c2 topic 2 notes

7/25/2019 C2 Topic 2 Notes

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Topic 2 Ionic compounds and analysis

IONIC BONDS

An ion is an atom or groups of atoms with a positive or negative charge

Ionic bonds form between positively and negatively charged ions (i.e between

cations and anions see below)

Atoms are most stable with full outer shells

However, atoms of most elements have incomplete (i.e not full) outer shellsthey

readily lose or gain electrons during chemical reactions to obtain full outer shells

When electrons are gained or lost, the atoms become ions

Cations:

!etal atoms readily lose their outermost electrons to form positively charged ions

called cations

"or elements in groups # and $, the number of outer electrons lost is the same as

their group number%

o &.g sodium is in group #has # electron in its outer shell ($.'.#). It can

lose one electron to become a acation with a full outer shell ($.')

o&.g$ magnesium is in group $has $ electrons in its outer shell ($.'.$). It

can lose two electrons to become a !g$cation with a full outer shell ($.')

Anions:

on*metal atoms can gain electrons to form negatively charged ions called anions

"or elements in groups + and , the number of electrons they gain is ' minus their

group number%

o &.g o-ygen is in group +has + electrons in its outer shell ($.'.+). It can

gain two electrons to become an $*anion with a full outer shell ($.'.')

o&.g$ chlorine is in group has electrons in its outer shell ($.'.). It can

gain one electron to become a /l*anion with a full outer shell ($.'.')

ote% when non*metal atoms form ions, their name changes to ide (e.g chlorine

atoms are called chloride ionso-ygen atoms are called o-ide ions)

IONIC COMPOUNDS

Ionic compounds form when a metal reacts with a non*metal%

o &lectrons lost by the metal are transferred to the non*metal

o both positive and negative ions that form end up with stable, full outer

shells

oppositely charged ions attract each other stronglyforming an ionic

compound held together by ionic bonds

&.g when sodium and chlorine react%

o &lectrons are transferred from sodium atoms to chlorine atoms

o 0his forms aand /l*ions (both have a stable, full outer shell)

o 0hese oppositely charged ions attract each other stronglyforming the

ionic compound a/l (table salt)

Worin! out a "ormula:

Ionic compounds are electrically neutral because they contain e1ual numbers of

positive and negative charges

If you 2now the charges of the cations and anions you can wor2 out the formula of

the ionic compound

&.g for sodium chloride, one ais needed for every /l*ionformula is a/l


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&.g$ the ionic compound aluminium o-ide%

o Aluminium o-ide contains Al3cations and $*anions

o "or charges to be e1ual you must have%

0wo Al3cations ( + charge)

0hree $*anions ( * + charge)

o formula of aluminium o-ide is Al$3(neutral charge)

Compound ions:

/ompound ions contain more than one element

&.g the nitrate ion 3*contains one nitrogen atom 4oined to three o-ygen atoms

plus an e-tra electron (negative charge)

If two or more compound ions of the same type are needed in a formula, the ion

must be written inside brac2ets (with the number on the outside)

&.g the ionic compound magnesium nitrate%

o !agnesium nitrate contains !g$and 3

*ions

o

"or charges to be e1ual you need two 3*

ions for every !g

$

iono formula of magnesium nitrate is !g(3)$

Nam#s o" ionic compounds:

ote%

oIonic compounds usually end in ide (e.g a/l * sodium chloride)

oHowever, compounds that contain o-ygen atoms end in ate (e.g

!g(3)$* magnesium nitrate)

T$# structur# o" ionic compounds:

0he ions in an ionic compound are pac2ed tightly together and arranged in a

regular way, called a lattice structure

0he lattice structure is held together by strong electrostatic forces of attraction (i.eby ionic bonds) between oppositely charged ions

P%OP&%TI&S O' IONIC COMPOUNDS

Conductin! #l#ctricity:

Ionic compounds don5t conduct electricity when solid

However, they do conduct electricity%

owhen molten (i.e when they5re heated until they turn into li1uid)

o when in a1ueous solution (i.e dissolved in water) this is why sea water

(a/l in a1ueous solution) conducts electricity

&-planation%

o 0wo conditions must be met for a substance to conduct electricity !ust contain charged particles

