Class 10 Science Chapter 1 Chemical Reactions and Equations

Key Concept: Reaction Condition Justification, Energy Source Identification

b) To increase the electrical conductivity of water
[Solution Description] The electrolysis of water involves decomposition through electricity, but pure water is a poor conductor of electricity. Adding an electrolyte like dilute sulfuric acid increases the conductivity of the solution, allowing electric current to pass through and decompose water into hydrogen and oxygen gases:
$ 2H_2O(l) \rightarrow{Electricity} 2H_2(g) + O_2(g)$
The presence of ions from the sulfuric acid facilitates the flow of electric current.

Key Concept: Complex Reaction Analysis, Advanced Concept Application

b) Aluminum is oxidized from 0 to +3
[Solution Description] Aluminum starts at an oxidation state of 0 in $2Al$. During the reaction, it forms $Al_2O_3$ where the aluminum has an oxidation state of +3. This means that aluminum is oxidized from 0 to +3.

Key Concept: Real-world Problem Solving, Advanced Concept Application

b) Using dilute acetic acid followed by a wax coating
[Solution Description] The green coating on copper is primarily due to the formation of basic copper carbonate, known as patina, which occurs when copper reacts with atmospheric carbon dioxide and moisture. To clean and prevent further corrosion, we must remove the patina and create a protective barrier. Cleaning can be done using dilute acetic acid (vinegar), which dissolves the patina without damaging the underlying metal. To prevent future oxidation, applying a sealant like a wax coating or lacquer can protect the surface from air and moisture.

Key Concept: Heat Release Recognition

a) $CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(g)$

[Solution Description] The given reactions must be analyzed for heat release. Option a) describes a combustion of methane which is known to release heat as it forms carbon dioxide and water vapor. Hence, option a) is an exothermic reaction.

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Key Concept: Coefficient Application

b) 2
[Solution Description] The coefficient of $H_2O$ is 2, as shown in the balanced equation.

Key Concept: Skeletal Equation

a) Skeletal
[Solution Description] In this equation, on the left: N=2, H=2; on the right: N=1, H=3. The counts of atoms are not equal, indicating the equation is skeletal (unbalanced).

Key concept: Physical States

b) Liquid
[Solution Description] In the given reaction, $H_2O$ is shown with the notation (l), indicating that it is in the liquid state.

Key Concepts: Balancing Steps

c) Balance carbon atoms

[Solution Description]

First, balance the carbon atoms by adjusting the coefficient of $CO_2$. Since there are three carbon atoms in $C_3H_8$, place a coefficient of 3 in front of $CO_2$: $C_3H_8 + O_2 \rightarrow 3CO_2 + H_2O$.

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Key Concept: Effects of Rancidity

b) Changes in smell and taste
[Solution Description] Rancidity affects food by altering its smell and taste due to the oxidative breakdown of fats and oils. This change impacts the overall quality and acceptability of the food product.

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Key Concepts: Coefficient Identification

b) 2

[Solution Description] To balance the equation $CH_4 + O_2 \rightarrow CO_2 + 2H_2O$, we require 2 molecules of $O_2$ to provide 4 oxygens for 2 $H_2O$ and 2 additional for 1 $CO_2$, thus needing a coefficient of 2 for $O_2$.

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Key Concept: Physical States

c) Gas
[Solution Description] The physical state of $CO_2$ is gaseous, indicated by the symbol $(g)$.

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Key Concept: Reaction Mechanism, Real-World Application

b) It is a decomposition reaction forming CaO and $CO_2$.

[Solution Description]

Heating calcium carbonate (limestone) results in thermal decomposition, producing calcium oxide (quicklime) and carbon dioxide gas. The reaction can be represented as:

$\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2$

This reaction is endothermic as it involves breakdown due to heating and is commonly used in industries to produce lime for construction and other purposes.

