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(i) (a) and (b)

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(d) The given reaction is an example of a displacement reaction.

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(a) Hydrogen gas and iron chloride are produced. The reaction is as follows:

$ Fe _{(s)}+2 HCl _{(a q)} \longrightarrow FeCl _{2(a q)}+H_2 \uparrow $

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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.

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(a) $3 H _{2(g)}+N _{2(g)} \longrightarrow 2 NH _{3(g)}$

(b) $2 H_2 S _{(g)}+3 O _{2(g)} \longrightarrow 2 H_2 O _{(l)}+2 SO _{2(g)}$

(c) $3 \mathrm{BaCl} _{2(a q)}+\mathrm{Al} _2\left(\mathrm{SO} _4\right) _{3(a q)} \longrightarrow 2 \mathrm{AlCl} _{3(a q)}+3 \mathrm{BaSO} _{4(s)}$

(d) $2 K _{(s)}+2 H_2 O _{(l)} \longrightarrow 2 KOH _{(a q)}+H _{2(g)}$

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(a) $2 HNO_3+Ca(OH)_2 \longrightarrow Ca(NO_3)_2+2 H_2 O$

(b) $2 NaOH+H_2 SO_4 \longrightarrow Na_2 SO_4+2 H_2 O$

(c) $NaCl+AgNO_3 \longrightarrow AgCl+NaNO_3$

(d) $BaCl_2+H_2 SO_4 \longrightarrow BaSO_4+2 HCl$

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(a) $Ca(OH)_2+CO_2 \longrightarrow CaCO_3+H_2 O$

(b) $Zn+2 AgNO_3 \longrightarrow Zn(NO_3)_2+2 Ag$

(c) $2 Al+3 CuCl_2 \longrightarrow 2 AlCl_3+3 Cu$

(d) $BaCl_2+K_2 SO_4 \longrightarrow BaSO_4+2 KCl$

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Chemical reactions that release energy in the form of heat, light, or sound are called exothermic reactions.

Example: Mixture of sodium and chlorine to yield table salt

$ Na _{(s)}+\dfrac{1}{2} Cl _{2(s)} \longrightarrow NaCl _{(s)}+411 kJ \text{ of energy } $

In other words, combination reactions are exothermic.

Reactions that absorb energy or require energy in order to proceed are called endothermic reactions.

For example: In the process of photosynthesis, plants use the energy from the sun to convert carbon dioxide and water to glucose and oxygen.

$ 6 CO _{2(g)}+6 H_2 O _{(l)} \xrightarrow{\text{ Sunlight }} \underset{\text{Glucose}}{C_6 H _{12} O _{6(a q)}}+6 O _{2(g)} $

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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.

$\underset{\text { Glucose }}{\mathrm{C} _6 \mathrm{H} _{12} \mathrm{O} _{6(a q)}}+\underset{\text { Oxygen }}{6 \mathrm{O} _{2(g)}} \longrightarrow \underset{\text { Carbon dioxide }}{6 \mathrm{CO} _{2(g)}}+\underset{\text{water} } {6 \mathrm{H} _2 {\mathrm{O} _{(l)}}}+\text { Energy }$

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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.

Decomposition reaction: $AB+$ Energy $\longrightarrow A+B$

$2 H_2 O _{(l)} \xrightarrow{\text{ Electrolysis }} 2 H _{2(g)}+O _{2(g)}$

Combination reaction: $A+B \longrightarrow AB+$ Energy

$2 H _{2(g)}+O _{2(g)} \longrightarrow 2 H_2 O _{(f)}+$ Energy

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(a) Thermal decomposition:

$\underset{\text { Ferric oxide }}{2 \mathrm{FeSO} _{4(s)} \xrightarrow{\Delta}} \underset{\text { Sulphur dioxide }}{\mathrm{Fe} _2 \mathrm{O} _{3(s)}}+\underset{\text { Sulphur trioxide }}{\mathrm{SO} _{2(g)}}+\underset{\text { SOlphate }}{\mathrm{SO} _{3(g)}}$

(b) Decomposition by light:

$ \underset {\text{Silver chloride}}{2 \mathrm{AgCl} _{(s)}}\xrightarrow{\text { Light }} \underset {\text{Silver}}{2 \mathrm{Ag} _{(s)}}+\underset{\text{Chlorine}}{\mathrm{Cl} _{2(g)}} $

(c) Decomposition by electricity:

$$\begin{aligned} & 2 \mathrm{Al} _2 \mathrm{O} _{3(\mathrm{aq})} \xrightarrow{\text { Electricity }} 4 \mathrm{Al} _{(s)}+3 \mathrm{O} _{2(\mathrm{~g})} \\ & \text { Aluminium oxide } \quad \text { Aluminium } \quad \text { Oxygen } \\ &\end{aligned}$$

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In a displacement reaction, a more reactive element replaces a less reactive element from a compound.

