Chemical Reactions and Equations
Multiple Choice Questions
- Which of the statements about the reaction below are incorrect?
2PbO(s) + C(s)-2Pb(s) + CO₂(g)
(i) Lead is getting reduced
(ii) Carbon dioxide is getting oxidised
(iii) Carbon is getting oxidised
(iv) Lead oxide is getting reduced
(a) (i) and (ii)
(c) (i), (ii) and (iii)
(b) (i) and (iii)
- Fe₂O3 +2A1-Al₂O3 +2Fe
(d) all of these.
The above reaction is an example of a
(a) combination reaction
(b) double displacement reaction
(c) decomposition reaction
(d) displacement reaction.
- What happens when dilute hydrochloric acid is added to iron fillings? Tick the
(a) Hydrogen gas and iron chloride are produced
(b) Chlorine gas and iron hydroxide are produced
(c) No reaction takes place
(d) Iron salt and water are produced.
- In the reaction 2FeCl₂ + Cl₂-2FeCl, Chlorine is a
(a) Reducing agent
(b) Oxidizing agent
(d) Inert medium.
- Rancidity can be prevented in food items by
(a) Mixing anti oxidant
(b) Keeping in
(c) Keeping away from light
(d) All of these. refrigerator
- Which among the following is not an oxidizing agent?
(b) Conc. Sulphuric acid
- Which among the following gases are used to keep the oily food items fresh ?
(a) Carbon dioxide and Oxygen
(b) Nitrogen and helium
(c) Helium and Oxygen
(d) Nitrogen and Oxygen.
- Chemically rust is
(a) Hydrated ferrous oxide
(b) Ferric oxide
(d) None of these.
(c) Hydrated ferric oxide
- An exothermic reaction in which glucose combines with oxygen in the
our body and provides energy
(a) Respiration (c) Excretion
- Formula of lead sulphate is
(d) None of these. (d) Nore of the
(a) Pb₂SO4 (c) Pb₂(SO4)3
(b) Pb(SO4)2 (d) PbSO4.
Ans. 1. (a), 2. (d), 3. (a), 4. (b), 5. (d), 6. (d), 7. (b), 8. (c), 9. (a), 10. (d).
Fill in the Blanks
1………….. coloured powder is formed on burning magnesium ribbon in air.
- The formation of water by the reaction of H₂(g) and O₂(g) is a ……….reaction.
- Silver articles turn black on keeping in open due to the formation of …………
- Silver chloride is stored in dark coloured bottles to prevent from…………..
- Oily and fatty food items become rancid due to………….
- Those reactions in which heat is released along with the formation of products arecalled ………… reactions.
Ans. 1. White, 2. Combination, 3. Silver sulphide, 4. Sunlight,
(2019) 5. Oxidation,
- Dissociation reaction is opposite of combination reaction.
- The reaction of zinc oxide and copper is a redox reaction. 3. Copper is a more reactive element as compared to zinc.
- Decomposition of vegetable matter to form compost is an endothermic reaction.
- On putting copper coins in silver nitrate solution, copper displaces silver.
- Corrosion and rancidity are redox reactions.
Ans. 1. True, 2. True, 3. False, 4. False, 5. True, 6. False.
Match the Columns
Column ‘A’. Column ‘B’
- Respiration. (a) Calcium carbonate
- Calcium oxide. (b) White powder
- Magnesium oxide. (c) Quick lime
- Marble. (d) Absorption of heat
- Endothermic reaction. (e) Exothermic reaction
Ans. 1.→→ (c), 2. → (c), 3. → (b), 4. →(a), 5.→ (d).
Answer in one Word/Sentence
- Name the chemical reaction in which heat is released.
Ans. Exothermic reaction.
- What is the loss of electrons by an element, in a reaction called?
- Name the chemical reaction in which one reactant is oxidised and the other is reduced.
Ans. Redox reaction.
- In a double displacement reaction, what is exchanged between the reactants?
- In a chemical reaction, what does the symbol represent? ↓
Ans. Formation of precipitate.
- What is the cause of change in taste and smell of oily and fatty food items?
