Form 4 Sc Chapter 5

7/28/2019 Form 4 Sc Chapter 5

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Chapter 5PHYSICAL AND CHEMICAL CHANGES.

5.1 Physical and Chemical Changes.

Changes that occur in matter are classified as:a) physical change b) chemical change

Physical change.

is a change that alters the form or appearance of material that does not

convert the material into new substances.

the chemical composition of the material produced remains the same.

a physical change usually involves little or no change in energy.

examples of physical changes are :

a) melting ice

b) evaporation of waterc) solubility of salt in water

Chemical change.

is a change in matter that forms one or more new substances.

the chemical properties and composition of the new substance are

different from those of the original.

the chemical change that occurs usually involves absorption or release of

heat. (sometimes light energy is also produced)

the new substances formed usually cannot be changed back to the

original material physically. examples of chemical changes are :

a) burning paperb) change in colour of sliced applesc) rusting of iron

5.2 Heat Changes in Chemical Reactions.

heat change occurs in most of the chemical reactions.

all occurring chemical reactions involve energy transformation.

when a chemical reaction occurs, heat energy is absorbed or released.

heat energy is absorbed to break the bond in a compound.

conversely, heat energy is released when the bond is formed.

chemical reaction can be classified into two type :

exothermic reactions

endothermic reactions


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Comparison between physical and chemical changes.

Similarities- both experience physical changes

like appearances.

Physical changes Differences Chemical changes

No Formation of new substance Yes

Same Properties of composition of reactants and products

Different

Normally yes Reversible change Normally no

Usually requires a little energy Energy requirement Usually requires a lot energy

Same Mass of reactants andproducts

Different

Comparison between Endothermic and Exothermic reactions.Exothermic reaction Endothermic reaction

Definition Reactions in which heat energy isreleased to the surroundings. Reactions in which heat energy isabsorbed from the surroundings.

Reactant When this reaction occurs, thereactant will lose heat to thesurroundings.

When this reaction occurs, thereactant will gain heat from thesurroundings.

Energy content The total energy content of theproduct is less than the total energycontent of the reactant. The energytransfer can be shown in an energylevel diagram.

Reactant

Product

The total energy content of theproduct is more than the total energycontent of the reactant. The energytransfer can be shown in an energylevel diagram.

Product

Reactant

Surroundingtemperature

The surrounding temperature israised.

The surrounding temperature islowered.

Contents incontainer

The contents in the container becomehot.

The contents of the containerbecome cool.

Chemical reactions in industry.

usually occur under optimum conditions so that the time of reaction is very

short and the cost involved is minimal.

two important chemical process in industry are :

a) Haber process which produces ammonia.b) Contact process which produces sulphuric acid

in the Haber process, ammonia is produced from the mixture of nitrogen

and hydrogen.

Haber process is a reversible reaction.

Nitrogen + Hydrogen Ammonia


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when ammonia is produced, bond formation occurs between the atoms of

nitrogen and hydrogen, a lot of heat is released to the surroundings.

in the contact process, sulphuric acid is produced through three stages.

in the first stage, sulphur is burnt in the air to produce sulphur dioxide gas.

heat is released to form the bond when sulphur oxide is produced.

in the second stage, sulphur dioxide gas reacts with oxygen to form

sulphur trioxide gas at temperature of 400-500oC and pressure of 1atmosphere. Vanadium (V) oxide is used as a catalyst in this reaction.

this reaction is reversible.

heat is released to form the bond when sulphur trioxide is produce.

at the third stage, the reaction which occurs involves two steps.

the reaction at both steps releases heat to form the chemical bonds when

oleum and sulphuric acid are produce.

just like at the second stage, optimum conditions are maintained at this

stage.

