Chemical Reactions and Equations – Notes, MCQs, Quiz & Worksheet

When magnesium is kept in the open air for some time, it reacts slowly with the oxygen in the air to form a thin, white layer of magnesium oxide (MgO) on its surface. This layer sticks tightly to the metal and prevents the fresh magnesium underneath from coming in direct contact with oxygen.

Before burning magnesium in an experiment, this oxide layer must be removed by rubbing the ribbon with sandpaper or a file. This exposes the shiny surface of pure magnesium, which reacts easily and quickly with oxygen when heated.

When clean magnesium is burned, it reacts vigorously with oxygen to form magnesium oxide, producing a bright white flame and intense heat.

Reasons for cleaning the ribbon:

• To remove the oxide layer (MgO).
• To ensure smooth and complete combustion.
• To allow accurate observation of the reaction.

When an iron nail is dipped in copper sulphate (CuSO₄) solution, a chemical reaction takes place. Iron is more reactive than copper, so it displaces copper from the copper sulphate solution. As a result, the blue colour of the solution slowly fades and a reddish-brown layer of copper is deposited on the iron nail.

Chemical equation:

Fe (solid) + CuSO₄ (aqueous) → FeSO₄ (aqueous) + Cu (solid)

Explanation:

• Copper sulphate solution is blue because of copper ions (Cu²⁺).
• Iron replaces copper in the solution and forms iron sulphate (FeSO₄), which is greenish in colour.
• The copper that gets displaced is deposited on the nail.

Respiration is considered an exothermic reaction because it releases energy in the form of heat and ATP (a type of energy molecule used by cells).

In this process, glucose (a sugar) combines with oxygen to produce carbon dioxide, water, and energy. The energy released is used by the body for various activities, like muscle movement and cell growth.

Here’s how it works:

• The body takes in glucose from food and oxygen from the air.
• These react in the cells of the body, breaking down glucose to release energy.
• The by-products of this reaction are carbon dioxide and water.

Explanation:

• Glucose (C₆H₁₂O₆) reacts with oxygen (O₂).
• This produces carbon dioxide (CO₂) and water (H₂O).
• Energy (ATP and heat) is released during the process.

The reason it’s exothermic is that the energy released (ATP and heat) is greater than the energy required to start the reaction. So, it’s a reaction that releases more energy than it absorbs.

Refining of Silver – Recovery from Silver Nitrate Solution:

In the refining of silver, silver metal is recovered from its salt solution (silver nitrate – AgNO₃) by a displacement reaction using copper metal. This works because copper is more reactive than silver and can displace silver from its compound.

Type of Reaction:
Displacement Reaction (also called Single Displacement or Single Replacement Reaction)

Chemical Reaction:
Cu + 2AgNO₃ → Cu(NO₃)₂ + 2Ag

Explanation:

• Copper (Cu) is placed into a solution of silver nitrate (AgNO₃).
• Copper, being more reactive than silver, displaces silver from the silver nitrate solution.
• As a result, copper nitrate (Cu(NO₃)₂) is formed in the solution, and silver (Ag) is deposited as a solid.
• This reaction is used in the purification and recovery of silver in metallurgy.

Result:
Silver is obtained in pure form, and the solution now contains copper nitrate.

The substance ‘X’ is quicklime, also known as calcium oxide (CaO).

When calcium oxide is mixed with water, it reacts vigorously to form slaked lime, also known as calcium hydroxide [Ca(OH)₂]. This solution is used for whitewashing walls.

Chemical Reaction:

CaO (s) + H₂O (l) → Ca(OH)₂ (aq) + Heat

• Calcium oxide (CaO) is quicklime.
• Calcium hydroxide [Ca(OH)₂] is slaked lime (used in whitewashing).
• The reaction is exothermic (produces heat).

Note: When this slaked lime solution is applied on walls, it reacts with carbon dioxide in the air to form a thin layer of calcium carbonate, which gives a shiny white finish.

Ca(OH)₂ + CO₂ → CaCO₃ + H₂O

Decomposition by heat (Thermal decomposition):

CaCO3 → CaO + CO2
(Calcium carbonate decomposes into calcium oxide and carbon dioxide when heated.)

