Chapter 5

Chapter 5

PHYSICAL 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 water

c) 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 paper

b) change in colour of sliced apples

c) 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

Comparison between physical and chemical changes.

Similarities

- both experience physical changes

like appearances.

Physical changesDifferencesChemical changes

NoFormation of new substanceYes

SameProperties of composition of reactants and productsDifferent

Normally yesReversible changeNormally no

Usually requires a little energyEnergy requirementUsually requires a lot energy

SameMass of reactants and productsDifferent

Comparison between Endothermic and Exothermic reactions.

Exothermic reactionEndothermic reaction

DefinitionReactions in which heat energy is released to the surroundings.Reactions in which heat energy is absorbed from the surroundings.

ReactantWhen this reaction occurs, the reactant will lose heat to the surroundings.When this reaction occurs, the reactant will gain heat from the surroundings.

Energy contentThe total energy content of the product is less than the total energy content of the reactant. The energy transfer can be shown in an energy level diagram.

Reactant

ProductThe total energy content of the product is more than the total energy content of the reactant. The energy transfer can be shown in an energy level diagram.

Product

Reactant

Surrounding temperatureThe surrounding temperature is raised.The surrounding temperature is lowered.

Contents in containerThe contents in the container become hot.The contents of the container become 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. 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 1 atmosphere. 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.

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 metal oxide.

metals burn in oxygen with different reactivity. 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.MetalFlame

Very reactiveIgnite brightly

ReactiveGlows brightly

Less reactiveGlows dimly

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

WaterDilute acidOxygen

PotassiumReact with cold waterReact with dilute acidReactivity of metals in decreasing order

Sodium

Calcium

MagnesiumReact with steam

Aluminium

Zinc

Iron

TinDo not react with cold water or steam

LeadReact with hot dilute acid

CopperDo not react with dilute acid

Silver

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.

Position of carbon in reactivity series of metals.

although carbon is a non-metallic element, it reacts with excess oxygen to form 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.

5.4 Application of The Concepts of Reactivity Series of Metals.Relationship between the position of metals in the reactivity series and the method 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 the extraction 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. dirt which is unwanted.

Importance of reactivity. the importance of reactivity series of metals :

i. Reactivity series of metals enables the reactivity of metals to be compared. Metals with higher positions in the series are more reactive 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 the series. 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 in choosing 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 substances to their constituents.

electrical energy changes to chemical energy in electrolysis.

arrangement of apparatus for electrolysis process

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

electrolysis of copper(II) chloride solution.

copper(II) ion with positive charge will attract to cathode to discharge as a copper metal.

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

at anode, chloride ions lose of electrons, greenish-yellow bubbles of chlorine gas are released.

at the cathode, copper(II) ion receives electron, brown copper metal is deposited.

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.

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 metals

b) purification of metals

c) 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 are used 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 of aluminium oxide.

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

iii. at the cathode, aluminium ions receive electrons and are discharged. molten aluminium forms and settles at the base of the electrolytic cell.

iv. at the anode, oxide ions release electrons and are discharged. oxygen atoms are formed. the combination of two oxygen atoms forms 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.

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 impure copper 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 copper

sediment will settle on it. thus, the copper is purified.

Electroplating of metals.

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 of metal.

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 a sandpaper.ii. the iron nail and the copper metal are immersed in copper(II) sulphate solution as shown above. (the circuit is completed)iii. at the anode, the copper metal becomes thinner, this is because copper atoms at the anode release electrons to form copper cation.

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

v. the copper metal formed settles on the surface of the iron nail. the aims of electroplating are to1. prevent the metal from corrosion or rusting

2. 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

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

an alkaline battery is similar to a dry cell but it uses a different electrolyte and is lasts long.

the zinc casing is the negative terminal.

