CBSE Class 10 Science Chemical Reactions and Equations VBQs Set 05

CASE-BASED QUESTIONS

Question. Read the given passage and answer the questions based on passage and related studied concepts.
The metal sodium reacts with air and water. A student reacted sodium with water and measured the volume of gas at intervals of 30 seconds. The results are shown below: Time/s: 0, 30, 60, 90, 120, 150, 180 Volume/\( cm^3 \): 0, 40, 60, 74, 86, 96, 140
(a) Up to what time reaction was fastest? Why?
(b) Which gas is liberated in above reaction?
(c) What will be colour of universal indicator in the solution formed? Write balanced chemical reaction.
OR
(c) (i) What will be colour of phenolphthalein in solution formed?
(ii) Which ions are responsible for basic nature of \( NaOH \)?

Answer: (a) The reaction was fastest up to 30 seconds. This is because the concentration of reactants is highest at the beginning, leading to a higher rate of gas production (40 \( cm^3 \) in the first 30 seconds).
(b) Hydrogen gas (\( H_2 \)) is liberated.
(c) The universal indicator will turn purple or dark blue, indicating a strongly basic solution. The balanced chemical reaction is:
\( 2Na(s) + 2H_2O(l) \longrightarrow 2NaOH(aq) + H_2(g) \)
OR
(c) (i) The colour of phenolphthalein in the solution formed will be pink.
(ii) Hydroxide ions (\( OH^- \)) are responsible for the basic nature of \( NaOH \).

Question. Read the given passage and answer the questions based on passage and related studied concepts. Chemical reaction involves chemical changes in which new substances with new properties are formed, when two or more substances react with each other. Combination, decomposition (thermal, electrical and photochemical), displacement, double displacement (precipitation), redox (oxidation and reduction) are various type of chemical reactions.
(a) What is formula of marble?
(b) Which compound is used in photographic and X-ray films?
(c) What happens when potassium reacts with water? Write balanced chemical equation.
OR
(c) What happens when
(i) Iron reacts with chlorine
(ii) Copper reacts with oxygen on heating

Answer: (a) The chemical formula of marble is \( CaCO_3 \) (Calcium carbonate).
(b) Silver bromide (\( AgBr \)) or silver chloride (\( AgCl \)) is used in photographic and X-ray films.
(c) When potassium reacts with water, it reacts vigorously to form potassium hydroxide and hydrogen gas, which catches fire immediately. The balanced equation is:
\( 2K(s) + 2H_2O(l) \longrightarrow 2KOH(aq) + H_2(g) + heat \)
OR
(c) (i) Iron reacts with chlorine to form iron(III) chloride: \( 2Fe(s) + 3Cl_2(g) \longrightarrow 2FeCl_3(s) \)
(ii) Copper reacts with oxygen on heating to form a black layer of copper(II) oxide: \( 2Cu(s) + O_2(g) \xrightarrow{heat} 2CuO(s) \)

Question. Answer the questions on the basis of your understanding of the following paragraph and the related studied concepts. Redox reactions involves both oxidation as well as reduction. If we cut an apple, it turns reddish brown after some time because \( Fe^{2+} \) present in apple gets converted into \( Fe^{3+} \) by loss of electrons. Oxidation is a process in which loss of electrons takes place. Oxidation is carried out with the help of oxidising agent which can give \( O_2 \) or remove hydrogen or can gain electrons. Non-metals are good oxidising agents, \( KMnO_4 \), \( K_2Cr_2O_7 \) are also good oxidising agent in acidic medium. Combustion is oxidation reaction. Reduction involves gain of electrons. Reducing agent can add \( H_2 \) or remove \( O_2 \) or can lose electrons. Metals are good reducing agents. Reducing agents are used in extraction of metals.
(a) \( Fe_2O_3(s) + 3CO(g) \longrightarrow 2Fe(s) + 3CO_2(g) \)
Identify oxidising and reducing agents in the above equation.
(b) \( H_2S(g) + Cl_2(g) \longrightarrow 2HCl(g) + S(s) \)
Name the substance which are getting oxidised and reduced.
(c) \( Mg(s) + 2H^+(aq) \longrightarrow Mg^{2+}(aq) + H_2(g) \)
Why is magnesium acting as reducing agent and \( H^+ \) are oxidising agent?
OR
(c) (i) \( Cr_2O_3 + 2Al \longrightarrow Al_2O_3 + 2Cr \)
Identify the type of above reaction.
(ii) \( Fe_3O_4 + 4H_2 \longrightarrow 3Fe + 4H_2O \)
Which substance undergoes oxidation and reduction?

