16.1 Introducing bases and alkalis A BASE is a compound which reacts with an acid to form a salt and water only. Examples of base: Metal oxides Ammonia

16.1 Introducing bases and alkalis16.1 Introducing bases and alkalis

A BASE is a compound which reacts with an acid to form a salt

and water only.

Examples of base:

Metal oxidesMetal

oxides

Ammonia (NH3)

Ammonia (NH3)

Metal hydroxides

Metal hydroxides

Soluble Bases

Soluble Bases

AlkaliAlkaliAlkaliAlkali

This means that all alkalis are bases, but not all bases are alkalis.

Only a few bases are soluble in water.


Figure 16.1 All alkalis are bases, but not all bases are alkalis.


Basese.g. CuO, Cu(OH)2

Basese.g. CuO, Cu(OH)2

Alkalise.g. NaOHAlkalise.g. NaOH

COMMON ALKALIS IN DAILY LIFE

Figure 16.2 These household cleaners contain alkalis.


sodium hydroxide

sodium hydroxide

ammoniaammoniasodium

hydroxide

sodium hydroxide

COMMON ALKALIS IN LABORATORY

Figure 16.3 Alkalis commonly used in a laboratory.



How to prepare limewater?

How to prepare limewater?


Preparation of limewater

Calcium hydroxide is an alkali. It is a white solid, slightly soluble in

water.

Limewater is a saturated aqueous solution of calcium hydroxide.

Preparation of limewaterPreparation of limewater

1. Stirring solid calcium hydroxide with water for a long time.

3. The clear filtrate obtained is the limewater.3. The clear filtrate obtained is the limewater.

2. Filtering the suspension.


16.2 CHARACTERISTICS AND CHEMICAL PROPERTIES OF AQUEOUS ALKALIS

(Never try to check this yourself!)

Taste

Taste

Aqueous solutions of alkalis usually taste bitter.

Feel

Feel

Dilute solutions of alkalis have a soapy feel.

They change red litmus paper to blue colour.Effect on litmusEffect on litmus

Like acids, alkalis are electrolytes. They dissolve in water to form solutions that conduct electricity.

Electrical conductivityElectrical conductivity

Chemical properties of alkalisA. alkali + acidic substance salt + H2O(l) (neutralization)

(no heating)

e.g1. NaOH (aq) + HCl (aq) NaCl (aq) + H2O (l)

e.g2. NH3 (aq) + HCl (aq) NH4Cl (aq)

e.g3. 2NaOH (aq) + CO2 (g) Na2CO3 (aq) + H2O (l)

e.g4. Ca(OH)2 (aq) + CO2 (g) CaCO3 (s) + H2O (l) limewater test

All ammonium compounds (NH4+) are soluble in water

Chemical properties of alkalis

B. alkali + NH4+ compound salt + NH3(g) + H2O(l)

(heating)

e.g1. NaOH (aq) + NH4Cl(aq) NaCl(aq) + NH3(g) + H2O(l)

e.g2. KOH (aq) + NH4Cl (aq) KCl(aq) + NH3(g) + H2O(l)

NH3(g)+ NH4Cl(aq) NH4Cl(aq) + NH3(g) + H2O(l)

Same reactants and products

This is not a possible reaction.

NH3(g) can turn red limus paper blue.

Chemical properties of alkalis

C. alkali + metal ions (aq) hydroxides + salt (no heating)

e.g1. 2NaOH (aq) + CuSO4 (aq) Cu(OH)2(s) + Na2SO4(aq)

e.g2. NaOH(aq) + Cu(NO3)2 (aq) Cu(OH)2(s) + NaNO3(aq)

e.g3. 2H2O(l)+ 2NH3(aq)+ CuSO4(aq) Cu(OH)2(s)+ (NH4)2SO4(aq)

e.g4. NaOH(aq) + KNO3(aq) KOH(aq) + NaNO3(aq) No ppt formed.

2OH- (aq) + Cu2+ (aq) Cu(OH)2 (s)

2OH- (aq) + Cu2+ (aq) Cu(OH)2 (s)

Metal Hydroxides

• Gp 1 hydroxides can dissolve in water(no ppt formed).

• Hydroixdes of main group metals are white, while those are transition metals are coloured.

• In excess NaOH:

Pb(OH)2 (s) , Al(OH)3 (s), Zn(OH)2 (s), are soluble in excess NaOH.

• Water is important to alkalis (OH- ions)• e.g1 NaOH(aq) Na+(aq) + OH-(aq)

• e.g2 NH3(aq) + H2O(l) NH4+(aq) + OH–(aq)

Strong alkali

Weak alkali

A16.5

(a) (i) Fe2(SO4)3(aq) + 6NaOH(aq)

2Fe(OH)3(s) + 3Na2SO4(aq)

(ii) Fe3+(aq) + 3OH-(aq) Fe(OH)3(s)(b) (i) Cu(NO3)2(aq) + 2KOH(aq) Cu(OH)2(s) + 2KN

O3(aq)

(ii) Cu2+(aq) + 2OH-(aq) Cu(OH)2(s)

p32A Q14

(a) Zinc dissolved/ colourless gas bubbles evolved.

Zn(s) + 2HCl (aq) ZnCl2(aq) + H2(g)

(b) Dirty green precipitates formed.

