Chemical Reactions and Redox Reactions – NDA Chemistry Notes

A chemical reaction is a process in which substances are converted into new substances with different properties. The starting substances are called reactants. The new substances formed are called products.

You see chemical reactions every day: food cooking, iron rusting, fuel burning, fruits browning after cutting. Chemistry gives us a way to classify these reactions, understand what is happening at the atomic level, and predict what will happen.

NDA tests this chapter through reaction type identification, match-list questions pairing reactions with their types, exothermic vs endothermic classification, redox identification (who is oxidised, who is reduced), and specific factual questions about rusting, galvanisation, CO poisoning, and AgBr in photography. NDA 2025-II asked four questions from this chapter in a single paper.

1. Signs of a Chemical Reaction

How do we know a chemical reaction has occurred? Look for these signs:

• A new substance is formed
• Change in colour
• Gas is produced (bubbles or smell)
• A precipitate forms (solid appears in a liquid)
• Change in temperature (heat released or absorbed)
• Change cannot be easily reversed

2. Types of Chemical Reactions

NDA tests reaction types through match-list questions. You must be able to look at a reaction and identify its type.

Combination Reaction

Two or more substances combine to form one new substance.

A + B → AB

Examples:

• CaO(s) + H₂O(l) → Ca(OH)₂(aq): quicklime + water → slaked lime
• C + O₂ → CO₂: burning of carbon
• N₂ + 3H₂ → 2NH₃: Haber process (making ammonia)
• 2Na + Cl₂ → 2NaCl: sodium + chlorine → common salt

Burning of coal is a combination reaction: C + O₂ → CO₂. [NDA 2023-II]

Decomposition Reaction

One substance breaks down into two or more simpler substances.

AB → A + B

Examples:

• CaCO₃ → CaO + CO₂: limestone decomposes on heating
• 2H₂O → 2H₂ + O₂: electrolysis of water
• 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂: lead nitrate decomposes on heating
• 2KClO₃ → 2KCl + 3O₂: potassium chlorate decomposition

Electrolysis of water is a decomposition reaction. [NDA 2023-II]

Displacement Reaction (Single Displacement)

A more reactive element displaces a less reactive element from its compound.

A + BC → AC + B (where A is more reactive than B)

Examples:

• Fe + CuSO₄ → FeSO₄ + Cu: iron displaces copper
• Zn + CuSO₄ → ZnSO₄ + Cu: zinc displaces copper
• Pb + CuCl₂ → PbCl₂ + Cu: lead displaces copper

Iron nail immersed in copper sulphate solution: iron displaces copper. This is a displacement reaction. [NDA 2023-II]

When iron displaces copper from copper sulphate:

• The solution turns from blue to green (CuSO₄ is blue; FeSO₄ is green)
• Iron is the reducing agent (it loses electrons)
• Copper is deposited on the iron nail
• Iron is more reactive than copper [NDA 2019-I]

Fe is the reductant in Fe + 2HCl → FeCl₂ + H₂. [NDA 2025-II]

Cu is being reduced in Zn + CuSO₄ → ZnSO₄ + Cu (Cu²⁺ gains electrons → Cu⁰). [NDA 2025-II]

Double Displacement Reaction

Two compounds exchange their ions to form two new compounds. Often results in a precipitate, gas, or water.

AB + CD → AD + BC

Examples:

• Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl: white precipitate of BaSO₄ forms
• AgNO₃ + NaCl → AgCl↓ + NaNO₃: white precipitate of AgCl forms
• HCl + NaOH → NaCl + H₂O: neutralisation (also a double displacement)

Addition of barium chloride solution to aluminium sulphate solution: double displacement reaction producing BaSO₄ precipitate. [NDA 2023-II]

When two salt solutions are mixed and a precipitate forms, it is a double displacement reaction. [NDA 2010-II]

Precipitation Reaction

A special type of double displacement where an insoluble solid (precipitate) forms when two solutions are mixed.

The precipitate appears as a solid inside the liquid. The liquid becomes cloudy or milky.

Example: Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄↓ (white precipitate) + 2NaCl(aq)

Reaction	Type
CaO + H₂O → Ca(OH)₂	Combination
CaCO₃ → CaO + CO₂	Decomposition
Pb + CuCl₂ → PbCl₂ + Cu	Single displacement
Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl	Double displacement
Electrolysis of water	Decomposition
Burning of coal	Combination
Fe nail in CuSO₄ solution	Displacement

[NDA 2025-II Q107, NDA 2023-II]

3. Exothermic and Endothermic Reactions

Exothermic reaction: Releases heat to the surroundings. The temperature of the surroundings increases. Products have less energy than reactants.

