric Basic
2.2.3 – Le Chatelier’s Principle
In a closed system when there is no more reactions at a macroscopic level the system is known to be at a static equilibrium, however there are still reactions being undertaken at the microscopic level hence the system is in a dynamic equilibrium.
Le Chatelier’s Principle states that if a change occurs in one of the conditions of a system initially in equilibrium, the system would adjust, tending to nullify the change and return to equilibrium. The key features of the principle include:
- Must be a closed system
- NO macroscopic changes
- Reactant and product concentrations remain the same
- Microscopic changes occur
- Rate of forward and reverse reactions are the same
- There will always be reactants and products
2.2.4 – Factors Affecting Equilibrium
Various factors can affect the equilibrium point in a reversible reaction:
Catalysts increase the rate of the reaction but don’t change the equilibrium point.
2.2.5 – Solubility of Carbon Dioxide
Carbon dioxide exists on Earth in gaseous form and dissolved in water. It forms 0.03-0.04% of the Earth’s atmosphere by volume. It is formed by the combustion of carbon and carbon containing matter and also the respiration of plants and animals.
It dissolves sparingly in water:
The dissolved carbon dioxide reacts with water and produces a hydrogen ion, making it acidic.
Rain water is neutrally acidic due to the CO2. It is also part of the natural carbon-cycle where plants use CO2 for photosynthesis. Over the past 150 years CO2 has increased by 30% due to burning of fossil fuels, resulting in global warming. It will continue to increase because:
- More animals and humans putting out CO2
- Rainforests being torn down, hence less plants to take in CO2
- Increasing number of machines and factories hence more pollution of CO2
- Warming oceans results in reduced solubility of CO2
2.2.6 – Sulfur Dioxide and Nitrogen Oxides
The concentration of sulfur dioxide is 0.001 ppm. It can occur naturally and also the product of human activity, including:
- Emissions from volcanoes and hot springs
- Burning organic matter
- Oxidisation of hydrogen sulfide produced during the decay of organic matter
- Combustion of fossil fuels
- Smelting of sulfide ores
Natural sources account for 25% of the 108 tonnes produced each year. It can irritate eyes and the respiratory tract. It can also cause lung damage and asthma and is a major component of smog.
The three main nitrogen oxides are N2O (nitrous oxide), NO (nitric oxide) and NO2 (nitrogen dioxide). The latter two are known as NOx. They are created by:
- High temperatures in car engines and power stations, nitrogen and oxygen in the atmosphere combine to produce
- Lightning strikes can join nitrogen and oxygen together
- Burning fossil fuels and vegetation produces NOx
- Nitrous oxide is formed by soil bacteria and by vehicle catalytic converters. It also contributes to greenhouse gases.
All NOx can damage the respiratory system, damaging lungs and allowing infections to take hold. Photochemical smog is generally made up of NOx. During lightning strikes, the nitrogen atoms are bonded with oxygen atoms to form NO.
NO can then be synthesised into NO2 by naturally reacting with O2.
2.2.7 – Reactions with SO2 and Nitrogen Oxides
Reactions Releasing SO2 into the Atmosphere
Smelting of zinc and lead ores produce SO2:
Burning of FeS2 to produce SO2:
Reactions releasing nitrogen oxides into the Atmosphere
At high temperatures – lightening or internal combustion engines – nitric oxide can form
Nitric acid can react with oxygen to produce nitrogen dioxide
Nitric acid is produced via the catalytic oxidisation of ammonia
2.2.8 – Increases in Concentrations of Oxides of Sulfur and Nitrogen
The Industrial Revolution has resulted in a dramatic increase in the demand for coal and petroleum; hence as a result, there has been a dramatic increase in the production of acidic oxides. However, presently, governments have started to take action due to the increase detrimental health effects that have arisen as a result of the pollution. The extent of the emissions varies with the greatest emissions being close to cities and mining areas, largely due to the vehicles which are sources of sulfur and nitrogen oxides.
