CDS Chemistry · Chemical Reactions

Types of Reactions and Energy Changes

Physical against chemical change, the main reaction types, what heating lead nitrate gives, and the energy ideas behind whether a reaction goes: state functions and Gibbs energy.

All Chemical Reactions notes

Why this matters

Eight CDS questions. Lead nitrate on heating alone was asked three times (2020, 2021, 2023). The two energy questions are HARD Class 11 thermodynamics, the only ones of their kind in CDS chemistry.

Concept 1 of 4: Physical and chemical changes

A physical change alters form but makes no new substance, and can usually be undone. A chemical change makes a new substance. Some everyday events do both at once: a burning candle melts (physical) and burns (chemical).

Definition

The test:

Physical change: no new substance; usually reversible (melting, freezing, dissolving, evaporating).
Chemical change: a new substance forms; usually not reversible (burning, rusting, cooking).
Both at once: a burning candle — the wax melts and vaporises (physical), and the vapour burns to CO₂ and water (chemical).

Event	Kind of change
Freezing of water	Physical
Rusting of iron	Chemical
Cooking food	Chemical
Burning candle	Both physical and chemicalQ

Practice this concept3 quick reps

The same idea in a real exam question:

CDS · 2023 · CDS (II) 2023 — General Knowledge · Q39Moderate

Example 1 · Chemical Reactions · Types of Reactions and Energy Changes

In which one of the following cases do both physical and chemical changes take place ?

The candle is the 'both' answer

Freezing is only physical and rusting only chemical. A burning candle is the standard example of both at once: melting wax plus burning vapour.

Concept 2 of 4: Combination, decomposition, displacement and double displacement

Reactions are named by what happens to the reactants. Two join into one: combination. One breaks into several: decomposition. One element pushes another out: displacement. Two compounds swap partners: double displacement.

Definition

The four types:

Combination: A + B → AB. Magnesium burning in air, 2Mg + O₂ → 2MgO, is combination and combustion: a white powder forms and heat and light are given out.
Decomposition: AB → A + B, by heat (thermal), light (photochemical) or electricity.
Displacement: A + BC → AC + B (a more reactive element takes the place of a less reactive one).
Double displacement: AB + CD → AD + CB. When one product is insoluble it is also a precipitation reaction: K₂SO₄ + BaCl₂ → 2KCl + BaSO₄↓.

Type	Pattern	Example
Combination	A + B → AB	2Mg + O₂ → 2MgO (white powder, heat and light)Q
Decomposition	AB → A + B	CaCO₃ → CaO + CO₂ (on heating)
Displacement	A + BC → AC + B	Zn + CuSO₄ → ZnSO₄ + Cu
Double displacement	AB + CD → AD + CB	K₂SO₄ + BaCl₂ → 2KCl + BaSO₄↓Q

Practice this conceptself-check · 3 quick reps

The same idea in a real exam question:

CDS · 2025 · CDS (II) 2025 — General Knowledge · Q40Easy

Example 2 · Chemical Reactions · Types of Reactions and Energy Changes

The chemical reaction

\mathrm{K_2SO_4\,(aq.)} + \mathrm{BaCl_2\,(aq.)} \longrightarrow 2\mathrm{KCl\,(aq.)} + \mathrm{BaSO_4}\downarrow\,\text{(solid)}

is an example of :

Swapping partners is double displacement

In K₂SO₄ + BaCl₂ both compounds exchange ions. That is double displacement, not a single displacement or an addition reaction.

Drill 1 more on combination, decomposition, displacement and double displacement

Concept 3 of 4: Heating lead nitrate

Lead nitrate is the textbook thermal decomposition. Heated in a tube it crackles and gives off brown fumes. The brown gas is nitrogen dioxide; a colourless gas, oxygen, comes with it, and yellow lead monoxide is left.

Definition

The reaction:

2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂, on heating.
NO₂ is the brown gas. O₂ is colourless. Lead monoxide, PbO, is left behind.
It is a thermal decomposition, not an oxidation reaction. NO, not NO₂, is colourless, so 'brown fumes of NO' is wrong.

Product	Formula	Seen as
Lead monoxide	PbO	Yellow solid left in the tubeQ
Nitrogen dioxide	NO₂	Brown fumesQ
Oxygen	O₂	Colourless gasQ CDS 2023 (I): the correct statements were 'colourless O₂ is released' and 'thermal decomposition producing NO₂'.

