Science & Tech Part 1 Chapter 3 Chemical Reactions And Equations Solutions

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Calcium carbonate undergoes a decomposition reaction in the first test tube forming calcium oxide powder and carbon dioxide gas.

\[\ce{\underset{\text{Limestone}}{CaCO_{3(s)}} ->[\Delta] \underset{\text{Calcium oxide}}{CaO_{(s)}} + \underset{\text{Carbon dioxide}}{CO2\uparrow}}\]

Carbon dioxide gas formed in the first test tube turns the lime water milky in the second test tube due to the formation of calcium carbonate, which is insoluble in water.

\[\ce{\underset{\text{Calcium hydroxide}}{Ca(OH)_{2(aq)}} + \underset{\text{Carbon dioxide}}{CO_{2(g)}} -> \underset{\text{Calcium carbonate}}{CaCO_{3(s)}} + \underset{\text{Water}}{H2O_{(l)}}}\]

The calcium oxide powder, remains behind in the first test tube.

Coefficient ‘2’ is added to the reactant ‘NaOH’ and the product ‘H2O’ to balance the equation.

The final balanced equation is:

\[\ce{2NaOH + H2SO4 -> Na2SO4 + 2H2O}\]

Elements in the chemical reaction are: Na, O, H and S.

Reduction is gain of electrons.

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Names of reactants: Sodium hydroxide and Sulphuric acid

Names of products: Sodium sulphate and water

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Magnesium ribbon when burned in air, it forms magnesium oxide- Exothermic reaction

\[\ce{\underset{\text{sulpate}}{\underset{\text{Copper}}{CuSO_{4(aq)}}} + \underset{\text{Iron}}{Fe_{(s)}} -> \underline{\underset{\text{sulphate}}{\underset{\text{Ferrous}}{FeSO_{4(aq)}}}} + \underline{\underset{\text{Copper}}{Cu_{(s)}}}}\]

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The factors that affect the rate of a chemical reaction are as follows:

Nature of the reactants: Nature or reactivity of reactants influences the rate of a chemical reaction.

Size of the particles of the reactants: Smaller the size of the reactant particles, higher is the rate of the reaction.

E.g. When HCl is added to pieces of Shahabad tile, the CO2 effervescence is formed slowly. However, when HCl is added to Shahabad powder, the CO2 effervescence is formed at a faster rate.

Concentration of the reactants: The rate of a chemical reaction increases with increase in the concentration of the reactants.

E.g. Dilute HCl reacts slowly with calcium carbonate, CaCO3 whereas concentrated HCl reacts rapidly with CaCO3.

Temperature of the reaction: Higher the temperature, faster is the rate of a chemical reaction.

E.g. The temperature in the refrigerator is low. So, the rate of decomposition of perishable foodstuffs is low and it remains fresh for a longer time.

Catalyst: Presence of catalyst increases the rate of a chemical reaction.

E.g. The decomposition of hydrogen peroxide into water and oxygen takes place slowly at room temperature. However, the same reaction occurs at a faster rate on adding manganese dioxide (MnO2) powder in it.

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Zinc, on treatment with dilute hydrochloric acid, forms zinc chloride with release of hydrogen gas.

\[\ce{\underset{\text{Zinc}}{Zn_{(s)}} + \underset{\text{Hydrochloric acid}}{2HCl_{(aq)}} -> \underset{\text{Zinc chloride}}{ZnCl_{2(aq)}} + \underset{\text{Hydrogen}}{H2↑}}\]

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When aluminium is treated with dilute hydrochloric acid, it forms aluminium chloride with release of hydrogen gas.

\[\ce{\underset{\text{Aluminium}}{2Al_{(s)}} + \underset{\text{Hydrochloric acid}}{6HCl_{(aq)}} -> \underset{\text{Aluminium chloride}}{2AlCl_{3(aq)}} + \underset{\text{Hydrogen}}{3H2↑}}\]

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Ammonia gas and hydrogen chloride gas react to form the salt ammonium chloride. It is a combination reaction.

\[\ce{\underset{\text{Ammonia}}{NH_{3(g)}} + \underset{\text{Hydrogen chloride}}{HCl_{(g)}} -> \underset{\text{Ammonium chloride}}{NH4Cl_{(s)}}}\]

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Hydrogen sulphide and sulphur dioxide react to form sulphur and water. It is a redox reaction.

