chemical reactions & chapter 1 equationsdirector: sushant arora (bca, pgdca, mca) 9818753877...

CHAPTER 1

CHEMICAL REACTIONS & EQUATIONS Sushant Arora

Director: Sushant Arora (BCA, PGDCA, MCA)

9818753877

Chemical Reactions & Equations

Chemical Reaction: A chemical reaction is a process in which one or more reactants undergo a chemical

change. Example, burning of Magnesium Ribbon in air.

**Chemical reactions involve the breaking and making of bonds between atoms to produce new

substances.

Q. What happens when we burn magnesium ribbon?

When we burn magnesium ribbon in the presence of air, it burns with a white dazzling flame and gets

converted into a white powder. This powder is magnesium oxide which is formed due to the reaction between

magnesium and oxygen present in air.

2Mg + O2 → 2MgO

Q. How do we know that a chemical reaction has taken place?

A chemical reaction has taken place if any of the following observations is seen:

1. Change in state

2. Change in colour

3. Evolution of a gas

4. Change in temperature

Example. When we add zinc granules to a test tube containing hydrochloric acid or sulphuric acid, the

following reaction takes place:

Zn + 2HCl → ZnCl2 + H2

Similarly,

Zn + H2SO4 → ZnSO4 + H2

In both the reactions hydrogen is liberated which may be tested by bringing a burning candle near the mouth

of the test tube. A pop sound indicates the presence of hydrogen.

Define:

Reactants: The substances that undergo chemical change in a reaction.

Products: The new substance which is formed as a result of the reaction is known as product.

2Mg + O2 → 2MgO

In this reaction, Magnesium and oxygen are the reactants and Magnesium oxide is the product.

Q. What is a chemical equation?


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A chemical equation is a way to represent the chemical reactions.

Eg. 2Mg + O2 → 2MgO

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NOTE:

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The number of atoms of each element must be the same on both the sides. If not, then the equation is

unbalanced because the mass is not the same on both sides of the equation.

Such unbalanced chemical equation is called as a skeletal chemical equation for a reaction.

Mg + O2 → MgO

On left hand side (Reactants Side) the number of oxygen atoms is 2 whereas on the right hand side(Product

Side) the number of oxygen atoms is just 1.

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Balanced Chemical Equations

Need to balance a chemical equation?

The total mass of the elements present in the products of a chemical reaction has to be equal to the total

mass of the elements present in the reactants. In other words, the number of atoms of each element remains

the same, before and after a chemical reaction. Hence, we need to balance skeletal chemical equation.

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

Magnesium is an extremely 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 by sand paper for removing this layer so that the underlying

metal can be exposed to air.

Q. Write the balanced equation for the following chemical reactions.

i. Hydrogen + Chlorine → Hydrogen chloride

H2 + Cl2 → 2HCl

ii. Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride

3BaCl2 + Al2(SO4)3 → 3BaSO4 + 2AlCl3


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iii. Sodium + Water → Sodium hydroxide + Hydrogen

2Na + 2H2O → 2NaOH + H2

Q. Write a balanced chemical equation with state symbols for the following reactions.

• Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and

the solution of sodium chloride.

• Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce

sodium chloride solution and water.

TYPES OF CHEMICAL REACTIONS

1. Combination Reaction

2. Decomposition Reaction

3. Displacement Reaction

4. Double Displacement Reaction

5. Redox Reactions

Combination Reaction

Combination Reaction is a reaction in which two or more reactants combine or react with each other to form

a single product.

Eg. Calcium oxide reacts vigorously with water to produce slaked lime

(calcium hydroxide) releasing a large amount of heat.

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A solution of slaked lime produced in the above reaction is used for white washing walls. Calcium hydroxide

reacts slowly with the carbon dioxide in air to form a thin layer of calcium carbonate on the walls. Calcium

carbonate is formed after two to three days of white washing and gives a shiny finish to the walls.

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Some more examples of combination reaction:

1. Burning of coal


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2. Formation of water from H2 and O2.

Q. What are exothermic and endothermic reactions?

Exothermic Reactions: These are the reactions in which a huge amount of heat energy is released along with

the formation of products.

E.g. Burning of natural gas

Respiration

The decomposition of vegetable matter into compost is also an example of an exothermic reaction.

Endothermic Reactions: These are the reactions in which heat energy is absorbed along with the formation of

products.