Endothermic reaction: Absorbs heat from the surroundings. The temperature of the surroundings decreases. Products have more energy than reactants.

Reaction	Type	Why
CaO + H₂O → Ca(OH)₂	Exothermic	Releases heat (hissing sound, hot to touch)
Burning of fuel (combustion)	Exothermic	Releases heat and light
Neutralisation	Exothermic	Acid + base always releases heat
Respiration	Exothermic	Releases energy for body
Decomposition of Pb(NO₃)₂	Endothermic	Requires heat input to break down
Photosynthesis	Endothermic	Absorbs light energy
Dissolving NH₄Cl in water	Endothermic	Solution becomes cold
N₂ + 3H₂ → 2NH₃	Exothermic	Haber process releases heat

CaO + H₂O → Ca(OH)₂ is an exothermic reaction. [NDA 2021-I]

Decomposition of lead nitrate (2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂) is endothermic (requires heat). [NDA 2025-II]

4. Redox Reactions

Redox = Reduction + Oxidation

In a redox reaction, both oxidation and reduction happen at the same time. One cannot happen without the other.

Oxidation and Reduction

There are three ways to understand oxidation and reduction.

Old definition (oxygen-based):

• Oxidation = addition of oxygen (or removal of hydrogen)
• Reduction = removal of oxygen (or addition of hydrogen)

New definition (electron-based):

• Oxidation = loss of electrons
• Reduction = gain of electrons

Modern definition (oxidation state):

• Oxidation = increase in oxidation state
• Reduction = decrease in oxidation state

Memory trick: OIL RIG: Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons).

Oxidising Agent and Reducing Agent

Oxidising agent:

• Causes oxidation in another substance
• Gets reduced itself (gains electrons, oxidation state decreases)
• Examples: Cl₂, O₂, H₂O₂, KMnO₄, HNO₃, ozone

Reducing agent:

• Causes reduction in another substance
• Gets oxidised itself (loses electrons, oxidation state increases)
• Examples: H₂, C (coke), CO, Na, Zn, Fe, N₂H₄

Critical NDA trap about reducing agents: In the reaction 2FeBr₃ + 3Cl₂ → 2FeCl₃ + 3Br₂:

• Cl₂ is the oxidising agent
• FeBr₃ is the reducing agent
• Reducing agents are themselves OXIDISED (they lose electrons)
• Reducing agents do NOT gain electrons. They LOSE electrons

The incorrect statement is: “Reducing agents gain or appear to gain electrons.” This is WRONG. Reducing agents LOSE electrons. [NDA 2012-I]

Oxidising agent decreases in oxidation state (is reduced). Reducing agent increases in oxidation state (is oxidised). [NDA 2012-I]

N₂H₄ as reducing agent: N₂H₄ + 2H₂O₂ → N₂ + 4H₂O. N₂H₄ is the reducing agent. It loses hydrogen (is oxidised) → N₂. H₂O₂ is the oxidising agent. It gains hydrogen (is reduced) → H₂O. [NDA 2025-II]

Identifying the reducing agent in given reactions (NDA 2025-II ):

• Fe + 2HCl → FeCl₂ + H₂ → Fe is reductant (Fe loses electrons)
• Zn + CuSO₄ → ZnSO₄ + Cu → Cu is being reduced (Cu²⁺ gains electrons)
• Br₂ + 2I⁻ → 2Br⁻ + I₂ → Br₂ is oxidant (NOT reductant, Br₂ gains electrons)
• CH₄ + 2O₂ → CO₂ + 2H₂O → C atom in CH₄ is oxidised (oxidation state goes from −4 to +4)

Correct statements: I, II, and IV. Statement III is wrong. Br₂ is the oxidant, not the reductant. [NDA 2025-II]

Which reaction is NOT redox: AlCl₃ + 3H₂O → Al(OH)₃ + 3HCl. This is a hydrolysis / double displacement reaction. No change in oxidation states occurs. NOT a redox reaction. [NDA 2022-I]

Oxidation States of Elements in Compounds

Rules for calculating oxidation state:

• Free element = 0 (Na, O₂, Cl₂ all = 0)
• Monatomic ion = charge (Na⁺ = +1, Cl⁻ = −1, Fe³⁺ = +3)
• Oxygen = −2 (except in peroxides where it is −1)
• Hydrogen = +1 (except in metal hydrides where it is −1)
• Sum of oxidation states in a neutral compound = 0
• Sum of oxidation states in a polyatomic ion = charge of ion