Ans. Becoming rancid due to oxidation.
- Why silver and gold items do not corrode ?
Ans. Silver and gold are less reactive, hence they do not corrode.
- Write the name of equation in which there is an exchange of ions between the reactants. Ans. Double displacement reaction.
Very Short Answer Type Questions
- 1. Write the balanced equation for the following chemical reactions :
(i) Hydrogen + Chlorine Hydrogen chloride
(ii) Barium chloride +Aluminium sulphate-Barium sulphate +Aluminium chloride
(iii) Sodium + Water→Sodium hydroxide+ Hydrogen
Ans. (i)H₂(g) + Cl₂ (g)→→→→ 2HCl(g)
(ii) 3BaCl2(s) + Al₂(SO4)3(8) 3BaSO4(s) +
(III)2AlCl3(s) 2Na(s) + 2H₂O()2NaOH(aq) + H₂(g)
- 2. does the colour of copper sulphate solution change when an iron nailis dipped in it?
Ans. When an iron nail is dipped in the copper sulphate solution then iron displaces copper from the copper sulphate because iron is more reactive than copper. Therefore the colour of the copper sulphate solution changes.
Fe(s) + CuSO4(aq) →→→ FeSO4(aq) + Cu(s)
- 3. Identify the substances that are oxidised and the substances that are reduced in the following reactions:
(1)4Na(s) + O₂(g) →→→ 2Na₂O(s)
(ii)CuO(s) + H₂(g) Cu(s) + H₂O(l)
Ans. (i) Sodium (Na) is oxidised as it gains oxygen and oxygen gets reduced.
(ii) Copper oxide (CuO) is reduced to copper (Cu) while hydrogen (H₂) gets oxidised to water (H₂O).
- 4. Write the balanced chemical equations for the following reactions :
(a) Calcium hydroxide + Carbon dioxide → Calcium carbonate +
(b) Zine + Silver nitrate Zinc nitrate + Silver+water
(c) Aluminium + Copper chloride → Aluminium chloride + Copper
(d) Barium chloride + Potassium sulphate → Barium sulphate +Potassium chloride
Ans. (a) Ca(OH)₂ + CO₂ → CaCO3 + H₂O
(b) Zn+ 2AgNO3 Zn(NO3)2 + 2Ag
(c) 2A1+ 3CuCl₂ 2AlCl3 + 3Cu
(d) BaCl₂ + K₂SO4BaSO4 +2KCI
- 5. Why is respiration considered an exothermic reaction ? Explain.
Ans. Respiration is considered an exothermic reaction because in respiration, oxidation of glucose takes place which produces large amount of energy.
C6H12O6(aq) + 60₂(g) 6CO₂(g) + 6H₂O(l) + Energy
- 6. Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.
2AgCl(s)———2Ag(s) + Cl₂(g)
2H₂O(1)————-2H₂(g) + O₂(g)
- 7. In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.
Ans. 2AgNO3(aq) + Cu(s) ——–Cu(NO3)2(aq)
Silver nitrate. Copper. Copper nitrate
+ 2Ag(s) Silver
Q.8. A shiny brown coloured element ‘X’ on heating in air becomes black in colour. Name the element ‘X’ and the black coloured compound formed.
Ans. ‘X’ is copper (Cu) and the black coloured compound formed is copper oxide(CuO). The equation of the reaction involved on heating copper is
2Cu. +. O₂ ———2CuO
Shiny brown colour. Oxygen. Black in colour
- 9. Why do we apply paint on iron articles ?
Ans. Iron corrodes or gets rusted on coming in contact with moisture and oxygen present in air. Iron articles are painted because it prevents them from rusting. When painted, the contact of iron article with moisture and oxygen of air is cut off. Hence, it prevents rusting.
- 10. Oil and fat containing items are flushed with nitrogen. Why?
Ans. Oil and fat containing food items become rancid on coming in contact with oxygen due to oxidation. They are flushed with nitrogen because nitrogen is a less reactive gas and it acts as an antioxidant. It prevents the food items from getting oxidised or becoming rancid.
- 11. Define rancidity.
Ans. See long answer type Q. No. 2(b).
