﻿ Air and combustion

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## Air and combustion

AIR AND COMBUSTION

AIR AND COMBUSTION

The approximate percentages by volume of air of the components of air are as shown in the pie chart

OBJECTIVES

#### By the end of the lesson you should be able to:

1.State the percentage composition of air by volume

2.Carry out simple experiments to show that Oxygen is the active part of air

3.Determine percentage composition of Oxygen in air using suitable methods

QUIZ

DETERMINING PERCENTAGE OF OXYGEN IN AIR USING COPPER TURNINGS.

The apparatus is set as shown

The Copper turnings are heated strongly until they are red hot.Air is passed from the syringe backward and forward over the red hot metal until no change in the volume of air remaining in the syringe.Allow the glass tube to cool.

SAMPLE RESULTS

Copper metal uses part of the air during combustion.The percentage of air by volume used by the copper can be estimated as follows

From the results 20% by volume of air is used up when copper is heated

The 20% by volume of air used up is oxygen hence oxygen is the active part of air

In this experiment copper metal combines with oxygen to form copper (II) oxide which is black in colour.

WHAT HAPPENS WHEN A CANDLE BURNS IN A FIXED VOLUME OF AIR?

Half fill a water trough with sodium hydroxide solution. Stick a candle on a beehive shelf and place it in the trough containing sodium Hydroxide.Cover the candle with a graduated gas jar. Mark the level of the solution in the jar. Record the volume.Light the candle. Cover it slowly with the graduated gas jar. Observe how the level of the solution changes.

Record the final volume of air in the graduated gas jar.

The candle lights for a while and then goes off. The level of the solution in the gas jar rises.

Therefore the solution in the gas jar rises to occupy the volume of air used up during combustion

.

Sample results

CALCULATIONS

20% by volume of air is used up when the candle burns.

The part that supports combustion is the active part of air.

The active part is oxygen which forms about 20% of dry air by volume

The part that remains in the gas jar does not support combustion.

This component of air is inactive and is mainly nitrogen.

FRACTIONAL DISTILLATION OF LIQUID AIR

RUSTING

RUSTING

LESSON OBJECTIVES

#### By the end of the lesson you should be able to:

1.Carry out experiments to investigate the conditions necessary for rusting

2.State the composition of rust

3.State methods of preventing rusting

CONDITIONS NECESSARY FOR RUSTING

Two clean nails each are placed in Five separate boiling tubes labelled 1, 2, 3, 4 and 5 To the first tap water is added. To the second tube 10cm3 of boiled water followed by oil are added. To the third a piece of cotton wool is placed half way the tube containing nails. Anhydrous Calcium chloride is placed on the wool .To the fourth the boiling tube is left as it was set up initially.To the fifth water containing salt is added to the nails.The tubes are Corked and left for about a week.Observe the following animation and note what happens to the nails.

CONCLUSION

From the observations made we can conclude that water and air (Oxygen) are necessary for the rusting of the nails.Rust contains mainly Iron (III) oxide which is hydrated, Fe2O3. XH2O

Although rusting occurred in boiling tube 1 and 4 it occurred much faster in tube 5 containing salty water.Salt accelerates the rate of rusting.Note: Salt is not a condition for rusting.Rusting destroys machinery, equipment and roofs made of iron. Prevention of rusting is therefore of great importance

METHODS USED TO PREVENT RUSTING
Rusting destroys machinery, equipment and roofs made of iron. Prevention of rusting is therefore of great importance.The following methods are widely used to prevent rusting of Iron.Click to play the video to observe how painting takes place.

Plastic coating

REFRIGERATOR

GALVANIZING/ELECTROPLATING

This is a process of coating a material with Zinc.Coating Iron with Zinc slows down rusting because Zinc is more reactive than Iron. The Zinc undergoes corrosion instead of the Iron.

IRON SHEETS COATED WITH ZINC

ALLOYING

Alloying is mixing of iron with one or more metals e.g. Chromium, steel, Nickel. The presence of Carbon reduces the rate at which steel rusts in comparision to pure Iron but does not eliminate it. Some metals are added to steel to form stainless steel which is resistant to rusting. Stainless steel is used to make surgical blades, cutting tools and non rust cutlery.

OILING AND GREASING

Oiling and greasing is another method used to prevent rusting. This method is used for door hinges and moving parts of a machine.

Click to play the video and observe how oiling and greasing takes place

OXYGEN GAS

Oxygen is one of the components of air.

Objectives

#### By the end of the lesson you should be able to:

1.Prepare and collect Oxygen gas

2.Investigate the properties of Oxygen

3.State the uses of Oxygen

PREPARATION OF OXYGEN IN THE LABORATORY

Oxygen gas is prepared in the laboratory by use of Manganese (IV) Oxide and Hydrogen peroxide.

REAGENTS

The reagents used to prepare Oxygen gas in the laboratory include manganese (IV) Oxide and Hydrogen peroxide. Click to play the video to observe these reagents.

PREPARATION

Manganese(IV) oxide powder is placed in a flat bottomed flask and Hydrogen peroxide is added drop by drop using a dropping funnel.

Play the video to observe how Oxygen gas is prepared

Effervescence occurs producing Oxygen gas.Before the gas is collected a few bubbles are allowed to escape.The gas jar is removed by covering its mouth with the gas jar lid while still under water.

The apparatus is set up.

EQUATION

Oxygen gas can be collected dry in the laboratory by passing it through Concentrated Sulphuric acid.

Concentrated Sulphuric acid is referred to as drying agent. A drying agent is a substance that removes water from another Substance.

Oxygen gas can also be collected dry in the laboratory by passing it through anhydrous calcium Chloride.

PROPERTIES OF OXYGEN GAS

Oxygen gas has various properties. When a glowing splint is put into a gas jar containing oxygen gas it relights.

Click to play the video to observe this property

When a red and blue litmus paper are put in a gas jar containing Oxygen there is no effect on the litmus papers.

1.REACTION OF OXYGEN GAS WITH NON -METALS

Non metals react with Oxygen gas.Reaction of Oxygen with Sulphur.

NB: Oxides of Sulphur and Phosphorus are poisonous and so the experiment is carried out in a fume chamber.

REACTION OF OXYGEN GAS WITH NON -METALS

Sulphur burns in Oxygen gas .

NB: Oxides of Sulphur and Phosphorus are poisonous and so the experiment is carried out in a fume chamber.Click to play the video to observe what happens when Sulphur is burned in Oxygen gas.

Sulphur burns in oxygen with a blue flame producing misty fumes of Sulphur (IV) oxide.

Sulphur (IV) oxide dissolves in water. Click to play the video to observe what happens when red and blue litmus paper are placed into the solution.

The Oxide dissolves in water forming Sulphurous acid and Sulphuric acid. Blue litmus paper turns red while red litmus paper remains red. The Oxides formed are acidic.These are examples of acidic Oxides.

REACTION OF OXYGEN WITH METALS

Magnesium burns with a very bright white flame to produce a white ash. The product formed is magnesium oxide.

Magnesium oxide dissolves in water to form Magnesium hydroxide.

Sodium burns with a bright yellow flame to produce sodium peroxide.

This oxide dissolves readily in water to produce oxygen gas and sodium hydroxide which turns red litmus paper blue. This shows that the solution is alkaline.

Iron burns with yellow sparks to form brown black iron (III) oxide which is insoluble in water.

Copper burns with a blue flame in a gas jar of Oxygen to form black Copper (II) Oxide

This oxide is not soluble in water.

Calcium burns vigorously in oxygen producing a bright red flame. It forms white calcium oxide.

Calcium oxide dissolves in water to form calcium hydroxide solution.

REACTION OF OXYGEN WITH METALS

Magnesium burns in Oxygen with a very bright white flame to produce a white ash. Click to play the video to observe how Magnesium burns in Oxygen gas.

Magnesium burns with a dazzling flame when lowered into a gas jar containing Oxygen gas. A white solid is produced. This product is Magnesium (II) oxide

ACTIVITY

Magnesium (II) Oxide dissolves in water to form Magnesium Hydroxide.Click to play the video to observe what happens when blue and red litmus paper is placed into this solution.

The solid is sligthly soluble in water. The red litmus paper turns blue while the blue litmus paper remains blue.The solution formed is Magnesium hydroxide. This solution is alkaline.

Sodium burns with a bright yellow flame to produce sodium peroxide. Click to play the video to observe the product formed when sodium is burnt in Oxygen gas

This oxide dissolves readily in water to produce oxygen gas and sodium hydroxide which turns red litmus paper blue. This shows that the solution is alkaline. Click the video to observe what happen when red and blue litmus paper are placed into this solution

Iron burns with yellow sparks to form brown black iron (III) oxide which is insoluble in water.

Copper burns with a blue flame in a gas jar of Oxygen to form black Copper (II) Oxide

This oxide is not soluble in water.

