Chapter 7a Acids & Bases

8/8/2019 Chapter 7a Acids & Bases

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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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Chapter 7a Acids & Bases

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