Short Answer Type Questions
- 1. Why should a magnesium ribbon be cleaned before it is burnt in air ? (2020)
Ans. Magnesium is a very reactive metal. When stored, it reacts with oxygen to form a layer of magnesium oxide on its surface. This layer of magnesium oxide is quite stable and prevents further reaction of magnesium with oxygen. The magnesium ribbon is cleaned before it is burnt in air, for removing this layer so that the underlying metal can be exposed to air.
- 2. What is a balanced chemical equation? Why should chemical equations be balanced?
Ans. A chemical equation is balanced when the number of atoms of each element remains the same, before and after a chemical reaction. Chemical equation should be balanced because of law of conservation of mass. In a chemical reaction, mass can neither be created. nor destroyed. That is, the total mass of the elements present in the products of a chemical reaction has to be equal to the mass of the elements present in the reactants.
- 3. Write the balanced chemical equations for the following reactions and identify the type of reaction in each case :
(a) Potassium bromide (aq) + Barium iodide (aq) →→→ Potassium iodide (aq) +Barium bromide (s)
(b) Zine carbonate (s)-Zinc oxide (s) + Carbon dioxide (g)
(c) Hydrogen (g) + Chlorine (g) Hydrogen Chloride (g)
(d) Magnesium (s) + Hydrochloric acid (aq) →→→ Magnesium Chloride (aq)
Ans.(a) 2KBr (aq) + Bal₂ (aq) →→→ 2K1 (aq) + BaBr₂(s) It is a double displacement reaction.
(b)ZnCO3(s)→→→→→ ZnO(s) + CO₂(g)
+ Hydrogen (g) It is a dissociation reaction.
©H₂(g) + Cl₂(g) →→→→→ 2HCl(g)
It is a combination reaction..
(d)Mg(s) + 2HCl(aq)→→→ MgCl₂(aq) + H₂(g)
It is a displacement reaction.
- 4. Define exothermic and endothermic reactions with example.
Define exothermic chemical reaction.
Ans. Exothermic reaction-Reaction in which heat is released along with the formation
of products is called exothermic reaction. Example (i)Burning of natural gas. CH₂(g) +20₂(g) – CO₂(g) + 2H₂O(g) + Energy
(ii) The decomposition of vegetable matter into compost is also an exothermic reaction.
Endothermic reaction-A reaction in which energy is absorbed is known as
Example-(i) Decomposition of calcium carbonate into calcium oxide (quick lime)
and carbon dioxide.
(Limestone). (Quick lime)
(ii) Decomposition of lead nitrate into lead oxide and nitrogen dioxide. Heat
2Pb(NO3)₂(s)——-2PbO(s) + 4NO₂(g) + O₂(g) Lead nitrate. (Lead oxide) (Nitrogen dioxide)(Oxygen)
- 5. What is the difference between displacement and double displacement reactions ? Write equations for these reactions. Ans. In a displacement reaction, a more reactive element replaces a less reactive element from a compound.
Example CuSO4(aq) + Zn(s)→→→→→→ ZnSO₂(aq) + Cu(s)
In a double displacement reaction, there is an exchange of ions between the reactants.
Example-Na₂SO4(aq) + BaCl₂(aq) BaSO4(s) + 2NaCl(aq)
- 6. Why are decomposition reactions called the opposite of combination reactions?
Also write equations for combination reactions. Ans. Decomposition reactions are those in which a compound breaks down to form
two or more substances. These reactions require energy to proceed.
Example (i) CaCO3(s)————-CaO(s)+CO,(g)
(ii) 2FeSO4(s) ————Fe₂O3+ SO₂(g) + 5O3(g)
In a combination reaction, contrary to decomposition reaction, two or more substances combine to give a new substance with a release of energy. Hence, decomposition reactions.
are opposite of combination reactions.
Examples of Combination reactions
(I) 2H₂(g) + O₂(g) →→→2H₂0 (1)
(ii)C(s) + O₂(g) →→→ CO₂(g)
- 7. What do you mean by a precipitation reaction ? Explain by giving examples
Ans. A reaction in which an insoluble salt or a precipitate is formed, is called-ecipitation reaction.
