TOPIC TWO

THE ACIDIC ENVIRONMENT

Contextual Outline

Acidic and basic environments exist everywhere. The human body has a slightly acidic skin

surface to assist in disease control and digestion occurs in both acidic and basic environments

to assist the breakdown of the biopolymers constituting food. Indeed, micro-organisms found in

the digestive system are well adapted to acidic or basic environments.

Many industries use acidic and basic compounds for a wide range of purposes and these

compounds are found in daily use within the home. Because of this, an awareness of the

properties of acids and bases is important for safe handling of materials. Currently, concerns

exist about the increased release of acidic and basic substances into the environment and the

impact of these substances on the environment and the organisms within those environments.

This module increases students’ understanding of the history, nature and practice of chemistry,

the applications and uses of chemistry and implications of chemistry for society and the

environment.

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Section ONE

Indicators were identified with the observation that the colour of some flowers depends on soil composition

Acidic Substances Basic Substances Neutral Substances

Vinegar Ammonia cleanser Water

Citric juices Caustic soda solution NaCl solution

Lemon juices Washing Soda Glucose solution

Carbonated soft drinks Oven cleaner Lactose solution

Lactic acid Drain Cleaner

An indicator is a chemical dye that changes colour depending on the concentration of hydrogen

of the solution.

Indicator Colour in

acidic solution

Colour in

pure water

Colour in

basic

solution

pH range Suitable

titration

Methyl orange Red Orange –

yellow

Yellow 3.1 – 4.4 strong acid –

weak base

Bromothymol

blue

Yellow Green Blue 6.2 – 7.6 strong acid –

strong base

Litmus Red -------- Blue 6.0 – 8.0 ---------

Phenolphthalei

n

Colourless Colourless Crimson 8.3 – 10.0 Weak acid –

strong base

These indicators specialise in the classification of certain acids and bases. E.g. Methyl orange

would be able to distinguish between a strong acid and a weak acid, whereas phenolphthalein

would be able to distinguish between a weak base and a strong base.

However these indicators also have limitations e.g. methyl orange cannot distinguish between

a neutral and a base.

-3-

Classify common substances as acidic, basic or neutral

Identify that indicators such as litmus, phenolphthalein, methyl orange and bromothymol blue can be used to determine the acidic or basic nature of a material over a range, and that the range is identified by change in indicator colour

Identify data and choose resources to gather information about the colour ranges of a range of indicators

Identify and describe some everyday uses of indicators including the testing of soil acidity/basicity

Indicators provide a cheap and convenient way to determine the acidity or alkalinity of

substances

Indicators are used to test the soil acidity/basicity

o Some plants grow best in slightly acidic (azaleas and camellias) soil while others prefer

slightly alkaline soils(most annual flowers and vegetables)

o A sample of soil to be tested is mixed with water and then white barium sulfate powder

added to the soil sample. This allows the colour changes of the indicators to be seen.

Testing home swimming pools

o These need to be approximately neutral to prevent eye or skin irritation

Monitoring wastes

o Discharges to the sewerage system must be nearly neutral as wastes can cause sinks

and pipes to corrode e.g. photographic solutions are highly alkaline

Chemical analysis

o Indicators are often used in analytical work such as titration

Red cabbage was chopped up and then grinded with a pestle in a mortar. As this was done,

methanol was added to the mixture. The resulting clear purple solution was extracted and used

as an indicator. This indicator turned green in a base e.g. oven cleaner and pink in an acid e.g.

lemon juice

-4-

Perform a first hand investigation to prepare and test a natural indicator

Solve problems by applying information about the colour changes of indicators to classify some household substances as acidic, neutral or basic

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Section TWO

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

The oxides of non-metals are usually acidic oxides which

Reacts with water to form an acidic solution or/and

Reacts with bases to form a salt and water

Carbon dioxide + water carbonic acid

CO2 (g) + H2O (l) H2CO3 (aq)

Carbon dioxide + sodium hydroxide sodium carbonate + water

CO2 (g) + NaOH (aq) Na2CO3 (aq) + H2O (l)

The oxides of metals are usually basic oxides which

Reacts with water to form a basic solution or/and

Reacts with acid to form a salt and water

Sodium oxide + water Sodium hydroxide

Na2O(s) + H2O(l) 2NaOH(aq)