Answer: (a) Oxidising agent: \( Fe_2O_3 \); Reducing agent: \( CO \).
(b) Substance oxidised: \( H_2S \); Substance reduced: \( Cl_2 \).
(c) Magnesium (\( Mg \)) is acting as a reducing agent because it loses electrons (oxidation), and \( H^+ \) ions are oxidising agents because they gain electrons (reduction) to form \( H_2 \).
OR
(c) (i) This is a displacement reaction and also a redox reaction.
(ii) \( H_2 \) undergoes oxidation to form \( H_2O \), and \( Fe_3O_4 \) undergoes reduction to form \( Fe \).

Question. Read the given passage and answer the questions based on passage and related studied concepts. Chemistry in Automobiles: For an internal combustion engine to move a vehicle down the road, it must convert the energy stored in the fuel into mechanical energy to drive the wheels. In your car, the distributor and battery provide this starting energy by creating an electrical “spark”, which helps in combustion of fuels like gasoline. Below is the reaction depicting complete combustion of gasoline in full supply of air: \( 2C_8H_{18}(l) + 25O_2(g) \longrightarrow 16X + 18Y \)
(a) What products are obtained in above reaction?
(b) Identify the type of reaction taking place during combustion of fuel.
(c) On the basis of evolution/absorption of energy which of the following processes are similar to combustion of fuel?
I. Photosynthesis is plants
II. Respiration in human body
III. Decomposition of vegetable matter
IV. Decomposition of Ferrous sulphate
OR
(c) (i) Why is there rich smoke in the exhaust of moving trucks on the road?
(ii) Why does \( N_2 \) not take part in combustion?

Answer: (a) The products are carbon dioxide (\( X = CO_2 \)) and water vapor (\( Y = H_2O \)).
(b) It is an exothermic oxidation reaction (combustion).
(c) Respiration in human body (II) and decomposition of vegetable matter (III) are both exothermic processes, similar to combustion.
OR
(c) (i) Rich smoke (black smoke) is due to the incomplete combustion of fuel, leading to the formation of unburnt carbon particles.
(ii) \( N_2 \) does not take part in combustion because it is a relatively inert gas due to the presence of a strong triple bond between nitrogen atoms, which requires very high energy to break.

Question. Two students decided to investigate the effect of water and air on iron object under identical experimental conditions. They measured the mass of each object before placing it partially immersed in 10 ml of water. After a few days, the object were removed, dried and their masses were measured. The table shows their results. Student A: Object - Nail, Mass before - 3.0 g, Mass after - 3.15 g Student B: Object - Thin plate, Mass before - 6.0 g, Mass after - 6.33 g
(a) What might be the reason for the varied observations of the two students?
(b) In another set up the students coated iron nails with zinc metal and noted that, iron nails coated with zinc prevents rusting. They also observed that zinc initially acts as a physical barrier, but an extra advantage of using zinc is that it continues to prevent rusting even if the layer of zinc is damaged. Name this process of rust prevention and give any two other methods to prevent rusting.

Answer: (a) The variation in mass increase is due to the difference in surface area. The thin plate has a larger surface area exposed to air and water compared to the nail, leading to more extensive rusting.
(b) The process is called Galvanisation. Two other methods are painting and alloying (creating stainless steel).
OR
(b) Rusting will occur most in C (Iron hinges on a gate). Hinges are subject to friction which can wear away any protective coating, and they are constantly exposed to both air and moisture, unlike electroplated buckets, covered cables, or painted fences which have more durable protective layers.

NCERT ZONE - NCERT INTEXT QUESTIONS

Question. Why should a magnesium ribbon be cleaned before burning in air?

