Performing and observing the following reactions and classifying them into:

(a) Combination Reaction

(b) Decomposition Reaction

(c) Displacement Reaction

(d) Double displacement Reaction

(i) Action of water on quicklime.

(ii) Action of heat on ferrous sulphate crystals.

(iii) Iron nails kept in copper sulphate solution.

(iv) Reaction between sodium sulphate and barium chloride solutions.

CLASSIFICATION OF CHEMICAL REACTIONS Experiment

22

17Classification of Chemical Reactions

APPARATUS AND MATERIALS REQUIRED : Test tubes, test tube stand, spatula, quick lime, distilled water, ferrous sulphate crystals, zinc metal, dilute H SO , iron fillings, copper sulphate solution, sodium sulphate solution, 2 4

barium chloride solution.

PROCEDURE

A 1. Take a little sodium sulphate solution in a clean test tube and add Barium chloride solution to it.

2. Note down the observation.

B 1. Take a little quick lime in a boiling tube and add water.

2. Observe whether the boiling tube has become warm or not.

3. Record your observations.

C 1. Heat ferrous sulphate crystals in a clean dry test tube.

2. Observe the changes taking place and note down the observations.

D 1. Take a little copper sulphate solution in a test tube and add a pinc of iron fillings to it.

2. Note down the colour changes taking place.

E 1. Burn a piece of Mg ribbon using a pair of tongs and record your observations.

F 1. Add dilute H SO to zinc pieces in a test tube.2 4

2. Observe the liberation of any gas.

OBSERVATIONS

A. When Na SO solution is added to BaCl solution, a ___________ precipitate is formed.2 4 2

B. The reaction mixture becomes____________ on adding water to _____________

C. On heating FeSO crystals, the colour changes from______________to____________4

D. The colour of copper sulphate solution changes from __________ to ________ when iron fillings are added.

E. Mg ribbon burns with a __________ light forming a __________ powder of __________

F. ____________ gas is evolved on addition of dilute H SO to zinc metal.2 4

RESULT

1. The reaction in A is ______________

2. The reaction in B is ______________

3. The reaction in C is ______________

4. The reaction in D is ______________

5. The reaction in E is ______________

6. The reaction in F is _______________

PRECAUTIONS AND SOURCES of error

1. All chemicals should be handled carefully.

2. Solid chemicals should be put into the test tube bring a spatula.

3. Mg ribbon should be burnt carefully.

SECTION - A

VERY SHORT QUESTIONS

Q.1 Two test tubes X and Y contain a solution of Sodium Sulphate and Barium Chloride respectively. The two solutions are mixed. What would be observed and why?

Ans. A curdy white precipitate would be formed due to the formation of Barium Sulphate which is insoluble in water.

Na SO (aq) BaCl g 2NaCl (aq) + BaSo (s)2 4 2 4

Curdy white precipitate

Q.2 What do you observe when water is added to quick lime? What is the type of reaction taking place? Give reason for your answer.

Ans. When water is added to quick lime, a white suspension of calcium hydroxide is formed with liberation of heat. It is a combination reaction as a single product is formed from two reactants.

Q.3 What is the type of reaction taking place when aqueous sodium sulphate solution is added to an aqueous solution of Barium Chloride. Give reason for your answer. Name the insoluble compound formed.

Ans. When sodium sulphate solution is added to Barium Chloride solution, a double displacement reaction takes place because exchange of ions takes place between two reactants. The insoluble substance formed is Barium Sulphate.

Q.4 What happens when a little iron fillings are added to an aqueous solution of copper sulphate? Write the equation for the reaction taking place.

Ans. A reddish brown metal is deposited. The blue colour of copper sulphate solution gradually fades and turns pale green.

The reddish brown metal is copper and the pale green solution is of iron (II) sulphate.

Fe(s) + CuSO (aq) g FeSO (aq) + Cu (s)4 4

Q.5 What is the type of reaction taking place when iron fillings are dropped in ferrous sulphate solution? Justify your answer.

Ans. The type of reaction taking place is displacement. Iron is more reactive than copper, so it displaces copper from copper sulphate solution.