Sodium oxide + hydrochloric acid Sodium chloride + water

Na2O(s) + HCl (aq) 2NaCl(aq) + H2O (l)

The oxides of semi-metals are usually amphoteric

This means they can react with an acid or a base

Zinc oxide + hydrochloric acid Zinc chloride + water

ZnO(s) + HCl (aq) ZnCl2 (aq) + H2O (l)

Zinc oxide + sodium hydroxide + water sodium zincate

ZnO(s) + 2NaOH (aq) + H2O (l) Na2[Zn(OH)4] (aq)

Acidic oxides are generally oxides of non-metals.

o They are all covalent compounds.

o These are elements towards the right and top of the periodic table

Basic oxides are generally oxides of metals

o They are ionic compounds

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Identify oxides of non-metals which act as acids and describe the conditions under which they act as acids

Analyse the position of these non-metals in the Periodic Table and outline the relationship between position of elements in the Periodic table and acidity/basicity of oxides

o These are elements towards the left and bottom of the periodic table

Oxides of semi-metals are amphoteric

In going down a group of the periodic table, the oxides of the elements become more basic

due to the increasing metallic character of the elements

In going across a group from left to right of the periodic table, the oxides of the elements

become more acidic to the decreasing metallic characteristic of the elements.

Le chatelier’s principle states that if a system at equilibrium is disturbed then the

system will adjust itself in such a measure to counteract this disturbance

Chemical equilibrium is a particular state of a reversible reaction when

The rate of forward reaction is equal to the rate of backward reaction

The concentrations of the reactants are constant (not necessarily equal)

It is in a closed system

It has the following properties of

Macroscopic properties such as colour, concentration, pressure etc remain unchanged

The same equilibrium is obtained both ways (evident in graph)

Factors that affect chemical equilibrium are

Temperature

Pressure

Concentration

Concentration

Increasing the concentration of one sides shifts it to the other by Le Chatelier’s principle

Decreasing the concentration of one side shifts it to the same side in order to produce more

of it

2NO2 (g) N2O4 (g)

o Increasing the concentration of NO2 would shift the equilibrium to the right while

increasing the concentration of N2O4 would shift it left*

-7-

Define Le Chatelier’s principle

Identify factors which can affect the equilibrium in a reversible reaction

o Decreasing the concentration of NO2 would shift the equilibrium to the left and

similarly decreasing N2O4 would shift it right

*Note: Even though increasing the concentration of either gases increases the pressure, the

concentration shift is more significant!!!

*Gases have concentration!!!

The addition or removal of solid and/or liquid has no effect on an equilibrium as the

concentration has not changed

Pressure

Varying the pressure of an equilibrium only affects it if

o There exists gaseous molecules within the equilibrium

o The total number of moles of gases on each side is different

Increasing the pressure would shift the equilibrium to the side with less gas molecules by

LCP in order to counteract the change by producing less gaseous molecules.

Decreasing the pressure of the system would shift it to the side with more gas molecules

2NO2 (g) N2O4 (g)

o Increasing the pressure would shift it to the right

o Decreasing the pressure would shift it to the left

Temperature

Increasing the temperature would cause the system to favour the endothermic reaction and

shift to that side accordingly. This is to decrease the heat energy by using it up in the

reaction

Decreasing the temperature would cause the system to favour the exothermic reaction and

shift it to that side. This would increase the temperature from the heat given off by the

exothermic reaction

N2 (g) + 3H2(g) 2NH3 (aq) + 92 KJ/mol

OR

N2 (g) + 3H2(g) 2NH3 (aq) ∆H = - 92 KJ/mol

This signifies the forward reaction is exothermic while the backward reaction is endothermic.

Therefore if the temperature is increased, then the system shifts left, if the temperature is

decreased, the system shifts right.