Practice this concept3 quick reps

The same idea in a real exam question:

CDS · 2023 · CDS (I) 2023 — General Knowledge · Q13Moderate

Example 3 · Chemical Reactions · Types of Reactions and Energy Changes

Which of the following statements with regard to heating of lead nitrate powder over a flame are correct? 1. Brown fumes of NO are released. 2. Colourless

\mathrm{O_2}

gas is released. 3. It is an example of oxidation reaction. 4. It is an example of thermal decomposition used for the production of

\mathrm{NO_2}

gas. Select the correct answer using the code given below.

Lead nitrate gives PbO, not PbO₂

The solid left is lead monoxide, PbO, and the brown gas is NO₂. Options with PbO₂ or with NO are wrong.

Drill 2 more on heating lead nitrate

Concept 4 of 4: State functions and Gibbs energy

A state function depends only on where a system starts and ends, like the height between two floors, whatever stairs you take. Heat and work depend on the route, but their sum does not. Gibbs energy combines heat and disorder and tells you whether a reaction goes on its own.

Definition

The ideas:

Exothermic reactions give out heat (ΔH negative); endothermic ones take it in (ΔH positive).
State functions: internal energy U, enthalpy H, entropy S, Gibbs energy G = H − TS. Heat q and work w are path functions, but q + w = ΔU is a state function.
A reaction is spontaneous when ΔG is negative.
Where ΔG changes sign, ΔG = 0, so T = ΔH ÷ ΔS. If ΔH and ΔS are both negative, the reaction goes below that temperature; if both positive, above it.

Gibbs energy

\Delta G = \Delta H - T\Delta S \qquad T_{\text{switch}} = \frac{\Delta H}{\Delta S}

$\Delta H$ enthalpy change (heat at constant pressure)
$\Delta S$ entropy change
$T$ temperature in kelvin

Worked example

For a reaction, ΔH = +40 kJ/mol and ΔS = +100 J/(mol·K). Above or below what temperature is it spontaneous?

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

CDS · 2019 · CDS (I) 2019 — General Knowledge · Q79Hard

Example 4 · Chemical Reactions · Types of Reactions and Energy Changes

Which of the following is/are state function/functions? 1.

q + w

q

w

H - TS

Select the correct answer using the code given below.

q and w are path functions, their sum is not

Heat and work each depend on the route taken, but q + w = ΔU, the change in internal energy, depends only on the start and end states.

Both negative means 'below', not 'above'

When ΔH and ΔS are both negative, the reaction is spontaneous below T = ΔH ÷ ΔS. That temperature is a maximum, not a minimum.

Drill 1 more on state functions and gibbs energy

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

State functions and Gibbs energy
Gibbs energy
$\Delta G = \Delta H - T\Delta S \qquad T_{\text{switch}} = \frac{\Delta H}{\Delta S}$

Reference tables (3)

Physical and chemical changes4 rows

Event	Kind of change
Freezing of water	Physical
Rusting of iron	Chemical
Cooking food	Chemical
Burning candle	Both physical and chemicalQ

Combination, decomposition, displacement and double displacement4 rows

Type	Pattern	Example
Combination	A + B → AB	2Mg + O₂ → 2MgO (white powder, heat and light)Q
Decomposition	AB → A + B	CaCO₃ → CaO + CO₂ (on heating)
Displacement	A + BC → AC + B	Zn + CuSO₄ → ZnSO₄ + Cu
Double displacement	AB + CD → AD + CB	K₂SO₄ + BaCl₂ → 2KCl + BaSO₄↓Q

Heating lead nitrate3 rows

Product	Formula	Seen as
Lead monoxide	PbO	Yellow solid left in the tubeQ
Nitrogen dioxide	NO₂	Brown fumesQ
Oxygen	O₂	Colourless gasQ CDS 2023 (I): the correct statements were 'colourless O₂ is released' and 'thermal decomposition producing NO₂'.

Watch out for (5)

The candle is the 'both' answer
→ Physical and chemical changes
Swapping partners is double displacement
→ Combination, decomposition, displacement and double displacement
Lead nitrate gives PbO, not PbO₂
→ Heating lead nitrate
q and w are path functions, their sum is not
→ State functions and Gibbs energy
Both negative means 'below', not 'above'
→ State functions and Gibbs energy

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