Chemistry All Topics Intros

Introduction

SEPARATING MIXTURES AND PURE SUBSTANCES

http://www.elateafrica.org/elate/chemistry/separation/separarationintro.html
An important skill in chemistry is to be able to separate mixtures of substances into their separate parts. If a mixture is made up of separate bits of each substance then they can often be separated using physical methods. Once they have combined together to form chemical compounds then other methods of separation have to be used.

This unit will help you learn alternative methods of separating mixtures and pure substances based on their chemical characteristics and properties. For example, some substances are either soluble or insoluble in water. What are some of the substances used in our daily lives that are either soluble or insoluble in water?
This unit will also help you appreciate the practical application of the basic methods of separating mixtures and pure substances in the real world situation. You will be able to appreciate that basic knowledge of principles of separating mixtures and pure substances are fundamental in our lives.
You will learn about basic methods to
(i) separate insoluble solids from liquids
(ii) separate soluble solids from liquids
(iii) separate coloured substances in solution
(iv) separate liquid-liquid mixtures
(v) separate solid-solid mixtures
(vi) methods of drying solids. 
Introduction

OBSERVATIONS AND EXPERIMENTS

HEATING SUBSTANCES

This unit is about the effect of heat on substances. When we subject some substances to heat they catch fire or burn to form completely different products that are not similar to the original substance. When a substance burns in the presence of air/oxygen we say it has undergone combustion.

Other substances do not catch fire but undergo a permanent change forming new substance(s). These processes are examples of a chemical change. Yet again there are other substances, such as water and iodine, that merely change their physical appearance or state when subjected to heat. This change is non-permanent and is called a physical change.

Furthermore, as some substances breakdown, gases are given off. The properties of those liberated gases help us to identify the original substance.

This knowledge can be applied to separating mixtures of iodine, ammonium chloride, aluminium chloride, iron(III) chloride from other substances, such as common salt. Furthermore, the effect of heat on substances is also useful in qualitative analysis tests to identify unknown compounds.

In industry or at home the effect of heat on substances such as wax is put to use when making candles. The effect of heat is also used to make the following: metallic doors and windows by welding, saucepans from molten aluminium, and various steel products. The effect of heat is also used in the disposal of solid waste, for example burning used syringes in hospitals and clinics, rubbish in homes and sterilising medical tools, babies feeding bottles etc. Burning of plant materials like banana and sweet potato peelings is used to produce local salt (sodium hydrogen carbonate), which is used to speed up the cooking of beans, peas and greens.

Ionic Chemistry

INTRODUCTION

Ionic Chemistry deals with the chemistry of ions. In nature substances exist mainly as ionic or covalent compounds. Ionic compounds consist of ions held together by electrostatic forces of attraction, known as ionic or electrovalent bonds while covalent compounds consist of atoms held together by forces of attraction called covalent bonds. Some other substances combine through coordinate or dative bonding. A dative bond is a form of a covalent bond in which only one species (donor) donates both electrons to be shared to the acceptor.
When ionic compounds dissolve in water they dissociate to form the component particles, ions, in solution. The ions formed can react with oppositely charged ions from other ionic compounds to form products that are either in solid form (precipitate) or in solution (aqueous).
In this subtopic we shall consider reactions leading to formation of insoluble hydroxides. Knowledge of such reactions is useful in qualitative analysis that will help the student in identification of unknown substances.
This knowledge is examined in the Uganda Certificate of Education chemistry practical paper 545/3 or 545/4 that every student offering the subject has to sit for. Furthermore, the skills acquired in this topic are useful in research work.
Several reagents are normally used to test for cations and anions. It is imperative that students are guided to master the use of specific reagents used during tests and also confirmatory test for ions such as Pb²⁺, Fe²⁺ and Fe³⁺. 

QUALITATIVE ANALYSIS

The sub-topic deals with:

1. Introduction to qualitative analysis
2. Recognising the precipitates and complex ions produced by specified cation-anion reactions.
3. Differentiating between ions using a series of ionic reactions.
4. Writing ionic equation(s) for the reaction(s) taking place between solutions.