Example (i) In the reaction of barium chloride (BaCl₂) and sodium sulphate (Na₂SO4)white precipitate of barium sulphate (BaSO4) is formed. It is a precipitation reaction.
Na₂SO4(aq) + BaCl₂(aq) →→→ BaSO4(s) + 2NaCl(aq)
(ii) In the reaction of sodium carbonate (Na₂CO3) and calcium chloride (CaCl₂), calcium arbonate is obtained as a precipitate.
Na₂CO3(aq) +CaCl₂(aq) →→→ CaCO3(s) + 2NaCl(aq)
- 8. A solution of substance ‘X’ is used for white washing.
(i) Name the substance ‘X’ and write its formula.
(ii) Write the reaction of the substance ‘X’ with water.
Ans. (i) The substance ‘X’ is calcium oxide. Its chemical formula is CaO.
(ii) Calcium oxide reacts vigorously with water to form calcium hydroxide:
CaO(s)+H,O(1) — Ca(OH),(aq) Slaked lime
Quick lime water. Slaked lime
The white suspension of slaked lime when applied on the walls, combines with carbon de dioxide (in air) forming a thin shining layer of calcium carbonate.
Ca(OH₂)+CO₂(g) →→→ CaCO3(s) + H₂O Slaked lime Calcium Carbonate
- 9. What information does a chemical equation convey ?
Ans. A chemical equation conveys the following information
(i) It represents the names of products and reactants.
(ii) It tells the relative number of molecules (or atoms or moles or masses) of products (=
(iii) It represents the relative volumes of gaseous products and reactants.
(iv) It represents the chemical reaction.
(v) It represents the physical states of the reactants and products.
- 10. Define displacement and double displacement reaction with example.
Ans. Displacement reaction : “The reaction in which an element displaces another
element from its compound is called the displacement reaction.”.
Fe+ CuSO4 (aq) → FeSO4(aq) + Cu
Double displacement reaction: The reaction in which two different atoms or groups of atoms (ions) are exchanged, is known as the double displacement reaction.”
Example : Na₂SO4 (aq) + BaCl₂ (aq) →→ BaSO4(s) + 2NaCl(aq)
Long Answer Type/ Analytical Questions
- 1. Explain the following terms of gain or loss of oxygen with two examples each:
(a) Oxidation, (b) Reduction.
QWhat do you mean by oxidation-reduction (Redox) reactions? Explain with
Ans. (a) Oxidation A chemical reaction in which gain of oxygen or loss of hydrogen takes place is called oxidation reaction and the process is called oxidation. When a substance gains oxygen during a reaction, it is said to be oxidised.
Example (1) 2Cu(s) + O₂(g)——–2CuO (s)
Here, copper is oxidised to copper oxide.
(ii)2Mg(s) + O₂(g)—— 2MgO(s)
Here, magnesium is oxidised to magnesium oxide.
(b) Reduction A chemical reaction in which loss of oxygen or gain of hydrogen takes place is called reduction reaction and this process is called reduction. A substance is reduced when it either gains hydrogen or loses oxygen.
Example (i) CuO + H₂——-Cu + H₂O
Here, Copper oxide is reduced to copper.
(ii)ZnO+C——-Zn + CO
Here, Zinc oxide is reduced to zinc.
- 2. Explain the following terms with one example each :
Ans.(a) Corrosion-Corrosion is defined as a process where materials, usually metals, deteriorate as a result of chemical reaction with air, moisture, chemicals etc. Corrosion causes damage to car bodies, bridges, ships and all objects made of metals, specially those of iron. Example Iron, in the presence of moisture, reacts with oxygen to form hydrated iron oxide, which is called rust.
4Fe + 30, + nH,O — 2Fe,O.nH,O
(b) Rancidity-The process of oxidation of fats and oils due to which there is change in taste and smell is known as rancidity. Hence, substances which prevent oxidation (antioxidants) are added to foods containing fats and oil. Keeping food in air tight containers helps to slow down oxidation.
Change of taste and smell of butter on keeping it for a long time is an example of
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