SALTS

By NORINI BT JAAFAR

 

1.Definition: Salt is an ionic compound formed when the H⁺ ion in an acid is replaced by a metal ion or an ammonium, NH₄⁺ ion.

e.g. HCl + NaOH → NaCl + H₂O

 

 

2. Salts can be classified into two main groups shown in Figure 1.

 

SALTS

 

Soluble Insoluble

 

 

 

 

Figure 1

 

3. (a) Effects of heat on carbonate salts.

 

Carbonate salt → Metal oxide + CO₂

Δ

 

Example: Heating of copper(II) carbonate.

 

Chemical equation: CuCO₃ → CuO + CO₂

Observations: i. Green solid of copper(II) carbonate turns black solid of copper(II)

oxide.

ii. Gas passed through lime water will turn lime water chalky.

Other examples: (a) PbCO₃ → PbO + CO₂

Observations: i. Lead(II) oxide, PbO deposit is orange when hot, turns yellow when

cooled.

        ii. Lime water turns chalky.

         (b) ZnCO₃ → ZnO + CO₂

 

Observations: i. Zinc oxide, ZnO deposit is yellow when hot, turns white when cooled.

ii. Lime water turns chalky.

Note: All carbonates salt will be decomposed by heat to produce metal oxide and carbon dioxide except sodium carbonate, Na₂CO₃ and potassium carbonate, K₂CO₃ which cannot be decomposed by heat.

 

3(b) Effects of heat on nitrate salt.

        Nitrate salt → metal oxide + NO₂ + O₂

Δ

 

Example: Heating of lead(II) nitrate, Pb(NO₃)₂.

 

 

 

 

 

Chemical equation: 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂

Observations: i. Lead(II) oxide, PbO deposit is orange when hot, turns yellow when

cooled.

ii. When glowing wooden splinter is put inside the test tube, it

rekindles.

Oxygen, O₂ gas produced.

iii. Brown gas released turns damp blue litmus paper to red.

Nitrogen dioxide, NO₂ gas produced.

Other examples: 2Zn(NO₃)₂ → 2 ZnO + 4NO₂ + O₂

2Cu(NO₃)₂ → 2 CuO + 4NO₂ + O₂

Note: Sodium nitrate, NaNO₃ and potassium nitrate, KNO₃ when heated will decompose to produce nitrite salt and oxygen gas.

2NaNO₃ → 2NaNO₂ + O₂

2KNO₃ → 2KNO₂ + O₂

 

4. Preparation of Soluble Salts

Soluble salts can be prepared by the following methods:

    (i) Neutralization reaction: Acid + Alkali → Salt + H₂O

This method is suitable for sodium, potassium and ammonium salts.

    (ii) Acids + Metal → Salts + H₂

This method is not suitable for salts when the metal is below hydrogen in the

Electrochemical Series, e.g copper.

    (iii) Acid + Metal Oxide → Salt + H₂O

    (iv) Acid + Metal Carbonates → Salt + H₂O + CO₂

Example: To
prepare of copper(II) sulphate salt

Substances: copper(II) oxide, sulphuric acid 1.0 mol dm^-3 solution.

Materials: beaker, 250 ml, filter funnel, glass rod, filter paper, evaporating dish, Bunsen burner, tripod stand, wire gauze.

Procedures:

1. Pour (50 – 100) cm³ of sulphuric acid solution into a beaker.
   
2. Heat the acid.
   
3. Copper(II) oxide powder is added into the acid until excess (to ensure that all the acid has completely reacted).
   
4. Stir the mixture.
   
5. Filter the mixture to remove the excess copper(II) oxide powder.
   
6. Transfer the filtrate to an evaporating dish and heat the solution until saturated.
   

(To ensure the solution is saturated, dip a glass rod into the solution and take out. If there are crystals formed at the glass rod, this shows that the solution is saturated)

1. Cool the saturated solution to room temperature.
   
2. Filter the salt crystals formed and dry the salt with filter paper.
   

 

 

1. Chemical equation: CuO + H₂SO₄ → CuSO₄ + H₂O
   

 

5. Preparation of insoluble salts.

Insoluble salts can be prepared by the precipitation or the double decomposition method. The general equation:

Salt solution of X + Salt solution of Y → Insoluble salt XY + Salt solution

Example: Pb(NO₃)₂ + 2NaCl → PbCl₂ + 2NaNO₃

(White precipitate)

 

Ionic equation: Pb²⁺ + 2Cl^- → PbCl₂

Example: To prepare silver chloride salt (an insolube salt)

 

Substances: silver nitrate solution, sodium chloride solution.