Aims:

By the end of this sub-section you should be able to:

• carry out tests for cations.
• identify ions from a series of specified reactions.
• carry out tests for anions.
• write formulae of ions, complexes and ionic compounds.
• write ionic equation(s) for the reaction(s) taking place.
• Make accurate observations.
• state observation(s) made during tests.
• define the following terms: Cation, anion, and precipitate.       

• SULPHUR DIOXIDE

  INTRODUCTION

  
  This unit highlights the preparation of sulphur dioxide in the laboratory and on a large industrial scale. Discusses the properties of sulphur dioxide and its uses such as bleaching of straws and sponges, fumigation of our houses to kill insects (i.e. black ants), preservation of some liquids (i.e. orange juice). Sulphur dioxide is also used in the manufacture of sulphuric acid. Sulphuric acid is widely utilized to make paints, plastics, detergents (i.e. Omo, Nomi, Aerial, etc.) and fertilizers.
  Sulphur dioxide could be obtained on a large scale from burning sulphur that is itself obtained from natural gas, a component of crude petroleum. In Uganda, the discovery of oil in the western part of the country will in the near future become a source of sulphur.
  Sulphur dioxide is a major pollutant of atmospheric air. It is toxic, causes respiratory ailments, damages plant life and corrodes metals. The gas is soluble in water forming what is known as ‘acid rain’. Acid rain erodes marble and limestone. In industries where this gas is evolved it is recommended to bubble the gas through an alkali to form sulphites and thus control its polluting effect on the environment.
  Sulphites are useful as mild oxidising agents but more often they act as reducing agents. They are oxidised to sulphates by substances such as potassium manganate (VII), potassium dichromate (VI), chlorine, bromine, hydrogen peroxide, oxygen and iron (III) chloride.
  A mixture of sodium sulphite and sulphur when heated react to form sodium thiosulphate. A solution of a ‘fixer’ containing sodium thiosulphate is added to developed films to remove unreduced silver ions to form a soluble complex, which is washed away. This prevents unreduced silver ions from becoming reduced and darkening slowly over time. This is useful in photography, which is a very vital practical application.
  

  USES AND PROPERTIES OF SULPHUR DIOXIDE

  This sub-topic deals with

• Methods of preparing sulphur dioxide in the laboratory and on a large scale.
• The physical and chemical properties /reactions of sulphur dioxide gas.
• Uses of sulphur dioxide
• Preparation and properties of sulphite salts.

Demonstration Experiments:

(a) Laboratory methods of preparing sulphur dioxide

Sulphur dioxide in the laboratory is prepared by:
(i) Reacting dilute hydrochloric acid or dilute sulphuric acid with sodium sulphite crystals
Diagram of the apparatus used

Equation of reaction taking place

(ii) Heating a mixture of copper turnings and concentrated sulphuric acid
Diagram of Apparatus used is the same as the one above except a source of heat must be applied.

Equation of reaction taking place

Uses of sulphur dioxide:

Sulphuric acid used to make soap, paints and pigments
(i) Soap and Detergents

(ii) Fertilizers

(iii) Bleaching agent for wool, straws and sponges.

(iv) Preservative for some liquids e.g. orange juice.

(v) Preparation of calcium hydrogen sulphite used for bleaching wood-pulp in the manufacture of papers.

(vi) Rrefrigerant.

(g) Preparation of sulphite salts for example sodium sulphite and zinc sulphite.

A: Learning Outcome

By the end of this sub-section a student (you) should be able to:
(a) Describe the two methods by which sulphur dioxide is prepared in the laboratory.
(b) State possible methods of preparation of sulphur dioxide gas on a large scale.
(c) State the method of collection of sulphur dioxide gas in the laboratory.
(d) State the physical and chemical properties of sulphur dioxide.
(e) Describe a confirmatory test for sulphur dioxide.
(f) State the observations made in the oxidising and reducing properties of sulphur dioxide.
(g) Write equation(s) for the reaction(s) of sulphur dioxide with various substances.
(h) State the uses of sulphur dioxide.
(i) Describe the preparation of sodium sulphite and zinc sulphite in the laboratory
(j) State chemical properties of sulphite salt.