Sulphur + Oxygen Sulphur Oxide

Sulphur Dioxide + Oxygen Sulphur Trioxide

Sulphur Trioxide + Concentrated Sulphuric Acid OleumOleum + Water Sulphuric Acid

Nitrogen

Hydrogen

Sulphur

Oxygen

Haber Process

Mixture ofnitrogen andhydrogen iscompressed

temperature : 450oCpressure : 200 atmcatalyst : iron filings

Ammonia gas

Ammonia (liquid)

condensed

combustionSulphurDioxide

temperature : 400- 500oCpressure : 1 atmcatalyst : vanadium(V) oxide

SulphurTrioxide

dissolved inconcentratedsulphuric acid

OleumLiquid

Sulphuricacid

Contact Process


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5.3 Reactivity Series of Metals.

Reactivity of Metals with Water.

some metals react with water more vigorously than others.

metals like sodium and potassium react very vigorously with water. metals like calcium react less vigorously with water as compared to

sodium and potassium.

reactivity of metals with water can be represented by the following

equations :

metals like magnesium, aluminium, zinc, and iron react less vigorously

with water. (this metals only react vigorously with water)

metals like lead, copper, silver, and gold do not react with water and

steam.

Reactivity of Metals with Acid.

some metals react with dilute acid to produce salt and release hydrogen

gas.

reactive metals like magnesium, aluminium, zinc, and iron react vigorously

with dilute acid.

metals like copper silver and mercury do not react with dilute acid because

these metals are not reactive.

Reactivity of Metals with Oxygen.

when a metal is heated in oxygen, it combines with oxygen to form metaloxide.

metals burn in oxygen with different reactivity.

diluted withwater

Reactive metal + water metal hydroxide + hydrogenex : Sodium + water sodium hydroxide + hydrogen

Metal + steam metal oxide + hydrogenex : Magnesium + steam magnesium oxide + hydrogen

Metal + dilute acid salt + hydrogenex : Magnesium + dilute hydrochloric acid magnesium

chloride + hydrogen

Metal + oxygen metal oxideex : Magnesium + oxygen magnesium oxide


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when a metal is heated, its reactivity can be determined through the

brightness of the flame that is produced.

Reactive of metal can be determined by the brightness of its flame.

Metal Flame

Very reactive Ignite brightlyReactive Glows brightly

Less reactive Glows dimly

Comparison of reactivity of metals with water, dilute acid and oxygen.

Metal Reactivity of metals when reacting with

Water Dilute acid Oxygen

PotassiumReact with cold water

React with dilute

acid

Reactivity of metalsin decreasing order

Sodium

Calcium

Magnesium

React with steamAluminiumZinc

Iron

Tin

Do not react withcold water or steam

Lead React with hotdilute acid

Copper Do not react withdilute acidSilver

Gold

Reactivity Series of Metals.

reactivity series of metals is a series that shows the order of metal

reactivity.

the series is formed based on metals reactivity with oxygen.

metals that react vigorously with oxygen are placed at the top of the

series.

metals that react less vigorously are placed at the bottom of the series.

Reactivity series of metals

Potassium

SodiumCalciumMagnesium

AluminiumZincIronTinLeadCopperSilverGold

reactivity ofmetals in

decreasingorder


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Position of carbon in reactivity series of metals.

although carbon is a non-metallic element, it reacts with excess oxygen toform carbon dioxide.

if carbon is more reactive than metal X, a bright flame or glow will be seen

when a mixture of carbon and oxide of metal X is heated.

for example, carbon can eliminate zinc from zinc oxide. therefore, the

position of carbon is higher than that of zinc in the reactivity series of

metals.

if carbon is less reactive than metal Y, a flame or glow is not seen when a

mixture of carbon dioxide of metal Y is heated.

for example, no reaction takes place when a mixture of carbon and

aluminium oxide is heated. therefore, the position of carbon is lower than that of aluminium in the

reactivity series of metals.

by conducting reactions between carbon and oxides of metals, the

position of carbon in the reactivity series of metals can be determined.

Oxide of metal X + carbon metal X + carbon dioxideheated

Zinc oxide + carbon zinc + carbon dioxide

Carbon + oxide of metal Y no reaction

Aluminum oxide + carbon no reaction

Potassium

SodiumMagnesium

Aluminium

Carbon

ZincIronTinLeadCopperSilver

Gold

reactivity of metals indecreasing order

position of carbon in the reactivity series of metals.


