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CHAPTER 7: ACIDS AND BASES Meaning of acid, base and alkali Fill in the blanks with the correct words: 1

An acid is a chemical substance which ionises in………………. to produce

………….ions.

The hydrogen ion combines with a water molecule, H2O to form a ………………………..

ion, H3O+.

2 Acid can be classified as a……………………….acid or a ………………….acid based

on its basicity.

3 Basicity is the number of ionisable …………………. atoms per molecule of an acid.

4 A base is a substance that reacts with an acid to form a …………... and water only.

Bases include metal hydroxides and metal oxides which dissolve in water which are

known as……………..

5 Give the names of four common acids and write out their formulae and basicity:

Name of acid Formula of acid Basicity

(i)

(ii)

(iii)

(iv)

6 Complete the equations below :

a)

b)

c)

d)

HCl (aq) → . . .…….(aq) + Cl- (aq)

……… (aq) → H+ (aq) + NO3 - (aq)

H2SO4 (aq) → ……. …… + ………..

CH3COOH (aq) …………. + CH3COO- (aq)

(Refer to page 117 -118 - F4 Chemistry textbook)

7 An alkali is a water-soluble base which ionises in water to produce………..………..,OH-

8 Give the names of two common alkalis and write out their formulae.

Name of alkali Formula of alkali

(i)

(ii)

9 Complete the equations below :

NaOH (aq) → …….(aq) + OH- (aq)

2

……… (aq) + H2O (l) NH4+ (aq) + …………. (aq)

10 Uses of acids, bases and alkalis

(a) To use as …………………. … reagent

Example: sodium hydroxide solution, sulphuric acid, hydrochloric acid

(b) To manufacture and preserve ……………

Example: Ethanoic acid (vinegar), benzoic acid

(c)To make various ………………..

Example: Magnesium oxide antacid medicine, Ascorbic acid vitamin C

(d)To produce …………., detergent and ………………

Example: sodium hydroxide to make soap and detergent

Magnesium hydroxide added to tooth-paste

(e)To manufacture dyes,……………………. and drugs

Example: methylamine

(f)Used in rocket fuel

Example:…………………………………………………………………………………………

(Refer to page 118 - F4 Chemistry textbook)

Role of water and its properties of acids / alkalis Fill in the blanks with the correct words. 1 An acid only shows its …………..…… properties when dissolve in ……………………….

2 In the presence of water, the acid ionises to form ……………………………………..ions .

3 Without water, an acid still exists as…………………and there are no H+ ions present.

4 Complete the following table.

Condition of ethanoic acid Effect on the blue litmus paper

Inference

Glacial ethanoic acid, CH3COOH

Ethanoic acid , CH3COOH in water

Ethanoic acid , CH3COOH in dry propanone, CH3COCH3

5 An alkali only shows its ……………….. properties when dissolve in ………………………..

6 In the presence of water, the base dissociates to produce …………………………... ions

that are responsible for the ………………… properties.

3

7 Complete the table below.

Condition of ammonia Effect on the red litmus paper Inference

Dry

Aqueous (dissolved in water)

Dissolved in propanone

(Refer to page 118 -119 F4 Chemistry textbook and page 84 – 86 F4 Chemistry Practical Book )

Properties of acids and alkalis Place a tick in the box next to each of the following statements to indicate which are True and which are False.

Statement True False

1. All acids are dangerous

2. All alkalis are dangerous

3. Acids taste sweet

4. Alkalis taste bitter

5. Acids taste sour

6. Most acids can burn skin

7. Alkalis feel soapy

8. Acids produce H+ ions in solution

9. Acids produce OH- ions in solution

10. Acids can corrode

11. Acids have a pH above 7

12. Acids have a pH below 7

13. Alkalis turn moist red litmus paper blue

14. Acids turn moist red litmus paper blue

Chemical properties of acids

1

Acids react with bases to form salts and water only.

4

Write an equation for the reaction between sulphuric acid and copper(II) oxide.

……………………………………………………………………………………………………...

2

Acids react with reactive metals to produce salts and hydrogen gas.

Write an equation for the reaction between hydrochloric acid and zinc .

……………………………………………………………………………………………………...

3

Acids react with carbonates to produce salts, water and carbon dioxide gas.

Write an equation for the reaction between nitric acid and calcium carbonate.

……………………………………………………………………………………………………...

(Refer to page 119 - F4 Chemistry textbook and page 87 – 90 - F4 Chemistry Practical Book ) Chemical properties of alkalis 1

Alkalis react with acids to form salts and water only

5

Write an equation for the reaction between sodium hydroxide and benzoic acid.