Decomposition by light (Photodecomposition):

2AgCl → 2Ag + Cl2
(Silver chloride decomposes into silver and chlorine gas when exposed to light.)

Decomposition by electricity (Electrolysis):

2H2O → 2H2 + O2
(Water decomposes into hydrogen and oxygen gases when electricity is passed through it.)

The shiny brown coloured element ‘X’ is copper (Cu).
When copper is heated in air, it reacts with oxygen to form a black coloured compound called copper(II) oxide (CuO).

Reaction:
Cu + O₂ → 2CuO

Explanation:

• Copper (Cu), which is shiny and brown, reacts with oxygen in the air when heated.
• This forms copper(II) oxide (CuO), which is black in colour.
• The surface of the copper turns black due to the formation of this oxide layer.

Answer Summary:

• Element X: Copper (Cu)
• Black compound formed: Copper(II) oxide (CuO)

Difference between Displacement and Double Displacement Reactions:

Displacement Reaction:

One element displaces another element from a compound. Usually involves a more reactive element replacing a less reactive one. For example:
            Zn + CuSO4 → ZnSO4 + Cu
            (Zinc displaces copper from copper sulfate to form zinc sulfate and copper.)

Double Displacement Reaction:

Two compounds exchange their ions to form two new compounds. Often occurs in aqueous solutions and may form a  precipitate, gas, or water. For example:
             NaCl + AgNO3 → NaNO3 + AgCl
             (Sodium chloride reacts with silver nitrate to form sodium nitrate and silver chloride, which precipitates.)

Key Difference:

• Displacement reaction involves one element replacing another.
• Double displacement reaction involves exchange of ions between two compounds.

Exothermic & Endothermic reactions refer to the flow of energy (usually in the form of heat) during a chemical reaction.

Exothermic Reactions:

An exothermic reaction releases energy, usually in the form of heat, to its surroundings. In these reactions, the energy of the products is lower than that of the reactants. For example:

• Combustion (burning) of fuel like methane:--      CH₄ + 2O₂ → CO₂ + 2H₂O + energy (heat)
• In this reaction, energy is released as heat when methane burns.

Endothermic Reactions:

An endothermic reaction absorbs energy from the surroundings. In these reactions, the energy of the products is higher than that of the reactants. For example:

• Photosynthesis in plants:--        6CO₂ + 6H₂O + energy (sunlight) → C₆H₁₂O₆ + 6O₂
• Plants absorb sunlight to convert carbon dioxide and water into glucose and oxygen.

In summary:

1. Exothermic = Energy released (e.g., burning fuels).
2. Endothermic = Energy absorbed (e.g., photosynthesis).

A balanced chemical equation is a chemical equation where the number of atoms of each element is the same on both sides of the equation — that is, the number of atoms in the reactants equals the number in the products.

Example of balanced chemical equation:-

Balanced equation:

2H₂ + O₂ → 2H₂O

Now:

• On the left: 4 hydrogen atoms, 2 oxygen atoms
• On the right: 4 hydrogen atoms, 2 oxygen atoms

Now the number of atoms on both sides is equal — the equation is balanced.

Why should chemical equations be balanced?

• To follow the law of conservation of mass, which says that matter cannot be created or destroyed in a chemical reaction.
• To show the correct amounts of substances involved in the reaction.
• To make accurate calculations for experiments and industrial processes.

Balancing ensures that the reaction makes sense scientifically and practically.

Oil and fat containing food items are flushed with nitrogen gas to prevent rancidity.

What is Rancidity?

Rancidity is the condition in which fats and oils in food get oxidized, usually by the oxygen present in air. This leads to:

• Bad smell
• Unpleasant taste
• Change in color or texture

Rancid food is not only unappetizing but can also be harmful to health if consumed.

Explanation:

• Oils and fats can react with oxygen in the air, leading to a process called oxidation.
• Oxidation of fats and oils produces unpleasant smells and tastes, which is known as rancidity.
• To prevent this, food packets are flushed with nitrogen, an inert gas that does not react with the oils and fats.
• Nitrogen displaces oxygen inside the packaging and helps in preserving the freshness and shelf life of the food.
• Nitrogen is an inert gas, meaning it does not react easily with other substances.
• By flushing the food package with nitrogen, the oxygen inside the pack is removed or displaced.
• Without oxygen, the oxidation of oils and fats is minimized or completely prevented.
• This helps to keep the food fresh for a longer time, maintaining its taste, smell, and nutritional quality.

