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Acids & Bases
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What are acids ?
Arr henius : An acid is a substance which
ionises or dissociates in waterto produce
hydrogen ions, H
+
.
HCl(aq) H+(aq) + Cl(aq)
HCl(l) + H2O(l) H3O+
(aq) + Cl
(aq)
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Basicity of Acids
Number ofionisable hydrogen atoms per
acid molecule.
Monoprotic acid 1
Diprotic acid 2
Triporotic acid 3
Polyprotic acids
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Mineral Acid Organic Acid
Carbonic acid H2CO3 Methanoic acid HCOOH
Hydrochloric acid HCl Ethanoic acid CH3COOH
Hydrochlorous acid HClO Propanoic acid C2H5COOH
Nitrous acid HNO2 Ascorbic acid C 6H8O5
Nitric acid HNO3 Citric acid C6H8O7
Sulphuric acid H2SO4 Lactic acid C3H6O3
Sulphurous acid H2SO3 Malic acid C4H6O5
Phosphoric acid H3PO4 Ethanedioic
acid
H2C2O4
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Strength of Acids
Depends on the degree of dissociation of acids in water.
Strong acids ionise completely in water to produce high
concentration of H+ ions.HCl, H2SO4, HNO3
HCl(l) + H2O(l) H3O+(aq) + Cl(aq)
Weak acids ionise partially in water to produce lowconcentration of H+ ions.
H2CO3, H2SO3, CH3COOH
CH3COOH + H2O(l) CH3COO(aq) + H3O
+(aq)
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Strength of acids
depends on the degree of dissociation of acid in water
There are no hydrogen chloride
molecules in the acid.
There are still many ethanoic acid
molecules in the acid.
Strong acid
Acid that ionise
completely in water
Weak acid
Acid that ionise
partially in water
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Acidity of an acid can be measured using the pH scale.
pH < 7 acidic
pH = 7 neutral
pH > 7 alkaline
The lower the pH values, the higher the concentration of
H+ ions.
0.10 mol dm3 HCl : pH = 1
0.10 mol dm3 CH3COOH : pH = 3
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What are bases ?
Arr henius : A base is a substance which
ionises in waterto produce hydroxide ions,
OH.
NaOH(s) Na+(aq) + OH(aq)H2O
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Types of Base
Ionic bases : metal oxides & metal
hydroxides.
CaO(s) + H2O(l) Ca(OH)2(aq)Ca(OH)2(aq) Ca
2+(aq) + 2OH(aq)
Covalent bases : NH3NH3(l) + H2O(l) NH4
+(aq) +
OH(aq)
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Bases vs Alkalis
Bases that are soluble in water are called
alkalis.
Bases
Alkalis
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Soluble Base Insoluble Base
Ammonia NH3 Magnesium
hydroxide
Mg(OH)2
Sodium oxide Na2O Magnesium oxide MgO
Sodium hydroxide NaOH Aluminium
hydroxide
Al(OH)3
Potassium oxide K2O Aluminium oxide Al2O3
Potassium hydroxide KOH Zinc hydroxide Zn(OH)2
Calcium oxide CaO Zinc oxide ZnO
Calcium hydroxide Ca(OH)2 Copper (II)
hydroxide
Cu(OH)2
Barium oxide BaO Copper (II) oxide CuO
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Strength of Alkalis
Strong alkalis ionise completelyin water to produce high
concentration of OH.
NaOH, KOH, Ba(OH)2
NaOH(s) Na+(aq) + OH(aq)
Weak alkalis dissociate partially in water to produce low
concentration of OH.NH3, Hydrazine (N2H4), methylamine (CH3NH2)
NH3(l) + H2O(l) NH4+(aq) + OH(aq)
H2O
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Liquid ammonia
Covalent molecule
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pH values can be used to compare the strength of
different alkalis.
For 2 different alkalis of the same concentration, alkali
withhigher pH value is the stronger alkali.
0.10 mol dm3 NaOH : pH = 13
0.10 mol dm3 NH3 : pH = 11
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Concentration of Acids / Alkalis
)(dmsolutionofvolume
(g)soluteofmassionConcentrat
3!
)(dmsolutionofvolume
(mol)soluteofmass
Molarity 3!
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)mol(gmassmolar
)dm(gionconcentrat
Molarity 1-
-3
!
= mol dm3
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Example 1
4.0 g of sodium hydroxide, NaOH is dissolved in enoughwater to make 200 cm3 of solution. What is the
concentration of the solution in
(a) g dm3
(b) mol dm3
[ Relative atomic mass : Na, 23 ; O, 16 ; H, 1 ]
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Example 2
Vinegar contains ethanoic acid, CH3COOH. Analysisdone on a sample of the vinegar solution shows its
concentration to be 1.5 mol dm3. What is the
concentration of the vinegar solution in g dm3 ?
[ Relative atomic mass : H, 1 ; C, 12 ; O, 16 ]
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1000
Vol !
M = molarity (mol dm3
)V = volume (cm3)
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Example 3
Calculate the mass ofhydrochloric acid in 300 cm3 of2 mol dm3 hydrochloric acid.