Answer: When magnesium ribbon is exposed to air, it forms a layer of magnesium oxide on its surface. This layer of magnesium oxide, being a stable compound, prevents further reaction of magnesium with oxygen. Hence, it should be cleaned before burning in air to remove this layer so that the metal can be exposed to air properly (the oxide layer may prevent or slow down the burning of magnesium).

Question. Write the balanced equation for the following chemical reactions.
(a) Hydrogen + Chlorine \(\longrightarrow\) Hydrogen chloride
(b) Barium chloride + Aluminium sulphate \(\longrightarrow\) Barium sulphate + Aluminium chloride
(c) Sodium + Water \(\longrightarrow\) Sodium hydroxide + Hydrogen

Answer: (a) \( H_2(g) + Cl_2(g) \longrightarrow 2HCl(g) \)
(b) \( 3BaCl_2(aq) + Al_2(SO_4)_3(aq) \longrightarrow 3BaSO_4(s) + 2AlCl_3(aq) \)
(c) \( 2Na(s) + 2H_2O(l) \longrightarrow 2NaOH(aq) + H_2(g) \)

Question. Write a balanced chemical equation with state symbols for the following reactions.
(a) Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride.
(b) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.

Answer: (a) \( BaCl_2(aq) + Na_2SO_4(aq) \longrightarrow BaSO_4(s) + 2NaCl(aq) \)
(b) \( NaOH(aq) + HCl(aq) \longrightarrow NaCl(aq) + H_2O(l) \)

Question. 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.

Answer: (a) The substance X is lime or quick lime. Its formula is \( CaO \).
(b) \( CaO + H_2O \longrightarrow Ca(OH)_2 + Heat \)

Question. Why is the amount of gas collected in one of the test tubes in Activity 1.7, (electrolysis of water) is double of the amount collected in the other? Name this gas.

Answer: Water (\( H_2O \)) contains two parts of hydrogen and one part of oxygen. Therefore, during the electrolysis of water, the amount of hydrogen gas collected in one of the test tubes is double the amount of oxygen produced and collected in the other test tube. The gas collected in double amount is hydrogen.

Question. Why does the colour of copper sulphate solution change when an iron nail is dipped in it?

Answer: Iron is more reactive than copper. So, when an iron nail is dipped in a copper sulphate solution, iron displaces copper from its solution to form iron sulphate, which is green in colour.
\( Fe(s) + CuSO_4(aq) \longrightarrow FeSO_4(aq) + Cu(s) \)
Hence, the blue colour of copper sulphate solution changes into green colour because of this displacement reaction. It is a redox reaction.

Question. Give an example of a double displacement reaction other than the one given in Activity 1.10.

Answer: Sodium carbonate and calcium chloride exchange ions to form two new compounds calcium carbonate and sodium chloride.
\( Na_2CO_3(aq) + CaCl_2(aq) \longrightarrow CaCO_3(s) + 2NaCl(aq) \)

Question. Identify the substances that are oxidized and the substances that are reduced in the following reactions:
(a) \( 4Na(s) + O_2(g) \longrightarrow 2Na_2O(s) \)
(b) \( CuO(s) + H_2(g) \longrightarrow Cu(s) + H_2O(l) \)

Answer: (a) Sodium (\( Na \)) is oxidized to \( Na_2O \), and oxygen is getting reduced to \( O^{2-} \) ion.
(b) \( CuO \) (Copper oxide) is reduced to \( Cu \), while \( H_2 \) gas is oxidized to \( H_2O \).

NCERT EXERCISES

Question. Which of the statements about the reaction below are incorrect?
\( 2PbO(s) + C(s) \longrightarrow 2Pb(s) + CO_2(g) \)
(a) Lead is getting reduced.
(b) Carbon dioxide is getting oxidised.
(c) Carbon is getting oxidised.
(d) Lead oxide is getting reduced.
(i) (a) and (b)
(ii) (a) and (c)
(iii) (a), (b) and (c)
(iv) All of the options

Answer: (i) (a) and (b) are incorrect statements because ‘Pb’ and ‘\( CO_2 \)’ are products and not reactants.

