Ch1 - Chemical Reactions & Equations
MCQs, Q & A, NCERT Solutions
Q1: A chemical reaction involves in
a. Only breaking of bonds
b. Only formation of bonds
c. Both breaking and formation of bonds
d. None of these
Answer: (c) Both breaking and formation of bonds
Q2: A balanced chemical equation always obeys
a. Law of conservation of mass
b. Law of thermal equilibrium
c. Law of conservation of energy
d. All of the above
Answer: (a) law of conservation of mass.
Q3: Single displacement reaction involves:
Answer: (c) redox
Q4: String of ants, bees contain
a. Formic acid
c. Succinic acid
d. Common Salt (NaCl)
Answer: (a) Formic acid
Q5: Some stale food gives a bad taste and a bad smell because of:
Answer: (d) rancidity
Q6: A red brown gas is released on heating lead nitrate. It is an example of
a. Combination reaction
b. Oxidation reaction
c. Decomposition reaction
d. Reduction reaction
Answer: (c) Decomposition
Q7: The sign ↓ indicates
a. release of gas
b. dissolution of gas
c. formation of a precipitate
d. lowering of temperature
Answer: (c) formation of a precipitate
Q8: Why do gold and silver do not corrode?
Answer:It is because they are less reactive.
Q9: What do you mean by balanced chemical equation?
Answer: An equation that has equal number of atoms of each element on both the sides of equation is called balanced chemical equation, i.e., mass of the reactants is equal to mass of the products.
e.g., 2Mg + O2 → 2MgO
Q10: Define rancidity.
Answer: When fats and oils are oxidized, they become rancid and their smell and taste
change. This process is known as rancidity.
Q11 (CBSE 2007): Write the type of reactions in the following:
i. Reaction between an acid and a base
ii. Rusting of iron
Answer: i. Neutralization reaction
ii. Oxidation reaction
Q12: Give an example of decomposition reaction where energy is supplied in form of light.
2AgBr(s) ------------------------> 2Ag (s) + Br2(g)
Q13: Why does copper vessel acquire green coating in rainy season?
Answer: It reacts with CO2 in the atmosphere and forms a layer of basic Copper Carbonate.
Q14: What is the name of the gas which burns with a pop sound?
Answer: Hydrogen gas.
Q15: Why is Hydrogen peroxide stored in coloured bottles?
Answer: It is done to prevent photolytic decomposition of hydrogen peroxide.
Q16: Name the oxidizing agent and reducing agent in the following equation.
3MnO2(s) + 4Al(s) → 3Mn(s) + 2Al2O3(s)
ORName a reducing agent that may be used to obtain manganese from manganese dioxide.
Answer: Reducing agent : Al
Oxidation Agent : MnO2
Q17: Give two example from everyday life situations where redox reactions are taking place.
Answer: Corrosion and Rancidity
Q18: In electrolysis of water, why is the volume of gas collected over one electrode double that of gas collected over the other electrode ?
Answer: In water (H2O), hydrogen and oxygen are present in the raito of 2:1 by volume.
Q19: Why should a magnesium ribbon be cleaned before burning in air?
Answer: Magnesium is a reactive metal. It combines with oxygen of air to form a layer of magnesium oxide on its surface. Hence, it should be cleaned with a sand paper before burning to remove the oxide layer formed on its surface.
Q20: Explain how respiration is an exothermic reaction.
Answer: During digestion, food is broken down into simpler substances. For example, rice, potatoes and bread contain carbohydrates. These carbohydrates are broken down to form glucose. This glucose combines with oxygen in the cells of our body and provides energy. Hence, respiration is an exothermic process.
C6H12O6(aq) + 6O2(aq) → 6CO2(aq) + 6H2O(l) + energy
Q21: What factors influence the rate of chemical reaction?
Answer: Following are the factors that can influence the rate of reaction:
- Nature of Reactants. e.g. Mg reacts faster in HCl than in acetic acid.
- Concentration of reactants
- Surface Area of reactants. E.g. Powdered CaCO3 reacts quickly than marble chips.
Q22: Define Endothermic reaction.
Answer: The reactions in which heat is absorbed are called endothermic reactions. In such reactions heat is shown as one of the reactants. If exact amount of heat absorbed is known then this amount is written otherwise simply the word heat is written.
E.g. N2(g) + 2O2(g) + heat → 2NO(g)
Q23: A silver ware is kept in a solution of CuSO4 (aq). What change do you expect?
Answer: No reaction. because Silver is less reactive than copper.
Q24(CBSE exam): Name one metal when placed in ferrous sulphate solution will discharge its green colour. Write a chemical equation and state the reasons also.
Answer: Potassium(K) is one example. K is more reactive than iron and will replace it.
2K + FeSO4 → K2SO4 + Fe
Potassium + Iron Sulphate → Potassium Sulphate + Iron
Q25: Why do we apply paint on iron articles?
Answer: Iron articles are painted to prevent them from rusting. Rusting is oxidation of iron in the presence of air and moisture. The paint layer cuts off the the contact of iron articles from moisture and air.
Q26(CBSE): A solution of a substance ''X'' is used for white washing
(i) Name the substance ''X'' and write its formula
(ii) Write the reaction of the substance ''X'' named in (i) above with water.
(i) The substance ''X'' used for white washing is quick lime (calcium oxide). Its formula is CaO.
