8.1INVESTIGATING ELECTRICAL CONDUCTIVITY OF COMPOUNDS All pure metals conduct an electric current....

8.1 INVESTIGATING ELECTRICAL

CONDUCTIVITY OF COMPOUNDS

All pure metals conduct an electric current.

With the exception of carbon (in the form of graphite), all pure

non-metals do not conduct electricity.

Figure 8.1 Investigating the electrical conductivity of compounds.

6 V battery+ _

light bulb

crocodile clipsolid piece

under test

(a) (b)

_+

crucible

liquid(molten)/aqueous under test

carbon (graphite) rods

chlorine, carbon, hydrogen

1,1,1-trichloroethanesodium, hydrogen, oxygen

Sodium hydroxide

chlorine, carbonTetrachloromethanepotassium, manganese, oxygen

Potassium permanganate

carbon, hydrogenHexanecopper, sulphur, oxygen

Copper(II) sulphate

carbon, hydrogen, oxygen

Ethanol (an alcohol)magnesium, sulphur, oxygen

Magnesium sulphate

hydrogen, oxygenDistilled watercalcium, nitrogen, oxygen

Calcium nitrate

carbon, hydrogenPolythene (a plastic)potassium, iodinePotassium iodide

carbon, hydrogen, oxygen

Sugarlead, bromineLead(II) bromide

carbon, hydrogenWaxsodium, chlorineSodium chloride

Constituent elements

(Non-conductors whether solid, liquid or in aqueous solution)Constituent elements

(Non-conductors when solid, but conductors when molten or in aqueous solution)

Table 8.1 Electrical conductivity and constituent elements of some compounds.

ELECTROLYTES are compounds which allow electricity to pass

through them only when molten or in aqueous solution.

Ionic compounds

MetalsMetalsMetalsMetals

Non-Non-metalsmetals


1 Compounds

solid state

molten or aqueoussolution

(conduct electricity)

(do not conduct electricity)

Electrolyte

Compounds2

(do not conduct electricity)

solid, liquid or in aqueous solution



Non-Non-

metalsmetalsNon-Non-

metalsmetals

Non-Electrolyte

• Why ionic compound is an electrolyte ?

• How do ionic compound conduct electricity in molten or aqueous state?

Solid lead(II) bromide

Molten lead(II) bromide

does not conduct electricity

conducts electricity

heat

molten lead(II) bromide

positive electrode (anode)

crocodile clip

clamp

boiling tube

negative electrode (cathode)

Glass tubing (connected to a rubber tubing which leads to the fume cupboard)

light bulb

6 V battery+ _

Thus electric current is carried through molten lead(II) bromide by

mobile (freely moving) ions.

Reddish brown brReddish brown bromine vapour is liomine vapour is li

beratedberated

At positive electrode At positive electrode (anode)(anode)

At positive electrode At positive electrode (anode)(anode)

At negative electrode At negative electrode (cathode)(cathode)

At negative electrode At negative electrode (cathode)(cathode)

A silvery grey substance (lead)

is deposited

ELECTROLYTES AND IONS

Electrolytes


through them only when molten or in aqueous solution. There is

chemical decomposition during the conduction.


through them only when molten or in aqueous solution.

Molten lead(II) bromide is thus decomposed to the two elements

lead and bromine.

The whole process is called electrolysis (meaning

'decomposition by electricity').

Lead(II) bromide does not conduct electricity in solid state. This is

because ions in solids are not mobile (i.e. unable to move

freely).

Pb2+

Pb2+

Pb2+

Pb2+

Pb2+

Pb2+

Pb2+

Pb2+

Pb2+Br -

Br -

Br -

Br -

Br - Br -

Br -

Br - Br -

Conclusions• Ionic compounds in molten or aqueous states can

conduct electricity. It is because there are mobile ions.

• Ionic compounds in solid state cannot conduct electricity. It is because ions are not mobile.

• Electrolytes are compounds which allow electricity to pass through them only when molten or in aqueous solution. There is chemical decomposition during the conduction.