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5.4 Application of The Concepts of Reactivity Series of Metals.

Relationship between the position of metals in the reactivity series and themethod of metal extraction.

knowledge of the position of metals in the reactivity series of metal can be

applied in the method of extracting metals from their ores.

there are two methods of extracting metal :

(a) reduction by carbon(b) electrolysis on smelting metal ore

carbon is used in the extraction process because

(a) it is cheap(b) easily obtained

(c) the side product of carbon (carbon dioxide) during theextraction process is a type of gas which is easily eliminated.

Extracting metal from metal ore.

in nature, metal tends to react with oxygen to form metal oxide. (ore)

metal in the form of ore does not have much use and needs to be

extracted.

the method of extracting metal depends on the position of the metal in the

reactivity series of metals.

Extraction of tin.

tin ore (cassiterite) is tin oxide which exists in the earths crust.

tin oxide is washed with water to remove dirt.

then, tin oxide is roasted to remove impurities like sulphur and oil.

after that, tin oxide is extracted by heating tin oxide with carbon and

limestone in a high temperature blast furnace.

the function of the limestone is to remove impurities.

during heating, carbon which is more reactive than tin removes oxygen

from the tin oxide to produce pure tin and carbon dioxide.

molten tin is poured into moulds to form tin ingots.

at the same time, the limestone )calcium carbonate) decomposes into

quicklime (calcium oxide) which reacts with impurities to form slag, i.e. dirtwhich is unwanted.

Importance of reactivity.

Tin oxide + carbon tin + carbon dioxide

(tin ore)


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the importance of reactivity series of metals :

i. Reactivity series of metals enables the reactivity of metals to becompared. Metals with higher positions in the series are morereactive than those below them.

ii. The series is used to determine whether a reaction can occur.For example, sodium has a higher position than iron in theseries. This means that sodium is more reactive than iron.Therefore, sodium can remove oxygen from iron oxide.

iii. Knowledge of reactivity series of metals can be applied inchoosing the method of metal extraction from its ore.

5.5 Electrolysis.

is a dissociation process of chemical substances in aqueous solution or

molten state to its constituents by using electricity.

a dry cell or battery supplies electricity to dissociate chemical substancesto their constituents.

electrical energy changes to chemical energy in electrolysis.

arrangement of apparatus for electrolysis process

electrical energy chemical energy

+-

-- +

+

dry cell/battery

ammeter Arheostat

+ -

cathodeanode

cathode

cation anion

electrolyte


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Electrolysis.

is usually used in electrolysis because carbon is inert and does not take

part in reaction.

during electrolysis, cation moves towards the cathode while anion moves

towards the anode.

at the cathode, cation receives electron from the cathode and is

discharged to form a neutral atom.

at the anode, anion releases electron and is discharged to form a neutralatom.

discharge is a charge neutralisation process in ions to form neutral atoms.

electrolysis of copper(II) chloride solution.

o copper(II) ion with positive charge will attract to cathode to

discharge as a copper metal.o chloride ion will attract to anode to discharge as a chlorine gas.

o at anode, chloride ions lose of electrons, greenish-yellow bubbles

of chlorine gas are released.o at the cathode, copper(II) ion receives electron, brown copper metal

is deposited.o the blue colour of copper(II) chloride solution fades.

Electrolysis of molten lead(II) bromide.

lead(II) bromide in a crucible is heated until it melts.

two carbon electrodes are put in the molten lead(II) bromide.

at the anode, brown vapour is released, i.e. bromine vapour.

at the cathode, shiny grey solid is produced, i.e. lead.

this is because lead(II) ion and bromide ion move freely when lead(II)

bromide is melted.

during electrolysis, lead(II) ion which is positively charged moves towards

the cathode to receive electrons and form lead.

positive ion + electron neutral atom

negative ion neutral atom + electron

copper(II) chloride copper + chlorineelectrolysis


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bromide ion which is negatively charged moves towards the anode to

release electron and form bromine vapour.

thus, electrolysis of lead(II) bromide produces lead and bromine gas.

