Chemical Reactions and Equations — Class 10 Science

Why must a chemical equation be balanced?

Because matter is conserved — atoms are neither created nor destroyed, so the number of atoms of each element must be equal on both sides.

Why is respiration considered an exothermic reaction?

Because the breakdown of glucose during respiration releases energy, which the body uses to stay warm and do work.

What is a chemical reaction? Give one everyday example.

A change in which reactants are converted into new substances (products) with different properties; for example, the rusting of iron or the souring of milk.

What information does a balanced chemical equation give?

It shows the reactants and products, their physical states, and the relative number of atoms/molecules, while obeying the law of conservation of mass.

Define a combination reaction with an example.

A reaction in which two or more substances combine to form a single product, e.g. calcium oxide reacting with water to form calcium hydroxide.

What is a decomposition reaction? Name its three types.

A reaction in which a single compound breaks into two or more simpler substances, brought about by heat (thermal), light (photolytic) or electricity (electrolytic).

Distinguish between displacement and double displacement reactions.

In displacement, a more reactive element replaces a less reactive one from its compound; in double displacement, two compounds exchange ions, often forming a precipitate.

What are oxidation and reduction in terms of oxygen and hydrogen?

Oxidation is the gain of oxygen or loss of hydrogen; reduction is the loss of oxygen or gain of hydrogen. Both occur together in a redox reaction.

What is corrosion, and how can rusting of iron be prevented?

Corrosion is the slow eating away of a metal by reaction with air and moisture; rusting is prevented by painting, oiling, greasing, galvanising or coating with a less reactive metal.

What is rancidity, and how can it be slowed down?

Rancidity is the oxidation of fats and oils in food, spoiling its taste and smell; it is slowed by adding antioxidants, refrigeration, airtight packing or flushing with nitrogen.

What do you observe?

The magnesium ribbon burns with a dazzling white flame and changes into a white powder of magnesium oxide. The reaction is

What do you observe?

A bright yellow precipitate of lead iodide is formed when lead nitrate solution is mixed with potassium iodide solution. The reaction is

Do you observe anything happening around the zinc granules?

Bubbles of hydrogen gas are produced around the zinc granules because zinc reacts with the dilute acid. With dilute sulphuric acid, the reaction is

Touch the conical flask or test tube. Is there any change in its temperature?

Yes. The conical flask or test tube becomes warm because the reaction releases heat and is exothermic.

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

A magnesium ribbon develops a thin coating of magnesium oxide on exposure to air. It is cleaned with sandpaper to remove this coating so that magnesium can burn readily in oxygen.

2. 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

3. Write a balanced chemical equation with state symbols for the following reactions. (i) Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride.

(ii) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.

Do you feel any change in temperature?

Yes. The beaker becomes hot because calcium oxide reacts vigorously with water and releases heat.

Note the colour of the ferrous sulphate crystals.

The ferrous sulphate crystals are pale green.

Observe the colour of the crystals after heating.

The crystals first lose water and become whitish. On stronger heating, a brown solid of ferric oxide is formed.

What do you observe? Note down the change, if any.

Brown fumes of nitrogen dioxide are evolved, a yellow residue of lead oxide remains, and oxygen gas is also produced.

Is the volume of the gas collected the same in both the test tubes?

No. The volume of hydrogen collected is twice the volume of oxygen because water contains hydrogen and oxygen in a ratio.

What happens in each case?

The gas collected at the cathode burns with a popping sound, showing that it is hydrogen. The gas collected at the anode supports combustion and makes the candle flame burn more brightly, showing that it is oxygen.

Which gas is present in each test tube?

Hydrogen gas is collected at the cathode, while oxygen gas is collected at the anode.

What is its colour?

Silver chloride is white.

Observe the colour of the silver chloride after some time.

The white silver chloride turns grey because sunlight decomposes it into silver and chlorine gas.

Take about 2 g barium hydroxide in a test tube. Add 1 g of ammonium chloride and mix with the help of a glass rod. Touch the bottom of the test tube with your palm. What do you feel? Is this an exothermic or endothermic reaction?

The bottom of the test tube feels cold because the reaction absorbs heat from its surroundings. It is an endothermic reaction.

1. A solution of a substance ‘X’ is used for whitewashing. (i) Name the substance ‘X’ and write its formula.

The substance ‘X’ is calcium oxide, also called quicklime. Its formula is . Its reaction with water produces the calcium hydroxide solution used for whitewashing.

(ii) Write the reaction of the substance ‘X’ named in (i) above with water.

2. Why is the amount of gas collected in one of the test tubes in Activity 1.7 double of the amount collected in the other? Name this gas.

During electrolysis, water decomposes according to Therefore, hydrogen and oxygen are produced in a volume ratio of . The gas collected in double the amount is hydrogen.

Compare the intensity of the blue colour of copper sulphate solutions in test tubes (A) and (B).

The solution in test tube A remains blue, while the blue colour in test tube B becomes lighter because iron displaces copper from copper sulphate and forms pale-green iron sulphate.

Also, compare the colour of the iron nails dipped in the copper sulphate solution with the one kept aside.

The iron nails dipped in copper sulphate solution acquire a brown coating of deposited copper. The nail kept aside retains its original grey appearance.

What do you observe?

A white insoluble precipitate of barium sulphate is formed when sodium sulphate and barium chloride solutions are mixed.

(i) What was the colour of the precipitate formed? Can you name the compound precipitated?

A bright yellow precipitate was formed. The precipitated compound was lead iodide, .