• Electrolysis means 'decomposition by electricity'.

• You do not need to remember the observations in electrolysis.

ELECTROLYSIS OF AQUEOUS COPPER(II)

CHLORIDE SOLUTION

Aqueous copper(II) chloride conducts electricity.

During electrolysis, copper(II) chloride is decomposed to form

copper (reddish brown solid) at cathode and chlorine gas (with

smell of swimming pool) at anode.

Electrolysis of aqueous copper(II) chloride solution.

Q8.2

(a) What charged particles does copper(II) chloride

consist of?

(b) Explain why solid copper(II) chloride does not

conduct electricity, but its aqueous solution does.

Q8.2

(a) What charged particles does copper(II) chloride

consist of?

(b) Explain why solid copper(II) chloride does not

conduct electricity, but its aqueous solution does.

A8.2

(a) Positively charged copper(II) ions and negatively charged

chloride ions.

(b) Solid copper(II) chloride consists of ions which are not

mobile, therefore it does not conduct electricity. In aqueous

solution, the ions become mobile and thus the solution

conducts electricity.

Q8.3

Which of the following are electrolytes?

Wax, sodium chloride, sodium, mercury, ethanol,

potassium iodide, sugar, carbon monoxide

Q8.3

Which of the following are electrolytes?

Wax, sodium chloride, sodium, mercury, ethanol,

potassium iodide, sugar, carbon monoxide

Sodium chloride and potassium iodide only.

A8.3

An ION is an atom or a group of atoms having an overall electric

charge.

A simple (monoatomic) ion is derived from a single atom. A pol

yatomic ion is derived from a group of atoms.

There are two kinds of ions: positively charged ions (cations) and

negatively charged ions (anions).

Cations and anions

cationscationscationscationsare attracted towards

anionsanionsanionsanionsare attracted towards

cathodecathode

anodeanode

MIGRATION OF IONS

We can observe the migration (movement) of coloured ions durin

g electrolysis.

Figure 8.7 To show the migration of coloured ions during electrolysis (using a U-tub

e).

carbon cathodecarbon anode

dilute hydrochloric acid

this region slowly becomes blue due to the migration of positive copper(II) ions towards the negative cathode

dilute hydrochloric acidthis region slowly becomes orange due to the migration of negative dichromate ions towards the positive anode

a gel containing copper(II) ions and dichromate ions

+ _

+ _

20V d.c. supply

To show the migration of purple permanganate ions during under the influence of an

electric field(using a strip of filter paper on a microscope slide).

Q8.6

Refer to again.

(a) Towards which electrode are potassium ions

migrating? Why?

(b) Can we see the movement of potassium ions?

Why?

(c) If a chromium(III) sulphate crystal was used

instead of a potassium permanganate crystal,

what would be observed? Why?

Q8.6

Refer to again.

(a) Towards which electrode are potassium ions

migrating? Why?

(b) Can we see the movement of potassium ions?

Why?

(c) If a chromium(III) sulphate crystal was used

instead of a potassium permanganate crystal,

what would be observed? Why?

Figure 8.8

A8.6

(a) The cathode. Potassium ions are positively charged. They

are thus attracted towards the negative electrode

(cathode).

(b) No. Potassium ions are colourless.

(c) A green patch would move towards the negative electrode

(cathode). Chromium(III) ions are green in colour and

positively charged. They are attracted towards the negative

electrode.

ObjectivesObjectives• To define what is chemical bondingTo define what is chemical bonding• To define what is ionic bondingTo define what is ionic bonding- Electrostatic attractions of opposite ionsElectrostatic attractions of opposite ions- Transfer of electron(s)Transfer of electron(s)- Non-directional forceNon-directional force- Obey the rule of stabilityObey the rule of stability

• To draw electron diagramsTo draw electron diagrams• To name and give chemical formula of the ionic compound To name and give chemical formula of the ionic compound

( both from mono or polyatomic ions)( both from mono or polyatomic ions)• To plan and describe experiments to realize what are electrTo plan and describe experiments to realize what are electr

olytes and non-electrolytes.olytes and non-electrolytes.• To realize the nature of ions of an ionic compound.To realize the nature of ions of an ionic compound.• To figure out the colour of an ionic compound in To figure out the colour of an ionic compound in aqueousaqueous s s

tate.tate.