Uses of electrolysis in industry.

Electrolysis is widely used in industry for the following purposes :a) extraction of metalsb) purification of metalsc) electroplating of metals

Extraction of metals.

metals that are more reactive than carbon are extracted from their ores by

electrolysis.

extraction of aluminium from bauxite :

i. molten aluminium oxide (bauxite) and carbon electrodes areused in the extraction of aluminium.

ii. the steps are as follows :1. Aluminium oxide with cryolite is heated until it melts. The

function of cryolite is to lower the melting point ofaluminium oxide.

2. Aluminium oxide dissociates into aluminium ions (cation)and oxide ions (anion).

iii. at the cathode, aluminium ions receive electrons and aredischarged. molten aluminium forms and settles at the base ofthe electrolytic cell.

iv. at the anode, oxide ions release electrons and are discharged.oxygen atoms are formed. the combination of two oxygen atomsforms an oxygen molecule. thus, oxygen gas is released.

Purification of metals.

metals can be purified through electrolysis.

in this process, the impure metal becomes the anode while the pure metal

becomes the cathode.

electrolyte is a salt solution of that respective metal.

lead(II) ion + electrons lead

bromine ion bromine + electron

aluminium ions + electrons aluminium ions

oxide ion oxygen atom + electrons


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purification of copper :

purification of metal through electrolysis.

i. anode is an impure copper plate while cathode is a pure copper plate.copper(II) sulphate solution is used as electrolyte.

ii. at the anode, the impure copper plate will dissolve to form copper(II)ion.

a. impurities will be left at the base of the beaker when the impurecopper plate dissolves.

b. the impure copper plate will become thinner after a while.iii. at the cathode, copper(II) ion will move towards the cathode to receive

electrons and is discharged. copper metal is formed.

eventually, the cathode will become thicker because pure coppersediment will settle on it. thus, the copper is purified.

Electroplating of metals.

A

+ -

impure copper plate pure copper plate

copper(II) sulphatesolution

copper atom copper(II) ion + electrons

copper(II) ion + electrons copper atom


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in electroplating process,

the metal used for electroplating becomes the anode.

the object to be plated becomes the cathode.

the electrolyte is a salt solution of that metal.

during electrolysis, the anode dissolves to form metallic ions.

these ions then move towards the cathode and settle as a thin layer ofmetal.

thus, the metallic object (cathode) is coated with a thin layer of metal from

anode.

electroplating iron nail with copper :

electroplating iron nail with cooper

i. the surface of the iron nail and the copper metal are rubbed with asandpaper.

ii. the iron nail and the copper metal are immersed in copper(II) sulphatesolution as shown above. (the circuit is completed)

iii. at the anode, the copper metal becomes thinner, this is becausecopper atoms at the anode release electrons to form copper cation.

iv. at the cathode, the surface of the iron nail is coated with a browncopper layer. this is because copper ions in the solution move towardsthe cathode to receive electrons and are discharged. copper metal isformed.

v. the copper metal formed settles on the surface of the iron nail.

A

+ -

copper plate

copper(II) sulphatesolution

iron nail

copper atom copper(II) ion + electrons

copper(II) ion + electrons copper atom


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the aims of electroplating are to

1. prevent the metal from corrosion or rusting2. make the metal look more attractive

5.6 Production of Electrical Energy From Chemical Reactions.

Production of Electrical Energy by simple voltaic cell.

electrical energy can be produced from chemical reactions.

simple voltaic cell consists of two different metals, or one of it is carbon,

that is immersed into a electrolyte.

chemical changes will occur to produce electrical energy.

in a simple voltaic cell, the energy transformation which occurs is as

follow:

Various types of cells and their uses.

Dry cell

chemical energy electrical energy

G

+ -copper plate

iron plate

dilute sulphuric acid

Iron plate can bereplaced by other

conductors like lead,zinc, and carbon

galvanometer is used todetect the production ofelectric current.

can be replaced by avoltmeter or ammeter.

dilute sulphuric acid canbe replaces by other

electrolytes like sodiumchloride solution and

dilute nitric acid.


