chemical reactions and equations

Ans The reactions in which more reactive element displaces less reactive element from its salt solution are called displacement reactions.. Write a balanced chemical reaction for each of

CHEMICAL REACTIONS AND EQUATIONS (Q1.) Identify the component oxidised in the following reaction :

H2S + Cl2 S + 2HCl (1 Mark)

(Ans) H2S

oxidising agent

(Q3.) Which of the following is a combination and which is a displacement

reaction?

(a) Cl2 + 2KI → 2KCl + I2

(b) 2K + Cl2 → 2KCl (1 Mark)

(Ans) (a) Displacement reaction

(b) combination reaction

(Q4.) What is catalytic decomposition? (1 Mark)

the presence of catalyst are called catalytic decomposition reactions Example:- Digestion of food in human body

(Ans) The reactions in which more reactive element displaces less reactive element from its salt solution are called displacement reactions

2KI (aq) + Cl2 (g) → 2KCl (aq) + I2(g)

(Ans) It is defined as (i) the addition of oxygen or electronegative atom to an element or

compound (ii) the removal of hydrogen or electropositive atom from a compound

(Ans) The condition produced by aerial oxidation of fats and oils in foods marked by unpleasant smell, taste and makes food unfit for eating is called rancidity

(Q9.) What type of reaction is represented by the digestion of food in our body?

(1 Mark)

(Ans) Decomposition reaction

(Q10.) Why does the colour of copper sulphate solution change when an iron nail is

(Ans) Iron displaces copper from copper sulphate solution and form iron sulphate solution The colour of iron sulphate solution is green

(Q11.) What are double displacement reactions? Give an example (1 Mark)

atoms or ions displace each other

are called combination reactions

(Ans) It is defined as

(i) addition of hydrogen or electropositive element to a compound

(ii) removal of oxygen or electronegative element from a compound

Examples: (i)2Na + H2→ 2NaH

(ii) CuO + H2 → Cu + H2O

(Ans) It is defined as the process of slow and gradual conversion of a metal in to their

oxides,sulphides,sulphates by the attack of air, water in the atmosphere Example:- Rusting of iron

(Q15.) Why do we apply paint on iron articles? (1 Mark)

(Ans) We paint iron articles for protection from rusting

(Q16.) Write the term used for the symbolic representation of a reaction (1 Mark)

(Ans) Chemical equation

(Q17.) In the following reaction between lead sulphide and hydrogen peroxide: PbS(s) + 4H2O2(aq) → PbSO4(s) + 4H2

(a) Which substance is reduced?`(b) Which substance is oxidised? (1 Mark)

(Q18.) Identify the substance which is oxidised and which is reduced in the following

(Ans) In this reaction, ZnO is reduced to Zn and C is oxidised to CO

(Q19.) Translate the following statements into chemical equations and then balance them

a)Sodium chloride with sulphuric acid giving sodium sulphate and hydrochloric acid b)Zinc reacting with silver chloride to give zinc chloride and silver (2 Marks)

(Ans) a) 2NaCl + H2SO4 Na2SO4 +2 HCl

b) 2Zn + AgCl2 2ZnCl2 + Ag

(Q20.) Packets of potato chips are filled with a gas Name this gas and explain its use

(Ans) Packets of potato chips are filled with nitrogen gas Oil and fat containing food items are

flushed with nitrogen to prevent them from being oxidized They become rancid and their smell and taste change

(Q21.) What happens when an iron nail is dipped in copper sulphate solution?

(2 Marks)

(Ans) When an iron nail is dipped in copper sulphate solution, the blue colour of copper

sulphate changes to light green due to the formation of Ferrous sulphate and copper is formed Iron is more reactive than copper, so, iron displaces copper from its salt solution

Fe (S) + CuSO4 (aq) FeSO4 (aq) + Cu (S)

(Iron) (Blue Solution) (Light Green) (Copper)

(Q22.) Write two uses of decomposition reactions (2 Marks)

(Ans) 1.These reactions are used to extract several metals from their oxides or other salts When the oxide or salt is electrolysed, metal is obtained For example, aluminium metal is

obtained by passing electricity in molten aluminium oxide

2 Digestion of food in our body is also an example of decomposition reaction The starch

(carbohydrates) present in food is decomposed to give simple sugar (glucose) and the proteins are broken down into amino acids

(Q23.) Why should magnesium ribbon be cleaned before burning in air? (2 Marks)

layer of oxide on the surface of magnesium at ordinary temperature This layer prevents

magnesium from further oxidation

(Q24.) A solution of a substance ‘X’ is used for white washing

(a) Name the substance ‘X’ and write its formula

(b) Write the reaction of the substance ‘X’ named in (a) above with water (2 Marks)

(Ans) (a) The substance ‘X’ is calcium oxide and its formula is CaO

(b) CaO (s)+ H2O (l) Ca(OH)2 (aq)

(Calcium oxide) (Water) (Calcium hydroxide)

Or (Slaked Lime)

(Q25.) Write a balanced chemical reaction for each of the following reactions:

(a) Carbon dioxide is passed through lime water which results in the formation of

precipitate of calcium carbonate and water

(b) Precipitate of silver metal is formed when a strip of copper metal is dipped in the

(Ans)

(Q26.) (a)Respiration is considered an exothermic reaction Why?