Q.6 What would happen if copper turnings are dropped in iron (II) sulphate solution. Give reason for your answer.

Ans. No change will take place if copper turnings are added to iron (II) sulphate solution. This is because copper is less reactive than iron and it cannot displace iron from iron (II) sulphate solution.

Q.7 Identify the following samples with the help of their colours by matching the columns.

Blue Palegreen

A B

Colourless

CReddishbrown metalD

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18 Classification of Chemical Reactions

Iron nails

CuSO solution4

Ans. I II

A Copper sulphate solution

B Iron sulphate solution

C Barium chloride

D Copper metal

Q.8 In the following set up for an experiment, what are the visible changes taking place. Justify your answer.

Ans. A reddish brown metal which is copper is deposited on the iron nails. The blue colour of the solution gradually fades and turns pale green due to the formation of iron (II) sulphate solution. This is because iron is more reactive than copper and can displace copper from copper sulphate solution.

Q.9 What would be the reaction (if any) taking place when aqueous solution of sodium chloride is dropped over solid Barium Sulphate? Give reason for your answer.

Ans. No reaction will take place as Barium Sulphate is solid. In solid Barium Sulphate ions are not free to react. Barium Sulphate is insoluble in water.

Q.10 How will you classify a reaction in which two or more reactants combine to form a single product? Give one such example of a reaction with chemical equation in which a white suspension is formed as a product.

Ans. A combination reaction is a reaction in which two or more reactions combine to form a single product.

When water is put into quick lime a combination reaction takes place and a white suspension of calcium hydroxide is formed along with liberation of heat.

Q.11 What determines whether a displacement reaction will take place between two substances or not? Give an example to justify your answer.

Ans. The relative reactivity of the two substances determines whether a displacement reaction will take place between them or not. For example if iron filings are dropped in copper sulphate solution, copper metal is displaced

along with the formation of iron (II) sulphate because iron is more reactive than copper.

Q.12 Pale green crystals of a substance X were heated first gently and then strongly as shown in the diagram. A brown residue was left on strong heating and the gas (es) evolved turned moist blue litmus red.

(a) Identify the pale green crystals and the brown residue.

(b) Write the equation for the reaction taking place under strong heat.

(c) What is the type of reaction taking place?

Ans. (a) Pale green crystals are of FeSO . 7H O 4 2

and the brown residue is ferric oxide Fe O .2 3

heat (b) 2FeSO (s) Fe O (s)+SO (g)+SO (g).4 2 3 2 3

(c) It is a thermal decomposition reaction.

Q.13 Give one example each of: (a) an exothermic reaction which is also a

combination reaction. (b) an endothermic reaction which is also

a thermal decomposition reaction.Ans. (a) The reaction between calcium oxide

and water is a combination reaction which is exothermic.

CaO + H O g Ca(OH)2 2

(b) Thermal de- composition reaction takes place when ferrous sulphate crystals are heated.

heat 2FeSO (s) Fe O (s)+SO (g)+SO (g)4 2 3 2 3

This is also an endothermic reaction.

Q.14 Name the type of reaction taking place when iron fillings are added to an aqueous solution of copper sulphate. Would a similar reaction take place when copper turnings are dropped in an aqueous solution of iron (II) sulphate? Explain your answer.

Ans. When iron fillings are added to an aqueous solution of copper sulphate, a displace-ment reaction takes place because iron is more reactive than copper and it can displace copper from copper sulphate solution.

A similar reaction cannot take place when copper turnings are dropped into iron (II) sulphate solution because copper is less reactive than iron and it cannot displace iron from iron (II) sulphate solution.

Colourlessgas

Test tubeholder

X

Bunsen burner

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Classification of Chemical Reactions 1 9

Q.15 (a) What are quick lime and slaked lime? (b) How can quick lime be converted into

slaked lime? Write the chemical equation for the reaction taking place.

Ans. (a) Quick lime is calcium oxide and slaked lime is calcium hydroxide.

(b) Quick lime can be converted into slaked lime by adding water to it.

CaO + H O �  Ca(OH)2 2

quicklime slaked lime

Q.16 Which out of the following will not give a precipitate when an aqueous solution of Barium Chloride is added to it? Give reason.