THE EFFECT OF ELECTRICITY ON SUBSTANCES

INTRODUCTION

In chemistry, you might have learnt that some reactions proceed easily (spontaneously) either naturally or on application of heat. Other reactions can only occur (need to be forced to proceed) by supplying energy with an externally applied electric current, for example, a battery. This process is called electrolysis.
Electrolysis is important in that it is used in the extraction of reactive metals, such as potassium, sodium, calcium, magnesium and aluminium; electroplating metals to improve their appearance and reduce corrosion/prevent rusting and in refining metals such as copper. For example, copper is mined in Kilembe and is transported to Jinja for purifying electrolytically. Electroplating can be done at home once you have the necessary substances to use and intend to start a small business, for example, coating iron or aluminium bangles with copper.
See Table 1: Summary of the effects of electrolysis
ELECTROLYSIS

This unit deals with
(i) Characterisation of substances as electrolytes, non – electrolytes, conductors and non-conductors.
(ii) Selective discharge of an ion at an electrode.
(iii) Reactions at the electrodes during electrolysis.
(iv) Laws of electrolysis.
(v) Electrochemical cells.

In dealing with this unit the teacher should have knowledge on the following:

(i) Definitions of strong and weak electrolytes, conductors, non-  conductors and electrolysis.

(ii) Examples of strong and weak electrolytes, non-electrolytes, conductors and non-conductors.

 (iii) The conditions required for electrolysis to take place such as:             

• Source of current (direct current)
• Electrodes (Points at which electricity enter and leave the solution)
• Electrolyte (in solution or molten state).

Figure 1- Diagram showing the requirements for electrolysis

(iv)   The factors which affect discharge of an ion at an electrode. These include:

       Position of the ion in the activity series: Illustrate this factor using copper(II) sulphate with carbon  electrodes or dilute sulphuric acid using platinum electrodes.

       Concentration of electrolytes: Illustrate this factor using dilute and concentrated hydrochloric acid, dilute and concentrated sodium chloride solutions.

       Caution: if you are performing the experiment take note that chlorine is poisonous. Therefore, you should avoid inhaling too much of it. In order to do so keep the windows open to ensure proper ventilation.

       Nature of electrodes: This could be illustrated by using copper(II) sulphate solution with copper electrodes.

• Reactions at the electrodes in terms of electron transfer. Explain the reactions taking place in view of the ions discharged using dilute sulphuric acid, copper(II) sulphate solution,  sodium chloride solution, concentrated sodium chloride solution (brine) to establish the role of water. For example, electrolysis of copper(II) sulphate solution.

Figure 2- Electrolysis of copper(II) sulphate using graphite electrodes

Sources of ions   

CuSO₄ (aq)        Cu²⁺(aq) + SO₄^2-(aq)

H₂O(l)       H⁺(aq)  + OH^-(aq)

Reactions

At the anode hydroxide, OH^- and sulphate, SO₄^2– ions are present.

Since the hydroxide ions, OH^- is lower in the activity series than SO₄^2– .

It is preferentially selected for discharge. The equation for the reaction is:

4OH^– (aq)      2H₂O (l)  +  O₂(g)  +   4e

OH^– ions give up their excess electrons to the anode to form oxygen gas
At the cathode hydrogen, H⁺ and copper(II), Cu²⁺ ions are present but since Cu²⁺ is lower in the activity series than H⁺ it is discharged. Copper(II) ions take up electrons to form copper metal.

Cu²⁺(aq) +  2e   Cu(s)

Figure 3 - Movement of ions in an incomplete and complete circuit

The arrows indicate the direction of movement of ions when charged plates are placed in solution.