……………………………………………………………………………………………………...

2

When a mixture of an alkali and an ammonium salt is heated, ammonia gas is liberated.

Write an equation for the reaction sodium hydroxide and ammonium chloride .

……………………………………………………………………………………………………...

3

Alkalis react with most metal ion solutions to produce the insoluble metal hydroxides (precipitation reaction)

Write an equation for the reaction between sodium hydroxide and iron(II) sulphate.

……………………………………………………………………………………………………...

(Refer to page 120 - F4 Chemistry textbook and page 91- F4 Chemistry Practical Book ) The strength of acids and alkalis The pH scale Fill in the blanks with the correct words 1 The pH scale 0 to 14, is used to indicate the degree of ……………… or ……………. of a

solution. 2 pH value less than 7 , indicates an…………………………solution

pH value equal to 7 , indicates a…………………………solution

pH value more than 7 , indicates an…………………………solution

3 pH value can be determined by using…………..,pH paper or…………………indicator.

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4

Acids Neutral Alkalis

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

Very………………. Very…………….

(Refer to page 121 – Chemistry text book)

Strong and weak acid 1. Complete the flowchart below.

Strong acid Weak acid

Degree of dissociation Ionization in water Concentration of ions

pH value

Examples

2 Write the equation for the ionisation of the following substances in water :

(a) nitric acid ……………………………………………………………………………………………………..

(b) sulphuric acid ……………………………………………………………………………………………………..

(c) hydrochoric acid …………………………………………………………………………………………………….

(d) ethanoic acid ……………………………………………………………………………………………………..

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Strong and weak alkali 1. Complete the flowchart below.

Strong alkali Weak alkali

Degree of dissociation Ionization in water Concentration of ions

pH value

Examples

2 Write the equation for the ionisation of the following subtances in water :

(a) sodium hydroxide ……………………………………………………………………………………………………..

(b) potassium hydroxide ……………………………………………………………………………………………………..

(c) ammonia …………………………………………………………………………………………………….

(Refer to page 122 - F4 Chemistry textbook )

CONCENTRATIONS OF ACIDS AND ALKALIS Concentration of acids and alkalis Fill in the blanks with the correct answers. 1 The ……………………………. of a solution refers to the quantity of solute in 1dm3 of

solution can be defined in two ways :-

(a)

Concentration (g dm-3 ) =

(b) Concentration (mol dm-3) also known as molarity or molar concentration (M) =

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2

The two units of concentration can be inter-converted:

Work this out. 3 5.0 g of copper(II) sulphate is dissolved in water to form 500 cm3 solution. Calculate the

concentration of copper(II) sulphate solution in g dm-3 ?

[Answer: 10.0 g dm-3]

4

What is the mass of sodium carbonate required to dissolve in water to prepare 200 cm3 solution that contains 50 g dm-3 ?

[Answer: 10 g]

(Refer to page 123 -124 - F4 Chemistry textbook )

5 4.0 g of sodium carbonate powder, Na2CO3, is dissolved in water and made up to 250

cm3. What is the molarity of the sodium carbonate solution? [Relative atomic mass: C, 12; O, 16; Na, 23]

[Answer: 0.15 mol dm

-3]

Number of mole (n)

The number of moles of solute in a given volume of solution can be calculated using:

n = Number of moles of solute M = Molarity of solution (mol dm-3) V = Volume of solution (dm3)

If the volume in in cm3 – convert the volume of solution from cm3 to dm3

……………... (mol dm-3

)

Concentration (………..)

X Molar mass

÷ Molar mass

n = MV

n = M x V

1000 n = MV

1000 or

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6 Calculate the number of moles of ammonia in 150 cm3 of 2 mol dm-3 aqueous ammonia.

[Answer: 0.3 mol]

7 A student pipetted 20.0 cm3 of potassium hydroxide, KOH solution into a conical flask. The concentration of the alkali was 1.5 moldm-3. Calculate the number of moles of potassium, KOH in the flask.