Iron (II) sulphate solution (Aqueous) Copper sulphate solution (Aqueous) Sodium (II) sulphate solution (Aqueous) Sodium Chloride solution (Aqueous)Ans. When an aqueous solution of Barium Chloride

is added to Sodium Chloride solution no precipitate is formed because there are no such

2+ions that would react with Ba ions to form an insoluble salt. If Barium Sulphate solution is added to any of the aqueous solution of FeSO , 4

CuSO or Na SO , then BaSO will get 4 2 4 4

precipitated as it is insoluble.

Q.17 Explain giving reason as to how would you separate the mixture of products formed when an aqueous solution of Barium Chloride is added to an aqueous solution of Sodium Sulphate.

Ans. BaCl (aq)+Na SO (aq)gBaSO + NaCl(aq)2 2 4 4

Curdy white precipitate In this reaction, the products are Barium

Sulphate which is insoluble in water and Sodium Chloride which is soluble in water. The two can be separated by filtration. Barium Sulphate will be obtained as residue and filtrate is an aqueous solution of Sodium Chloride which can be evaporated to dryness.

Q.18 How can sulphur dioxide gas be identified physically?

Ans. It is a colourless gas having smell of burning sulphur. It turns moist blue litmus paper red. It also bleaches a rose petal.

Q.19 What is the nature of the tiny droplets seen on the cooler parts of the test tube when ferrous sulphate crystals are heated gently? What is the liquid?

Ans. The liquid is water. The droplets are colourless, odourless and neutral to litmus. It is the water of crystallization present in ferrous sulphate crystals.

Q.20 What is the formula for copper sulphate crystals? How many molecules of water of crystallization does it have? How can it be converted into anhydrous copper sulphate?

Ans. Copper sulphate crystals have the formula CuSO . 5H O. It has five molecules of water of 4 2

crystallization and it can be converted into anhydrous copper sulphate by heating it when it loses its water of crystallization.

Q.21 Name the substance getting oxidised and the substance getting reduced when iron nail is dipped in a solution of copper sulphate.

2+Ans. In this reaction, Fe gets oxidised to Fe in FeSO as it 4

loses electrons. This gain of positive charge by losing electrons is oxidation.

2+ Cu in CuSO is getting reduced to Cu as it gains 4

electrons. This loss of positive charge due to loss of electrons is reduction.

Q.22 Why does the colour of copper sulphate solution changes when an iron nail is dipped in it?

Ans. This happens because FeSO solution is formed 4

which is pale green in colour.

CuSO (aq) + Fe(s) g FeSO (aq) + Cu(s) 4 4

blue pale green

Q.23 Fill in the blanks. (a) Sodium sulphate and barium chloride

are ____________ (ionic/covalent) compounds.

(b) As the white precipitate of barium sulphate is formed (immediately/ sometime after mixing the two solutions), the reaction between _______ (ionic/ covalent) compound is _____________ (instantaneous/slow).

Ans. (a) ionic

(b) immediately, ionic, instantaneous.

Q.24 Write in brief the procedure to show that iron is more reactive than copper.

Ans. Prepare ferrous sulphate and dip copper strip in it. No reaction takes place showing copper is less reactive than iron. Prepare copper sulphate solution in water and add iron fillings to it. Iron displaces copper showing iron is more reactive than copper.

Q.25 Explain giving reason, whether a displacement reaction will take place in each of the following beakers I, II, III, and IV.

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20 Classification of Chemical Reactions

Ans. In beaker I, no reaction takes place as Cu is less reactive than Fe.

In beaker II, no reaction takes place as Cu does not react with water.

In beaker III, iron displaces copper as iron is more reactive than Cu.

In beaker IV, no reaction takes place as iron cannot displace iron from FeSO .4

Q.26 An aqueous solution of Barium chloride was added to an aqueous solution of sodium sulphate. The correct observation at that instant would be shown in the test tube:

Ans. The correct observation at that instant is shown in beaker III as barium sulphate precipitates out in the form of a suspension as soon as it is formed.

Q.27 A student took about 3 g of ferrous sulphate crystals in a dry test tube and heated it over a bunsen burner flame, first gently and then strongly. Explain giving reason all that he would observe.