Conclusion:

Flushing with nitrogen prevents rancidity by protecting oils and fats from oxidation.

A precipitation reaction is a type of chemical reaction in which two soluble substances (usually aqueous solutions of salts) react to form an insoluble solid called a precipitate. This solid separates out from the solution as it does not dissolve in water.

General Form of the Reaction:
Soluble salt A (aq) + Soluble salt B (aq) → Insoluble salt (s) + Soluble salt (aq)

Example 1:
Na₂SO₄ (aq) + BaCl₂ (aq) → BaSO₄ (s) + 2NaCl (aq)

• Sodium sulfate reacts with barium chloride.
• Barium sulfate (BaSO₄) is formed as a white precipitate because it is insoluble in water.
• Sodium chloride remains dissolved in the solution.

Example 2:
AgNO₃ (aq) + NaCl (aq) → AgCl (s) + NaNO₃ (aq)

• Silver nitrate reacts with sodium chloride.
• Silver chloride (AgCl) is formed as a white precipitate.
• Sodium nitrate remains in solution.

Key Points:

• Precipitation reactions help in detecting the presence of certain ions in a solution.
• They are commonly used in qualitative chemical analysis and purification processes.

Experiment: Heating Copper Powder and Passing Hydrogen Gas

When copper powder is heated:

• Observation: The reddish-brown copper powder turns black.
• Reason: Copper reacts with oxygen in the air to form copper(II) oxide, which is black in color.
• Chemical Equation:    2Cu (s) + O₂ (g) → 2CuO (s)
• Copper (Cu) is reddish-brown.
• Copper(II) oxide (CuO) is black.

When hydrogen gas is passed over the heated copper(II) oxide:

• Observation: The black color of copper(II) oxide changes back to reddish-brown.
• Reason: Hydrogen reduces copper(II) oxide to copper metal. Water vapor is formed.
• Chemical Equation:   CuO (s) + H₂ (g) → Cu (s) + H₂O (g)
• Copper(II) oxide (CuO) is black.
• Copper (Cu) is reddish-brown.
• Water (H₂O) is formed as vapor.

Summary of Color Changes:

• Reddish-brown (Cu) → Black (CuO) on heating.
• Black (CuO) → Reddish-brown (Cu) when hydrogen gas is passed over it.

Water is a compound formed by the chemical combination of hydrogen and oxygen in a fixed ratio (H₂O). When hydrogen and oxygen combine to form water, they lose their individual properties. The properties of a compound are completely different from the properties of the elements that form it.

1. Hydrogen is highly inflammable.
2. Oxygen supports combustion.
3. But water is neither inflammable nor supports combustion.

Water is used to extinguish fire because:

1. It cools the burning material below its ignition temperature.
2. It forms a barrier between the fire and oxygen in the air, cutting off the supply of oxygen.

Thus, despite being made from hydrogen and oxygen, water acts as an effective fire extinguisher due to its completely different chemical properties.

Decomposition reactions and combination reactions are called opposites because they involve opposite processes.

Decomposition Reactions:

In a decomposition reaction, a single compound breaks down into two or more simpler substances. It requires energy to break the bonds in the compound.

General form:
AB → A + B

Example:

Decomposition of calcium carbonate (CaCO₃):
CaCO₃ → CaO + CO₂

Combination Reactions:

In a combination reaction, two or more simpler substances combine to form a more complex compound. This reaction releases energy.

General form:
A + B → AB

Example:

Formation of water from hydrogen and oxygen:
2H₂ + O₂ → 2H₂O

Summary:

• Decomposition reaction: One compound breaks down into two or more simpler substances (requires energy).
• Combination reaction: Two or more substances combine to form a more complex compound (releases energy).

A chemical reaction is a process in which one or more substances (called reactants) are converted into new substances (called products) with different chemical properties.