/harged particles must be free to move

oIonic compounds contain charged particles (ions) but these are only free to

move when molten or in a1ueous solution

o In the solid form, ions can5t move (they5re strongly held together in a

lattice structure by ionic bonds)can5t conduct electricity

M#ltin! points and (oilin! points:

0he melting point of a substance is the temperature at which it changes from a

solid to a li1uid

0he boiling point of a substance is the temperature at which it changes from ali1uid to a gas (at its fastest possible rate)


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Ionic bonds holding ionic compounds together in a lattice are very strong%

o 0his means lots of (heat) energy is needed to brea2 the ionic bonds

o ionic compounds have high melting and boiling points

o ionic compounds are usually solids at room temperature

SO)UBI)IT*

If a substance dissolves well in a particular li1uid, it is said to be 6soluble5

If a substance doesn5t dissolve at all in a particular li1uid, it is said to be

6insoluble5

A 6salt5 (notNaCl which is table salt) is a substance that can be made by

reacting an acid and an al2ali

Solu(ility rul#s "or salts in +at#r:

Solu(l# in +at#r Insolu(l# in +at#r

All common sodium, potassium and ammonium salts

All nitrates

!ost chlorides 7ilver8lead chlorides

!ost sulfates lead8barium8calcium sulfates

sodium8potassium8ammonium carbonates !ost carbonates

sodium8potassium8ammonium hydro-ides !ost hydro-ides

Pr#cipitation r#actions:

A reaction in which at least one insoluble solid (called the 6precipitate5) is

produced from two soluble substances is called a precipitation reaction%

o &.g lead nitrate potassium iodidelead iodide potassium nitrate

o 9alanced e1uation% :b(3)$(a1) $;I (a1):bI$(s) $;3 (a1)

o 7tate symbols show that all substances are dissolved in water (a1 *

a1ueous) e-cept for lead iodide, which is insoluble (so it5s shown as a solid

s)forms a precipitate

In precipitation reactions, the precipitate can be separated from the unreacted ions

by filtration. It is washed on filter paper and then dried in a warm oven

P%&CIPITAT&S


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o As the barium sulfate passes through the patient5s digestive system, -*ray

photos are ta2en

o 9arium (li2e bone) absorbs -*raysshows up as white on the photosany

problems with the patient5s digestive system can be seen

!ost barium salts are to-ic, however%

o barium sulphate is insolublecan5t enter the patient5s blood

o 0his ma2es it safe to swallow

ION T&STS

'lam# t#sts:

>ifferent metal ions produce different coloured flames when held over a 9unsen

burner flame%

o7odium (a) yellow

o:otassium (;) lilac

o /alcium (/a$) red

o /opper (II) (/u$) green8blue

ote% the most intense colours are obtained from solids, but flame tests also wor2

when solids are dissolved in water (as a1ueous solutions)

"lame tests led to the discovery of new elements%

o /hemists in the #'??s did flame tests of different samples of mineral water

and then used a prism to separate the colours of light given off * method is

called 6spectroscopy5

o0hey saw a grey*blue colour that hadn5t been seen before and realised they

had discovered a new element later called it caesium

o A year later, using the same method, rubidium was discovered (gave off a

dar2 red colour in a 9unsen flame)

Pr#cipitation t#sts:

7ome anions can be identified by precipitation tests%

o /hloride ions (/l*)%

Add dilute nitric acid and silver nitrate to the solution

If the sample contains chloride ions, a white precipitate of silver

chloride will form

o7ulfate ions (7@

$*)%

Add dilute hydrochloric acid and barium chloride to the solution

If solution contains sulfate ions, a white precipitate of barium

sulfate will form

T#st "or car(onat# ions: Add a dilute acid to the solution

If solution contains carbonate ions (/3*), carbon dio-ide gas will be given off,

which when bubbled through limewater will turn the limewater mil2y

c2 topic 2 notes

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