$A+BX \longrightarrow AX+B$; where $A$ is more reactive than B

In a double displacement reaction, two atoms or a group of atoms switch places to form new compounds.

$ AB+CD \longrightarrow AD+CB $

For example:

Displacement reaction:

$ CuSO _{4(a q)}+Zn _{(s)} \longrightarrow ZnSO _{4(a q)}+Cu _{(s)} $

Double displacement reaction:

$ Na_2 SO _{4(a q)}+BaCl _{2(a q)} \longrightarrow BaSO _{4(s)}+2 NaCl _{(\alpha q)} $

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$\underset{\text{Silver nitrate}}{2 AgNO _{3(a q)}}+ \underset{\text{Copper}} {Cu _{(s)}} \rightarrow \underset{\text{Copper nitrate}}{Cu(NO_3) _{2(a q)}}+\underset{\text{Silver}}{2 Ag _{(s)}}$

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A reaction in which an insoluble solid (called precipitate) is formed is called a precipitation reaction.

For example:

$ Na_2 CO _{3(a q)}+CaCl _{2(a q)} \longrightarrow CaCO _{3(s)}+2 NaCl _{(a q)} $

Sodium carbonate Calcium chloride Calcium carbonate Sodium chloride

In this reaction, calcium carbonate is obtained as a precipitate. Hence, it is a precipitation reaction.

Another example of precipitation reaction is:

$\underset{\text { Sodium sulphate }}{\mathrm{Na} _2 \mathrm{SO} _{4(a q)}}+\underset{\text { Barium chloride }}{\mathrm{BaCl} _{2(a q)}} \longrightarrow \underset{\text { Barium sulphate }}{\mathrm{BaSO} _{4(s)}}+\underset{\text { Sodium chloride }}{2 \mathrm{NaCl} _{(a q)}}$

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

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For example:

(i) $ \mathrm{CO} _2+\underbrace{\mathrm{H} _2 \longrightarrow \mathrm{CO}+\mathrm{H} _2 \mathrm{O}} _{\text {Addition of oxgyen - oxidation }} $

(ii) $\underbrace{2 \mathrm{Cu}+\mathrm{O} _2 \longrightarrow 2 \mathrm{CuO}} _{\text {Gain of oxgyen - oxidation }}$

In equation (i), $H 2$ is oxidized to $H 2 O$ and in equation (ii), $Cu$ is oxidised to $CuO$.

(b) Reduction is the loss of oxygen.

For example:

(i) $ \underbrace{\mathrm{CO} _2+\mathrm{H} _2 \longrightarrow \mathrm{CO}} _{\text{Removal of oxgyen - reduction}}+\mathrm{H} _2 \mathrm{O} $

(ii) $ \underbrace{\mathrm{CuO}+\mathrm{H} _2 \xrightarrow{\Delta} \mathrm{Cu}} _{\text{Loss of oxgyen - reduction }}+\mathrm{H} _2 \mathrm{O} $

In equation (i), $CO 2$ is reduced to $CO$ and in equation (ii), $CuO$ is reduced to $Cu$.

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’ $X$ ’ 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.

$\underset{\text{Shiny brown in colour}} {2Cu}+ O_2 \xrightarrow{Heat} \underset{\text{Black in color}}{2CuO}$

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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 their presence is essential for rusting to take place.

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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 to remove oxygen inside the pack. When oxygen is not present inside the pack, rancidity of oil and fat containing food items is avoided.

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(a) Corrosion: Corrosion is defined as a process where materials, usually metals, deteriorate as a result of a chemical reaction with air, moisture, chemicals, etc.

For example, iron, in the presence of moisture, reacts with oxygen to form hydrated iron oxide.

$ \begin{aligned} & 4 Fe+3 O_2+n H_2 O \longrightarrow 2 Fe_2 O_3 \cdot n H_2 O \\ & \text{ Hydrated iron oxide } \end{aligned} $

This hydrated iron oxide is rust.

(b) Rancidity:

The process of oxidation of fats and oils that can be easily noticed by the change in taste and smell is known as rancidity.

For example, the taste and smell of butter changes when kept for long.

Rancidity can be avoided by:

1. Storing food in air tight containers

2. Storing food in refrigerators

3. Adding antioxidants

4. Storing food in an environment of nitrogen