(vi)      State the Faraday's laws of electrolysis and solve problems involving mass or the volume of product.

(vii)    State applications of electrolysis, for example,
            -           In extraction of metals – Sodium and aluminium. The first five metals in the activity series are extracted by electrolysis.

            -           In electroplating.

            -           Purification of metals for example copper.

            -           Manufacture of sodium hydroxide and chlorine.

(viii)   Construct and explain the working of a simple electrochemical cell.

For example, the Daniel cell consisting of Zinc rod dipping in zinc(II) sulphate solution and copper rod dipping in copper(II) sulphate solution.

Figure  4- A simple Daniel Cell

CARBON

INTRODUCTION

Elements are substances that cannot be split into anything simpler through chemical means. Elements may be solid, like iron and carbon; liquid like mercury and bromine; or gaseous like oxygen and nitrogen. Many of them, like those mentioned, are common in the pure state. The chemist divides elements into two broad classes; metal and non-metals. The metals include potassium, sodium, calcium, magnesium, aluminium, tin, zinc, iron, lead, copper, silver, gold and mercury. The non-metals include carbon, sulphur, oxygen, nitrogen, hydrogen and chlorine.

In this unit we are going to learn more about carbon. Carbon occurs in the pure state as two allotropic forms; diamond and graphite; and in many impure forms such as coal, coke, soot, and charcoal (which could be wood, animal or sugar charcoal).
Diamond is the hardest known natural substance because of its structure. 90% of the world’s diamonds come from South Africa. Diamonds are used in the manufacture of cutting tools and drill bits. Graphite on the other hand occurs in small quantities in Borrow dale in Cumbria and in many other countries. Graphite is one of the softest solids and is used as a lubricant, in electrodes (for electrolytic cells), as a ‘moderator’ in atomic reactors, and as ‘lead’ in pencils (graphite mixed with clay).

Powder forms of carbon have adsorbing power and are used to purify substances. For example, boiling using ‘activated animal charcoal’ and then filtering off the suspended charcoal often purify solutions contaminated with a coloured impurity. The same principle is made use of in gas masks. Granular carbon contained in gas masks has the ability to absorb many gases, particularly many of the toxic ones. Carbon is also used as a reducing agent in the extraction of metals such as zinc, iron and lead.
Carbon burns in oxygen to form compounds such as carbon monoxide (in limited air) and carbon dioxide (in plentiful of air).

In industries, carbon dioxide is made by heating limestone (calcium carbonate) in a limekiln and it is also obtained as a by-product of fermentation reactions. Large quantities of the gas come from steam reforming of natural gas in the production of hydrogen for ammonia synthesis. Human beings and animals also produce carbon dioxide during the process of respiration. In the atmosphere carbon dioxide is present up to 0.03% by volume. It is removed from the atmosphere by photosynthesis, a process in which green plants make their food.

Carbon dioxide is useful in many areas such as the manufacture of carbonated drinks, fire extinguishers, refrigeration, special effects at pop concerts, heat transfer in nuclear reactors, baking and health salts.

Content and concepts to emphasise

This unit covers the following content:

(i) Preparation and properties of carbon dioxide.

(ii) Reaction of carbon dioxide with water, limewater and alkalis.

(iii) Uses of carbon dioxide e.g. in the manufacture of soft drinks, refrigeration, baking and fire extinguishers.

Principles and methods of extinguishing different types of fires.

(i)In addition, while teaching this unit the teacher should have knowledge of the following:

(ii)Equations for the burning of carbon in oxygen to form carbon monoxide and carbon dioxide.

(iii)Action of acids on carbonates.

(iv)Methods of collection of gases such as oxygen and hydrogen.
Solubility of gases in water.

(v)Properties of acidic oxides i.e. reaction with alkalis to form a salt and water.

(vi)Solubility of salts in water.

