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CHEMICAL REACTIONS - Questions & Answers

EXERCISE

Introduction

State what is a chemical reaction. A chemical reaction is often accompanied by external indications or characteristics which include -

a] Colour change

b] Effervescence or gas evolved

c] Evolution or absorption of heat

d] Formation of a precipitate.

With reference to each of the above indications, state the external indication seen during -

i] Addition of dilute acid to an active metal

ii] Addition of dilute hydrochloric acid to silver nitrate

iii] Addition of water to quicklime

iv] Thermal decomposition of mercury [II] oxide.

A chemical reaction is a chemical change in which matter changes into a new substance or substances. It involves the breaking of existing bonds and the formation of new bonds.

External indications for the given reactions:
i] Effervescence / Evolution of gas: Hydrogen gas is evolved with effervescence when a dilute acid is added to an active metal like zinc.
ii] Formation of a precipitate: A milky white precipitate of silver chloride is formed.
iii] Evolution of heat: A large amount of heat energy is evolved (exothermic reaction) when water is added to quicklime.
iv] Colour change: Red mercury [II] oxide decomposes to give silvery liquid mercury.

Main reactions

State why a direct combination reaction is called a - 'synthesis reaction'.

A direct combination reaction is called a synthesis reaction because it involves the "synthesis" or building up of a single new compound from two or more simpler substances (elements or compounds).

Differentiate between - a] Direct combination reaction & a decomposition reaction b] Displacement reaction & a neutralization reaction.

a] Direct Combination vs Decomposition: In a direct combination reaction, two or more reactants combine to form one new compound (A + B → AB). In a decomposition reaction, a single chemical compound breaks down into two or more simpler substances (AB → A + B).

b] Displacement vs Neutralization: A displacement reaction is one where a more reactive element displaces a less reactive element from its compound (A + BC → AC + B). A neutralization reaction is a specific type of double decomposition reaction where an acid reacts with a base to form salt and water.

Classify the following reactions into - a] Direct combination b] Decomposition c] Displacement d] Double decomposition - The reactions are - i] Zinc hydroxide on heating gives zinc oxide & water ii] Zinc reacts with copper [II] sulphate to give zinc sulphate & copper iii] Zinc sulphate reacts with ammonium hydroxide to give ammonium sulphate & zinc hydroxide iv] Molten zinc at high temperatures, burns in air to give zinc oxide.

i] Decomposition reaction: A single compound (zinc hydroxide) breaks into two simpler substances.
ii] Displacement reaction: Zinc displaces copper from copper [II] sulphate.
iii] Double decomposition (Precipitation) reaction: Both reactants exchange their radicals to form new compounds.
iv] Direct combination (Synthesis) reaction: Zinc and oxygen combine to form a single compound.

Types of reactions

Give balanced equations for - i] A direct combination reaction involving two elements, one of which is a non-metal ii] A thermal decomposition reaction involving heat on limestone [calcium carbonate] iii] An electrolytic decomposition reaction involving a neutral liquid iv] A displacement reaction involving a metal above hydrogen in the activity series with copper [II] sulphate solution v] A double decomposition neutralization reaction involving an acid & a base vi] A white precipitate obtained during a double decomposition reaction involving a silver salt with a sodium salt.

i] 2Na + Cl₂ → 2NaCl
ii] CaCO₃ Δ → CaO + CO₂
iii] 2H₂O electric current → 2H₂ + O₂
iv] Zn + CuSO₄ → ZnSO₄ + Cu
v] NaOH + HCl → NaCl + H₂O
vi] AgNO₃ + NaCl → AgCl↓ + NaNO₃

State what is meant by 'reactivity series of metals'. With reference to - a] Water b] Acids explain with suitable examples how the reactivity of the metals could be differentiated.