Question. \( Fe_2O_3 + 2Al \longrightarrow Al_2O_3 + 2Fe \). The above reaction is an example of a
(a) combination reaction.
(b) double displacement reaction.
(c) decomposition reaction.
(d) displacement reaction.

Answer: (d) Al is displacing iron from iron (III) oxide. Therefore, it is a displacement and redox reaction.

Question. What happens when dilute hydrochloric acid is added to iron fillings? Tick the correct answer.
(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.

Answer: (a) \( Fe(s) + 2HCl(dil.) \longrightarrow FeCl_2(aq) + H_2(g) \). Hydrogen gas and Iron (II) chloride are produced.

Question. What is a balanced chemical equation? Why should chemical equations be balanced?

Answer: Balanced chemical equation means total number of atoms of each element should be equal on both sides of the reaction. For example, magnesium and oxygen combine, when heated to form a single compound, magnesium oxide.
\( 2Mg(s) + O_2(g) \longrightarrow 2MgO(s) \)
The reaction should be balanced because matter can neither be created nor be destroyed. The total mass of reactants should be equal to the total mass of products.

Question. Translate the following statements into chemical equations and then balance them.
(a) Hydrogen gas combines with nitrogen to form ammonia.
(b) Hydrogen sulphide gas burns in air to give water and sulphur dioxide.
(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.
(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.

Answer: (a) \( 3H_2(g) + N_2(g) \longrightarrow 2NH_3(g) \)
(b) \( 2H_2S(g) + 3O_2(g) \longrightarrow 2H_2O(l) + 2SO_2(g) \)
(c) \( 3BaCl_2(aq) + Al_2(SO_4)_3(aq) \longrightarrow 3BaSO_4(s) + 2AlCl_3(aq) \)
(d) \( 2K(s) + 2H_2O(l) \longrightarrow 2KOH(aq) + H_2(g) \)

Question. Balance the following chemical equations.
(a) \( HNO_3 + Ca(OH)_2 \longrightarrow Ca(NO_3)_2 + H_2O \)
(b) \( NaOH + H_2SO_4 \longrightarrow Na_2SO_4 + H_2O \)
(c) \( NaCl + AgNO_3 \longrightarrow AgCl + NaNO_3 \)
(d) \( BaCl_2 + H_2SO_4 \longrightarrow BaSO_4 + HCl \)

Answer: (a) \( 2HNO_3(aq) + Ca(OH)_2(aq) \longrightarrow Ca(NO_3)_2(aq) + 2H_2O(l) \)
(b) \( 2NaOH(aq) + H_2SO_4(aq) \longrightarrow Na_2SO_4(aq) + 2H_2O(l) \)
(c) \( NaCl(aq) + AgNO_3(aq) \longrightarrow AgCl(s) + NaNO_3(aq) \)
(d) \( BaCl_2(aq) + H_2SO_4(aq) \longrightarrow BaSO_4(s) + 2HCl(aq) \)

Question. Write the balanced chemical equations for the following reactions.
(a) Calcium hydroxide + Carbon dioxide \(\longrightarrow\) Calcium carbonate + Water
(b) Zinc + Silver nitrate \(\longrightarrow\) Zinc nitrate + Silver
(c) Aluminium + Copper chloride \(\longrightarrow\) Aluminium chloride + Copper
(d) Barium chloride + Potassium sulphate \(\longrightarrow\) Barium sulphate + Potassium chloride.

Answer: (a) \( Ca(OH)_2(aq) + CO_2(g) \longrightarrow CaCO_3(s) + H_2O(l) \)
(b) \( Zn(s) + 2AgNO_3(aq) \longrightarrow Zn(NO_3)_2(aq) + 2Ag(s) \)
(c) \( 2Al(s) + 3CuCl_2(aq) \longrightarrow 2AlCl_3(aq) + 3Cu(s) \)
(d) \( BaCl_2(aq) + K_2SO_4(aq) \longrightarrow BaSO_4(s) + 2KCl(aq) \)