2NaOH (aq) + FeSO4 (aq) Fe(OH)2(s) + Na2SO4(aq)

or 2OH- (aq) + Fe2+ (aq) Fe(OH)2(s)

• metal oxides• ____________• ____________

Concept map

Complete the following concept map.

examples

Concept mapConcept map

Bases

Insoluble bases

ammonia

metal hydroxides

Alkalis (soluble bases)

Dilute alkalis• taste bitter• feel soapy• turn red litmus _______• conduct ____________• react with ______ to give salt and water• react with __________ oxides to give

salts and water• react with ammonium compounds to give

salt,__________ and water• react with ______ ions to form insoluble

hydroxide (except K+ and Na+ ions)

examplesdissolve in water to give properties

blueelectricity

acids

metal

non-metal

ammonia

Concept mapConcept map

Alkalis (soluble bases)

OH-(aq)

Concentrated alkalis

• _____________very corrosive

• KOH(aq)• NaOH(aq)

• NH3(aq)

• Ca(OH)2(aq)

How can we tell a solution is

acidic, neutral or alkaline?



17.1 pH AND INDICATORS

THE pH SCALE

All aqueous solution contain

All aqueous solution contain

H+(aq)H+(aq)

OH-(aq)OH-(aq)





It depends on the difference in H+(aq) concentration

and OH-(aq) concentration.

It depends on the difference in H+(aq) concentration

and OH-(aq) concentration.

17.1 pH and indicators17.1 pH and indicators


H+(aq) concentration > OH-(aq) concentration

H+(aq) concentration = OH-(aq) concentration

H+(aq) concentration < OH-(aq) concentration

acidic

neutral

alkaline

Difference in H+(aq) and OH-(aq) concentrationDifference in H+(aq) and OH-(aq) concentrationSolution

Table 17.1 Difference in H+(aq) and OH-(aq) concentration for acidic, neutral and alkaline solution.Table 17.1 Difference in H+(aq) and OH-(aq) concentration for acidic, neutral and alkaline solution.

H2O H+ + OH-

HCl H+ + Cl-

pH valuepH value

H+ (aq) concentrationH+ (aq) concentration


express by

Increasingly alkaline

Increasingly alkaline

Increasingly acidic

Increasingly acidic

NeutralNeutral


pH scale : a measure of acidity and alkalinity pH scale : a measure of acidity and alkalinity

7700 1414

On the pH scale:

An acidicacidic solution has a pH value less than 7.

An neutralneutral solution has a pH value of exactly 7.

An alkalinealkaline solution has a pH value greater than 7.

Q17.1

what can you tell about the relationship between H+(aq) and

OH-(aq) concentration of the following substances?

(a) limewater (b) rainwater (c) soft drinks

Q17.1

what can you tell about the relationship between H+(aq) and

OH-(aq) concentration of the following substances?

(a) limewater (b) rainwater (c) soft drinks


A17.1

(a) H+(aq) concentration < OH-(aq) concentration

(b) H+(aq) concentration > OH-(aq) concentration

(c) H+(aq) concentration > OH-(aq) concentration

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

pH1 M NaOH

0.1 M NaOH

limewater

Na2CO3 solution

Milk of Magnesia,household ammonia

soap solution

NaHCO3 solution

blood

rainwater

pure water

ordinary distilled water

(NH4)2SO4 solution

soft drink

orange juice

vinegar, 0.1 M CH3COOH

lemon juice

0.1 M HCl

1 M HCl

neutral

increasingly alkaline

increasingly acidic

Q17.3

State whether the pH value increases or decreases in each

of the following cases:

(a) Add a piece of magnesium ribbon to dilute hydrochloric

acid.

(b) Dissolve hydrogen chloride gas in water.

Q17.3

State whether the pH value increases or decreases in each

of the following cases:

(a) Add a piece of magnesium ribbon to dilute hydrochloric

acid.

(b) Dissolve hydrogen chloride gas in water.


A17.3

(a) Increases

(b) Decreases

COMMON ACID-BASE INDICATORS

An acid-base indicator is a special dye added to the solution to be tested.An acid-base indicator is a special dye added to the solution to be tested.

Figure 17.2 Three

common acid-base

indicators.

Figure 17.2 Three

common acid-base

indicators.


Different acid-base indicators show different colours in different p

H ranges.

Figure 17.3 Different colours of acid-base indicators in different pH ranges.


Methyl orange

Indicator

pH scale

Litmus red purple blue

red orange yellow3.1 4.4

5 8

Phenolphthaleinvery pale

pinkcolourless 8.3 10 red

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

MEASURING pH OF A SOLUTION

Universal Universal indicatorindicator

Universal Universal indicatorindicator

pH meterpH meterpH meterpH meter


Measuring pH value

Universal indicatorUniversal indicator

Gives different colours in different pH r

anges

A mixture of several

indicators

Available as a solution or

paper (pH paper)


Figure 17.4 pH paper and pH colour chart.Figure 17.4 pH paper and pH colour chart.


universal indicator solution

Figure 17.5 Finding the pH value of a soluti

on by universal indicator solution. Put the s

olution in front of a piece of white paper to

see the colour easily.

Figure 17.5 Finding the pH value of a soluti

on by universal indicator solution. Put the s

olution in front of a piece of white paper to

see the colour easily.



Q17.4

Do not dip the pH paper into the solution to be tested! Why?

Q17.4

Do not dip the pH paper into the solution to be tested! Why?

A17.4

The dyes on the pH paper may go into the solution and

contaminate it.

pH meterpH meter


To measure pH values accurately, use a pH meter.

electrodes

IMPORTANCE OF pH


Keeping a constant pH value is important in many cases, for example:

Normal functioning of body fluids

Food production

Water supply

Documents

16.1 Introducing bases and alkalis A BASE is a compound which reacts with an acid to form a salt and water only. Examples of base: Metal oxides Ammonia