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Describe the solubility of carbon dioxide in water under various conditions as an equilibrium process and explain in terms of Le Chatelier’s principle

The reaction between carbon dioxide and water reaches equilibrium

CO(g) + H2O(l) H2CO3 (aq) ∆H <0

If the temperature increases, the reaction shifts to the left in order to use up the extra heat

energy and hence the solubility of carbon dioxide decreases. This accounts for the extra

fizzling when the temperature has increased

If the temperature decreases, the reaction shifts right and hence the solubility increases

When a bottle of soft drink is opened, the system goes from high pressure to lower RTP,

thus the system shifts left and causes fizzing

If the pressure is increased, the solubility increases and shifts left due to there being less

gaseous molecules

Sulfur dioxide

Naturally

o Volcanoes and geothermal hot springs are a major source of natural SO2, Since

volcanic activity varies, the concentration of SO2 also varies

o The oxidation of H2S produced during the decay of organic matter

2H2S(g) + 3CO(g) 2SO2 (g) + 2H2O(g)

Industrially

o Sulfur dioxide is produced from the combustio of fossil fuels in power plants due to

sulfur impurities present in coal

S(s) + O2(g) SO (aq)

o The smelting of sulfide ores

2ZnS(s) + 3O2(g) ZnO(s) + SO2 (g)

Nitrogen Dioxide

Naturally

o Lightning generates very high temperatures in which causes N2 and O2 to form

nitrogen monoxides which slowly oxides to form nitrogen dioxide

N2(g) + O2(g) 2NO(g)

2NO(g) + O2(g) 2NO2(g)

o The action of bacteria

Industrially

o Both in power stations and vehicles releases both NO and NO2 provide the high

temperatures

-9-

Identify natural and industrial sources of sulfur dioxide and oxides of nitrogen

Describe, using equations, examples of chemical reactions which release sulfur dioxide and oxides of nitrogen

Assess the evidence which indicates increases in atmospheric concentration of oxides of sulfur and nitrogen

Since the industrial revolution, levels of acidic oxides in the atmosphere have greatly

increased. This is evident from

o The increase burning of coal and fossil fuels in order to meet societies demands

from a larger population

o The usage of motor vehicles have increased levels of nitrogen oxides

o The pH of lakes and rivers have increased over time

o The quantitative analysis of trapped air bubbles in Antarctic ice and measurement of

carbon isotopes in old trees, grass seeds in museum collections and calcium

carbonate in coral.

However

o It is extremely difficult to have global areas and thus tests are done in a localised

basis

o Technology to measure low concentrations were only available in the past 30 years

o Emission controls have stabilised the concentration of acidic oxides

Assessment: Therefore the evidence is unreliable to an extent

When measured at the same temperature ad pressure, equal volumes of gases contain the

same number of molecules. At STP, the volar volume of gases is 22.74L while at RTP, it is

24.79L

Moles = Volume

Molar Volume

The bottle of soft drink was weighed, then shook and opened. This was done repeatedly over

intervals of 10 minutes with the decrease in mass recorded. The mass change was due to the

CO2 being released in the air as a result of the equilibrium decreasing in pressure as the bottle

opened.

The validity of the experiment could have been repeated by using the salt method that uses

the stronger attraction soluble salt ions have for water than dissolved gas molecules have for

water. However the results were not totally accurate as a new equilibrium was established,

thus not all the gas removed from the bottle

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Calculate volumes of gases given masses of some substances in reactions, and calculate masses of substances given gaseous volumes in reactions involving gases at 0oC and 100kpa or 25oC and 100kpa

Identify data, plan and perform a first hand investigation to decarbonate soft drink and gather data to measure the mass changes involved and calculate the volume of gas released at 25oC and 100kps

There was a great increase in emissions of sulfur dioxide in growing industrial cities as a result

of the development of motor cars and an increasing population leading to greater demand

The release of sulfur dioxide and oxides of nitrogen can cause considerable harm to both

humans and the environment

These oxides are soluble and form acidic rain

o Acid rain is rain that has a pH of less than 7

o The pollutants of sulfur dioxide and oxides of nitrogen form sulfuric and nitric acids

when dissolved in the rain

S(s) + O(2) SO2 (g)

2SO2(g) + O2 (g) 2SO3(g)

SO3(g) + H2O (l) H2SO4(aq)

N2(g) + O2(g) 2NO(g)

2NO(g) + O2(g) 2NO2(g)

2NO2(g) + H2O(l) HNO2(aq) +

HNO3(aq)