[Answer: 0.03 mol dm-3

]

8 Calculate the number of moles of hydrogen ions present in 200 cm3 of 0.5 mol dm-3 sulphuric acid.

[Answer: 0.2 mol of H+ ions]

Standard solutions –weighing method 1

What is a standard solution? ……………………………………………………………………………………………………

2

Preparation of standard solutions can be carried out by :- Weighing method (mass of solute)

Calculate the mass of solute needed mass = n X molar mass

= MV X molar mass 1000

Examples: To prepare 100 cm3 of 2.0 mol dm-3 sodium hydroxide solution Calculate the mass of NaOH needed. [Relative atomic mass: Na, 23 ; O, 16 ; H, 1] mass = n X molar mass

= MV X molar mass 1000 = 2.0 X 100 X 40 [23 + 16 + 1] 1000 = 8 g

n = mass

molar mass

n = MV

1000

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3 Try this: To prepare 250 cm3 of 1.0 mol dm-3 sodium carbonate solution. Calculate the mass of Na2CO3 needed. [Relative atomic mass: Na, 23 ; O, 16 ; C, 12]

[Answer : 26.50g]

4 0.25 mol dm-3 solution of sodium hydroxide was prepared by dissolving x g of sodium hydroxide in 750 cm3 of water. What is the value of x ? [Relative atomic mass: Na, 23 ; O, 16 ; H, 1]

[Answer : 7.5 g] (Refer to page 126 - F4 Chemistry textbook )

5 Match the descriptions/procedures with the correct diagram below.

Wash and rinse the weighing bottle ,small beaker and filter funnel to ensure no solute remains in any of the apparatus used.

Transfer the dissolved solute into a suitable volumetric flask.

Add water slowly by using a dropper

to bring the level of the solution to the calibration mark.

The volumetric flask is closed tightly and inverted several times to get a uniform or homogenous solution.

Calculate the mass of solute needed.

Weigh out the exact mass of solute needed in a weighing bottle.

Dissolved the solute in a small amount of distilled water.

Add more water carefully to the

volumetric flask and swirl gently.

Shake well to ensure thorough mixing.

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Standard solutions – dilution method 1 Dilution method Calculate the volume of stock solution required by using the equation:-

M1 = molarity of solution before dilution V1 = volume of solution before dilution M2 = molarity of solution after dilution V2 = volume of solution after dilution

Example: 50 cm3 of 0.1 mol dm-3 sodium hydroxide, NaOH solution from 2.0 mol dm-3

sodium hydroxide,NaOH solution.

Before dilution After dilution

M1 V1 M2 V2

2.0 mol dm-3 ? 0.1 mol dm-3 50 cm3

2.0 x V1 = 0.1 x 50 V1 = 0.1 x 50 2.0 = 2.5 cm3

2 Try this: 100 cm3 of 0.5 mol dm-3 potassium manganate(VII) ,KMnO4 solution from 1.0 mol dm-3 potassium manganate(VII) ,KMnO4 solution.

[Answer : 50 cm3]

3 Match the diagram with the correct descriptions below.

(a)

(b)

(c)

(d)

Add water slowly by using a dropper to bring the level of the solution to the calibration mark.

The volumetric flask is closed tightly and inverted several times to get a uniform or homogenous solution.

Transfer the stock solution to a suitable volumetric flask.

Calculate the volume of stock solution required.

Use a pipette to draw up the required volume of stock solution.

M1V1 = M2V2

When using the equation M1V1 = M2V2 , make sure that both V1 and V2 are of the same unit.

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The pH values and molarity of acids and alkalis Fill in the blanks with the correct words .Use the words in the box.

Increases decreases concentration hydrogen dissociation higher hydroxide alkali

1 The pH value of an acid or alkali depends on three factors, that is

(a) degree of…………………………………………………………………………………….

(b) molarity or ………………………………………………………………………………..

(c) ………………….. of the acid or …………………………………………………………..

2 The lower the pH value, the ……………….. the concentration of ……………………ions.

3 The higher the pH value, the …………….. the concentration of …………………... ions.

4 As the molarity of an acid increases , the pH value of the acid ………………………….

The pH value of an alkali increases when the molarity of the alkali ………………..…….

(Refer to page 128 - F4 Chemistry textbook )

NEUTRALISATION

1 What is the meaning of neutralisation? ………………………………………………………………………………………………..

2 What are the only products of neutralisation? ………………………………………………………………………………………………….

3 Write a balanced equation for the neutralisation of each of the following:-

(a) nitric acid and barium hydroxide ……………………………………………………………………………………………

(b) sulphuric acid and sodium hydroxide ……………………………………………………………………………………………

(c) phosphoric acid and calcium hydroxide ……………………………………………………………………………………………

(d) ethanoic acid and potassium hydroxide ……………………………………………………………………………………………

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4 Complete the flow chart below:-

(Refer to page 128 – 129 - F4 Chemistry textbook ) Acid and base : Titration

1 What is titration?

………………………………………………………………………………………………….

2 What is the function of an indicator?

…………………………………………………………………………………………………..