Ans. On gentle heating colourless water vapour is given off and the crystals turn white.

FeSO , 7H O FeSO +7H O .4 2 4 2

On strong heating, a colourless gas having smell of burning sulphur is evolved. It is sulphur dioxide. Another gas sulphur trioxide is also evolved in this reaction. a brown residue of ferric oxide is left in the test tube.

2FeSO (s) Fe O (s) + SO (g) + SO (g)4 2 3 2 3

Q.28 Four student A, B, C and D heated ferrous sulphate crystals strongly in a separate test tube and recorded their observation. Study the observation table carefully and explain giving reason which student report the correct observation.

Ans. Student B reported correctly. A brown residue of Fe O is left in the test tube and sulphur 2 3

dioxide evolved has pungent odour and is colourless.

Q.29 A student took a pale green substance in a dry test tube and heated it strongly on a bunsen burner flame. He observed a brown residue and evolution of two gases with smell of burning sulphur. Identify the substance heated and explain the observation.

Ans. The substance heated was ferrous sulphate crystals as they are pale green in colour and leave a brown residue on heating strongly. The brown residue is of ferric oxide, the gases evolved are sulphur dioxide and sulphur trioxide. The chemical equation for the reaction is as follows.

FeSO 7H O(s) FeSO (g) + 7H O(g)4 2 4 2

2FeSO Fe O (s) + SO (g) + SO (g)4 2 3 2 3

Q.30 Write one difference between a displacement and a double displacement reaction. Give one example of each of these types of reaction.

Ans.

Test tube

FeSO solution4

Copper pieces

Water

Copper pieces

CuSO solution4

Iron pieces

FeSO solution4

Iron pieces

I II

III IV

I II

Precipitate

Precipitate

III

Precipitate

IV

Clear solution

Classification of Chemical Reactions 21

Laboratory Manual Science PBQ-X

gentle heat

heat

Colour ofthe residue

Student

A Green Pungent, colourless

B Brown Pungent, colourless

C Grey Odourless, brown

D Black Pungent, colourless

Odour and colour ofthe gases evolved

gentle heat

strong heat

Displacement ReactionIn these reactions, a more reactive element displaces a less reactive element from its salt solution. Eg. Fe(s) + CuSO (aq) ® 4

FeSO (aq) + Cu(s)4

Double displacement ReactionIn these reactions an exchange of ions between solutions of two reactants takes place. Eg.

BaCl (aq) + Na SO (aq)�2 2 4

®�BaSO (s)+2NaCl (aq)4

Q.31 A solution of a substance X is used for testing carbon dioxide. Identify X. What will be its reaction with carbon dioxide. Write a balanced chemical equation for the reaction taking place.

Ans. The substance X is calcium hydroxide Ca(OH) . 2

Aqueous solution of calcium hydroxide is commonly called lime water. When carbon dioxide is passed through lime water, it turns milky due to the formation of insoluble calcium carbonate.

Ca(OH) (aq) + CO (g)g CaCO (s) + H O(l)2 2 3 2

Q.32 A few grams of quicklime are added to water in a beaker.

(a) Name and define the type of reaction taking place.

(b) State two observation that you will make in this reaction.

Ans. It is a combination reaction. It is defined as a reaction in which a single product is formed from two or more reactants.

A white suspension of Ca(OH) is formed and a 2

large amount of heat is released in the reaction.

CaO(s) + H O(l) g Ca(OH) (aq)2 2

Q.33 When aqueous solution of barium chloride is added to an aqueous solution of aluminum sulphate, a curdy white precipitate of barium sulphate is formed.

(a) Write the chemical equation for the reaction taking place.

(b) State two types into which this reaction can be classified.

Ans. (a) 3BaCl (aq) + Al (SO ) (aq)g2AlCl (aq) 2 2 4 3 3

+ 3BaSO (s)4

(b) (i) Double displacement Reaction (ii) precipitation reaction.

Q.34 Dilute sulphuric acid was taken in a test tube and zinc pieces were added to it. What would you observe in this reaction? Write the chemical equation for the reaction taking place. Name the type of reaction.