Effects of Acid Rain

Increasing acidity of lakes and rivers which has a detrimental effect on fish population and

hence affects other parts of the ecosystem e.g. the humans that feed on them

Damage to forests

Decrease in soil that ruins crops and plants as some can only grow in certain conditions

Erosion of the marble and limestone of building surfaces because they contain primarily

CaCO3

Corrosion when it comes into contact with metals, paints and similar substances

Health Issues

SO2 irritates the respiratory system and causes breathing difficulty

NO2 irritates the respiratory tract and causes breathing discomfort

o Can do extensive tissue damage

o Photochemical smog causing poor visibility

o Unpleasant odour

o Produces O3 with sunlight

-11-

Explain the formation and effects of acid rain Analyse information from secondary sources to summarise the

industrial origins of sulfur dioxide and oxides of nitrogen and evaluate reason for concern about their release into the environment

Section Three

Acids occur in many foods, drinks and even within our stomaches.

Acids in aqueous solutions are proton donors. The arrhenius definition describes acids as

producing hydrogen ions when dissolved in water, however H+ does not exist in aqueous

solution. When acids dissolve in water, they react and ionise to form H3O+ ions.

e.g. HCl(aq) + H2O(l) H3O+(aq) + Cl-(aq)

Naturally Occuring acids

Acetic acid (ethanoic acid) – CH3COOH is present in vinegar which is commonly made from

wine by the oxidation of ethanol

Citric acid (2-hydroxypropane-1,2,3-tricarboxylic acid) – This acid is found in citrus fruits

Hydrochloric acid – This acid is produced by the glands in the lining of our stomaches to form

an acidic environment for the efficient operation of enzymes that break food molecules into

other molecules for easy transportation to the intestine and so on.

Industrial acids

Sulfuric acid – This acid is manufactured worldwide more than any other chemical. It is used to

make synthetic fibres, industrial ethanol, fertilisers and car batteries

Naturally occurring Bases

Ammonia – This is present in the stale urine of humans

Amines (NH2CH3) – these are formed during the anaerobic decomposition

pH = -log10[H+] and thus [H+] = 10-pH

In measuring the concentration of acids and bases, a pH scale is used because it is a lot more

convenient

If the pH is <7 then the substance is an acid

If the pH is > 7 then the substance is a base

If the pH = 7 then the substance is neutral

-12-

Define acids as proton donors and describe the dissociation of acids in water

Identify acids including acetic (ethanoic), citric (2-hydroxypropane-1,2,3-tricarboxylic), hydrochloric and sulfuric acid

Identify data, gather and process information from secondary sources to identify examples of naturally occurring acids and bases and their chemical composition

Describe the use of pH scales in comparing acids and bases Identify pH as –log10[H+] and explain that a change in pH of 1

means a ten-fold change in [H+]

From the equation it can be seen that increasing the a change in pH of 1 means a 10- fold

change in [H+] due to the log10

A strong acid is one which is fully ionised in solution. There are no neutral acid molecules

present

HCl(aq) H+(aq) + Cl-(aq) (this shows HCl has fully ionised)

A weak acid is one which is partially ionised in solution. These acids are involved in an

equilibrium process

CH3COOH(aq) CH3COO-(aq) + H+

(aq)

A concentrated solution is one which there is a large number of particles of the solute

compared to water molecules e.g. 1M HCl

A diluted solution is one where there is a small amount of particles compared to the water

molecules. E.g. 0.00001M HCl

The following table contains the pH of acids for equal 0.1M concentrations of their solutions

Acid Hydrochloric acid Acetic acid Citric acid

pH 1 2.9 2.1

This is because hydrochloric acid is a strong acid meaning it will completely ionise, hence the

pH of 1. Since Acetic and Citric acids are both weak acids, they will both partially ionise,

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Describe acids and their solutions with the appropriate use of the terms strong, weak, and concentrated and dilute.

Compare the relative strengths of equal concentrations of citric, acetic and hydrochloric acids and explain in terms of the degree of ionisation of their molecules

accounting for their lower pH of equal concentrations. Citric acid is stronger than acetic acid

and thus ionises more, producing more hydrogen ions and having a lower pH.