The NO2 and SO2 are regularly flushed out of the atmosphere by rain hence no large build-up during the 20th century, but there still is a lack of reliable data, as concentrations are below 0.1 ppm and only modern technologies are capable of detecting them. However, recently, data has shown that in the past 150 years, N2O has increased by 15% and CO2 by 30%.
Regulations are now in place to have catalytic converters in cars to minimise acidic oxide exhaust, whilst factories also need to monitor their emissions. But increased population concentration, greater emission controls have only kept place with the increasing pollution. However, in Australia, the situation is far better than other parts of the world, as the coal used to generate electricity contains far less sulfur. Australian population is centralised along the coast, hence most pollution would be blown away.
2.2.9 – Acid Rain
Rainwater can be acidic due to the presence of CO2, SO2 and NO2 dissolving into the water and forming acids.
Rainwater that has CO2 has pH of 5.5-6 whilst rainwater with NO2 and SO2 has pH of 3-5.
Effects of acid rain include:
- Increased acidity of lakes and river systems
- Defoliation of trees
- Severe damage around mine and smelter sites
- Soil chemistry can change, leading to the death of important micro-organisms and release of normally insoluble aluminium and mercury into soil water.
- As pH drops, plants find it difficult to absorb sufficient calcium or potassium
- Aquatic animals can die if pH drops below 5
- Erosion of marble and limestone structures as a result of acidity
2.3 – Acids Occur in Many Foods, Drinks and Even within Our Stomachs
2.3.1 – Acids as Proton Donors
An acid is a substance that in solution produces hydrogen ions (H+). However they don’t exist like this in solution, instead they exist as hydronium ions (H3O+). The Brønsted-Lowry states that acids are proton donors; hence all acids must contain hydrogen. These ions also explain why acids are good conductors of electricity.
2.3.2 – Some Common AcidsAcid Name / Formulae / Use
Hydrochloric Acid / HCl / Produced as lining in stomach
Used to clean brickwork
Used in industrial applications
Sulfuric Acid / H2SO4 / Used in explosives
Acetic Acid / CH3COOH / Vinegar
Citric Acid / C6H8O7 / Found in citrus fruits
Ascorbic Acid / C6H8O6 / Vitamin C
Important role in healing, blood cell formation and bone and tissue growth
2.3.3 – Acids: Strong, Weak, Concentrated and Dilute
Strong and weak acids depend on the level of ionisation of the acid. HCl in water for example ionised almost completely, as no matter how hard the solution is searched, it is nigh on impossible to find any unionised HCl.
Concentrated and dilute refers to the amount of solute dissolved, rather the percentage of that acid in the solution.
2.3.4 – Using the pH Scale
The pH scale is a scale used to measure the degree of acidity or alkalinity of a solution. The higher the number the most basic it is, the lower the number the more acidic it is. The pH of an acidic solution depends on:
- Concentration of the acid
- Strength of the acid
- Whether the acid is a monoprotic, diprotic, etc.
- The temperature of the solution
2.3.6 – Relative Strengths of Acids
The strength of various acids differ, hence their degree of ionisation also differs:
- Hydrochloric acid is strong
- Citric acid is weak
- Acetic acid is weaker
Their degree of ionisation is as follows:
- Hydrochloric acid – 100% ionised in solution
- Citric acid – 8.6% ionised in solution
- Acetic acid – 1.3% ionised in solution
2.3.7 – Equilibrium and Acid Concentrations
Hydrochloric acid is a strong acid, and in solution, the forward reaction is virtually complete, hence no equilibrium.
Acetic acid is a weak acid, and in solution, there is only partial ionisation, hence equilibrium can be established.
Strong acids cannot establish equilibrium, but weak acids can.
Explaining the Use of Acids as Food Additives
Acids are added to food to improve taste and/or to preserve them. Bacteria cannot survive in acidic conditions and if the acids are weak enough, they are not harmful for human consumption. Common acids include: acetic acid, citric acid, tartaric acid and phosphoric acid.
2.4. – Because of the Prevalence and Importance of Acids, They Have Been Used and Studied for Hundreds of Years. Over Time, the Definitions of Acid and Base Have Been Refined
2.4.1 – Historical Developments of Acids
Antoine Lavoiser (1743-1794) concluded from his experiments that all acids contain oxygen, believing that oxygen gave rise to their acidity. But it was later discovered that many basic oxides also contained oxygen.