Oxidation states in KMnO₄: K = +1, O = −2 (×4 = −8), so Mn = +7 (because +1 + Mn + (−8) = 0, Mn = +7). K = +1, Mn = +7, O = −2. [NDA 2012-II]

H₂ as reducing agent: H₂ reduces CuO to Cu: CuO + H₂ → Cu + H₂O. H₂ reduces PbO to Pb: PbO + H₂ → Pb + H₂O. H₂ is a reducing agent. It gets oxidised from 0 to +1. [NDA 2013-I, NDA 2012-I]

5. Important Specific Reactions

Browning of apple (oxidation): When an apple is cut, the inner surface is exposed to oxygen. Polyphenol oxidase enzyme converts polyphenolic compounds to dark-coloured quinones. This is oxidation, a chemical change. [NDA 2012-I]

CO poisoning: Carbon monoxide (CO) is poisonous because it forms a complex with haemoglobin (carboxyhaemoglobin). This complex is approximately 250 times more stable than the oxygen-haemoglobin complex. CO blocks oxygen transport, causing suffocation and death. [NDA 2018-I]

AgBr in photography: Silver bromide (AgBr) is used in black and white photography. When light falls on AgBr, it decomposes to silver metal (dark) and bromine. The pattern of light and dark silver creates the image. [NDA 2021-II]

The reaction is: 2AgBr → 2Ag + Br₂ (light-induced decomposition).

Galvanisation (zinc protects iron): Zinc is used to protect iron from rusting (galvanisation). This is because zinc is more reactive (more electropositive) than iron. When both metals are exposed to moisture and air, zinc corrodes preferentially, protecting the iron beneath. [NDA 2013-II, NDA 2017-II, NDA 2021-II]

Thermite reaction: Fe₂O₃ + 2Al → 2Fe + Al₂O₃ + heat

• Fe₂O₃ acts as oxidising agent (Fe is reduced from +3 to 0)
• Al acts as reducing agent (Al is oxidised from 0 to +3)
• Produces enormous heat, used to join railway tracks
• It is an exothermic displacement reaction [NDA 2023-II]

Rusting of iron: 4Fe + 3O₂ + xH₂O → 2Fe₂O₃·xH₂O (rust)

• Rusting requires both oxygen AND water
• Rust is a compound (hydrated iron oxide)
• Rusting INCREASES the weight of iron (oxygen and water are added)
• Rusting is a chemical change, cannot be reversed

Zinc used in galvanisation: Zinc is more electropositive (more reactive) than iron. Zinc corrodes first, protecting iron. [NDA 2017-II]

Hydrogenation of oils: Vegetable oil + H₂ → Vanaspati (solid fat), using Ni as catalyst. This is an addition reaction. [NDA 2021-I]

6. Catalyst

A catalyst is a substance that changes the rate of a chemical reaction without being permanently changed itself.

• Positive catalyst: Increases the rate of reaction. Example: MnO₂ in decomposition of KClO₃; Ni in hydrogenation of oils; Fe in Haber process.
• Negative catalyst (Inhibitor): Decreases the rate of reaction. Example: small amounts of alcohol slow down H₂O₂ decomposition.

The role of a positive catalyst is to increase the rate of reaction.

A catalyst does not change the equilibrium position or the products of a reaction. It only helps reach equilibrium faster.

7. H₂O₂: Both Oxidising and Reducing Agent

H₂O₂ (hydrogen peroxide) can act as BOTH an oxidising agent AND a reducing agent. This is because the oxygen in H₂O₂ has an oxidation state of −1, intermediate between 0 (O₂) and −2 (H₂O).

• As oxidising agent: H₂O₂ oxidises KI → I₂ (H₂O₂ gets reduced to H₂O)
• As reducing agent: H₂O₂ reduces KMnO₄ in acidic solution (H₂O₂ gets oxidised to O₂)

Which reducing agent can also act as an oxidising agent? H₂O₂.

8. Not a Chemical Change

NDA tests which processes are physical (NOT chemical):

• Crystallisation of salt from seawater = physical
• Sublimation of iodine = physical
• Dissolution of salt in water = physical
• Melting of ice = physical
• Evaporation of petrol = physical

Chemical changes:

• Burning = chemical
• Rusting = chemical
• Browning of apple = chemical
• Frying an egg = chemical
• Souring of milk = chemical
• Mixing NaOH and HCl (neutralisation) = chemical [NDA 2022-I, NDA 2024-II]