(b) Write an example of exothermic reaction (2 Marks)

(Ans) (a) During respiration, food is oxidised and energy is released Therefore, it is considered

an exothermic reaction

(b) Reaction of quick lime with water is an exothermic reaction

2CaO +2H2O 2Ca(OH)2

(Q27.) Explain the term combination reaction with the help of an example (2 Marks)

reaction

(Q28.) Write the name of the chemical and its reaction which is used in black and

sunlight

(Q29.) Give two examples of redox reactions (2 Marks)

(b) MnO2 +4 HCl MnCl2 +2 H2O + Cl2

(Q30.) Na2SO4 (aq) +BaCl2(aq) 2 NaCl (s) + BaSO4 (aq)

Is this reaction an example of precipitation reaction? Also give reason for your

formed which is insoluble in water The insoluble substance is called precipitate and this reacion

is called precipitation reaction

(Q31.) Write any four observations that help us to determine whether a chemical

(c) Evolution of a gas (d) Change in tempreature

(Q32.) Differentiate between combination reaction and decomposition reaction Give one example of each of these reactions (2 Marks)

(Ans)

Combination reaction Decomposition reaction

A reaction in which two or more substances

combine to give a single substance is

known as combination reaction

A + B AB

A reaction in which a compound is broken into two or more simpler substances is known as decomposition reaction

AB A + B

(Q33.) ASO4 + B BSO4 + A

(i) Name this reaction (ii) Name the more reactive element (2 Marks)

(Ans) (i) It is an example of displacement reaction

(ii) Element B is more reactive because it displaces element A from its salt

(Q34.) Write one difference between

(a) Oxidation and reduction (b) Exothermic and endothermic reactions (2 Marks)

(Ans) (a)Addition of oxygen is called oxidation, while addition of hydrogen is called reduction (b) Heat is evolved during exothermic reaction, while heat is absorbed during endothermic

reaction

(Q35.) In the following reaction, identify the species that undergoes –

(i) Oxidation (ii) Reduction

Give reasons also Fe(s) + Ni 2+ (aq) Fe 2+ (aq) + Ni(s) (2 Marks)

(Ans) Fe(s) – 2e - Fe 2+ (aq)

Here, iron loses its two electrons to form Fe2 + ions As we know that oxidation is the loss of electron(s), so iron undergoes oxidation

Ni 2+ (aq) + 2e - Ni(s)

Here, Ni2 + ions gain two electrons to form nickel As we know that reduction is gain of

electron(s), so nickel undergoes reduction

(Q36.) When sodium sulphate solution is treated with barium chloride solution, a

white precipitate is formed Name the reaction and also write the balanced chemical

compounds is called double displacement reaction

Na2SO4 (aq) + BaCl2 (aq) BaSO4 (s) + 2NaCl (aq)

(Q37.) What do you understand by corrosion? (3 Marks)

attack of air and water present in atmosphere is known as corrosion Corrosion, in case of iron is called rusting Rust is hydrated iron III oxide ( Fe2O3 .H2O)

Presence of oxygen and water (moisture) is essential for rusting

(Q38.) Write the type of chemical reaction represented by the following equations (i) 2Al(s) + 6HCl(aq) 2AlCl 3 (aq) + 3H 2 (g)

(ii) 2KBr(aq) + Cl2(g) 2KCl(g) + Br2(g)

(Q39.) What is a precipitation reaction? Give example (3 Marks)

the products, the residue is known as the precipitate and the reaction is called as precipitation reaction

Example- When a solution of silver nitrate is added to a solution of sodium chloride, a white

precipitate of silver chloride is obtained and sodium nitrate is formed

NaCl + AgNO3 AgCl + NaNO3

Sodium Chloride Silver nitrate Silver Chloride Sodium nitrate

(Q40.) What is the difference between combination and decomposition reaction? Give

compounds) combine to form a new single product is known as combination reaction

For example- 2H2 (g) + O2(g) 2H2 O(l)

Decomposition reaction – The reaction in which a single substance decomposes to give two or

more substances is known as decomposition reaction.For ex- 2NH3 (g) N2 (g) +3H2(g)

Decomposition reactions are opposite to combination reactions

(Q41.) (i)What is repiration?