[ Relative atomic mass : H, 1 ; Cl, 35.5 ]
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Example 4
A student pipetted 25.0 cm3 of NaOH into a conical flask.The concentration of the alkali is 1.2 mol dm3. Calculate
the number of moles of NaOH in the flask.
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2211 VMVM !
M = molarity
V = volume
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Example 5
Calculate the volume of concentrated acid needed toprepare 5.0 dm3 of 2 mol dm3 HNO3 from 18 mol dm
3
HNO3.
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Example 6
Calculate the number of moles ofhydrogen ions in 500cm3 of 1.0 mol dm3 sulphuric acid.
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Chemical Properties of Acids
Acid reacts with reactive metals to produce salt &
hydrogen gas.
Acid + reactive metal salt + hydrogen gas2HCl(aq) + Mg(s) MgCl2(aq) + H2(g)
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Acid reacts withbase to produce salt & water.
Acid + base salt + water
HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)
Neutralisation
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Acid reacts withcarbonates to produce salt, carbon
dioxide gas & water.
Acid + carbonates salt + carbon dioxide + water
HCl(aq) + CaCO3(s) CaCl2(aq) + CO2(g) + H2O(l)
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Preparation of Standard Solution
Standard solution a solution whose concentration isaccurately known.
A standard solution can be prepared in the following way
(a) the mass of solute needed to give the required
molarity is weighed.
(b) solute is completely dissolved in some distilled water.
(c) the solution is then transferred to a volumetric flask
partially filled with distilled water.
(d) just enough water is added to give the requiredvolume & the flask is shaken to ensure thoroughmixing.
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1.0 dm3 of 0.5 mol dm3 sodium chloride :
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Question 1
Calculate the volume of concentrated acid needed toprepare 5.0 dm3 of 2.0 mol dm3 HNO3 from 18 mol dm3
HNO3.
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Question 2
What volume of 2.0 mol dm3
sulphuric acid is needed toprepare 100 cm3 of 1.0 mol dm3 sulphuric acid ?
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Neutralisation :
Procedure :
Burette is rinsed with small
amount of H2SO4 to remove the
water on the interior surface. Burette then is filled with H2SO4.
Initial reading is recorded.
25.0 cm3 of KOH is pipetted into
a conical flask.
Two drops of phenolphthaleinare added into KOH & is shaken
(pink).
KOH is titrated slowly with
H2SO4
Conical flask is swirled. Addition of H2SO4 is stopped as
soon as the solution turns
colourless.
Final burette reading is recorded.
The above procedure is repeated
White
tile
Acid-base Titration :
Quantitative analysis to determine the
end point (volume of an acid required
to exactly neutralise a fixed volume ofan alkali) with the help of a suitable
indicator.
25.0 cm3 KOH + phenolphthalein
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Titration number 1 2 3
Final readings (cm3) 24.25 25.25 24.65
Initial readings (cm3) 0.15 1.20 0.60
Volume of H2SO4 (cm3) 24.10 24.05 24.05
Average volume of H2SO4 used
3cm24.07
3
05.2405.2410.24
!
!
31
1
2
1
22
11
dmmol0.193M
12
7)(0.1)(24.0(25)M
n
n
M
M
!
!
!
H2SO4 + 2KOH K2SO4 + 2H2O
Molarity of KOH
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d
dd
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End point can also be determined by using a
(a) pH meter can be interfaced with a computer to allow a graph of pH
against time to be plotted.
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(b) Conductivity cell
Acidic & alkaline solutions
conduct electricity because
they contain ions.
When an alkali is slowly added
to an acid, electrical
conductivity decreases.
H+ ions & OH ions combine to
form water molecules which
cannot conduct electricity.
The concentration of mobile
ions decreases, making the
solution less conducting.
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Question 1
What volume of 0.20 mol dm3 nitric acid is required to
completely neutralise 0.14 g of potassium hydroxide ?
[RAM : O, 16 ; K, 39]
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Question 2
What volume of ammonia gas, measured at room
conditions, is required to completely neutralise 30 cm3 of1.20 mol dm3 sulphuric acid ?
[Molar volume : 24 dm3 mol1 at room conditions]
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Question 3
Calculate the mass of barium hydroxide required to
completely neutralise 100 cm3 of 2.0 mol dm3 ethanoicacid ?
[RAM : H, 1 ; O,16 ; Ba, 137]
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Question 4
A student used a standard solution of sodium hydroxide
to determine the concentration of a solution of
hydrochloric acid.
Concentration of sodium hydroxide =0.1 mol dm3
Volume ofhydrochloric used = 25.00 cm3
Burette reading :
Titration number Rough 1 2 3
Final reading (cm3) 23.30 47.20 23.25 46.10
Initial reading (cm3) 1.00 25.10 1.20 24.00
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Question 5
The diagram below shows a titration apparatus used for
determining the molarity of a strong acid, H2X.
Sodium hydroxide
+ indicator
H2X
Whitetile
Retort stand
It was observed that 25.0 cm3 of H2X is needed
to neutralise completely 50.0 cm3 of 0.1 mol
dm3 sodium hydroxide solution.
(a) State the colour of phenolphthalein solutionin the conical flask
(a) b
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