E.g. Photosynthesis

Decomposition Reaction

Decomposition Reaction is a reaction in which a single reactant breaks down to give simpler products.

Ferrous sulphate crystals (FeSO4, 7H2O) lose water when heated and the green colour of the crystals changes.

It then decomposes to ferric oxide (Fe2O3) which is reddish-brown in colour, sulphur dioxide (SO2) and sulphur

trioxide (SO3). Ferric oxide is a solid while SO2 and SO3 are gases.


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Decomposition of calcium carbonate to calcium oxide and carbon dioxide on heating is an important

decomposition reaction used in various industries. Calcium oxide is called lime or quick lime. It is used to

manufacture cement.

When lead nitrate powder is heated over flame then you will observe emission of brown fumes. These fumes

are of nitrogen dioxide (NO2).

Q. What is a thermal decomposition reaction?

When a decomposition reaction is carried out by heating, it is called thermal decomposition.

Q. Explain electrolysis of water with the help of a diagram.

1. Take a plastic mug. Drill two holes at its base and fit rubber stoppers in these holes. Insert graphite

electrodes in these rubber stoppers.

2. Connect these electrodes to a 6 volt battery. 3. Fill the mug with water such that the electrodes are immersed. Add a few drops of dilute sulphuric acid

to the water. 4. Take two test tubes filled with water and invert them over the two carbon electrodes. 5. Switch on the current and leave the apparatus undisturbed for some time. 6. Bubbles would be formed at both the electrodes signifying formation of gas.


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The following reaction has taken place: 2H2O → 2H2 + O2

Hydrogen gas is liberated at cathode and Oxygen gas is liberated at anode.

Q. What happens when silver chloride is kept in sunlight? White coloured silver chloride decomposes to form grey coloured silver and chlorine gas is liberated.

This reaction is used in black and white photography. Q. A solution of a substance ‘X’ is used for white washing.

I. Name the substance ‘X’ and write its formula. The substance ‘X’ is calcium oxide. Its chemical formula is CaO.

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

Q. Why is the amount of gas collected in one of the test tubes in electrolysis of water double of the amount collected in the other? Name this gas. Water (H2O) contains two parts hydrogen and one part oxygen. Therefore, the amount of hydrogen and oxygen produced during electrolysis of water is in a 2:1 ratio.

Displacement Reaction Displacement Reaction is a reaction in which a highly reactive metal displaces a less reactive metal from its salt solution. If Iron nails are immersed in blue coloured copper sulphate solution, the following reaction takes place:

Iron nail becomes brownish in colour due to the deposition of copper over its surface. Other examples of displacement reactions are:

Double Displacement Reaction Reactions in which there is an exchange of ions between the reactants are called double displacement reactions.


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The white precipitate of BaSO4 is formed by the reaction of SO42- and Ba2+. The other product formed is

sodium chloride which remains in the solution. Example

In this reaction, sodium carbonate and calcium chloride exchange ions to form two new compounds. Hence, it is a double displacement reaction Q. What do you mean by a precipitation reaction? Explain by giving examples. A reaction in which an insoluble solid (called precipitate) is formed is called a precipitation reaction. For example

In this reaction, calcium carbonate is obtained as a precipitate. Hence, it is a precipitation reaction. Another example of precipitation reaction is:

In this reaction, barium sulphate is obtained as a precipitate. ________________________________________________________________________

Oxidation and Reduction Oxidation is a reaction in which oxygen is added to a substance. OR It is a reaction in which hydrogen is removed from a substance. In other words, addition of oxygen or removal of hydrogen to/from a substance is termed as oxidation. Reduction is the process of removal of oxygen from a substance or addition of hydrogen to a substance. Example when we heat copper powder in a test tube, it reacts with oxygen and forms black coloured copper oxide.

The above reaction is an example of oxidation since, oxygen is added to copper. If hydrogen gas is passed over this heated material (CuO), the black coating on the surface turns brown as the reverse reaction takes place and copper is obtained.

In the above reaction, copper(II) oxide is losing oxygen and is being reduced. The hydrogen is gaining oxygen and is being oxidised. In other words, one reactant gets oxidised while the other gets reduced during a reaction. Such reactions are called oxidation-reduction reactions or redox reactions.


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Some other examples of redox reactions are:

Oxidizing Agent: The substance that oxidizes a reactant and itself gets reduced. Reducing Agent: The substance that accepts oxygen from the oxidizing agent and gets oxidized. Substance Oxidized: If a substance gains oxygen or loses hydrogen during a reaction, it is said to be oxidised. Substance Reduced: If a substance loses oxygen or gains hydrogen during a reaction, it is said to be reduced.