(ii)Is resiration exothermic reaction or endothermic? Explain with reason (3 Marks)

(Ans) (i)We get energy from food In digestion, food is broken down into simple substances Carbohydrates present in food are broken down to form glucose This glucose combines with Oxygen in the cells of our body and provides energy This reaction is known as respiration

C6 H12O6 + 6 02 (g) 6 CO2 (g) + 6H2O (l) + energy

Glucose

(ii)Since energy is released during the process, respiration is considered as an exothermic

reaction

(Q42.) Write the balanced equation for the following chemical reactions-

(a) Sodium + Water Sodium hydroxide + Hydrogen

(b) Barium Chloride + Aluminium Sulphate Barium Sulphate + Aluminium Chloride (c) Hydrogen sulphide + Oxygen Water + Sulphur dioxide (3 Marks) (Ans) (a) 2Na + 2H2O 2NaOH + H2

(b) 3BaCl2 + Al2(SO4)3 3 BaSO4 + 2AlCl3

(c) 2H2S +3O2 2H2O +2SO2

(Q43.) What are exothermic and endothermic reactions? Give examples (3 Marks)

are known as exothermic reactions

For example,

(1) Burning of natural gas CH4 (g) + 2O2 (g) CO2 (g) + 2H2O (g) + Heat

(2) Formation of Ammonia in presence of Ni N2 + 3H2 2NH3 + Heat

Nitrogen Hydrogen Ammonia

Endothermic reactions The reactions in which heat is absorbed are known as Endothermic

reactions.For example,

(1) Formation of carbondisulphide

(2) Formation of nitric oxide

(Q44.) How a double displacement reaction is different from simple displacement

reaction? Explain with the help of an example (3 Marks)

while in displacement reaction more reactive metal displaces less reactive metal from its salt Example of displacement reaction:

Fe + CuSO4 Cu + FeSO4

Here, Fe displaces Cu from CuSO4 because Fe is more reactive than Cu

Example of double displacement reaction:

BaCl2(aq) + Na2SO4(aq) 2NaCl(aq) + BaSO4(s)

Here, exchange of ions between reactants takes place

(Q45.) Balance the following chemical equations

(b) NaCl + H2O NaOH + Cl2 + H 2

(c) (NH4) 2CO3 2NH3 + H2O + CO2 (3 Marks)

(b) 2NaCl + H2O 2NaOH + Cl2 + H 2

(c) (NH4) 2CO3 2NH3 + H2O + CO2

(Q46.) Identify the substances that are oxidised and the substances that are reduced

in the following reactions

(a) Fe + CuSO4 FeSO4 + Cu

(b) Fe2O3 +2Al Al2O3 +2Fe

(c) MnO2 + 4HCl MnCl 2 + 2H2O +Cl2 (3 Marks)

(Ans) (a) Fe - Oxidised Cu - Reduced

(b) Fe2O3 - Reduced Al - Oxidised

(c) MnO2 - Reduced HCl - Oxidised

(Q47.) Explain the terms oxidizing and reducing agents with an example of each

(5 Marks)

oxidizing agent For example:

4 Fe + 3O2 2 Fe2 O3

Hence, oxygen is an oxidizing agent It oxidizes Iron (Fe3 + ion)to ferric oxide ( Fe2O3 )

Reducing agent: The substance which reduces an element or a compound is known as

reducing agent For example:

Stannous chloride is a reducing agent It reduces ferric chloride (Or Fe3 + ion) to ferrous chloride (or Fe2 +) SnCl2 + 2FeCl3 SnCl4 + 2FeCl2

(Q48.) What is the difference between displacement and double displacement

reactions? Write example of each reaction (5 Marks)

from its compound is known as displacement reaction In these reactions, more reactive element displaces less reactive element from its compound Example-

2AgNO3 (aq) + Cu(s) Cu (NO3)2 (aq) + 2Ag (s)

In the above reaction, silver is replaced by copper because copper is more reactive than silver

Double displacement reaction: The reaction in which two different atoms or group of atoms

(ions) are displaced by other atoms or group of atoms (ions) is known as double displacement reaction Example:

Na2SO4(aq) + BaCl2 (aq) BaSO4(s) + 2NaCl(aq)

(Sodium Sulphate) (Barium chloride) (Barium Sulphate) (Sodium Chloride) Here, SO42- displaces Cl- ions and Cl- ions displace SO42- ions or both ions exchange each other These reactions occur in ionic compounds

(Q49.) What do you understand by a balanced chemical equation? Why should the

sides of the equation is known as balanced chemical equation

The chemical equation should be balanced due to the following reasons -

(a) The number of various types of atoms in reactants must be equal to the number of same type of atoms in products

(b) The total mass of all the reactants must be equal to the total mass of the products

(c) The number of different types of atoms must be equal on both sides of a chemical

equation

So, the chemical equation is balanced to satisfy the law of conservation of mass in chemical reaction