Chemical bonding (attractionChemical bonding (attraction))

Ionic bondingCovalent bonding

Forms between Forms between

metals and nonmetals nonmetals and nonmetals

Metallic bonding

Ionic bonding• The reaction of sodium and chlorine .

Sodium Chloride

Na

loses 1 e-

+

Na

unstable and unstable and reactivereactive


sodium atom (Na) 2, 8, 1sodium atom (Na) 2, 8, 1 Na ionNa ion+

stablestablestablestable

CATIONCATIONCATIONCATION

Clgains 1 e-

_



chlorine atom (Cl) 2, 8, 7chlorine atom (Cl) 2, 8, 7 Cl ionCl ion-

stablestablestablestable

xx

xx

x

x

x

xx

xx

xx

xx

xx

Cl

xx

xx

x

x

x

xx

xx

xx

xx

xx

ANIONANIONANIONANION

When a sodium atom and a chlorine atom react, the sodium atom

loses one electron to the chlorine atom. As a result of this

transfer of electron, two ions are formed.

FORMATION OF IONIC BOND

IONIC BONDING AND IONIC SUBSTANCES

Sodium Chloride (Salt)

NaNa+ ClCl_attracted by ionic bond

Nature of ionic bond

•The attraction force is non-directional

ClCl_

In sodium chloride

Figure 8.15 Crystal of an ionic compound, sodium chloride.

Sodium chloride has a giant ionic structure. It consists of Na+ and Cl– ions held

together by ionic bonds.

IONIC BOND is the strong non-directional electrostatic force of

attraction between oppositely charged ions.

An ionic bond is formed by the transfer of one or more electrons

from one atom (or group of atoms) to another.

Monoatomic ion (consists of 1 atom only)

Polyatomic ion (consists of 2 or more atoms )

Na+ OH-

K+ NO3-

Ag+ SO42-

Hg+

I-

Cl-

Br-

1) All simple (single) metal ions are cations.

2a) All simple (single) non-metal ions are anions (except H+).

2b) All polyatomic ions are anions (except NH4+)

3) There is only one common polyatomic cation - NH4+.

4) Polyatomic ions usually consist of non-metals only, but some consist of a metal and a non-metal.

5) When a metal forms only one cation, the ion has the same name as the metal.

6) Transition metals are rather unusual- the same metal can form more than one simple cation with different charges. To name each ion, a Roman numeral indicating the charge is written in brackets after the name of the metal.

7) Simple (single) anion names ending in –ide

8) Polyatomic anions containing oxygen ofter have names ending in –ate or –ite. The polyatomic anion with more oxygen is named as –ate, and that with less oxygen as –ite.

9)Ions with 4+ or 4- are uncommon. (unstable)

Questions

• Lead(II) dichromate• Iron(III) oxide• Potassium permanganate• Zinc hydroxide• KNO3

• CaCO3

• Ca(HCO3)2

• Fe2(SO4)3

ObjectivesObjectives• To define what is chemical bondingTo define what is chemical bonding• To define what is ionic bondingTo define what is ionic bonding- Electrostatic attractions of opposite ionsElectrostatic attractions of opposite ions- Transfer of electron(s)Transfer of electron(s)- Non-directional forceNon-directional force- Obey the rule of stabilityObey the rule of stability

• To draw electron diagramsTo draw electron diagrams• To name and give chemical formula of the ionic To name and give chemical formula of the ionic

compound ( both from mono or polyatomic ions)compound ( both from mono or polyatomic ions)

ENDENDENDEND

8.1INVESTIGATING ELECTRICAL CONDUCTIVITY OF COMPOUNDS All pure metals conduct an electric current....

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Transcript of 8.1INVESTIGATING ELECTRICAL CONDUCTIVITY OF COMPOUNDS All pure metals conduct an electric current....