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Key Concept: Reaction Prediction, Reaction Explanation

c) Magnesium displaces silver because it has higher reactivity
[Solution Description] Magnesium is more reactive than silver according to the reactivity series. Therefore, it will displace silver from its compound. The reaction can be represented as:
$\text{Mg(s)} + 2\text{AgNO}_3\text{(aq)} \rightarrow \text{Mg(NO}_3\text{)}_2\text{(aq)} + 2\text{Ag(s)}$
This occurs because magnesium has lower Gibbs free energy compared to silver, making the displacement thermodynamically favourable.

Key Concept: Chemical Reaction Analysis, Corrosion Mechanism

a) $4Fe + 3O_2 + 6H_2O \rightarrow 4Fe(OH)_3$
[Solution Description] The rusting of iron involves the reaction of iron with oxygen in the presence of water. This can be represented by the overall reaction:
$4Fe + 3O_2 + 6H_2O \rightarrow 4Fe(OH)_3$
Initially, iron reacts with water and oxygen to form hydrated iron(III) oxide, commonly known as rust.

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Key Concepts: Rancidity Identification

b) Rancidity

[Solution Description] When oils become oxidized, they develop an unpleasant taste and smell, which is referred to as rancidity.

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Key Concept: Complex Reaction Analysis, Thermodynamic Consideration

a) $C(s) + O_2(g) \rightarrow CO_2(g)$
[Solution Description] A combination reaction occurs when two or more reactants combine to form a single product. For a reaction to be spontaneous and exothermic, it must release energy in the form of heat. Among the given reactions, $C(s) + O_2(g) \rightarrow CO_2(g)$ is a classic example of an exothermic combination reaction. This process releases a significant amount of energy, making it spontaneous under standard conditions.

Key Concept: Complex Reaction Analysis, Non-Straightforward Path

a) $Pb(NO_3)_2(aq) + 2KI(aq) \rightarrow PbI_2(s) + 2KNO_3(aq)$
[Solution Description]
When $Pb(NO_3)_2(aq)$ is mixed with $KI(aq)$, a double displacement reaction occurs:

$Pb(NO_3)_2(aq) + 2KI(aq) \rightarrow PbI_2(s) + 2KNO_3(aq)$

This shows that lead(II) iodide $(PbI_2)$ precipitates out as a solid while potassium nitrate remains in aqueous form.

Key Concepts: Real-life Application

c) Whitewashing with slaked lime

[Solution Description] Whitewashing with slaked lime involves a combination reaction where calcium oxide and water form calcium hydroxide, which is used in whitewashing.

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Key Concept: Identify Balanced Equation

a) Yes
[Solution Description] To determine if an equation is balanced, count the number of atoms of each element on both sides:

– Left side: $N_2$ has 2 nitrogen atoms; $3H_2$ has 6 hydrogen atoms.
– Right side: $2NH_3$ has 2 nitrogen atoms and 6 hydrogen atoms.

Since there are an equal number of nitrogen and hydrogen atoms on both sides, the equation is balanced.

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Key Concept: Corrosion Mechanism

b) Copper
[Solution Description] The green coating on corroded copper is due to the formation of basic copper carbonate, commonly known as patina. This process occurs when copper reacts with moisture and carbon dioxide in the air.

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Key Concept: Observable Changes

c) Change in taste and smell
[Solution Description]
When milk sours, it undergoes fermentation due to bacterial action. This results in a noticeable change in taste and smell, indicating a chemical change.

Key Concept: Reaction Analysis, Complex Balancing

c) 5

[Solution Description] Begin by balancing carbon atoms. Propane contains 3 carbons, therefore three $\text{CO}_2$ molecules are needed:

$C_3H_8(g) + O_2(g) \rightarrow 3CO_2(g) + H_2O(l)$

Next, balance hydrogen atoms. Propane has 8 hydrogens, so 4 water molecules are required:

$C_3H_8(g) + O_2(g) \rightarrow 3CO_2(g) + 4H_2O(l)$

Now, count the total oxygen atoms required; 3 $\times$ 2 from $\text{CO}_2$ and 4 from $\text{H}_2O$, totaling 10 oxygens. Therefore, 5 $\text{O}_2$ molecules are needed:

$C_3H_8(g) + 5O_2(g) \rightarrow 3CO_2(g) + 4H_2O(l)$

Hence, the coefficient of $O_2$ is 5.

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