HA + H2O H3O+ + A-

The stronger an acid is, then the stronger the dissociation and thus the equilibrium will lie

to the right. Strong acids such as HNO3 and HCl will completely ionise, thus HCl(aq) H+(aq)

+ Cl-(aq)

The weaker an acid is, then the lower the dissociation percentage and thus the equilibrium

will lie to the left. Weak acids such as CH3COOH will partially ionise and thus have more

intact molecules than HCl. As a result an equilibrium is formed CH3COOH(aq)

CH3COO-(aq) + H+

(aq)

How does a pH meter work?

Millivoltmeter and a pH electrode which is sensitive to the concentration of hydrogen ions

The pH electrode is a concentrated cell consisting of 2 silver-silver chloride electrodes.

The outer of 2 electrodues is in contact with a 0.1M HCl solution saturated with silver

chloride and makes electrical contact with a thin glass membrane

The difference in hydrogen concentration causes changes in voltages between the two

electrodes and the mV scale converts the pH into a scale on the motor

The pH electrode must be calibrated with a known solution, thus a buffer (usually 7)

Aim: to use indicators and a pH meter to distinguish between acidic, basic and neutral

chemicals and measure the pH of substances with equal concentration

Method:

A couple of drops of universal indicator were added to the substances and the colour changes

were observed. These colour changes were compared against a chart and thus the pH was

estimated

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Describe the difference between a strong and a weak acid in terms of an equilibrium between the intact molecule and its ions

Solve problems and perform first-hand investigation to use pH meters/probes and indicators to distinguish between acidic, basic and neutral chemicals

Plan and perform a first hand investigation to measure the pH of identical concentrations of strong and weak acids

Gather and process information from secondary sources to write ionic equations to represent the ionisation of acids

Also, by using a pH meter, the pH measured was much more accurate. However between each

substance being tested, the pH probe needed to be washed to remove remnants from the

previous substance tested. This is also a non-destructive means of measuring pH

250ml of 0.1M solutions were gathered and poured into individual beakers.

Results:

Substance Strength pH meter Universal indicator

Hydrochloric acid Strong 1.5 Red

Ethanoic acid Weak 3.5 Red/orange

Citric acid Weak 2.81 Red/orange

Sulfuric acid Strong 1.88 Red

Sodium hydroxide Strong 12 Blue/purple

Water Neutral 6.67 Green

Discussion:

Strong acids have low pH because they completely ionise, thus having a large amount of

hydrogen ions in the solution. Weak acids partially ionise, thus have higher pH

Acids are frequently added to food in order to

Improve the taste by giving a sharper taste e.g. in drinks, sweets, tarts, jams

Preserve the food by increasing it’s acidity because many bacteria cannot survive in acidic

conditions e.g. canned fruits, vegetables

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Gather and process information from secondary sources to explain the use of acids as food additives

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Section Four

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

Lavoisier

o Proposed that acids were substances that contained oxygen. He proposed this after

observing acids such as sulfuric acid or nitric acid, but his theory was soon disproved by

basic substances containing oxygen and others such as HCl

SO3 + H2O H2SO4

Davy

o Suggested that acids contained replaceable hydrogen. This was proposed from

observations where hydrogen gas was released from the reaction between a metal and

an acid

o However it did not explain the properties of the acids and it did not explain why

methane was not acidic

Zn + 2HCl ZnCl2 + H2

Arrhenius

o Proposed that an acid was a substance which ionised in solution to produce hydrogen

ions.

o It was important in explaining the differences between strong and weak acids and the

development of the pH scale

o However there existed oxides that were acidic and did not fit the definition or

substances such as NH3

Bronsted and Lowry proposed that an acid is a substance that donates a proton and a base

is one that accepts a proton. Thus in a neutralisation reaction, the acid donates a proton to

the base. In this way, the Bronsted Lowry concept relates acidity and basicity to the

structure of both the substances and the solvent.

An acid gives up a proton to form its conjugate base. Similarly a base accepts a proton to

form its conjugate acid. They differ by that proton

e.g. H3O+ / H2O

The hydronium ion is the acid and the water is the conjugate base

     HCl     +     H2O             Cl-          +         H3O+     acid1          base2       conjugate base1    conjugate acid2

Hydrochloric acid and chloride ion is a conjugate acid-base pair.

