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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:

H+¿+H2O→H 3O+¿¿ ¿

Properties of AcidsSour in taste Acid + Metal Salt + H2Sting and burn skin Acid + Carbonate Salt + H2 + CO2Conducts and burns skin Acid + Base Salt + H2OTurns 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 BasesBitter taste Soapy and slippery feelConducts electricity Acid + Base Salt + H2OTurns 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 BlueBromothymol Blue

Yellow Yellow Green Blue Blue

Phenolphthalein

Colourless Colourless Colourless Pink Crimson

Methyl Red Red Pink Yellow Yellow YellowMethyl 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 BasicRed Orange-Yellow Green Blue Deep Violet

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2.1.3 – EVERYDAY USES OF INDICATORS

Phenol red is used in the testing of swimming pools. o The right level is required, because if the water is too neutral, it will allow for the

breeding of bacterium and minute organisms. o 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. o By adding bromothymol blue to the aquarium, the used is able to determine

whether the levels are right for the fish and plants. o Yellow is acidic at below pH of 6, whilst blue with a pH of 7.6 or higher. o It turns bluish-green when the substance is neutral.

Universal indicator is used to determine the acidity of soil. o Gardeners require this as the acidity of the soil can determine what plants can

be grown and whether they are able to live. o The universal indicator has a colour chart that is followed. o Red is the most acidic and gradually changes to green, which is neutral and then

finally purple which is extremely basic. o 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.

o Amphoteric oxides react with acids and bases.o 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 AcidicThe overall trend of acidity down the groups is:Acidic Amphoteric Basic

2.2.3 – LE CHATELIER’S PRINCIPLE

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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:

CO2(g)⇌CO2 (aq )

The dissolved carbon dioxide reacts with water and produces a hydrogen ion, making it acidic.

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Increase concentrati

on of reactants

Decrease concentrati

on of reactants

EndothermicIncreased temperature

ExothermicIncrased temperature

Increased PressureMore moles in PRODUCTS

Increased PressureMore moles in REACTANTS


CO2(aq)+H 2O(l)⇌H (aq)+¿+H2CO3

−¿¿ ¿

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.4 By: Raymond Chen


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.

N2 (g)+O2(g )→2NO(g)

NO can then be synthesised into NO2 by naturally reacting with O2.2NO(g)+O2 (g)→2NO2 (g)

2.2.7 – REACTIONS WITH SO2 AND NITROGEN OXIDES

REACTIONS RELEASING SO2 INTO THE ATMOSPHERESmelting of zinc and lead ores produce SO2:

PbS(s)+O2 (g)→Pb(s)+SO2 (g)

Burning of FeS2 to produce SO2: 2 FeS2(g )+O2(g)→2Fe(s)+SO2(g )

REACTIONS RELEASING NITROGEN OXIDES INTO THE ATMOSPHEREAt high temperatures – lightening or internal combustion engines – nitric oxide can form

N 2 (g)+O2 (g )→2NO(g )

Nitric acid can react with oxygen to produce nitrogen dioxide2NO(g)+O2 (g)→2NO2

Nitric acid is produced via the catalytic oxidisation of ammonia

4 NH3 (g )+5O2( g)catalyst→

4NO(g)+6H 2O(l)

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%.

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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.

SO2(g)+H2O(l)→H 2SO3 (aq) H 2SO3 (aq )+O2(g)→H2SO 4(aq)

H 2SO3 (aq )+O 2(g)→H 2SO 4(aq) H 2SO3 (aq )+O2(g)→H2SO 4(aq)

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 ACIDS

Acid Name Formulae

Use

Hydrochloric Acid

HCl Produced as lining in stomachUsed to clean brickworkUsed in industrial applications

Sulfuric Acid H2SO4 Used in explosivesFertilisersCar batteries

Acetic Acid CH3COOH

VinegarFood preservatives

Citric Acid C6H8O7 Found in citrus fruitsAscorbic Acid C6H8O6 Vitamin C

Important role in healing, blood cell formation and bone and

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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.

HCl(aq)+H2O(l )→H 3O(aq)+¿+Cl( aq)

−¿¿ ¿

Acetic acid is a weak acid, and in solution, there is only partial ionisation, hence equilibrium can be established.