Reactivity series of metals is a series of arrangement of metals in the order of their decreasing reactivity. The most active metal is at the top and the least active is at the bottom.

a] Reactivity with Water: Metals like Potassium (K) react violently with cold water, whereas Calcium (Ca) reacts less vigorously. Metals like Magnesium (Mg) react only with boiling water or steam, while metals below Hydrogen (like Cu, Au) do not react with water at all.

b] Reactivity with Acids: Metals like Sodium (Na) react explosively with dilute acids. Metals like Iron (Fe) react smoothly/less vigorously with dilute HCl. Metals below Hydrogen in the series do not react with dilute acids to displace hydrogen.

A chemical reaction may be 'reversible' in nature. State the meaning of the term in italics. Give a reason why a catalyst is used in certain chemical reactions. Give a balanced equation for the following - a] A reversible catalytic reaction involving - i] nitrogen as one of the reactants ii] sulphur dioxide as one of the reactants.

Reversible: It means a reaction in which the products formed can react together to reform the original reactants under certain conditions.

Catalyst: A catalyst is used to alter (usually speed up) the rate of a chemical reaction without itself taking part in the reaction.

Balanced equations:
a] i] N₂ + 3H₂ Fe/Mo, 450°C ⇌ 2NH₃
ii] 2SO₂ + O₂ V₂O₅, 450°C ⇌ 2SO₃

State which type of chemical reactions proceed with - a] Evolution of heat energy b] Absorption of heat energy. State in each of the following reactions whether heat is evolved or absorbed - i] water is added to quicklime ii] two neutral gases on passage through an electric arc give nitric oxide iii] two neutral gases combine to give - a basic gas.

a] Exothermic reactions proceed with the evolution of heat.
b] Endothermic reactions proceed with the absorption of heat.

Heat evolution/absorption in given reactions:
i] Heat is evolved (Exothermic).
ii] Heat is absorbed (Endothermic).
iii] Heat is evolved (Formation of ammonia is Exothermic).

Certain thermal decomposition reactions, result in formation of oxides. Give balanced equations for the thermal decomposition of the following, which result in formation of a metallic oxide. a] Limestone b] Lead carbonate c] Calcium nitrate d] Calcium hydroxide.

a] CaCO₃ → CaO + CO₂
b] PbCO₃ → PbO + CO₂
c] 2Ca(NO₃)₂ → 2CaO + 4NO₂ + O₂
d] Ca(OH)₂ → CaO + H₂O

Oxides

State the meaning of the term 'oxide'. Give a balanced equation for formation of the following oxides - a] Sulphur dioxide from a non-metal b] Zinc oxide from a metal c] Lead oxide from a mixed oxide.

An oxide is a binary compound formed by the reaction of a metallic or non-metallic element with oxygen.

Balanced equations:
a] S + O₂ → SO₂
b] 2Zn + O₂ → 2ZnO
c] 2Pb₃O₄ Δ → 6PbO + O₂

Give two examples each of the following oxides - a] Acidic oxides b] Basic oxides c] Amphoteric oxides d] Neutral oxides. State which of the following oxides i.e. a] to d] - i] React with water to give a base ii] React with a base to give salt & water iii] React with acids & bases to give salt & water.

Examples:
a] Acidic: SO₂, CO₂
b] Basic: K₂O, CaO
c] Amphoteric: ZnO, PbO
d] Neutral: NO, CO

Properties:
i] Basic oxides react with water to give a base.
ii] Acidic oxides react with a base to give salt and water.
iii] Amphoteric oxides react with both acids and bases to give salt and water.

Give one example each of - a] A peroxide b] A mixed oxide c] A dioxide.

a] Peroxide: Sodium peroxide (Na₂O₂)
b] Mixed oxide: Red lead (Pb₃O₄)
c] Dioxide: Lead dioxide (PbO₂)

CRITICAL THINKING OBJECTIVE TYPE QUESTIONS

Q.1 Select the correct answer from A, B, C, D and E for each statement given below: A: Iron B: Carbonic acid C: Hydrogen D: Oxygen E: Carbon monoxide
The product formed during direct combination reaction of carbon dioxide & water.