(ii) When quick lime is mixed with water, the following reaction takes place:
CaO (s) + H2O(l) → Ca(OH)2(aq)
Quick Lime Water Slaked Lime
(Calcium Oxide) (Calcium Hydroxide)
The white suspension of slaked lime when applied on the walls, combines with carbon dioxide (in air) forming a thin shining layer of calcium carbonate.
Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l)
Slaked Lime Calcium Carbonate
Q27(CBSE): Why is it important to balance a chemical equation?
Answer: The balancing of chemical equation is done to satisfy the law of conservation of mass, i.e. "total mass of all the products of reaction in a chemical reaction is equal the total mass of all the reactants".
Q28: What are the coefficients of the correctly balanced equation?
? Fe2O3 + ? CO → ? Fe + ? CO2
(a) 0, 2, 2, 3
(b) 1, 3, 2, 3
(c) 1, 2, 2, 2
(d) 2, 6, 4, 3
Answer: (b) Ferric Oxide + Carbon Mono-oxide → Iron + Carbon DiOxide
Q29: What are the coefficients of the correctly balanced equation?
? BaCl2 + ? Al2(SO4)3 → ? BaSO4 + ? AlCl3 (aq)
(a) 1, 1, 1, 2
(b) 3, 2, 3, 2
(c) 3, 1, 3,2
(d) 2, 1, 1, 2
Answer: (c) 3 BaCl2 + 1 Al2(SO4)3 → 3 BaSO4 + 2 AlCl3 (aq)
Barium Chloride + Aluminium Sulphate → Barium Sulphate + Aluminium Chloride
Q30(CBSE2011): What is the colour of FeSO4.7H2O crystals ? How does this colour change upon heating ? Give balanced chemical equation for the changes.
Answer: Hydrated ferrous sulphate crystals are green in colour. On heating, single reactant breaks down to give simpler products. This is a decomposition reaction.
Ferrous sulphate crystals (FeSO4.7H2O) lose water when heated and the colour of the crystals changes. It then decomposes to ferric oxide (Fe2O3), sulphur dioxide (SO2) and sulphur trioxide (SO3). Ferric oxide is a solid, while SO2 and SO3 are gases.
You may use Wolfram Chemical Reaction Calculator E.g. Type lime + Water --> Calcium Hydroxide
Q31: Quick like (CaO - Calcium Oxide) reaction with water is regarded as exothermic. A student mixes these two products in a test tube and touches its side surface. Which of the following statement correctly describes the student's observation?
(a) The test tube becomes cold due to release of heat energy.
(b) The test tube becomes hot due to release of heat energy.
(c) The test tube becomes hot due to absorption of heat energy.
(d) The test tube becomes cold due to absorption of heat energy.
Answer: (b) The test tube becomes hot due to release of heat energy.
Q32: The decomposition of vegetable matter into compost is an example of _____.
(Choose the correct option).
(a) endothermic reaction.
(b) exothermic reaction.
Answer: (b) exothermic.
Q33(CBSE): Write a balanced chemical equation to represent the following reaction:
Iron reacts with steam to form Iron(II,III) oxide and hydrogen gas.
3Fe(s) + 4H2O(l) ▬▬▬▬▬▶ Fe3O4(s) + 4H2(g)
Iron Water Iron Oxide Hydrogen
Q34(CBSE): Identify the substance oxidized, substance reduced, oxidising agent and reducing agent.
MnO2 + 4HCl → MnCl2 + 2H2O + Cl2
Substance Oxidised: HCl
Substance reduced: MnO2
Reducing Agent: HCl
Oxidising Agent: MnO2
Q35: Bromine gas (Br2) reacts with Sodium Carbonate (Na2CO3) in aqueous solution and gives sodium bromide (NaBr), sodium bromate (NaBrO3) and carbon-dioxide gas. How many number of sodium bromide molecules obtained in the balanced chemical equation.
Answer: From the question, we infer the unbalanced chemical equation is:
Br2 + Na2CO3 → NaBr + NaBrO3 + CO2
Balanced equation is:
3 Br2 + 3 Na2CO3 → 5 NaaBr + NaBrO3 + 3 CO2
It means 5 molecules of NaBr are obtained.
Q36: A numismatist (coin collector) has been collecting gold coins, silver coins and copper coins for a long time. One day he observed a black coating on silver coins and green coating on copper coins. What chemical process is responsible for these coatings. Also name the chemical formula of the black and green coatings.
Answer: The coating occurred due to a chemical process called corrosion. Black coating is due to deposit of Silver Sulphide (Ag2S). Green coating is due to formation of Copper Carbonate (CuCO3)
(✪Note: Silver usually is corrosion resistant and does not oxidize. However presence of sulphur gases in atmosphere can react and cause sulphur-corrosion. )
Q37: Methane gas (CH4) when burns with oxygen (O2) gives carbon dioxide and water. Write the balanced chemical equation and provide the following information in terms of reactants and products in the equation.
(a) no. of molecules
(b) no. of moles
(c) molar mass
(d) total mass of reactants and products
What does statement (d) infer?
Answer: The balanced chemical equation is:
CH4 (g) + 2 O2(g) → 2H2O (l) + CO2(g)
Methane + Oxygen → water + carbon di-oxide
|a. No. of moldecules||CH4 (g), O2(g)||H2O, CO2(g)|
|b. no. of moles||1 mole of CH4+ 1 mole of O2(g)||2 moles of H2O + 1 mol of CO2|
|c. molar mass||16g of CH4+ 64g of O2||44g of H2O + 36g of CO2|
Statement (d) infers that law of conservation of mass is obeyed.
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