CH 3COOH (l)+H2O(l)⇌H 3O(aq )+¿+CH3COOH (aq)

−¿ ¿¿

Strong acids cannot establish equilibrium, but weak acids can.

EXPLAINING THE USE OF ACIDS AS FOOD ADDITIVES

7 By: Raymond Chen


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 LAVOISIERAntoine 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 DAVYSir Humphrey Davy (1778-1829) showed that hydrochloric acid doesn’t contained oxygen. He proposed that acids were substances that contained hydrogen.

SVANTE ARRHENIUSSvante 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 Base8 By: Raymond Chen


Strong Base Weak Acid

CH 3COOH (aq )+OH (aq)−¿⇌CH3COO( aq)

−¿+H2O(l) ¿ ¿

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 SALTBasic salts are the salts of strong bases and weak acids. When hydrolysed in water, they have a pH of above 7.

Weak Acid+StrongBase→Basic Salt+Water

ACIDIC SALTAcidic salts are the salts of strong acids and weak bases. When hydrolysed in water, they have a pH of below 7.

Weak Base+Strong Acid→Acidic Salt+Water

NEUTRAL SALTThe hydrolysis of a neutral salt in water will have a pH of roughly 7. Neutral salts include, NaCl, KBr, and NaNO3.

Strong Acid+StrongBase→Neutral Salt+Water

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:Acid+Base→ Salt+Water

Neutralisation reactions are proton transfer reactions. They are also exothermic.

HCl+OH−¿→Cl−¿+H 2O ¿ ¿

HNO3 (aq )+NH 3 (aq)→NO3 (aq)−¿+NH4 (aq)

+¿¿ ¿

2.4.7 – CONDUCTING TITRATIONS

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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.

Titration curves depend on the strengths of the acid and bas used. When acid is titrated by base, the shape of the curve is ↑. When base is titrated by acid, the shape of the curve is . The equivalence point is in the steep, near vertical part of the curve.

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Titrations with weak acids and weak bases no don’t yield credible results as there is no sudden change in pH around the equivalence point.

Strong Acid and Strong Base Strong Acid and Weak Base

Strong Base and Weak Acid Weak Acid and Weak Base

ASSESSING THE USE OF NEUTRALISATION REACTIONS AS A SAFETY MEASURE OR TO MINIMISE DAMAGE IN ACCIDENTS AND CHEMICAL SPILLS

Neutralisation reactions are exothermic. The enthalpy change is roughly 57kJ. When there are chemical spills, they are dealt differently depending on the place, hence the strength of the acids and bases that are likely to be encountered:

Laboratories – acid is added to large volumes of water, hence the heat generated will be minimised.

Spills of acids on bench tops or floors – neutralised with weak bases such as sodium carbonate or weak acids such as acetic acid. Neutralisation must be done quickly as acids and bases can burn.

Legal requirements specify that acids must not be discharged into waterways until they have been neutralised and diluted. Settling tanks are used to ensure the heat has dissipated before it is released.

Sodium carbonate and sodium hydrogen carbonate are used to neutralise acidic spills as they are:

o Stable solids that can be handled and storedo Not dangerous when used in excess

Dilute hydrochloric and sulfuric acids are used to neutralise base spills or basic wastes, followed by dilution with water.

2.4.8 – BUFFERS IN NATURE

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A buffer is a solution that tends to resist any great change in pH when small quantities of acids or bas are added to it. A buffer consists of:

Weak acid and the salt of that weak acid Weak base and the salt of that weak base

BUFFERS IN BIOLOGICAL SYSTEMSThere are many buffer solutions in the living world. Enzymes, the biological catalysts, work best at very narrow pH ranges.

An important buffer in the bloodstream is carbonic acid/hydrogen carbonate.

CO2(aq)+H2O(l)⇌H 2CO3 (aq )

H 2CO3 (aq )⇌HCO3 (aq )−¿+H( aq )

+¿¿ ¿

As CO2 builds up in the plasma from respiration, more H2CO2 formed. This drives the second equilibrium to produce more hydrogen ions in solution, so pH falls – acidity increases. The kidney then releases more HCO3- into the bloodstream, driving the reaction in the opposite direction, hence reducing the level of H+ ions in the blood. In the meantime, CO2 is exhaled through the lungs. The rate of breathing is closely related to the concentration of CO2 in the bloodstream.