Question. Write the balanced chemical equation for the following and identify the type of reaction in each case.
(a) Potassium bromide(aq) + Barium iodide(aq) \(\longrightarrow\) Potassium iodide(aq) + Barium bromide(s)
(b) Zinc carbonate(s) \(\longrightarrow\) Zinc oxide(s) + Carbon dioxide(g)
(c) Hydrogen(g) + Chlorine(g) \(\longrightarrow\) Hydrogen chloride(g)
(d) Magnesium(s) + Hydrochloric acid(aq) \(\longrightarrow\) Magnesium chloride(aq) + Hydrogen(g)

Answer: (a) \( 2KBr(aq) + BaI_2(aq) \longrightarrow 2KI(aq) + BaBr_2(s) \)
Double displacement reaction.
(b) \( ZnCO_3(s) \longrightarrow ZnO(s) + CO_2(g) \)
Decomposition reaction.
(c) \( H_2(g) + Cl_2(g) \longrightarrow 2HCl(g) \)
Combination reaction and redox reaction.
(d) \( Mg(s) + 2HCl(aq) \longrightarrow MgCl_2(aq) + H_2(g) \)
Displacement reaction and redox reaction.

Question. What does one mean by exothermic and endothermic reactions? Give examples.

Answer: Exothermic reactions are those reactions in which heat is evolved, e.g.
\( C(s) + O_2(g) \longrightarrow CO_2(g) + heat \)
\( CH_4(g) + 2O_2(g) \longrightarrow CO_2(g) + 2H_2O(l) + heat \)
Endothermic reactions are those reactions in which heat is absorbed, e.g.
\( CaCO_3(s) \xrightarrow{heat} CaO(s) + CO_2(g) \)
\( N_2(g) + O_2(g) \xrightarrow{heat} 2NO(g) \)

Question. Why is respiration considered an exothermic reaction? Explain.

Answer: During respiration, glucose combines with oxygen in the cells of our body and provides energy. As energy is released during respiration, therefore, respiration is regarded as exothermic reaction.
\( C_6H_{12}O_6(aq) + 6O_2(g) \longrightarrow 6CO_2(g) + 6H_2O(l) + Energy \)

Question. Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.

Answer: In decomposition reaction, a compound is broken down into simpler compounds or elements, e.g.
\( CaCO_3(s) \xrightarrow{Heat} CaO(s) + CO_2(g) \)
Combination reaction is a reaction in which two or more elements or compounds combine to form new compound, e.g.
\( N_2(g) + 3H_2(g) \longrightarrow 2NH_3(g) \)
Thus, decomposition and combination reactions are opposite to each other.

Question. Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.

Answer: (i) \( CaCO_3(s) \xrightarrow{Heat} CaO(s) + CO_2(g) \)
(ii) \( 2AgCl(s) \xrightarrow{Sunlight} 2Ag(s) + Cl_2(g) \)
(iii) \( 2H_2O(l) \xrightarrow{Electricity} 2H_2(g) + O_2(g) \)

Question. What is the difference between displacement and double displacement reactions? Write equations for these reactions.

Answer: Displacement reaction: Those reactions in which more reactive metal displaces less reactive metal from its salt solution are called displacement reactions.
\( Fe(s) + CuSO_4(aq) \longrightarrow FeSO_4(aq) + Cu(s) \)
Double displacement reaction: Those reactions in which two compounds exchange their ions to form two new compounds are called double displacement reactions. It is redox reaction.
\( HCl(aq) + NaOH(aq) \longrightarrow NaCl(aq) + H_2O(l) \)

Question. In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.

Answer: \( Cu(s) + 2AgNO_3(aq) \longrightarrow Cu(NO_3)_2(aq) + 2Ag(s) \)
Copper + Silver nitrate \(\longrightarrow\) Copper nitrate + Silver
Thus, silver metal can be recovered.

Question. What do you mean by a precipitation reaction? Explain by giving examples.