B: Carbonic acid

The neutral gas obtained on thermal decomposition of potassium nitrate.

D: Oxygen

The displaced product of the displacement reaction of sodium with cold water.

C: Hydrogen

The catalyst used in the catalytic reaction involving the reactants nitrogen & hydrogen.

A: Iron

A neutral oxide which does not react with an acid or a base to give salt & water.

E: Carbon monoxide

Q.2 Complete the statements by filling in the blank with the correct word/s:

Direct combination reaction of phosphorus pentoxide with water gives H₃PO₄ [H₃PO₃/H₃PO₄]. Decomposition of silver salts in the presence of sunlight is an example of photochemical decomposition [double decomposition/photochemical decomposition]. The element molybdenum used in the reaction of nitrogen with hydrogen at elevated temperatures is an example of a promoter [promoter/catalyst]. The reaction of coke with steam to give water gas is an example of an endothermic [exothermic/endothermic] reaction. The metal which reacts with steam and the reaction is reversible is iron [calcium/iron].

Q.3 Give a balanced equation for each of the following types of reactions: A thermal decomposition reaction in which a compound decomposes to give two new compounds.

CaCO₃ → CaO + CO₂

A reaction of direct combination i.e. synthesis in which two gases combine to give another gas - which turns lime water milky.

2CO + O₂ → 2CO₂

A thermal decomposition reaction in which a metallic nitrate decomposes to give- a basic oxide.

2Ca(NO₃)₂ → 2CaO + 4NO₂ + O₂

A catalytic, reversible, exothermic reaction.

N₂ + 3H₂ Fe/Mo ⇌ 2NH₃ + Heat

A displacement reaction in which a metal above hydrogen in the reactivity series, displaces another metal from the solution of its compound.

Fe + CuSO₄ → FeSO₄ + Cu

Q.4 Differentiate between the following: Thermal decomposition & thermal dissociation.

Thermal decomposition is an irreversible chemical change brought about by heat. Thermal dissociation is a reversible decomposition reaction brought about by heat.

Neutralization reaction & a precipitation reaction.

Neutralization is a reaction between an acid and a base to form salt and water. Precipitation is a reaction between two compounds in aqueous state to form at least one insoluble product (precipitate).

Electrolytic decomposition & photochemical decomposition.

Electrolytic decomposition is brought about by the passage of electric current. Photochemical decomposition is brought about by the presence of light.

A catalyst & a promoter.

A catalyst alters the rate of a reaction. A promoter is a substance added to a catalyst to increase the efficiency of the catalyst.

An acidic oxide & a basic oxide.

An acidic oxide is a non-metallic oxide that reacts with water to form an acid. A basic oxide is a metallic oxide that reacts with water to form a base.

Q.5 Match the chemical reactions in List I with the appropriate answer in List II.

1. XY heat ⇌ X + Y : D: Thermal dissociation
2. XY → X + Y : E: Decomposition reaction
3. X⁺Y^- + A⁺B^- → X⁺B^- + A⁺Y^- : B: Double decomposition
4. X + YZ → XZ + Y : A: Displacement reaction
5. X + Y heat → XY - Δ : C: Endothermic reaction

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Quick Review Flashcards - Click to flip and test your knowledge!

Question

What is the definition of a chemical reaction?

Answer

A chemical change in which matter changes into a new substance or substances.

Question

Name four external indications that often accompany a chemical reaction.

Answer

Change of colour, evolution of a gas, evolution or absorption of heat, and formation of a precipitate.

Question

When red mercury [II] oxide ($2HgO$) is heated, what is the resulting colour change?

Answer

Red to silvery.

Question

Which gas is evolved when dilute hydrochloric acid ($HCl$) is added to zinc ($Zn$)?

