Sunday, 8 May 2011

Saturday, 7 May 2011

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 + 2NaClPbCl2 + 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.

Tuesday, 3 May 2011

Mid Year Exam 2011

Good luck to all my students in 5 Bestari, 5 Wawasan, 4 Cita and 4 Maju.

Alkali Metal + H2O


This shows how reactive Group 1 Alkali Metals when they react with water

 
 
What are the factor that contribute to this kind of reactivity??
 
Why does the reactivity of Group 1 Metals increases as you go down the Periodic Table of Elements?
 
*Try Answer It*