-17-

Outline the historical development of ideas about acids including those of:

o Lavoisiero Davyo Arrhenius

Outline the Bronsted-Lowry theory of acids and bases

Describe the relationship between an acid and it’s conjugate base and a base and it’s conjugate acid

Identify conjugate acid/base pairs

Identify a range of salts which form acidic, basic of neutral solutions and explain their acidic, neutral or basic nature

Salts can be acidic, basic or neutral depending upon the nature of the anions or cations

formed

Acidic salts

These salts are formed from a reaction between a strong acid and a weak base

e.g. NH4Cl + NH4+ + Cl-

NH4+ + H2O NH3 + H3O+

The salt formed contains a cation being a weak acid, this causes it to react with water to

form the hydronium ions accounting for it’s pH being less than 7

Basic salts

These salts are formed from a reaction between a weak acid and a strong base

The salt formed contains an anion which is a weak base and thus tends to react with water

CH3COONa CH3COO- + Na+

CH3COO- + H2O CH3COOH + OH-

This hydroxide ion causes the solution to have a pH of greater than 7

Neutral salts

These are formed by reacting a strong acid and a strong base or a weak acid with a weak

base

An amphiprotic substance is one that can accept or donate a proton

HCO3- + H2O H2CO3 + OH-

HCO3- + H2O CO3

2- + H3O

Neutralisation is a proton transfer reaction that consists of an acid donating a proton to a

base which produces salt and water. This reaction releases energy and thus is exothermic

Preparation of a primary standard solution

A primary standard solution is one with an accurately known concentration. It is the one

that goes in the conical flask and is titrated with a solution of unknown concentration.

-18-

Identify amphiprotic substances and construct equations to describe their behaviour in acidic and basic solution

Identify neutralisation as a proton transfer reaction which is exothermic

Describe the correct technique for conducting titrations and preparation of standard solutions

Properties of a primary standard

o Must be available in pure form

o Mass must not change when exposed to air

o Must be soluble in water

o Molecular mass should be relatively high to minimise weighing error

o It’s chemical formula must be accurately known

Firstly weigh the required mass of dried primary standard solid (after baking in a

desiccator)

Dissolve a small amount of distilled water

Add to a clean volumetric flask rinsed before hand with distilled water

Wash the remnants from the beaker into the flask

Fill the flask with more water and shake/invert until the solid has completely dissolved

Add more water up until the graduated mark

*Note: some substances are not suitable to be a primary standard solution as

Concentrated HCL fumes and loses it as gas

Concentrated sulfuric acid absorbs water from the atmosphere

NaOH reacts with CO2 or deliquescence’s

Acid standards Base standards

Potassium hydrogen phthalate Sodium carbonate

Benzoic acid Sodium hydrogen carbonate

Oxalic acid Borax

Secondary standard solutions

These are solutions whose concentrations are unknown and will be determined by titrating

against a primary standard

Using the equipment

Burette

This is an accurate piece of equipment used to transfer a large amount of solution. It is

used for the titrant and thus must be rinsed firstly by distilled water to remove any residue

waste and then with the titrant solution to prevent dilution and thus affecting the accuracy

of the results

Pipette

This is an accurate piece of equipment used to transfer small amounts of solutions of the

secondary standard solution to the conical flask. It must be rinsed out by distilled water and

then the secondary standard to avoid dilution

Indicators

-19-

The indicator used depends on the titration reactants. This is due to the different

equivalence point of the titration. The equivalence point is the point where the acid and

base neutralise. However due to the varying pH of salt solutions, it is not necessarily 7 and

thus an indicator with an end point within the range of the equivalence point is needed.

Type Indicator Colour change

Strong acid – strong base Bromothymol blue 6.2-7.6 Yellow -> Blue

Strong acid – weak base Methyl orange 3.1 – 4.4 Red -> Yellow

Weak acid – strong base Phenolpthalein 8.3 – 10 Colourless -> crimson

Note: Weak acid – weak base titrations are not done because there is no rapid change in pH,

thus we cannot detect the equivalence point using indicators because there will be a gradual

colour change

Techniques

The first titre is a rough measurement and thus discarded when using calculations

The end point is where the indicator changes colour

A white sheet of water was placed under the conical flask to see the colour change more

evidently

A buffer solution is one which contains a comparable amount of a weak acid and it’s conjugate

base which is therefore able to prevent a rapid change in pH.