Sir Humphrey Davy
Sir Humphrey Davy (1778-1829) showed that hydrochloric acid doesn’t contained oxygen. He proposed that acids were substances that contained hydrogen.
Svante Arrhenius (1859-1927) proposed that acids were neutral substances that, in solution, produced a positive hydrogen ion and a negative ion. He also proposed that bases ionise to produce hydroxide ions in solution. He believed that strength of the acid depended on its degree of ionisation and was able to explain neutralisation. However his ideas were limited:
- It only applied to aqueous solutions
- Didn’t explain that the ionisation of an acid is a reaction between the acid molecule and solvent
- It only takes into account substances that contained hydrogen and cannot explain why some salts can act as acids.
- It couldn’t explain why some acids didn’t ionise still reacted with bases to produce salts
- It can’t explain amphoteric acids.
2.4.2 – The Brønsted-Lowry Theory
In 1923, the Brønsted-Lowry theory was developed and it arrived at a new theory of acids and bases:
- Acids are proton donors
- Bases are proton acceptors
2.4.3 - Conjugates
Acid-base reactions are equilibrium situations and are reversible. Every base has a conjugate acid and every acid has a conjugate base.
- Strong Acid Weak Base
- Strong Base Weak Acid
2.4.4 – Acidic, Neutral or Basic Salts
Salt solutions can be basic or acidic, as well as being neutral. The reaction of a salt with water to produce a change in pH is called hydrolysis.
Basic salts are the salts of strong bases and weak acids. When hydrolysed in water, they have a pH of above 7.
Acidic salts are the salts of strong acids and weak bases. When hydrolysed in water, they have a pH of below 7.
The hydrolysis of a neutral salt in water will have a pH of roughly 7. Neutral salts include, NaCl, KBr, and NaNO3.
When weak acids and weak bases react, both the anion and cation react with water and cancel each other out.
2.4.5 – Amphiprotic Substances
Amphiprotic substances are molecules that can behave as either a Brønsted-Lowry acid or Brønsted-Lowry base in difference circumstances – they can either accept or donate a proton.
Examples of amphiprotic substances include: H2O, HS-, HCO3-, HPO4- and HSO4- - all the substances contain a hydrogen atom so that they can donate a proton.
2.4.6 – Neutralisation as a Proton Transfer Reaction
The neutralisation reaction is:
Neutralisation reactions are proton transfer reactions. They are also exothermic.
2.4.7 – Conducting Titrations
Neutralisation reactions are used in titration to determine the concentration of unknown samples. This is often known as volumetric analysis.
Initially, a primary or standard solution is made. The standard solution is created by what is known as the primary standard, they include:
- (Anhydrous) Sodium Carbonate – Base
- Oxalic Acid – Acid
The properties of the primary standard include:
- Is water soluble
- Unhygroscopic – doesn’t react with air or water
- High in purity
- Concentration known
- Unreactive with air
Volumetric flasks hold the primary/standard solution. The volume is indicated by a line etched onto the neck of the flask.
A pipette is used to accurately deliver a specific volume of solution into the conical flask prior to titration. The aliquot is put into the pipette.
A burette is used to deliver variable volumes of the solution. They are usually graduated from 0.0ml to 50.0ml. The difference between the initial and final readings on the burette indicates the volume of solution delivered into titration. The titrant is put into the burette.
Pipettes and burettes must be initially rinsed with distilled then with the substance that will be transferred. Water droplets remaining may alter the number of moles in the solution. Conical flasks and volumetric flasks only needs to be washed with distilled water and is not necessary for it to dry.
The equivalence point is the point where the volume of added titrant at which the number of moles of the titrant is equal to the number of moles in the aloqoiut.
The endpoint is the point where the colour of the indicator changes.
To determine the endpoint of a titration, a suitable indicator must be chosen, so that the point at which the indicator changes colour is as close to the endpoint as possible.