Corrosion When a metal is attacked by substances around it such as moisture, acids, etc., it is said to corrode and this process is called corrosion. Rusting of Iron is the process in which an iron article when attacked by substances such as moisture, acids forms a reddish brown layer over it. This reddish brown layer is called as Rust (Ferric Oxide) and the process of its formation is termed as Rusting. Examples of corrosion

1. Copper reacts with carbon dioxide present in the atmosphere to form a greenish layer of copper carbonate over its surface.

2. Similarly, silver reacts with sulphur present in the air to form a layer of silver sulphide which is black in colour.

**Corrosion causes damage to car bodies, bridges, iron railings, ships and to all objects made of metals, especially those of iron.

Rancidity When any fat or oil containing food item is kept in open, it reacts with oxygen present in the air and is said to be oxidized. Due to this reason the taste and smell of the food item changes and it becomes rancid. Q. How can be rancidity prevented? Rancidity can be prevented by the following:

1. Antioxidants might be added to the food item which will prevent oxidation and further prevent the food from getting rancid.

2. Food item shall be kept in air tight containers to slow down oxidation.

Q. What are antioxidants?

The substances which prevent oxidation are known as antioxidants. E.g. Nitrogen

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

When an iron nail is placed in a copper sulphate solution, iron displaces copper from copper sulphate solution

forming iron sulphate, which is green in colour.


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Therefore, the blue colour of copper sulphate solution fades and green colour appears.

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

A reaction which has an equal number of atoms of all the elements on both sides of the chemical equation is

called a balanced chemical equation.

The law of conservation of mass states that mass can neither be created nor destroyed. Hence, in a chemical

reaction, the total mass of reactants should be equal to the total mass of the products. It means that the total

number of atoms of each element should be equal on both sides of a chemical equation. Hence, it is for this

reason that chemical equations should be balanced.

Q. 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.

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

Energy is required to support life. Energy in our body is obtained from the food we eat. During digestion, large

molecules of food are broken down into simpler substances such as glucose. Glucose combines with oxygen in

the cells and provides energy. The special name of this combustion reaction is respiration. Since energy is

released in the whole process, it is an exothermic process.

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

reactions.

Decomposition reactions are those in which a compound breaks down to form two or more substances. These

reactions require a source of energy to proceed. Thus, they are the exact opposite of combination reactions in

which two or more substances combine to give a new substance with the release of energy.


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Combination Reaction: C+O2 → CO2

Decomposition Reaction: CO2 → C + O2

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

electricity.

Thermal Decomposition

Decomposition by light

Decomposition by electricity

2Al2O3 → 4Al + 3O2

Q. In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper

metal. Write down the reaction involved.

Q. Explain the following in terms of gain or loss of oxygen with two examples each.

(a) Oxidation

(b) Reduction

a. Oxidation is the gain of oxygen.

For Example

In equation (i), H2 is oxidized to H2O and in equation (ii), Cu is oxidised to CuO.

b. Reduction is the loss of oxygen.

For example:


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i.

ii.

In equation (i), CO2 is reduced to CO and in equation (ii), CuO is reduced to Cu.

Q. A shiny brown-coloured element ‘X’ on heating in air becomes black in colour. Name the element ‘X’ and

the black coloured compound formed.

‘X’ is copper (Cu) and the black-coloured compound formed is copper oxide (CuO). The equation of the

reaction involved on heating copper is given below.

Q. Why do we apply paint on iron articles?

Iron articles are painted because it prevents them from rusting. When painted, the contact of iron articles

from moisture and air is cut off. Hence, rusting is prevented. So presence of air and moisture is essential for

rusting to take place.

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

Nitrogen is an inert gas and does not easily react with these substances. On the other hand, oxygen reacts

with food substances and makes them rancid. Thus, bags used in packing food items are flushed with nitrogen

gas which act as an antioxidant and prevents the contact between food item and oxygen and hence rancidity

of oil and fat containing food items is avoided.

Q. Balance the following chemical equations.

(a)

(b)

(c)

(d)


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Q. 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

Q. 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)

b) Zinc 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) + Hydrogen(g)

chemical reactions & chapter 1 equationsdirector: sushant arora (bca, pgdca, mca) 9818753877...

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