Answer: Those reactions in which two compounds react to form insoluble compound, which is called precipitate, are called precipitation reactions, e.g.
When solutions of silver nitrate and sodium chloride are mixed, white precipitate of silver chloride is formed.
\( AgNO_3(aq) + NaCl(aq) \longrightarrow AgCl(s) + NaNO_3(aq) \)
On adding dilute hydrochloric acid to the aqueous solution of lead nitrate, precipitate of lead chloride is formed.
\( Pb(NO_3)_2(aq) + 2HCl(aq) \longrightarrow PbCl_2(s) + 2HNO_3(aq) \)

Question. Explain the following in terms of gain or loss of oxygen with two examples each. (a) Oxidation (b) Reduction

Answer: (a) Oxidation: It is a process in which gain of oxygen takes place, e.g.
\( 2Mg(s) + O_2(g) \xrightarrow{Burning} 2MgO(s) \)
\( 2Cu(s) + O_2(g) \xrightarrow{Heat} 2CuO(s) \)
(b) Reduction: It is a process in which removal of oxygen takes place, e.g.
\( CuO(s) + H_2(g) \xrightarrow{Heat} Cu(s) + H_2O(g) \)
\( Fe_2O_3(s) + 2Al(s) \longrightarrow Al_2O_3(s) + 2Fe(s) \)

Question. A shiny brown coloured element ‘X’ on heating in air becomes black in colour. Name the element ‘X’ and the black coloured compound formed.

Answer: ‘X’ is copper. Copper gets oxidised to copper oxide which is black in colour.
\( 2Cu(s) + O_2(g) \xrightarrow{Heat} 2CuO(s) \)
Copper (Reddish brown) + Oxygen \(\longrightarrow\) Copper oxide (Black)

Question. Why do we apply paint on iron articles?

Answer: Painting is done so as to prevent iron from rusting. When the surface of iron is coated with paint, its surface does not come in contact with oxygen and moisture and therefore, rusting does not take place.

Question. Oil and fat containing food items are flushed with nitrogen. Why?

Answer: When food items prepared in oil are kept for a long time, fat and oils present in them get oxidised by the oxygen. As a result of oxidation, some products are formed which have unpleasant smell and taste. To prevent the food items containing oil and fat from being oxidised and turned rancid, they are flushed with an unreactive gas like nitrogen.

Question. Explain the following terms with one example each. (a) Corrosion (b) Rancidity

Answer: (a) Corrosion: It is a process in which metal reacts with substances present in the atmosphere to form surface compounds, e.g. iron reacts with oxygen in presence of moisture to form rust, \( Fe_2O_3.xH_2O \) (hydrated Iron(III) oxide).
(b) Rancidity: When food items prepared in oil such as potato chips are kept for long time, fat and oil present in them get oxidised by the oxygen, and they start giving unpleasant smell and taste. This condition, produced by aerial oxidation of fats and oils in foods marked by unpleasant smell and taste, is called rancidity. Rancidity spoils the food materials and makes them unfit for eating.

SELECT NCERT EXEMPLAR PROBLEMS

Question. The following reaction is an example of a
\( 4NH_3(g) + 5O_2(g) \longrightarrow 4NO(g) + 6H_2O(g) \)
(i) displacement reaction
(ii) combination reaction
(iii) redox reaction
(iv) neutralisation reaction
(a) (i) and (iv)
(b) (ii) and (iii)
(c) (i) and (iii)
(d) (iii) and (iv)

Answer: (c) \( NH_3 \) is reducing agent and \( O_2 \) is oxidising agent.

Question. Which of the following are exothermic processes?
(i) Reaction of water with quick lime
(ii) Dilution of an acid
(iii) Evaporation of water
(iv) Sublimation of camphor (crystals)
(a) (i) and (ii)
(b) (ii) and (iii)
(c) (i) and (iv)
(d) (iii) and (iv)

Answer: (a)

Question. Barium chloride on reacting with ammonium sulphate forms barium sulphate and ammonium chloride. Which of the following correctly represents the type of the reaction involved?
(i) Displacement reaction
(ii) Precipitation reaction
(iii) Combination reaction
(iv) Double displacement reaction
(a) (i) only
(b) (ii) only
(c) (iv) only
(d) (ii) and (iv)

Answer: (d) \( BaCl_2(aq) + (NH_4)_2SO_4(aq) \longrightarrow BaSO_4(s) + 2NH_4Cl(aq) \)