(ii) Write the balanced chemical equation for this reaction.

(iii) Is this also a double displacement reaction?

Yes. It is a double displacement reaction because the ions of lead nitrate and potassium iodide exchange partners to form lead iodide and potassium nitrate. It is also a precipitation reaction.

What do you observe?

The reddish-brown copper powder becomes coated with a black layer of copper(II) oxide because copper combines with oxygen.

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

Iron is more reactive than copper, so it displaces copper from copper sulphate solution and forms iron sulphate. The blue colour fades because iron sulphate solution is pale green, while copper is deposited on the nail.

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

An example is the reaction between lead nitrate and potassium iodide solutions: The reactant ions exchange partners, and a yellow precipitate of lead iodide is formed.

3. Identify the substances that are oxidised and the substances that are reduced in the following reactions. (i)

Sodium is oxidised because it gains oxygen. Oxygen is reduced because it combines with sodium and gains electrons.

(ii)

Hydrogen is oxidised because it gains oxygen to form water. Copper(II) oxide is reduced because it loses oxygen to form copper.

1. Which of the statements about the reaction below are incorrect? (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

The incorrect statements are (a) and (b), so the correct option is (i). Lead oxide, not lead, is reduced to lead, while carbon is oxidised to carbon dioxide.

2. The above reaction is an example of a (a) combination reaction. (b) double displacement reaction. (c) decomposition reaction. (d) displacement reaction.

The correct answer is (d) displacement reaction. Aluminium displaces iron from iron(III) oxide.

3. 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.

The correct answer is (a) Hydrogen gas and iron chloride are produced.

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

A balanced chemical equation has an equal number of atoms of every element on the reactant and product sides. Chemical equations must be balanced to obey the law of conservation of mass, according to which matter is neither created nor destroyed during a chemical reaction.

5. 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 sulpur 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.

6. Balance the following chemical equations. (a)

(b)

(c)

(d)

7. Write the balanced chemical equations for the following reactions. (a) Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water

(b) Zinc + Silver nitrate → Zinc nitrate + Silver

(c) Aluminium + Copper chloride → Aluminium chloride + Copper

(d) Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride

8. Write the balanced chemical equation for the following and identify the type of reaction in each case. (a) Potassium bromide(aq) + Barium iodide(aq) → Potassium iodide(aq) + Barium bromide(s)

It is a double displacement reaction.

(b) Zinc carbonate(s) → Zinc oxide(s) + Carbon dioxide(g)

It is a thermal decomposition reaction.

(c) Hydrogen(g) + Chlorine(g) → Hydrogen chloride(g)

It is a combination reaction.

(d) Magnesium(s) + Hydrochloric acid(aq) → Magnesium chloride(aq) + Hydrogen(g)

It is a displacement reaction.

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

Exothermic reactions release heat into their surroundings. For example, Endothermic reactions absorb energy from their surroundings. For example,

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

Respiration is exothermic because glucose reacts with oxygen in body cells and releases energy needed for life processes.

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

In a combination reaction, two or more reactants form a single product, whereas in a decomposition reaction, one reactant breaks down into two or more products. Combination: Decomposition:

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

Heat: Light: Electricity:

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

In a displacement reaction, a more reactive element replaces a less reactive element from its compound. For example, In a double displacement reaction, two compounds exchange ions to form two new compounds. For example,

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

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

A precipitation reaction is a reaction in which two aqueous solutions react to form an insoluble solid called a precipitate. For example, Here, is a white precipitate. Another example is where is a yellow precipitate.

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

Oxidation is the gain of oxygen by a substance. Examples: in which copper is oxidised, and in which carbon is oxidised.

(b) Reduction

Reduction is the loss of oxygen by a substance. Examples: in which copper(II) oxide is reduced, and in which zinc oxide is reduced.

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

The element ‘X’ is copper, . The black compound formed is copper(II) oxide, .

18. Why do we apply paint on iron articles?

Paint forms a protective layer that prevents iron from coming into contact with oxygen and moisture. This protects the iron article from rusting and corrosion.

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

Nitrogen is relatively unreactive and replaces oxygen inside the package. This slows the oxidation of oils and fats and prevents rancidity.

20. Explain the following terms with one example each. (a) Corrosion

Corrosion is the gradual damage of a metal caused by its reaction with substances such as oxygen, moisture or acids in its surroundings. An example is the formation of reddish-brown rust on iron.

(b) Rancidity

Rancidity is the oxidation of fats and oils that causes an unpleasant smell and taste. For example, potato chips kept exposed to air for a long time may develop a stale smell and taste.

Perform the following activity. Take four beakers and label them as A, B, C and D. Put 25 mL of water in A, B and C beakers and copper sulphate solution in beaker D. Measure and record the temperature of each liquid contained in the beakers above. Add two spatulas of potassium sulphate, ammonium nitrate, anhydrous copper sulphate and fine iron fillings to beakers A, B, C and D respectively and stir. Finally measure and record the temperature of each of the mixture above. Find out which reactions are exothermic and which ones are endothermic in nature.

In beaker A, dissolving potassium sulphate generally causes a small fall in temperature, so the process is endothermic. In beaker B, dissolving ammonium nitrate produces a clear fall in temperature and is endothermic. In beaker C, anhydrous copper sulphate becomes hydrated and releases heat, so the temperature rises and the process is exothermic. In beaker D, iron displaces copper from copper sulphate solution and releases heat, so the reaction is exothermic: