Chapter 15 Chemical Reactions

Summary: Chemical Reaction Theory

1. Choose the best option for the following questions:

2. Give reason:

(i) Hydrogen peroxide is not stored in a transparent vessel.

Reason: Hydrogen peroxide ($H_{2}O_{2}$) is chemically unstable and slowly decomposes into water ($H_{2}O$) and oxygen ($O_{2}$) on its own. This decomposition process is significantly accelerated by exposure to light (photodecomposition). To slow down this reaction and preserve the chemical, it is stored in dark, opaque, or colored bottles to block out light.

(ii) Acid base reaction is also known as neutralization reaction.

Reason: In an acid-base reaction, the characteristic properties of both the acid (due to $H^{+}$ ions) and the base (due to $OH^{-}$ ions) are destroyed or “neutralized.” The $H^{+}$ ion from the acid combines with the $OH^{-}$ ion from the base to form a neutral water molecule ($H_{2}O$). Since the acidic and basic properties are nullified, the reaction is called a neutralization reaction.

(iii) The rate of chemical reaction is different according to type of the reaction.

Reason: The rate of a reaction depends on the nature of the reactants and the complexity of the bond-breaking and bond-forming processes.

Ionic reactions (like acid-base neutralization or double displacement) occur between ions in a solution. These reactions are extremely fast, almost instantaneous, because no complex electron sharing or bond breaking is needed; the ions simply attract and combine.

Molecular (covalent) reactions (like the combustion of methane or the rusting of iron) require the breaking of strong existing covalent bonds and the formation of new ones. This process requires more energy (higher activation energy) and time, making these reactions typically much slower than ionic reactions.

3. Write differences between:

(a) Single and double displacement reaction

Basis of Difference	Single Displacement Reaction	Double Displacement Reaction
Definition	A reaction where a more reactive element displaces a less reactive element from its compound.	A reaction where two compounds exchange their ions (cations and anions) to form two new compounds.
Reactants	Typically involves one element and one compound.	Involves two compounds, usually in an aqueous solution.
General Form	$A + BC \rightarrow AC + B$	$AB + CD \rightarrow AD + CB$
Driving Force	Based on the relative reactivity of the elements (guided by the reactivity series).	Often driven by the formation of an insoluble precipitate, a gas, or a neutral molecule like water.
Example	$Zn(s) + CuSO_{4}(aq) \rightarrow ZnSO_{4}(aq) + Cu(s)$	$AgNO_{3}(aq) + NaCl(aq) \rightarrow AgCl(s) + NaNO_{3}(aq)$

(b) Combination and decomposition reaction

Basis of Difference	Combination Reaction	Decomposition Reaction
Definition	A reaction where two or more simpler substances combine to form a single, more complex product.	A reaction where a single compound breaks down into two or more simpler substances.
Product(s)	Always results in a single product.	Always results in two or more products.
Reactant(s)	Always involves two or more reactants.	Always involves only one reactant.
General Form	$A + B \rightarrow AB$	$AB \rightarrow A + B$
Energy	These reactions are generally exothermic (release heat).	These reactions are generally endothermic (require an input of energy, like heat, light, or electricity).
Example	$C(s) + O_{2}(g) \rightarrow CO_{2}(g)$	$2H_{2}O(l) \xrightarrow{electricity} 2H_{2}(g) + O_{2}(g)$

4. Balance the following chemical reactions and mention their types.

(a) $KClO_{3} \rightarrow KCl + O_{2}$

Balanced Equation: $2KClO_{3} \rightarrow 2KCl + 3O_{2}$

Type: Decomposition Reaction

(b) $CH_{4} + O_{2} \rightarrow CO_{2} + H_{2}O$

Balanced Equation: $CH_{4} + 2O_{2} \rightarrow CO_{2} + 2H_{2}O$

Type: Combustion Reaction (which is a type of oxidation reaction)

(c) $Fe + O_{2} \rightarrow Fe_{2}O_{3}$

Balanced Equation: $4Fe + 3O_{2} \rightarrow 2Fe_{2}O_{3}$

Type: Combination (Addition) Reaction

(d) $Zn + H_{2}SO_{4} \rightarrow ZnSO_{4} + H_{2}$

Balanced Equation: $Zn + H_{2}SO_{4} \rightarrow ZnSO_{4} + H_{2}$ (Already balanced)

Type: Single Displacement Reaction

(e) $Al + H_{2}SO_{4} \rightarrow Al_{2}(SO_{4})_{3} + H_{2}$

Balanced Equation: $2Al + 3H_{2}SO_{4} \rightarrow Al_{2}(SO_{4})_{3} + 3H_{2}$

Type: Single Displacement Reaction

5. Answer the following questions:

(a) What do you mean by a chemical reaction? Explain with an example.