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are the most commonly used electrochemical cells.

the zinc casing is the negative terminal.

the carbon rod is the positive terminal.

the carbon rod is coated with a mixture of carbon powder and

manganese(IV) oxide.

the carbon powder reduces resistance in the cell.

the manganese(IV) oxide absorbs the hydrogen gas released during

reaction.

the electrolyte is paste of ammonium chloride mixed with zinc chloride.

Lead-acid accumulator

the car battery is a type of electrochemical cell called accumulator.

the lead-acid accumulator used in cars consists of six cells connected in

series.

this type of battery supplies 12 volts of electrical energy.

the lead electrode is the negative terminal.

the lead electrode coated with lead(IV) oxide is the positive terminal.

the electrolyte is concentrated sulphuric acid. the accumulator is a type of secondary cell which can be recharged to be

used repeatedly.

Alkaline battery


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an alkaline battery is similar to a dry cell but it uses a different electrolyte

and is lasts long.


the manganese(IV) oxide is the positive terminal.

the electrolyte is potassium hydroxide solution.

an alkaline battery is used in watches, torches, radios, electric shavers

and toys.

Silver oxide-zinc cell

the shape of this type of battery is like a button.


the silver oxide is the positive terminal.

the electrolyte is potassium hydroxide.

this battery is used in watches and electronic toys.

Nickel-cadmium battery

this battery operates on the same principle as the lead-acid accumulator,

but it uses different chemical substances.

the cadmium is the negative terminal.

the nickel(IV) oxide is the positive terminal.

the electrolyte is potassium hydroxide.

Advantages and Disadvantages of electrochemical cell

Type ofelectrochemical

cell

Advantages Disadvantage

Dry cell -light and can be easily -not long lasting


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carried along-supplies constant current

-not rechargeable

Lead-acidaccumulator

-rechargeable-supplies high voltage for along period

-heavy and expensive-electrolyte which corrodesspills over easily

Alkaline battery -long-lasting-supplies higher currentthan dry cell although thevoltage is same

-not rechargeable-more expensive thanordinary dry cell

Silver oxide-zinc cell -long-lasting-supplies constant current

-not rechargeable

Nickel-cadmiumbattery

-long-lasting-rechargeable-concentration of itselectrolyte does not change

-expensive

5.7 Chemical Reactions that Occur In The Presence of Light.

- some chemical reactions occur in the presence of light.- examples of such reactions are :

a. photosynthesis in green plantsb. decomposition of certain chemical substances in

photography

Photosynthesis.

in photosynthesis, green plants absorb sunlight and convert it into

chemical energy (glucose).

water and carbon dioxide are used in photosynthesis to produce glucoseand oxygen is released.

the light energy absorbed by the chlorophyll in green plants is used to

break water molecules into hydrogen and oxygen. (this process is calledphotolysis)

the hydrogen then combines with carbon and a part of the oxygen in

carbon dioxide to produce glucose. the energy transformation that takes place during photosynthesis is :

Effects of light on photosensitive chemicals.

photographic paper is covered with a thin layer of silver bromide or sliver

chloride.

sunlight

water + carbon dioxide glucose + oxygenchlorophyll

light energy chemical energy


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when a photographic paper is exposed to light, light energy decomposes

the silver bromide to silver atoms. (silver is a dark grey substance)

by the darker part of the photograph is caused by the formation of the

silver atoms.

Storing chemical substances.

chemical substances like chlorine water, sodium hypochlorite solution and

silver salt are very sensitive to light.

these chemical substances will decompose to other substance if exposed

to sunlight.

as a result, photosensitive chemicals must be stored in dark condition.

chlorine water and sodium hypochlorite solution must be stored in dark

bottles.

photographic paper is also stored is a black bag or a black box.

silver bromide silver + brominelight

Documents

Form 4 Sc Chapter 5