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 zinc casing is the negative terminal. 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 of electrochemical cellAdvantagesDisadvantage

Dry cell-light and can be easily carried along

-supplies constant current-not long lasting

-not rechargeable

Lead-acid accumulator-rechargeable

-supplies high voltage for a long period-heavy and expensive-electrolyte which corrodes spills over easily

Alkaline battery-long-lasting

-supplies higher current than dry cell although the voltage is same-not rechargeable-more expensive than ordinary dry cell

Silver oxide-zinc cell-long-lasting

-supplies constant current-not rechargeable

Nickel-cadmium battery-long-lasting

-rechargeable

-concentration of its electrolyte 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 plants

b. 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 glucose and 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 called photolysis) 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.

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.Nitrogen + Hydrogen Ammonia

Sulphur + Oxygen Sulphur Oxide

Sulphur Dioxide + Oxygen Sulphur Trioxide

Sulphur Trioxide + Concentrated Sulphuric Acid Oleum

Oleum + Water Sulphuric Acid

Nitrogen

Hydrogen

Oxygen

Sulphur

Haber Process

Mixture of nitrogen and hydrogen is compressed

temperature : 450oC

pressure : 200 atm

catalyst : iron filings

Ammonia gas

Ammonia (liquid)

condensed

combustion

Sulphur Dioxide

temperature : 400- 500oC

pressure : 1 atm

catalyst : vanadium(V) oxide

Sulphur Trioxide

dissolved in

concentrated

sulphuric acid

Oleum Liquid

Sulphuric acid

diluted with

water

Contact Process

Reactive metal + water metal hydroxide + hydrogen

ex : Sodium + water sodium hydroxide + hydrogen

Metal + steam metal oxide + hydrogen

ex : Magnesium + steam magnesium oxide + hydrogen

Metal + dilute acid salt + hydrogen

ex : Magnesium + dilute hydrochloric acid magnesium chloride + hydrogen

Metal + oxygen metal oxide

ex : Magnesium + oxygen magnesium oxide

Reactivity series of metals

Potassium

Sodium

Calcium

Magnesium

Aluminium

Zinc

Iron

Tin

Lead

Copper

Silver

Gold

reactivity of metals in decreasing order

Oxide of metal X + carbon metal X + carbon dioxide

heated

Zinc oxide + carbon zinc + carbon dioxide

Carbon + oxide of metal Y no reaction

Aluminum oxide + carbon no reaction

Potassium

Sodium

Magnesium

Aluminium

Carbon

Zinc

Iron

Tin

Lead

Copper

Silver

Gold

reactivity of metals in decreasing order

position of carbon in the reactivity series of metals.

Tin oxide + carbon tin + carbon dioxide

(tin ore)

electrical energy ( chemical energy

+

-

-

-

+

+

dry cell/battery

ammeter

A

rheostat

+ -

cathode

anode

cathode

cation anion

electrolyte

positive ion + electron ( neutral atom

negative ion ( neutral atom + electron

copper(II) chloride copper + chlorine

electrolysis

lead(II) ion + electrons lead

bromine ion bromine + electron

aluminium ions + electrons ( aluminium ions

oxide ion ( oxygen atom + electrons

+ -

A

impure copper plate

pure copper plate

copper(II) sulphate solution

copper atom ( copper(II) ion + electrons

copper(II) ion + electrons ( copper atom

copper(II) sulphate solution

copper plate

+ -

A

iron nail

copper atom ( copper(II) ion + electrons

copper(II) ion + electrons ( copper atom

chemical energy ( electrical energy

+ -

G

copper plate

iron plate

dilute sulphuric acid

Iron plate can be replaced by other conductors like lead, zinc, and carbon

galvanometer is used to detect the production of electric current.

can be replaced by a voltmeter or ammeter.

dilute sulphuric acid can be replaces by other electrolytes like sodium chloride solution and dilute nitric acid.

sunlight

water + carbon dioxide glucose + oxygen

chlorophyll

light energy ( chemical energy

silver bromide silver + bromine

light

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