Blood is an example of a buffer solution that maintains a pH of approximately 7.4 to allow the

proper functioning of enzymes. It consists of the buffer H2CO3 / HCO3- along with haemoglobin

to prevent rapid pH changes during cellular respiration

H2CO3 + H2O HCO3- + H3O+

If a strong acid is added the increase in H+ ions will cause the equilibrium to shift to the left

and maintain the pH

If a strong base is added to decrease the H+ ions, it will cause the equilibrium to shift to the

right

Lavoiser stated that acids contained oxygen. This was wrong, but stimulated research into the

composition and properties of acids

-20-

Qualitatively describe the effect of buffers with reference to a specific example in a natural example

Gather and process information from secondary sources to trace developments in understanding and describing acid/base reactions

Davy defined an acid as containing replaceable hydrogen. His definition helped to classify

substances in terms of it’s properties and reactions

The Arrhenius definition increased our understand by interpreting acid properties in terms the

hydrogen ions they produced and explained weak and strong acids in terms of the extent to

which the ionisation reaction proceeded.

The Bronsted Lowry definition increased our understanding further by showing that acidity

depends on both the structure of the substance itself and also those of the other reactant in

the solution. It showed neutralisation proceeded directly by a proton transfer and showed that

the hydrolysis of salt produced pHs different from 7 was nothing more than a simple acid or

base reaction

Each theory successively refined the previous

A 0.1mol/L 50ml solution of the following were gathered in beakers

Sodium hydrogen carbonate - acidic

Sodium chloride - neutral

Sodium acetate - basic

A pH meter was used to measure the pH of these solutions

HCO3- + H2O CO3

2- + H3O+

This hydronium ion causes sodium hydrogen carbonate to be acidic

CH3COO- + H2O CH3COOH + OH-

This hydroxide ion causes sodium acetate to be basic

Titration 1

Preparation of a standard solution

o Firstly solid sodium hydrogen carbonate was baked in a desiccator to dry and

then weighed on a mass balance. It was then transferred into a volumetric flask

where water was added to dissolve the solid by shaking and inverting. Once all

the solid had dissolved, more water was added to the graduated mark

-21-

Choose equipment and perform a first hand investigation to identify the pH of a range of salt solutions

Perform a first hand investigation and solve problems using titrations and including the preparation of standard solutions and use available evidence to quantitatively and qualitatively describe the reaction between selected acids and bases

Perform a first hand investigation to determine the concentration of a domestic acidic substance using computer based technology

Titration

o Nitric acid was titrated against a 0.05 mol/L primary standard of sodium

hydrogen carbonate

o The indicator used was methyl orange since the equivalence point was

approximately 3 – 5 due to the titration being a weak base + strong acid

o The concentration of nitric acid used was approximately 0,108 moles

Titration 2

NaOH was titrated against a primary standard solution of vinegar with a pH meter used

instead of an indicator

Neutralisations reactions are widely used for safety in laboratories and factories where acids or

bases are used since they are very corrosive.

Sodium carbonate is widely used to neutralise acid/base spills or effluents because

It is a stable solid which is easily and safely handled and stored

It is the cheapest alkali

If too much of it is used there is less danger than if a stronger acid/base was used to

neutralise it

It is amphiprotic and can neutralise both acids and bases

-22-

Analyse information from secondary sources to assess the use of neutralisation reactions as a safety measure or to minimise damage in accidents or chemical spills

It neutralises at a moderate speed so the release of energy from neutralisation being an

exothermic reaction is gradual

-23-

-24-

Section Five

Esterification is a naturally occurring process which can be performed in the laboratory

Alkanols are alcohols derived from alkanes that contain the hydroxyl group

e.g. methanol – CH3OH

ethanol – C2H5OH

(General formula of CnH2n+1OH)

Alkanoic acids contain the carboxylic functional group (COOH)

e.g. methanoic acid H-COOH

ethanoic acid – CH3 – COOH

(General formula of

The boiling and melting point of molecular substances depend upon the strength of the

intermolecular forces. The stronger the forces, the higher the boiling point.