Calcium burns vigorously in oxygen producing a bright red flame. It forms white calcium oxide.

Calcium oxide dissolves in water to form calcium hydroxide solution.

CONCLUSION

Non metals burn in Oxygen gas to form Oxides which turn moist blue litmus red. Such Oxides are said to be acidic.

Acidic Oxides can be defined as an Oxide which dissolves in water to form acidic solutions.

Some non metal Oxides are neutral and have no effect on moist red and blue litmus. For example Carbon (II) Oxide and water.

Metals burn in Oxygen gas to form Oxides which turn red litmus paper blue. Such Oxides are said to be Basic. Oxides of reactive metals such as Sodium, potassium, Magnesium and Calcium form soluble Oxides which turn red litmus blue. A soluble base is called an alkali.

Many metals burn in Oxygen at different rates.Metals burn in Oxygen faster than in air because air is made up of other constituents such as Nitrogen, Carbon (IV) oxide which do not support burning.

ACTIVITY

QUIZ

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When a burning magnesium ribbon on a deflagrating spoon is lowered into a gas jar containing carbon ( IV) Oxide a reaction takes place.

In this reaction Magnesium removes Oxygen from Carbon (IV) Oxide. This shows that Magnesium is more reactive than Carbon

When a mixture of magnesium powder and lead (II) oxide is heated in a crucible, the following observations are made. Observe the following reaction and note the colour changes.

In this reaction Magnesium removes Oxygen from Lead ( II) oxide . This shows that Magnesium is more reactive than lead.

The Magnesium oxide is a white powder. Magnesium combines with oxygen and is therefore oxidized. Lead (II) oxide losses oxygen and is said to be reduced.

The removal of oxygen from a substance is called reduction.

Addition of oxygen to a substance is called Oxidation.

When Magnesium powder is heated in a crucible containing Copper (II) oxide the following observation is made.

In this reaction Magnesium removes Oxygen from Copper( II) Oxide . This shows that Magnesium is more reactive than Copper

Magnesium Oxide is a white powder.Magnesium gains Oxygen and is therefore oxidized.Copper (II) Oxide losses Oxygen and is said to be reduced. The removal of oxygen from a substance is called reduction. Addition of oxygen to a substance is called Oxidation. The substance being oxidized is called a reducing agent and the one being reduced is called the oxidizing agent.

ACTIVITY

REACTIVITY SERIES

Using the competition for combined Oxygen, metals can be arranged in order of their reactivity.Reactivity series of metals and Hydrogen.

A metal will not remove combined Oxygen from an Oxide of another metal above it in the reactivity series.

It is possible therefore to arrange elements from the most reactive to the least reactive

ACTIVITY

POLLUTION EFFECTS OF BURNING SUBSTANCES

Non- metals burn in air to form gaseous oxides which causes environmental pollution. Some of the harmful gaseous Oxides are

Carbon (IV) Oxide

Carbon (II) Oxide

Nitrogen (IV) Oxide

Sulphur (IV) Oxide

Phosphorus (III) Oxide

Phosphorus (V) Oxide

All the three Oxides except Carbon (II) Oxide dissolve in rain water forming acid rain which corrodes Iron sheets and destroys plants and aquatic life.

Carbon (II) Oxide does not dissolve in water but is extremely poisonous. The gaseous Oxides of Sulphur and Phosphorous are also poisonous.

Effects of acid rain

Acid rain has various effects. It destroys buildings made from iron.

Click to play the video and observe how acid rain corrodes iron sheets.

Acid rain also destroys stone work

CORRODED STONE WORK

Plants are also affected by acid rain.

YELLOWING OF PLANTS.

USES OF OXYGEN

Oxygen has several uses.

Respiration Oxygen is used by mountain climbers.

MOUNTAIN CLIMBING

Deep sea Divers use Oxygen

DEEP SEA DIVING

Oxygen is also used in aeroplanes and high flying jets

HIGH ALTITUDE FLYING

In hospitals Oxygen is administered to patients with respiratory or breathing problems

IN HOSPITALS BY PATIENTS

Oxygen is used in welding and cutting metals e.g. Oxyacetylene flame

Liquid Oxygen is used to burn fuel for propelling rockets

In explosives, a mixture of charcoal, petrol and liquid Oxygen is used as an explosive.

Large quantities of Oxygen are used in steel making in steel making indusries.Oxygen removes impurities in steel making

The uses of Oxygen can be summarised as follows,

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