2.5 – ESTERIFICATION IS A NATURALLY OCCURRING PROCESS WHICH CAN BE PERFORMED IN THE LABORATORY

2.5.1 – ALKANOLS AND ALKANOIC ACIDS

ALKANOLSThe alkanols form a homologous series of carbon compounds containing the hydroxyl, -OH. The general formula for alkanols is CnH2n+1OH. Despite the hydroxyl group alkanols do not behave like bases or Arrhenius bases because no hydroxide ions are formed. Features of alkanols include:

The polar –OH group makes alkanols polar. Hydrogen bonding cab behave between –OH groups. Water soluble – decreases with increasing carbon-chain length They have higher melting and boiling points than corresponding alkanes and

alkenes. They react with alkanoic acids to form esters.

ALKANOIC ACIDS

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Alkanoic acids are also known as carboxylic acids form a homologous series of carbon compounds containing the form a homologous series of carbon compounds containing the carboxyl group, -COOH. The general formula for alkanoic acid is Cn-1H2n-1. Features of alkanoic acids include:

They are polar compounds due to the presence of the carboxyl group. Strong hydrogen bonds form between nearby alkanoic acid molecules. They are water soluble due to the hydrogen bonding. They have higher melting and boiling points than corresponding alkanes, alkenes

and alkanols. They are weak acids as they partly dissociate in water:

ROOCH (aq)⇌RCOO(aq)−¿+H(aq)

+¿¿ ¿

Salts of long-chain alkanoic acids are soaps.

2.5.2 – DESCRIBING ESTERS

A primary alkanols is one where the –OH is attached to a carbon and attached to another carbon. A secondary alkanols is where the –OH is attached to a carbon which is in turn attached to two other carbons. A tertiary alkanols is one where the –OH is attached to a carbon which is attached to other carbon atoms.

2.5.3 – MELTING POINTS AND BOILING POINTS OF ALKANOLS AND ALKANOIC ACIDS

Shorter alkane and alkene chains are gases at room temperature. Intermolecular bonds do not form to the same extent as they do for to other substances of similar masses that are liquid or solid at room temperature.

Alkanols contain C-O and O-H bonds, which is polar. Alkanols form hydrogen bonds with each other – hence higher boiling and melting points.

In carboxylic acids, the C=O bonds allow for even greater intermolecular forces hence resulting in even higher melting and boiling points.

2.5.4 – ESTERIFICATION

The reaction between alkanoic acid and an alkanols is called esterification.

alkanoic acid+alkanol⇌ester+water

Esterification is an example of a condensation reaction.

2.5.5 – ACID CATALYST IN ESTERIFICATION

Esterification is an equilibrium reaction which is slow at room temperature. A few drops of H2SO4 are used as a catalyst to increase the speed of the reaction. Boiling the reaction at close to the boiling point of the alcohol can also be used to speed it up. Strong acids may donate a proton to increase the reactiveness of the alkanols and alkanoic acid.

The reaction is often represented as:

alkanoic acid+alkanolH↔

+¿ester+water ¿

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Acid-catalysed esterification is reversible and is generally at equilibrium there are generally appreciable of both ester and alkanols present.

2.5.6 – REFLUXING

Direct esterification can be used to prepare esters in high yields by carefully controlling the reaction conditions. Examples can include:

Inexpensive alcohol – add more to drive the reaction If the boiling point of reactants and ester are higher than water, the temperature can

be raised above 100C to remove water.

Heating is necessary to cause the reaction to proceed. Increased volatility may result in some contents of the reflux being lost. By refluxing volatile components cool in an upright condenser and flow back in the reacting vessel. Boiling chips or porous pot is added to the reaction to provide nucleation centres and promote even boiling – prevents superheating and the sudden and explosive release of vapour. Water bath is used to ensure and promote even boiling.

2.5.7 – USES OF ESTERS

Esters have many uses they include:

Food – flavouring Soaps Solvents Medical uses

14 By: Raymond Chen

· web viewas co 2 builds up in the plasma from respiration, more h 2 co 2 formed. this drives the...

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