For the equivalent molecular mass alkanol and alkanoic acid

The molecular forces displayed in alkanol are

o dispersion forces

o dipole-dipole forces due to the high electronegativity of the oxygen atom

o hydrogen bonds due to the presence of the oxygen and the hydrogen.

There are similar forces displayed in the alkanoic acid, however there exists a greater degree

of hydrogen bonding due to the presence of two oxygen atoms. Thus this accounts for a

greater boiling and melting point of the equivalent massed alkanol and alkanoic acid

Alkanol alkanoic acid

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Describe the differences between the alkanol and alkanoic acid functional group in carbon compounds

Explain the difference in melting point and boiling point caused by straight chained alkanoic acid and straight chained primary alkanol structure

Note: for the equivalent number of carbons in the alkanol and alkanoic acid, the alkanoic acid

will have a greater molecular mass, thus stronger dispersion forces and hence a greater MP

and BP

Esterification:

An ester is a substance that contains the R-OO-R’ chain

Alkanoic acid + alkanol Alkyl Alkanoate (ester) + water

Note: The alkanol forms the alkyl part and the alkanoic acid forms the alkanoate

e.g.

Ethanoic acid + methanol methyl ethanoate + water

Ethanoic acid + ethanol ethyl ethanoate + water

Butanoic acid + propan-1-ol propyl butanoate + water

6 mol/L sulfuric acid is normally used as a catalyst during esterification because

o It acts as a catalyst by reducing the activation energy and thus increasing the

rate of reaction

o It acts as a dehydrating agent and absorbs the water, thus shifting the

equilibrium to the right and increasing yield of the ester

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Identify esterification as the reaction between an acid and an alkanol and describe using equations, examples of esterification

Identify the IUPAC nomenclature for describing the esters produced by reactions of straight chained alkanoic acids from C1 to C8 and straight chained primary alkanols from C1 to C8

Describe the purpose of using acid in esterification for catalyst

Explain the need for refluxing during esterification Identifying data, plan, select equipment and perform a first

hand investigation to prepare an ester using reflux

Refluxing is the process of heating a reaction mixture in a

vessel with a Liebig condenser to prevent loss of any

volatile reaction or product while allowing the reaction to

be carried out at a higher temperature. This is down by the

volatile components becoming a gas and then condensing

in the condenser to run back down

Reflux is used in esterification since it increases the rate of

reaction and also increases the yield by LCP as

esterification is a endothermic reaction

Reflux conditions also improves the safety of the operation

as the volatile components may be flammable e.g. the

alkanol

A stopper was not used at the top as it may cause an

explosion

10 ml of pure pentan-1-ol and 12ml of 1M ethanoic acid

were added to a round flask with 3 drops of 6M sulfuric acid.

C5H11OH(aq) + CH3COOH(aq) water + C4H11C-OO-C-CH3

Pentan-1-ol + ethanoic acid -> water + pentyl ethanoate

This mixture was heated under reflux for about 10 mins and then allowed to cool. The mixture

was then transferred to a separate funnel with 100ml of water added and shaken. The lower

layer was then discarded. Then 50ml of 1M Na2CO3 was added to remove the excess acid

Na2CO3 + CH3COOH 2NaCHCOO + CO2 + H2O

Na2CO3 + H2SO4 Na2SO4 + CO2 + H2O

The lower layer was then discarded once again. The remaining mixture was transferred into a

beaker with a few pellets of CaCl2 to absorb the water. The remaining liquid was then distilled

with temperatures maintained at 130. Pentyl ethanoate is a banana flavoured ester

-27-

Outline some examples of the occurrence, production and uses of esters

Process information from secondary sources to identify and describe the uses of esters as flavours and perfumes in processed foods and cosmetics

Esters have pleasant and fruity odours and occur widely in nature as perfumes and flavouring

agents.

Ester present Flavour

Methyl butanoate Apple

Pentyl ethanoate Banana

Ethyl heptanoate Grape

Butyl ethanoate Raspberry

Esters are developed artificially for use in flavouring and perfumes and are often cheaper than

natural extract. The natural odour and combination of crude taste contributes to the flavour of

the ester. Ethyl ethanoate is also used as a solvent in industry and is the common solvent in

nail polish remover.

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