Objectives

By the end of this lesson, learners should be able to:
Describe the laboratory preparation of carbon dioxide
State the properties of carbon dioxide.
Write equations for reactions.
State the uses of carbon dioxide

Activity organisation advice

Learners’ experiments can be done in groups. Each group should be able to carry out experiments outlined in activities 1, 2 and 3 above.
Ensure that the groups are small (5 – 6) and manageable. Maximum discipline must be observed. When grouping the learners, choose a leader for each group. He or she must be of an average IQ. Group leaders or one of the members must give a report to the whole class.
Practice team-teaching whenever possible.
Make prior arrangements to procure the necessary resource materials.
Try out the experiments before giving them to your learners. This will help you to get solutions to any shortcomings.
Improvise where necessary. Be innovative. A mineral water bottle could be used as a flat- bottomed flask to prepare a gas or even a gas jar to collect a gas. Straws can be used as delivery tubes and U-tube to hold the drying agent. Join the straws, if necessary using celotape.
Bear in mind the safety precautions. Concentrated hydrochloric acid is volatile and forms fumes when the bottle is opened. Learners must keep at some distance while you perform the experiment. The fumes from hydrochloric acid are choking.
Get into the mind of the learners and think about the questions they are likely to ask and the answers to such questions.
Teach learners to learn to make generalisations and conclusions from experiments carried out.
Formulate an activity on the reaction of carbon dioxide with an alkali, sodium hydroxide solution. Collect the gas in a boiling tube and invert it over a beaker containing sodium hydroxide solution.

Teaching / Learning Materials/Activities And Guidance

(a) The teacher should be able to perform the following experiment on preparation of carbon dioxide.

Apparatus

Flat-bottomed flask, corks, gas jars, water trough, Bee-hive shelve, card boards, dropping funnel or thistle funnel, delivery tube, U-tube.

Chemicals

carbonate lumps), blue litmus papers, calcium hydroxide solution (lime water), wooden splint, candle, anhydrous calcium chloride

Job related life skills

By the end of this topic, learners are expected to have acquired the following Job Mart related Skills:

Personal attributes – self-confidence, time management, creativity/imaginative, recording skills, enthusiasm, imagination and self-awareness.

Communication – observation & listening skills, reporting in writing.

Team work – task oriented leadership skills, group work.

Problem solving - information seeking, environmental protection & conservation, seeking for information (research).

Application of number - numeracy (as they compare crop yields in treated and untreated plots)

NITROGEN AND ITS COMPOUNDS

Introduction
Nitrogen occurs as combined gas in air. It forms 78% by volume of air. It exists as a diatomic molecule N₂.
It occurs naturally in compounds such as proteins, nitrates and ammonium salts.

LABORATORY PREPARATION OF NITROGEN

Apparatus – delivery tubes, furnace, beaker, trough, Bunsen burner, 2 wash bottles

Chemicals – Air, water, sodium hydroxide

Procedure  

Either:

Nitrogen is prepared from the air by removing oxygen and carbon dioxide. Water is used to push air through sodium hydroxide solution (caustic soda solution) which removes carbon dioxide.

2NaOH (ag) + CO₂ (g) Na₂CO₃ (g) + H₂O (i)

The remaining gas is passed over heated copper turnings to remove Oxygen.

2Cu (s) + O₂ (g)2CuO (s)

Nitrogen is collected over water as it is insoluble in water.

OR:
Nitrogen can also be prepared by b heating a mixture of ammonium chloride and sodium nitrite.

NaNO₂ (s) + NH₄Cl (s) NH₄NO₂ (s) +NaCl (s)

Nitrogen is formed by decomposition of ammonium nitrite

NH₄NO₂ (s)N₂ (g) + 2H₂O (i)

INDUSTRIAL PREPARATION (LARGE SCALE)

Industrially, Nitrogen is prepared by fractional distillation;Oxygen is obtained at the same time. Air is purified by removing gas from it.
Then carbondioxide is removed and lastly water vapour. The remaining air is removed and compressed. The liquidified air is allowed to evaporate. Because Nitrogen has a lower boiling point, it evaporates first leaving liquid oxygen.