Answer

Hydrogen ($H_2$).

Question

Term: Exothermic reaction

Answer

Definition: A chemical reaction that proceeds with the evolution of heat energy.

Question

Term: Endothermic reaction

Answer

Definition: A chemical reaction that proceeds with the absorption of heat energy.

Question

How is the evolution of heat represented in a chemical equation?

Answer

By the symbol $+\Delta$.

Question

How is the absorption of heat represented in a chemical equation?

Answer

By the symbol $-\Delta$.

Question

What type of reaction is the addition of water to calcium oxide ($CaO$)?

Answer

Exothermic reaction.

Question

The formation of nitric oxide ($2NO$) from nitrogen ($N_2$) and oxygen ($O_2$) at high temperatures is an example of an _____ reaction.

Answer

Endothermic

Question

Term: Precipitate

Answer

Definition: An insoluble solid substance formed during a chemical reaction.

Question

What precipitate is formed when silver nitrate ($AgNO_3$) reacts with dilute hydrochloric acid ($HCl$)?

Answer

Silver chloride ($AgCl$), which is a milky white precipitate.

Question

What is a Direct Combination (Synthesis) reaction?

Answer

A reaction in which two or more elements or compounds combine to give one new compound.

Question

Give an example of a direct combination reaction between two elements.

Answer

$Fe + S \rightarrow FeS$ (Iron + Sulphur $\rightarrow$ Iron sulphide).

Question

When a metal reacts with oxygen in a direct combination reaction, what is formed?

Answer

A metallic oxide, which is typically basic in nature.

Question

When a non-metal reacts with oxygen in a direct combination reaction, what is formed?

Answer

A non-metallic oxide, which is typically acidic in nature.

Question

What is the product when a metallic oxide like $Na_2O$ reacts with water?

Answer

A base or alkali, such as $2NaOH$.

Question

What is the product when a non-metallic oxide like $SO_2$ reacts with water?

Answer

An acid, such as $H_2SO_3$.

Question

What is a Decomposition reaction?

Answer

A reaction in which a chemical compound decomposes into two or more simpler substances.

Question

Which type of decomposition reaction is brought about specifically by heat?

Answer

Thermal decomposition.

Question

How does thermal dissociation differ from thermal decomposition?

Answer

Thermal dissociation is a reversible reaction, while thermal decomposition is generally not.

Question

What is the term for a decomposition reaction brought about by the passage of electric current?

Answer

Electrolytic decomposition.

Question

What is Photochemical decomposition?

Answer

A decomposition reaction that occurs in the presence of light.

Question

What is a Displacement (Substitution) reaction?

Answer

A reaction in which one element displaces another element from its compound.

Question

In a displacement reaction, which element can displace another from its solution?

Answer

An element placed higher in the activity series displaces an element placed below it.

Question

What is a Double Decomposition reaction?

Answer

A reaction in which both reactants are decomposed by exchanging their radicals to give two new compounds.

Question

Term: Neutralization reaction

Answer

Definition: A reaction between an acid and a base to give salt and water by interchange of radicals.

Question

What are the two specific types of double decomposition reactions?

Answer

Neutralization reactions and precipitation reactions.

Question

What is the reactivity series of metals?

Answer

A series where metals are arranged in the order of their reactivity, with the most active at the top.

Question

How do metals like potassium ($K$) and sodium ($Na$) react with cold water?

Answer

They react violently to form an alkali and hydrogen gas.

Question

Metals from magnesium ($Mg$) to iron ($Fe$) do not react with cold water but react with _____.

Answer

Steam

Question

Which metals in the reactivity series do not react with dilute acids to displace hydrogen?

Answer

Metals placed below hydrogen ($[H]$).

Question

What is a Reversible reaction?

Answer

A chemical reaction where the products can react together to reform the original reactants.

Question

Term: Catalyst

Answer

Definition: A substance that alters the rate of a chemical reaction without taking part in it.