Ans. Bubbles of a colourless, odourless gas which is hydrogen are evolved, A colourless solution of zinc sulphate is formed. It is a displacement reaction.

Zn(s) + H SO (aq) gZnSO (aq) + H (g)2 4 4 2

Q.35 What is the type of reaction taking place when lead zinc nitrate crystals are strongly heated in test tube? What would you observe in this reaction.

Ans. Decomposition reaction takes place when lead nitrate is heated in a dry test tube. A yellow residue is left in the test tube. A cracking sound is heard. A brown gas is evolved. A colourless gas is also evolved.

SECTION -B

MULTIPLE CHOICE QUESTIONS

Q.1 How is the reaction between calcium oxide and water classified?

(a) Combination reaction

(b) Neutralisation reaction

(c) Decomposition reaction

(d) Displacement reaction

Q.2 Heating of ferrous sulphate leads to the formation of a residue of __________ colour.

(a) white (b) black

(c) green (d) brown

Q.3 How will you classify the reaction taking place between an iron nail and copper sulphate solution?

(a) Combination reaction

(b) Displacement reaction

(c) Double displacement reaction

(d) Decomposition reaction

Q.4 On reaction with copper sulphate solution, the silver-grey colour of an iron nail becomes:

(a) blue (b) reddish-brown

(c) pale yellow (d) light green

Q.5 The reaction between aqueous solutions of sodium sulphate and barium chloride leads to the formation of a _____________ coloured precipitate.

(a) yellowish-brown (b) white

(c) black (d) light green

Q.6 A student immerses metal 'X' in a solution of copper sulphate. In some time, the colour of solution changes to light green. Identify 'X':

(a) Iron (b) Copper

(c) Zinc (d) Platinum

Q.7 To study double displacement reaction, a student is asked to select from the chemicals available in the laboratory. Identify the correct chemicals needed for the experiment:

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22 Classification of Chemical Reactions

(i) Solid ferrous sulphate

(ii) Aqueous solution of barium chloride

(iii) Powder of barium sulphate

(iv) Aqueous solution of sodium sulphate

(v) Calcium oxide powder

(vi) Zinc metal

(vii) Aqueous solution of copper sulphate

(a) (i), (v) and (vii) (b) (ii) and (vi)

(c) (vi) and (vii) (d) (ii) and (iv)

Q.8 Which of the following does not occur when water is added to quick lime?

(a) Quick lime reacts with water vigorously.

(b) Test tube becomes hot during the reaction.

(c) Slaked lime is a product of the reaction.

(d) The reaction takes place on heating the reactants.

Q.9 An aqueous solution of Barium chloride was added to an aqueous solution of sodium sulphate. The correct observation at that instant would be shown in the test tube:

(a) I (b) II

(c) III (d) IV

Q.10 On adding copper turnings to freshly prepared ferrous sulphate solution, a student observes that:

(a) A dull brown coating is formed

(b) A black coating is formed

(c) A greyish coating is formed

(d) No change takes place

Precipitate

Precipitate

PrecipitateClearsolution

I II

III IV

FeSO (aq)4

Cu pieces

ANSWER KEY (MCQ's)

Q.No. ExplanationAns.

1. (a) It is a combination reaction, that involves two reactants combining to form one product.

2. (d) Brown coloured ferric oxide is formed as residue after decomposition of ferrous sulphate.

3. (b) It is a displacement reaction in which the more reactive metal i.e. iron, displaces the less reactive metal, i.e., copper from its salt solution.

4. (b) During the reaction, copper metal is displaced and it gets deposited on the nail, with a reddish-brown colour.

5. (b) Barium sulphate is formed as precipitate in the reaction. It is white in colour and insoluble in water.

6. (a) Iron displaces copper, forming ferrous sulphate, which is light green in colour. Zinc would also displace copper, but zinc sulphate solution is colourless.

7. (d) Double displacement reaction occurs between aqueous solutions of barium chloride and sodium sulphate.

8. (d) The reaction takes place spontaneously and vigorously at room temperature.

9. (c) The precipitate is suspended in the medium.

10. (d) Copper is less reactive than iron.

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Classification of Chemical Reactions 2 3