PROPERTIES OF NITROGEN

Physical

1. it is colourless gas without smell
2. it is a reactive gas
3. it does not burn / doesn’t support combustion
4. it is neither acidic nor basic

Chemical properties

1. Nitrogen is inert unlike Oxygen, it reacts under special conditions for example
2. It reacts with some metals at very high temperatures forming nitrides e.g. Calcium and magnesium.

      3Mg (s) + N₂(g)Mg₃N₂(s)

3Ca (s) + N₂ (g) Ca₃N₂ (s)

Course content and outline

1. Learners are required to know ammonia, its state – covalent compound.
2. Its monocular mass (17).
3. Its formula NH₃.
4. Its preparation on small scale (in the laboratory).
5. The reagents used (calcium hydroxide and ammonium chloride.
6. Relevant equation for ammonia formation.
7. Precautions used in its preparation e.g. the tube in which ammonia is generated is fixed in a slanting position to prevent the water from running back and crack the whole tube.
8. Concentrated sulphuric acid and an hydrous calcium chloride are not used to dry ammonia because they react with it.
9. Its method of collection – by upward delivery – as it is lighter than air.
10. Diagramic illustration.

Its preparation on large (industrial scale) Haber process

1. The reagents used e.g. N₂ & H₂, catalyst finely divided iron,temp 350ºc - 400ºc,pressure  about 350 atmospheres.
2. N₂ (g) + 3H₂ (g)2NH₃ (g)
3. To explain the factors affecting the reaction in ammonia formation.
4. Pressure, temperature, catalyst,
5. To explain the properties of ammonia, using diagrams where possible, both physical and chemical properties.
6. To explain the uses of ammonia.

Important Points to Note:

1. Learners' experiments can be done in groups. Each group should be able to carry out assigned experiments perfectly.
2. Ensure that the group is small (5-6), manageable and maximum discipline must be observed. When grouping them, choose a leader for each group, s/he must be of average IQ (Intelligent quotient).
3. Practice team teaching whenever possible.
4. Make prior arrangements to produce the necessary resource materials.
5. Try out the experiment before giving them to Learners. This will help you (the teacher) to get solutions to any shortcomings.
6. Improvise where necessary.
7. Relate the experiments to daily life.
8. Organize project work on Haber process.
9. Bear in mind the safety precautions.
10. Get into the minds of Learners and think about the questions they are likely to ask.
11. Teach learners to make genuine conclusions from the experiments carried out.
12. Time should be set and followed.
    

    ACIDS,BASES AND SALTS

    INTRODUCTION
    Indicators are widely used in life. Motor vehicles have indicators which are used to alert road users of the intention to branch left or right.
    In chemistry, indicators are used to determine whether an unknown solution is acidic, basic or neutral. Indicators show different colours in acidic, alkaline, or neutral solutions.
    Indicators are also used in chemistry to determine the equivalence point of acid-base titration. Where appropriately chosen indicators change colour. 
    Indicators can be made locally by extraction from plant materials such as petals of flowers, beet roots and leaves like those of red cabbage.
    The knowledge is also applied in medicine to prepare intravenous injections and fermentation processes in the manufacture of alcoholic drinks e.g. beer
    
    
    A BRIEF DESCRIPTION OF THIS UNIT.
    This unit deals with
13. Definition of acids, bases and indicators
14.   Use of laboratory indicators to detect nature of solution.
15. Preparation and use of plant extracts as acid-base indicators.
16. Use of universal indicators to determine the pH of solution.
17. Use of indicators in the preparation salts in the laboratory.

Job related life-skills

• Team working: ability to work together and share tasks
• Communication skills: ability to report verbally and in writing
• Information skills: ability to seek relevant information, make proper observations and recording of results.
• Personal attributes: ability to follow instructions, develop self confidence, be self critical and invigorative, task oriented etc
• Application and implementation: ability to evaluate alternatives and follow instructions to achieve accurate results
• Application of Number: calculation, measurement and presentation of numerical data.