\[\ce{\underset{\text{Sulphur dioxide}}{SO_{2(g)}} + \underset{\text{Hydrogen sulphide}}{H2S_{(g)}} -> \underset{\text{Sulphur}}{S_{(s)}} + \underset{\text{Water}}{H2O_{(l)}}}\]

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When limestone is heated, calcium oxide and carbon dioxide gas are formed.

\[\ce{\underset{\text{carbonate}}{\underset{\text{Calcium}}{CaCO_{3(s)}}} + Heat -> \underset{\text{oxide}}{\underset{\text{Calcium}}{CaO_{(s)}}} + \underset{\text{dioxide}}{\underset{\text{Carbon}}{CO2\uparrow}}}\]

When carbon dioxide gas is passed through freshly prepared lime water, the solution turns milky due to the formation of calcium carbonate, which is insoluble in water.

\[\ce{\underset{\text{hydroxide}}{\underset{\text{Calcium}}{Ca(OH)2_{(aq)}}} + \underset{\text{dioxide}}{\underset{\text{Carbon}}{CO_{2(g)}}} -> \underset{\text{carbonate}}{\underset{\text{Calcium}}{CaCO_{3(s)}}} + \underset{\text{Water}}{H2O_{(l)}}}\]

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When edible oil is left aside for long time, it undergoes air oxidation.

Due to this, the taste and smell of oil changes and it becomes rancid. If food is cooked in this oil, its taste also changes.

Thus, the oil will become unfit for consumption.

The process of oxidation reaction of oil can be slowed down by storing it in air tight container.

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The rate of a chemical reaction depends upon the size of the reactant particles taking part in the reaction.

Smaller the size of the reactant particles, higher is the rate of reaction.

The size of reactant particles is more in pieces of Shahabad tile as compared to powder of Shahabad tile.

When HCl is added to pieces of Shahabad tile, the CO2 effervescence is formed slowly. However, when HCl is added to Shahabad powder, the CO2 effervescence is formed at a faster rate.

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Aluminium oxide (Al2O3) and zinc oxide (ZnO) are examples of amphoteric oxides.

The following reactions show the amphoteric character of zinc oxide:

\[\ce{\underset{(Basic nature)}{ZnO_{(s)}} + 2HCl_{(aq)} -> \underset{Zinc chloride}{ZnCl2_{(aq)}} + H2O_{(l)}}\]

\[\ce{\underset{(Acidic nature)}{ZnO_{(s)} + 2Na}OH_{(aq)} -> \underset{Sodium zincate}{Na2ZnO2_{(aq)}} + H2O_{(l)}}\]

\[\ce{2KClO_{3(s)}->[\Delta] 2KCl_{(s)} + 3O2}\]- Endothermic reaction

In a reaction, silver oxide is changing to silver. That is, oxygen is being removed from silver oxide. Removal of oxygen from a substance is called reduction, so silver oxide undergoes reduction.

In a reaction, magnesium is changing to magnesium oxide. That means, oxygen is being added to magnesium. Addition of oxygen to a substance is called oxidation, so magnesium undergoes oxidation.

In a reaction, nickel oxide is changing to nickel. That is, oxygen is being removed from nickel oxide. Removal of oxygen from a substance is called reduction, so nickel oxide undergoes reduction.

In a reaction, hydrogen is changing to H2O. That is, oxygen is being added to hydrogen. Addition of oxygen to a substance is called oxidation, so hydrogen undergoes oxidation.

The reactions in which oxidation and reduction take place simultaneously are called Redox reactions.

\[\ce{CaCO_{3(s)}->[\Delta] CaO_{(s)} + CO2}\]- Endothermic reaction

\[\ce{CaO + H2O -> Ca(OH)2 + Heat}\]- Exothermic reaction

Exothermic reaction

If edible oil is left aside for a long time, it will get rancid- Right

In the chemical equation the reactants are written on the left-hand side.

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The process shown in the figure is rusting (corrosion) of iron.

The process involves electrochemical reaction. Different regions on the surface of iron become anode and cathode. In the anode region, Fe is oxidized to Fe2+.

The electron released in the anode region flow through the metal surface to the cathode region where they reduce oxygen.

In the cathode region, O2 is reduced to form water. When Fe2+ ions migrate from the anode region, they react with water (or OH− ions) and further get oxidized to Fe3+ ions. Fe3+ ions form an insoluble hydrated oxide (Fe2O3.H2O), which is deposited as reddish-brown layer on the surface.