 

Apparatus: beaker, glass rod, filter funnel, evaporating dish, filter paper.

 

Procedures:

 

1. (30 – 50) cm³ of silver nitrate solution is poured into a beaker.
   
2. (30 – 50) cm³ of sodium chloride solution is added into the beaker.
   
3. Stir the mixture.
   
4. Filter the white precipate formed.
   
5. Wash the residue (white precipitate) with distilled water.
   
6. Dry the white powder with filter paper.
   
7. Chemical equation: AgNO₃ + NaCl → AgCl + NaNO₃
   
8. Ionic equation: Ag⁺ + Cl^- → AgCl
   
9. White salt of siver chloride formed.
   

Tips: When you are required to prepare a salt, the first thing to do is to determine whether the salt to be prepared is soluble or insoluble. Then choose the suitable method to prepare the salt.

6. Numerical Problems Related to Salts.

Example: Excess zinc is added to 50 cm³ of sulphuric acid solution 1.0 mol dm^-3 .

The chemical reaction is shown below:

Zn + H₂SO₄ → ZnSO₄ + H₂.

Calculate the mass of zinc sulphate salt formed from the reaction.

[ Relative atomic mass: O, 16, S, 32; Zn, 65;]

 

Solution: From the equation, 1 mol of H₂SO₄ ≡ 1 mol of ZnSO₄

Number of moles of H₂SO₄ given = MV

1000

= 1.0 x 50

1000

= 0.05

Therefore, mass of ZnSO₄ = 0.05 x (65 + 32 + 4(16))

= 8.05 g

Exercise

Objective Questions:

1. Which of the following salts is not soluble in water?
   

   A Sodium carbonate
   

   B Silver nitrate
   

   C Lead(II) chloride
   

   D Zinc sulphate
   

2. Which of the following equations is correct for the preparation of copper(II) sulphate?
   

   A Cu(NO₃)₂ + Na₂SO₄ → CuSO₄ + 2 NaNO₃
   

   B Cu(OH)₂ + H₂SO₄ → CuSO₄ + 2H₂O
   

   C Cu + H₂SO₄ → CuSO₄ + H₂
   

   D CuCO₃ + H₂SO₄ → CuSO₄ + H₂O + CO₂
   

3. When substance X is heated strongly, a type of gas and a black deposit were produced. The gas released turns lime water chalky. Which of the following is substance X?
   

   A Copper(II) oxide
   

   B Lead(II) carbonate
   

   C Copper(II) carbonate
   

   D Zinc carbonate
   

4. Which of the following nitrates will not releases brown gas when heated strongly?
   

   A sodium nitrate
   

   B magnesium nitrate
   

   C lead(II) nitrate
   

   D zinc nitrate
   

5. What will happen if an aqueous solution of sodium chloride is added to an aqueous solution of lead(II) nitrate?
   

   I The solution becomes acidic
   

   II Bubbles formed
   

   III Neutralisation reaction occurs
   

   IV White precipitate formed
   

   A I, II and III
   

   B I and II
   

   C II and IV
   

   D IV only
   

Structured Question.

The figure below shows some chemical reactions on X carbonate.

 

X carbonate Solution Y + H₂O + Gas Z

 

X carbonate is a green coloured powder. When added with acid R, blue coloured solution Y and gas Z that turns lime water chalky are produced.

(a) (i) Name one acid that could be R.

…………………………………………………………………………………………..

[1 mark]

1. Name solution X when acid in (a)(i) is used.
   

…………………………………………………………………………………………..

[1 mark]

(ii) Name gas Z.

…………………………………………………………………………………………..

[1 mark]

(b) X carbonate when heated strongly will also produce gas Z and a black deposit.

(i) Draw the apparatus set-up how the experiment can be carried out in the laboratory. Show also in your diagram how the gas can be tested.

[3 marks

(ii) Write the chemical equation for the heating of X carbonate.

…………………………………………………………………………………………..

[1 marks]

(c) You are required to prepare X carbonate salt in the laboratory.

(i) Name the two substances that are required to prepare the salt.

…………………………………………………………………………………………..

[2 marks]

(ii) Name the type of reaction for the preparation of the salt above.

…………………………………………………………………………………………..

[1 mark]

Answers:

Objective Questions.

1. C 2. D 3. C 4. A 5. D

Structured Questions

1. (i) sulphuric acid
   

   (ii) copper(II) sulphate
   

   1. carbon dioxide
      

(b)(i)

 

(ii) CuCO₃ → CuO + CO₂

(c)(i) Copper(II) nitrate and sodium carbonate

(ii) precipitation/ double decomposition method.