Key Features of a Chemical Reaction:

• A new substance is formed.
• There is a change in chemical composition.
• Energy may be absorbed or released (such as heat, light, or sound).
• Observable signs may include: 
  • Change in color
  • Formation of a gas
  • Change in temperature
  • Formation of a precipitate (solid)

Example of a Chemical Reaction:

Burning of magnesium in air:

2Mg + O₂ → 2MgO

• Reactants: Magnesium (Mg) and Oxygen (O₂)
• Product: Magnesium oxide (MgO), a white powder

When dilute hydrochloric acid is added to iron filings, a chemical reaction takes place in which iron reacts with the acid to form iron chloride and hydrogen gas.

The reaction is:
Fe + 2HCl → FeCl₂ + H₂

Correct answer: (a) Hydrogen gas and iron chloride are produced.

(i) The substance ‘X’ is calcium oxide, and its chemical formula is CaO.
(ii) The reaction of calcium oxide with water is:
         CaO (s) + H₂O (l) → Ca(OH)₂ (aq) + Heat

This reaction is exothermic and forms calcium hydroxide, which is used for whitewashing.

(i) 4Na(s) + O₂(g) → 2Na₂O(s)
Answer:
Sodium (Na) is oxidised (it loses electrons to form Na⁺).
Oxygen (O₂) is reduced (it gains electrons to form O²⁻ in Na₂O).

(ii) CuO(s) + H₂(g) → Cu(s) + H₂O(l)
Answer:
Hydrogen (H₂) is oxidised (it gains oxygen to form H₂O).
Copper(II) oxide (CuO) is reduced (Cu²⁺ gains electrons to become Cu).

The reaction:
Fe₂O₃ + 2Al → Al₂O₃ + 2Fe
is an example of a displacement reaction.
In this reaction, aluminum displaces iron from iron(III) oxide because aluminum is more reactive than iron. This type of reaction is called a displacement reaction.

Correct answer: (d) displacement reaction.

(i) Hydrogen + Chlorine → Hydrogen chloride
Answer:
H₂ + Cl₂ → 2HCl

(ii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride
Answer:
3BaCl₂ + Al₂(SO₄)₃ → 3BaSO₄ + 2AlCl₃

(iii) Sodium + Water → Sodium hydroxide + Hydrogen
Answer:
2Na + 2H₂O → 2NaOH + H₂↑

(i) Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride.
Balanced Equation with State Symbols:
BaCl₂ (aq) + Na₂SO₄ (aq) → BaSO₄ (s) ↓ + 2NaCl (aq)

(ii) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.
Balanced Equation with State Symbols:
NaOH (aq) + HCl (aq) → NaCl (aq) + H₂O (l)

Iron articles are painted to prevent rusting. The paint forms a protective layer, preventing the iron surface from coming into direct contact with oxygen and moisture in the air, which are necessary conditions for the corrosion (rusting) of iron.

In option B, there are 4 hydrogen atoms and 2 oxygen atoms on both the reactant and product sides, satisfying the Law of Conservation of Mass. Therefore, it is a balanced equation.

The formation of a green layer (verdigris) on a metal surface exposed to air and moisture is characteristic of copper. This green layer is typically a mixture of copper carbonate and copper hydroxide.

In this reaction, the copper ions (Cu2+) from copper sulphate exchange with hydrogen ions (H+) from hydrogen sulphide. Copper sulphide (CuS) precipitates, and sulphuric acid (H2SO4) is formed. This exchange of ions is characteristic of a double displacement reaction.

Respiration is a process where glucose reacts with oxygen to produce carbon dioxide, water, and energy in the form of ATP. This release of energy makes it an exothermic process.

In this reaction, CuO (copper oxide) loses oxygen to form Cu (copper). The loss of oxygen is defined as reduction, so copper oxide is reduced.

Reactions that form a precipitate are typically double displacement reactions, also known as precipitation reactions. In these reactions, the ions of two ionic compounds exchange places in aqueous solution to form two new compounds, at least one of which is insoluble in water and separates out as a solid (precipitate).

All listed observations—change in color, evolution of gas, and formation of a precipitate—are common indicators that a chemical reaction has occurred. Other indicators include change in temperature and change in state.