It is called rust.

\[\ce{2F^3+_{(aq)} + 4H2O_{(l)} -> \underset{\text{Rust}}{Fe2O3.H2O_{(s)}} + 6H+_{(aq)}}\]

Anode region: \[\ce{Fe_{(s)} -> Fe^2+_{(aq)} + 2e^-}\]

Cathode region: \[\ce{O_{2(g)} + 4H^+_{(aq)} + 4e^- -> 2H2O_{(l)}}\]

The chemical reaction in which two or more products are formed from a single reactant is called decomposition reaction.

Metals combine with oxygen to form basic oxides. Aluminium oxide and zinc oxide show the properties of both basic and acidic oxides. These oxides are known as amphoteric oxides.

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The substances formed as a result of a chemical reaction by the formation of new bonds are called products.

e.g. Decomposition of calcium carbonate by heating to form calcium oxide and carbon dioxide is a chemical reaction. In this reaction, calcium carbonate is the reactant while calcium oxide and carbon dioxide are the products.

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The substances taking part in chemical reaction are called reactants.

E.g. Formation of carbon dioxide gas by combustion of coal in air is a chemical reaction. In this reaction, coal (carbon) and oxygen (from air) are the reactants while carbon dioxide is the product.

Corrosion is the damage caused to the metal by the chemical reaction of air, water, and acids with the surface of the metal. Generally, reactive metals corrode easily, and non-reactive metals have good resistance to corrosion.

The cell which converts electrical energy into chemical energy is called an electrolytic cell, and the process which converts electrical energy into chemical energy is called electrolysis.

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The process in which oxidation and reduction occur simultaneously is called as redox reaction.

Oxidation is the loss of electrons or an increase in oxidation state by a molecule, atom, or ion.

Reduction is the gain of electrons or a decrease in oxidation state by a molecule, atom, or ion.

For example: If we add a stannous chloride solution to the yellow solution of ferric chloride then light green ferrous chloride solution and stannic chloride solution are produced.

\[\ce{2FeC l_{3(aq)} + SnC I_{2(aq)} -> 2FeC l_{2(aq)} + SnC I_{4(aq)}}\]

When the positive charge on an ion increases or the negative charge on them decreases it is called as oxidation.

The chemical composition of substance changes during chemical changes i.e., new substances are formed.

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A balanced equation is a symbolic representation of a chemical reaction where the number of atoms for each element is identical on both the reactant (left) and product (right) sides.

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The chemical reactions which are accompanied by absorption of heat are called endothermic reactions.

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Zinc is more reactive than copper.

\[\ce{\underset{\text{sulphate}}{\underset{\text{Copper}}{CuSO_{4(aq)}}} + \underset{\text{Zinc}}{Zn_{(s)}} -> \underset{\text{sulphate}}{\underset{\text{Zinc}}{ZnSO_{4(aq)}}} + \underset{\text{Copper}}{Cu_{(s)}}}\]

The blue colour of copper sulphate solution fades due to the formation of colourless zinc sulphate.

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Grills of doors and windows are generally made up of iron metal.

Before painting, iron metal is in direct contact with the moisture in the atmosphere.

Moist air attacks the surface of iron metal and causes rusting (i.e., corrosion of iron).

Contact between iron and moist air can be avoided by applying paint on grills of doors and windows. This prevents rusting and protects them from damage.

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Step I: Write the chemical equation:

For example, \[\ce{H2S2O_{7(l)} + H2O_{(l)} -> H2SO_{4(l)}}\]

Step II: Compare the number of atoms of the various elements present on the two sides of the equation.

Step III: Choose the reactant or product having a maximum number of atoms and equalize its atoms on both sides. Equalize the number of atoms of remaining elements in the reactants and products. H2S2O7 has a maximum number of atoms.

First balance ‘S’ as it requires a smaller factor.

Apply a factor ‘2’ to the product ‘H2SO4’.

\[\ce{H2S2O_{7(l)} + H2O_{(l)} -> 2H2SO_{4(l)}}\]

Check whether the equation is balanced.

The number of atoms of the elements in the reactants is same as the number of atoms of those elements in the product.

Writing a chemical reaction in brief by using chemical formulae is called as chemical equation.