(Q50.) Write balanced chemical reactions which represent the following:

(a) Lead nitrate powder is heated in a test tube

(b) Ferrous sulphate is heated

(c) Hydrogen gas combines with nitrogen to form ammonia

(d) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate

(e) Magnesium metal reacts with water to give magnesium hydroxide and hydrogen

(5 Marks)

(Ans)

(Q51.) Balance the following chemical equations (5 Marks)

(Ans)

(Q52.) Represent the following by balanced chemical equations

(a) Iron nail is kept in a copper sulphate solution

(b) Silver chloride is kept in sunlight

(c) Ferrous sulphate is heated at room temperature

(d) Silver nitrate reacts with hydrochloric acid

(Ans)

(Q53.) Write the chemical equation and its type for each of the following:

(a) Silver chloride is exposed to sunlight

(b) Manganese dioxide reacts with hydrochloric acid

(c) Zinc rod is dipped in copper sulphate solution

(d) Solution of sodium sulphate and barium chloride are mixed

(Ans) (a) It is decomposition reaction

(b) It is oxidation reaction

(c) It is displacement reaction

(d) It is double displacement reaction

(e) It is decomposition reaction

(Q54.) Write one example of each of the following type of reaction

(a) Double displacement reaction

(b) Redox reaction

(c) Decomposition reaction

(d) Combination reaction

(Ans) (a) Na2SO4 + BaCl2 BaSO4 + 2NaCl

(b) MnO2+ 4HCl MnCl2 + 2H2O + Cl2

(c) 2Pb(NO3)2 2PbO+ 4NO2 + O2

(d) CaO + H2O Ca(OH)2

(e) Pb +CuCl2 PbCl2 + Cu

(Q55.) Name the type of following reaction

(a) Na2SO4 + BaCl2 BaSO4 + 2NaCl

(b) Pb + CuCl 2 PbCl 2 + Cu

(c) MnO2 + 4HCl MnCl2 + 2H2O + Cl 2

(d) CaO + H2O Ca(OH)2

(Ans) (a) Double displacement reaction

(b) Displacement reaction

(c) Redox reaction

(d) Combination reaction

(e) Decomposition reaction

(Q56.) When a compound of lead is heated, brown coloured fumes evolve from it (i) Name the compound

(ii) Write the balanced chemical equation

(iii) Write the chemical name of brown coloured fumes

(iv) Name the type of reaction

(v) Is this reaction an exothermic or endothermic? (5 Marks)

(Ans) (i) The compound is lead nitrate

(ii) 2Pb(NO3)2 2PbO + 4NO2 + O2

(iii) Nitrogen dioxide

(iv) Decomposition reaction

(v) Endothermic reaction

MOST IMPORTANT QUESTIONS

(Q1.) Make two separate lists from the given data:-List A comprising of physical

changes and List B comprising of chemical changes

Conversion of water in ice, burning of paper, breaking of glass bottle, rusting of

iron, digestion of food ,falling of leaves from tree, melting of wax candle, Souring of milk

(Ans) List A (physical changes):-Conversion of water in ice, breaking of glass bottle, falling of leaves from tree, melting of wax candle

List B (chemical changes):-Burning of paper, rusting of iron, digestion of food , curdling of milk

(Q2.) Pick and write permanent change/s from following list

Powdering of sugar, sublimation of camphor, digestion of food, mixing of sand and salt, ripening of fruits

fruits are chemical changes

(Q3.) Is cutting vegetables into small pieces before cooking is physical or chemical change? Give reason for your answer also

(Ans) Yes, cutting vegetables into small pieces before cooking is a physical because no new compound is formed during this process and the vegetables don’t lose their identity

(Q4.) Why dissolution of sugar in water is physical change?

(Ans) Dissolution of sugar in water is physical change because both sugar and water don’t lose their identity and no new compound is formed

(Q5.) My friend says souring of milk is a chemical change Do you agree? Give

reasons to support your answer

(Ans) Yes, souring of milk is a chemical change because its taste and appearance is different from normal milk

(Q6.) In the following reaction identify reactant/s and products

X + Y Z

(Ans) X and Y are the reactants because they are taking part in a chemical reaction whereas Z

is the product because it is the new substance formed during reaction

(Q7.) Ammonia is prepared in laboratory according to the following reaction-

Write all the information you; get from this reaction

(Ans) (i)It tells about the formula, symbols, names & physical status of reactants ( nitrogen and hydrogen) and products (ammonia)

ii) It tells one mole nitrogen and three moles hydrogen react to form two moles of ammonia

iii) Nitrogen and hydrogen combine in the ratio of 1:3

(Q8.) Write balanced chemical equations for the following chemical reactions

(i) Potassium + Water Potassium hydroxide + Hydrogen

(ii) Magnesium oxide + Carbon Magnesium + Carbon dioxide

(Ans)