Answer: A chemical reaction is a process that involves the rearrangement of the molecular or ionic structure of substances, as opposed to a change in physical form. In a chemical reaction, substances called reactants are transformed into new substances called products, which have different chemical identities and properties. This transformation involves the breaking of existing chemical bonds and the formation of new ones.

Example: The burning of hydrogen gas in oxygen to form water.

$2H_{2} + O_{2} \rightarrow 2H_{2}O$

In this reaction, the reactants (hydrogen and oxygen) have different properties than the product (water).

(b) What is the rate of chemical reaction? Write any four factors which affect the rate of chemical reaction.

Answer: The rate of a chemical reaction is the speed at which reactants are converted into products over a specific period. It can be measured by how quickly a reactant is consumed or how quickly a product is formed.

Four factors affecting the rate of chemical reaction:

1. Temperature: Increasing the temperature generally increases the reaction rate as particles gain kinetic energy, move faster, and collide more frequently and forcefully.

2. Concentration: Increasing the concentration of reactants increases the frequency of collisions between particles, thus increasing the reaction rate.

3. Surface Area: For solid reactants, increasing the surface area (e.g., by grinding a solid into a powder) exposes more reactant particles, leading to more frequent collisions and a faster reaction rate.

4. Catalyst: A catalyst is a substance that increases the reaction rate by providing an alternative reaction pathway with lower activation energy, without being consumed in the reaction itself.

(c) What do you mean by a displacement reaction? Write its two examples with their balanced chemical equations.

Answer: A displacement reaction (or single displacement reaction) is a type of chemical reaction where a more reactive element takes the place of (displaces) a less reactive element in a compound.

Examples:

1. Zinc displaces hydrogen from sulphuric acid:

$Zn(s) + H_{2}SO_{4}(aq) \rightarrow ZnSO_{4}(aq) + H_{2}(g)$

2. Iron displaces copper from copper sulphate solution:

$Fe(s) + CuSO_{4}(aq) \rightarrow FeSO_{4}(aq) + Cu(s)$

(d) Write two examples of decomposition reactions with balanced chemical equations.

Answer:

1. Heating of calcium carbonate (limestone):

$CaCO_{3}(s) \xrightarrow{\Delta} CaO(s) + CO_{2}(g)$

2. Decomposition of potassium chlorate by heating:

$2KClO_{3}(s) \xrightarrow{\Delta} 2KCl(s) + 3O_{2}(g)$

(e) What is the type of chemical reaction occurring inside the test tube shown in the given figure? Write the balanced chemical equation of that reaction.

Answer: The figure shows an iron nail (Fe) being placed in a solution of copper sulphate ($CuSO_4$).

Type of reaction: This is a Single Displacement Reaction. Iron is more reactive than copper, so it displaces copper from the copper sulphate solution.

Balanced chemical equation:

$Fe(s) + CuSO_{4}(aq) \rightarrow FeSO_{4}(aq) + Cu(s)$

(f) Four students A, B, C, and D are allowed to carry out decomposition reactions in the laboratory. All of them perform it differently as given below.

i. Student A burnt magnesium(Mg) ribbon

ii. Student B mixed Zn in $FeSO_{4}$ solution

iii. Student C heated $KClO_{3}$ in a hard glass test tube

iv. Student D mixed Zn and HCl solution

Which student followed the correct procedure? Write the balanced chemical reaction of that process.

Answer:

Correct Student: Student C

Reason: The task was to perform a decomposition reaction (one reactant breaking down).

Student A’s reaction ($2Mg + O_2 \rightarrow 2MgO$) is a combination reaction.

Student B’s reaction ($Zn + FeSO_4 \rightarrow ZnSO_4 + Fe$) is a single displacement reaction.

Student C’s reaction (heating $KClO_{3}$) is a decomposition reaction.

Student D’s reaction ($Zn + 2HCl \rightarrow ZnCl_2 + H_2$) is a single displacement reaction.

Balanced chemical reaction (for Student C):

$2KClO_{3}(s) \xrightarrow{\Delta} 2KCl(s) + 3O_{2}(g)$

(g) Samir burns a magnesium ribbon which is like a small strip or a ribbon. It burns brightly and forms white ash’A’. Write the name of substance ‘A’. Write a balanced chemical reaction of the process.

Answer:

Name of substance ‘A’: The white ash ‘A’ is Magnesium Oxide.

Balanced chemical reaction: Magnesium (Mg) reacts with oxygen ($O_2$) from the air when it burns.

$2Mg(s) + O_{2}(g) \rightarrow 2MgO(s)$

Multiple Choice Questions:

Short Answer Questions:

7. Differentiate between:

(a) Physical change and chemical change

Physical Change	Chemical Change
Involves a change in the physical properties of a substance without altering its chemical composition	Involves a change in the chemical composition of a substance, resulting in the formation of new substances with different properties
Generally reversible, and the original substance can usually be obtained back	Generally irreversible, and the original substance cannot be easily obtained back
Examples: Melting ice, crushing a can, boiling water	Examples: Burning wood, digestion of food, rusting of iron

(b) Reactant and product

Reactant	Product
A substance that undergoes a chemical change or reaction	A substance that is formed as a result of a chemical reaction
Present at the beginning of a chemical reaction	Formed as a result of the chemical reaction
Participates in the chemical reaction and gets transformed into products	The outcome of a chemical reaction, formed from the reactants
Shown on the left side of a chemical equation	Shown on the right side of a chemical equation

8. What are the parts of a chemical equation?

The parts of a chemical equation include:

– Reactants: The starting substances

– Arrow Symbol (→): Indicates the direction of the reaction

– Products: The substances formed

– Coefficients: Numbers showing the ratio of molecules

– Subscripts: Numbers showing the number of atoms in a molecule

– State Symbols (optional): (s) solid, (l) liquid, (g) gas, (aq) aqueous

9. Name the chemical reaction in which two substances exchange ions.

The chemical reaction in which two substances exchange ions is called a “double displacement reaction.”

10. Define acid-base reaction with an example.

An acid-base reaction is a type of chemical reaction where an acid, which is a substance capable of releasing hydrogen ions (H⁺), reacts with a base, which is a substance capable of accepting hydrogen ions, to form water and a salt.

Example: $HCl$ (acid) + $NaOH$ (base) → $H_2O$ (water) + $NaCl$ (salt)

11. Identify the type of chemical reaction taking place when silver chloride turns black on exposure to sunlight.

The type of chemical reaction taking place when silver chloride turns black on exposure to sunlight is a “photochemical decomposition reaction.”

13. Name the gases that are evolved at anode and cathode during the electrolysis of acidified water.

During the electrolysis of acidified water:

– At Anode: Oxygen gas ($O_2$) is evolved

– At Cathode: Hydrogen gas ($H_2$) is evolved

14. Give one example of the reaction in which a catalyst increases the rate of reaction.

One example of a reaction in which a catalyst increases the rate of reaction is the decomposition of hydrogen peroxide. A catalyst like manganese dioxide ($MnO_2$) speeds up the breakdown of hydrogen peroxide ($H_2O_2$) into water ($H_2O$) and oxygen gas ($O_2$).

Chemical Equation: $2H_2O_2 \rightarrow 2H_2O + O_2$

15. What substance causes a chemical reaction without itself being affected?

The substance that causes a chemical reaction without itself being affected is called a “catalyst.”

16. Does a chemical reaction always result in a new substance being formed?

Yes, a chemical reaction usually creates new substances with different properties, although there are exceptions in certain equilibrium reactions.

17. How do you know if you have made a chemical reaction?

We can know if a chemical reaction has occurred by observing one or more of the following signs:

– Formation of Precipitate: A solid forms from a solution

– Color Change: A change in color indicating a transformation at the molecular level

– Gas Production: Formation of gas bubbles, often visible as fizzing or effervescence

– Temperature Change: A noticeable change in temperature, indicating energy changes during the reaction

– Odor Change: A distinct change in odor or the release of a new smell

– Change in Physical State: A change from solid to liquid, liquid to gas, etc.

