16.1 Introducing bases and alkalis A BASE is a compound which reacts with an acid to form a salt and...
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Transcript of 16.1 Introducing bases and alkalis A BASE is a compound which reacts with an acid to form a salt and...
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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
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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.
16.1 Introducing bases and alkalis16.1 Introducing bases and alkalis
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Figure 16.1 All alkalis are bases, but not all bases are alkalis.
16.1 Introducing bases and alkalis16.1 Introducing bases and alkalis
Basese.g. CuO, Cu(OH)2
Basese.g. CuO, Cu(OH)2
Alkalise.g. NaOHAlkalise.g. NaOH
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COMMON ALKALIS IN DAILY LIFE
Figure 16.2 These household cleaners contain alkalis.
16.1 Introducing bases and alkalis16.1 Introducing bases and alkalis
sodium hydroxide
sodium hydroxide
ammoniaammoniasodium
hydroxide
sodium hydroxide
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COMMON ALKALIS IN LABORATORY
Figure 16.3 Alkalis commonly used in a laboratory.
16.1 Introducing bases and alkalis16.1 Introducing bases and alkalis
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16.1 Introducing bases and alkalis16.1 Introducing bases and alkalis
How to prepare limewater?
How to prepare limewater?
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16.1 Introducing bases and alkalis16.1 Introducing bases and alkalis
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.
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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.
16.1 Introducing bases and alkalis16.1 Introducing bases and alkalis
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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
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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
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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.
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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)
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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.
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• 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
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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)
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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)
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• metal oxides• ____________• ____________
Concept map
Complete the following concept map.
examples
Concept mapConcept map
Bases
Insoluble bases
ammonia
metal hydroxides
Alkalis (soluble bases)
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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)
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How can we tell a solution is
acidic, neutral or alkaline?
How can we tell a solution is
acidic, neutral or alkaline?
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17.1 pH AND INDICATORS
THE pH SCALE
All aqueous solution contain
All aqueous solution contain
H+(aq)H+(aq)
OH-(aq)OH-(aq)
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How can we tell a solution is
acidic, neutral or alkaline?
How can we tell a solution is
acidic, neutral or alkaline?
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
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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-
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pH valuepH value
H+ (aq) concentrationH+ (aq) concentration
17.1 pH and indicators17.1 pH and indicators
express by
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Increasingly alkaline
Increasingly alkaline
Increasingly acidic
Increasingly acidic
NeutralNeutral
17.1 pH and indicators17.1 pH and indicators
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.
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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
17.1 pH and indicators17.1 pH and indicators
A17.1
(a) H+(aq) concentration < OH-(aq) concentration
(b) H+(aq) concentration > OH-(aq) concentration
(c) H+(aq) concentration > OH-(aq) concentration
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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
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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.
17.1 pH and indicators17.1 pH and indicators
A17.3
(a) Increases
(b) Decreases
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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.
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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.
17.1 pH and indicators17.1 pH and indicators
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
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MEASURING pH OF A SOLUTION
Universal Universal indicatorindicator
Universal Universal indicatorindicator
pH meterpH meterpH meterpH meter
17.1 pH and indicators17.1 pH and indicators
Measuring pH value
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Universal indicatorUniversal indicator
Gives different colours in different pH r
anges
A mixture of several
indicators
Available as a solution or
paper (pH paper)
17.1 pH and indicators17.1 pH and indicators
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Figure 17.4 pH paper and pH colour chart.Figure 17.4 pH paper and pH colour chart.
17.1 pH and indicators17.1 pH and indicators
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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.
17.1 pH and indicators17.1 pH and indicators
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17.1 pH and indicators17.1 pH and indicators
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.
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pH meterpH meter
17.1 pH and indicators17.1 pH and indicators
To measure pH values accurately, use a pH meter.
electrodes
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IMPORTANCE OF pH
17.1 pH and indicators17.1 pH and indicators
Keeping a constant pH value is important in many cases, for example:
Normal functioning of body fluids
Food production
Water supply