Question

What is the function of a positive catalyst?

Answer

It speeds up the rate of a chemical reaction.

Question

What is a negative catalyst?

Answer

A catalyst that reduces the rate of a chemical reaction.

Question

What is a promoter in a catalytic reaction?

Answer

A substance added to a catalyst to increase its efficiency.

Question

What are enzymes?

Answer

Biological catalysts present in the human body that accelerate biochemical reactions.

Question

Term: Oxides

Answer

Definition: Binary compounds formed by the reaction of a metallic or non-metallic element with oxygen.

Question

Classification: Amphoteric oxides

Answer

Definition: Oxides that react with both acids and bases to give salt and water. Example: $ZnO$ or $PbO$.

Question

Classification: Neutral oxides

Answer

Definition: Non-metal oxides that do not react with acids or bases. Example: $NO$ or $CO$.

Question

What characterizes a 'mixed oxide'?

Answer

An oxide that behaves as if it is a mixture of two simpler oxides, such as $Pb_3O_4$ or $Fe_3O_4$.

Question

How do the carbonates of potassium ($K$) and sodium ($Na$) behave when heated?

Answer

They do not decompose.

Question

What are the products of the thermal decomposition of metal carbonates from calcium ($Ca$) to copper ($Cu$)?

Answer

Metal oxide and carbon dioxide ($CO_2$).

Question

What are the products of the thermal decomposition of metal nitrates from calcium ($Ca$) to copper ($Cu$)?

Answer

Metal oxide, nitrogen dioxide ($NO_2$), and oxygen ($O_2$).

Question

Which metal nitrates decompose to give the metal nitrite and oxygen gas?

Answer

Potassium nitrate ($KNO_3$) and sodium nitrate ($NaNO_3$).

Question

What products are formed when silver nitrate ($AgNO_3$) is heated?

Answer

Silver metal ($Ag$), nitrogen dioxide ($NO_2$), and oxygen ($O_2$).

Question

Hydroxides of metals from calcium ($Ca$) to copper ($Cu$) decompose on heating to yield _____ and _____.

Answer

Metal oxide; water vapour

Question

What is the catalyst used in the reaction between nitrogen and hydrogen to form ammonia?

Answer

Iron ($Fe$).

Question

Which oxide is formed by the action of steam on red-hot iron?

Answer

Triferric tetroxide ($Fe_3O_4$).

Question

The catalyst vanadium pentoxide ($V_2O_5$) is used in the catalytic oxidation of which gas?

Answer

Sulphur dioxide ($SO_2$).

Question

What color is the precipitate formed by the reaction of ferric chloride ($FeCl_3$) and sodium hydroxide ($NaOH$)?

Answer

Reddish brown.

Question

What color is the precipitate formed by the reaction of zinc sulphate ($ZnSO_4$) and ammonium hydroxide ($NH_4OH$)?

Answer

Gelatinous white.

Question

Give an example of a mixed oxide mentioned in the text.

Answer

$Pb_3O_4$ (Lead tetroxide) or $Fe_3O_4$ (Triferric tetroxide).

Question

What is a dioxide example provided in the source material?

Answer

Lead dioxide ($PbO_2$).

Question

Under what condition does $Ca$ react with water?

Answer

It reacts less vigorously with cold water.

Question

Which factor initiates reacting particles to move faster and collide more frequently in a chemical change?

Answer

Heat

Question

In the reaction $N_2 + O_2 \rightleftharpoons 2NO - \Delta$, what does the $-\Delta$ indicate?

Answer

Heat is absorbed (Endothermic reaction).

Question

What is the product of the direct combination of phosphorus pentoxide ($P_2O_5$) with water?

Answer

Phosphoric acid ($H_3PO_4$).

Question

Molybdenum ($Mo$) acts as a _____ when added to the iron catalyst in ammonia synthesis.

Answer

Promoter