18. How can a chemical reaction be accelerated?

A chemical reaction can be accelerated or speeded up by employing the following methods:

– Catalysts: Substances that speed up reactions without being consumed

– Increase in Temperature: Higher temperatures increase molecular motion and collision frequency

– Pressure Changes: For gaseous reactions, increased pressure increases collision frequency

– Increase in Concentration: More reactant molecules lead to more frequent collisions

– Particle Size Reduction: Smaller particles have greater surface area for reaction

– Radiation: Some reactions are accelerated by light or other radiation

19. How does pressure affect the rate of gaseous reaction?

Higher pressure in a gaseous reaction increases the concentration of gas molecules, leading to more frequent and forceful collisions, ultimately accelerating the reaction rate.

21. What is a chemical reaction? What are the conditions required for a chemical Reaction?

Chemical Reaction: A chemical reaction is a process where substances, called reactants, undergo a transformation, resulting in the formation of new substances, known as products.

Conditions Required for a Chemical Reaction:

– Reactants: Chemical reactions require at least two or more reactants, which are substances undergoing the transformation

– Collision and Activation Energy: Reactant molecules need to collide with a certain minimum force called activation energy for the reaction to occur

– Orientation: The reactant molecules must collide with the correct orientation to form the desired product

22. Explain addition reaction and single displacement reaction with an example.

Addition Reaction: An addition reaction is like putting two things together to make a new thing. It’s when two reactants combine to form a single product.

Example: Formation of Water: $2H_2 + O_2 \rightarrow 2H_2O$

Single Displacement Reaction: A single displacement reaction is like a switch. One element in a compound is replaced by another element, resulting in a new compound and a different element.

Example: Zinc and Copper Sulfate: $Zn + CuSO_4 \rightarrow ZnSO_4 + Cu$

23. What is a catalyst? How does a catalyst affect the rate of reaction?

A catalyst is like a helper in a chemical reaction. It’s a substance that helps the reaction happen faster, but it doesn’t get used up in the process.

A catalyst makes the reactant molecules collide more effectively, sort of like giving them a nudge to react faster. It lowers the energy needed to start the reaction (activation energy), so the reaction happens faster.

24. Carbon reacts with oxygen to give carbon dioxide. What type of chemical reaction is this? Write its chemical equation.

The reaction of carbon with oxygen to produce carbon dioxide is a combustion reaction.

Chemical Equation: $C + O_2 \rightarrow CO_2$

25. Silver bromide is stored in dark bottles in the laboratories.

Silver bromide is stored in dark bottles in laboratories because it’s sensitive to light. If exposed to light, it can change or break down. Storing it in dark bottles protects it from light and helps to keep it stable and usable for experiments.

26. It is important to understand chemical reactions.

Understanding chemical reactions is important because it helps us know how substances interact and change, which is fundamental to understanding the world around us, from biological processes to industrial applications.

27. The conversion of magnesium ribbon into white powdered ash on burning is a chemical change.

When magnesium ribbon burns and turns into white powdered ash, it’s a chemical change because the magnesium atoms in the ribbon combine with oxygen from the air to create a new substance, magnesium oxide. The original magnesium ribbon is no longer there; it transformed into a different material with different properties.

28. The rate of a chemical reaction is important in real life.

The rate of a chemical reaction is important in real life because it tells us how fast or slow a reaction happens. In cooking, medicine, industry, and environmental processes, controlling reaction rates is crucial for efficiency, safety, and desired outcomes.

29. Iron fillings rust faster than an iron nail.

Iron fillings rust faster than an iron nail because of their size and surface area. The smaller particles have more surface area exposed to air and moisture, allowing more sites for the rusting reaction to occur simultaneously.

30. Acid base reaction is also called a neutralization reaction.

An acid-base reaction is termed a neutralization reaction because it involves the combination of an acid and a base, resulting in the formation of water and a salt, which neutralizes the acidic and basic properties.

31. The colour of copper sulphate solution changes when an iron nail is dipped into it.

The color of copper sulfate solution changes when an iron nail is dipped into it because of a chemical reaction between the iron and copper sulfate. The iron from the nail reacts with the copper sulfate, leading to the formation of a new substance (iron sulfate). This new substance has a different color than copper sulfate, causing the solution to change color.

32. We apply paints on iron articles.

We apply paints on iron articles because it helps to protect them from rusting and to enhance their appearance. This layer acts as a barrier, preventing direct contact between the iron and moisture in the air.

33. Finely divided solid reactants react more quickly than solid lump reactants.

Finely divided solid reactants react more quickly than solid lump reactants because of their increased surface area. The more surface area, the more places where the reaction can occur simultaneously.