3 Complete the table below.

Indicator

Colour in solution of:

Acid Alkali

Red litmus paper

Blue litmus paper

Phenolphthalein

Methyl orange

4 Write out the procedure for carrying out an acid-base titration to determine the

molarity of nitric acid (0.5 mol dm-3

) and label the diagram.

[ Alkali used : 25 cm3

(0.5 mol dm-3

) potassium hydroxide]

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(Refer to page 130 – F4 Chemistry textbook and Page 103 – F4 - Chemistry Practical Book)

Numerical problems involving neutralisation

Useful equations in solving numerical problems involving neutralisation.:

n = no of moles Ma = molarity of acid Mb = molarity of alkali Va = volume of acid Vb = volume of alkali a and b = mole ratio of acid to alkali (balanced equation)

Example: In an experiment, 25 cm3 of sodium hydroxide solution of unknown concentration

required 26.50 cm3 of 1.0 mol dm-3 sulphuric acid for complete reaction in titration. Calculate the molarity of sodium hydroxide.

Write out a balanced chemical equation:

2NaOH + H2SO4 Na2SO4 + 2H2O 2 mol 1 mol

MaVa 1 MbVb = 2 ,

1.0 X 26.50 = 1 Mb X 25.00 2

Mb = 2 X 1.0 X 26.50 = 2.12 mol dm-3 (Molarity of sodium hydroxide)

25.00

MaVa = a MbVb b

n = MV n = mass

molar mass

mass

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1 What is the volume of 1.5 mol dm-3 aqueous ammmonia required to completely neutralise 30.00 cm3 of 0.5 mol dm-3 sulphuric acid ?

2NH3 + H2SO4 (NH4)) 2SO4

[Answer: 20 cm3]

2 Calculate the volume in cm3 2.0 mol dm-3 hydrochloric acid that is required to react

completely with 2.65 g of sodium carbonate. [Relative atomic mass: Na, 23 ; O, 16 ; C, 12]

[Answer: 25 cm3]

3 25 cm3 of sulphuric acid was neutralised with 18.0 cm3 of sodium hydroxide 1.0 mol

dm-3. Calculate (a) the number of moles of sulphuric acid that is used in this reaction. (b) the molarity of sulphuric acid

[Answer (a) 0.009 mol (b) 0.36 mol dm-3

]

4 24 cm3 of 0.1 mol dm-3 NaOH is required to completely neutralise 20.0 cm3 of sulphuric acid. Calculate the concentration of sulphuric acid in (a) mol dm-3 (b) g dm-3

[Answer (a) 0.06 mol dm-3

(b) 5.88 g dm-3

]

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1

3

5 What is the molarity of phosphoric acid if 15 cm3 of the acid is neutralized by 38.5 cm3 of 0.15 mol dm-3 NaOH?

[Answer (a) 0.218 mol dm-3

] 6 A student carried out an experiment to determine the end-point for the titration of 25.0

cm3 of 1.0 mol dm-3 sodium hydroxide solution with hydrochloric acid. Phenolphthalein is used as the acid-base indicator. Table 1 shows the three titrations that were conducted and the magnification of the burette readings.

Titration No. I II III

Initial burette

reading

1

2

…………………..

13

14

……………….

26

27

………………..

Final burette

reading

13

14

……………………

26

27

……………………

38

39

…………………….

Table 1

(a) Record the burette readings for the three titrations in the space provided in Table 1.

hydrochloric

acid

hydrochloric

acid hydrochloric

acid hydrochloric

acid

hydrochloric

acid hydrochloric

acid

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(b) Construct a table and record the initial burette reading, final burette reading and the volume of acid used for each titration.

(c) Calculate the average volume of hydrochloric acid used in the experiment.

(d) Calculate the concentration of hydrochloric acid used in the experiment.

(e) If the experiment is repeated by replacing 1.0 mol dm-3 of hydrochloric acid with 1.0 mol dm-3 of sulphuric acid, predict the end-point of the titration.

………………………………………………………………………………………………….

(f) Acids can be classified into strong acid and weak acid. Classify the following acids into strong acids and weak acids.

(g) State the colour change of the phenolphthalein indicator at the end point of titration. …………………………………………………………………………………………………

(h) If phenolphthalein is replaced with methyl orange as the acid-base indicator, state the colour change. …………………………………………………………………………………………………

(i) Write a chemical equation for the neutralisation reaction between hydrochloric acid and sodium hydroxide. ………………………………………………………………………………………………….

Ethanoic acid, hydrochloric acid, sulphuric acid,

carbonic acid, nitric acid,

Strong acids Weak acids