Entropy ( S ) is the number of possible arrangements of particles and their energy in a system — a measure of disorder or chaos. Units: J K -1 mol -1 .

What happens to the entropy of a system when it becomes more disordered?

Entropy increases . A more disordered system has more possible arrangements of particles and energy, making it energetically more stable .

Why does enthalpy change alone not explain why endothermic reactions occur?

In endothermic reactions, products are at a higher energy state than reactants, so a decrease in enthalpy cannot be the driving force. The driving force is an increase in entropy , which makes the system energetically more stable.

Entropy ( S ) is related to the number of possible arrangements ( W ) by: S = ..........

S = k ln W where k is the Boltzmann constant (1.38 × 10 -23 J K -1 ). As W increases, entropy increases.

True or False? When ice melts to form liquid water, the entropy of the system increases.

True. In the liquid state, water molecules are arranged more randomly than in the fixed lattice of ice, so there are more possible arrangements and entropy increases .

What is a quantum of energy, and how does it relate to entropy?

A quantum is a discrete packet of energy. The more quanta and the more particles in a system, the greater the number of ways energy can be shared — so entropy increases .

Five molecules of bromine gas are confined to one gas jar. When a divider is removed, they spread into two jars. How many possible arrangements are there, and what does this show?

W = 2 5 = 32 possible arrangements. The molecules spread spontaneously because the number of possible arrangements — and therefore entropy — increases.

True or False? The entropy of an element in its standard state is zero.

False. Unlike standard enthalpy of formation, the entropy of elements in their standard state is not zero . Entropy values for elements can be found in data books.

What happens to entropy when a solid melts?

Entropy increases . The regular lattice of the solid becomes a more randomly arranged liquid, so disorder and the number of possible arrangements both increase.

What happens to entropy when a liquid boils?

Entropy increases significantly . Gas particles are far apart and move freely, giving a far greater number of possible arrangements than in the liquid state.

True or False? The thermal decomposition of CaCO 3 (s) → CaO (s) + CO 2 (g) results in a decrease in entropy.

False. Entropy increases . A gas molecule (CO 2 ) is produced, which is far more disordered than the solid reactant, increasing the number of possible arrangements.

Explain why dissolving ammonium nitrate in water is an endothermic process that is still spontaneous.

Dissolving NH 4 NO 3 results in a large increase in entropy as ions become free to move in solution, changing from a fixed lattice. The increase in disorder drives the process despite the endothermic enthalpy change.

In the reaction 2NH 4 Cl (s) + Ba(OH) 2 .8H 2 O (s) → BaCl 2 .2H 2 O (s) + 2NH 3 (g) + H 2 O (l), what is the sign of the entropy change and why?

The entropy change is positive (ΔS > 0). Two solids react to produce a gas (NH 3 ) and a liquid, greatly increasing disorder.

True or False? When a gas condenses to a liquid, entropy increases.

False. Entropy decreases when a gas condenses. Particles become more ordered and there are fewer possible arrangements of the particles.

Why is it difficult to predict whether dissolving an ionic solid in water is endothermic or exothermic?

Two competing processes occur: bond-breaking between ions (increases disorder, takes in energy) and bond-making between ions and solvent (decreases disorder, releases energy). The net outcome depends on the balance of these processes.

Give two changes of state that result in a decrease in entropy.

Condensation (gas → liquid) Freezing (liquid → solid) In both, particles become more ordered and the number of possible arrangements decreases.

The total entropy change for a reaction is given by: Δ S ꝋ total = .......... + ..........

Δ S ꝋ total = Δ S ꝋ sys + Δ S ꝋ surr where sys = system and surr = surroundings .

What are the units of standard entropy and why is entropy measured in joules rather than kilojoules?

Standard entropy has units of J K -1 mol -1 . Entropy changes are an order of magnitude smaller than enthalpy changes, so joules are used rather than kilojoules.

The standard entropy change of the system is calculated using: Δ S ꝋ system = .......... − ..........

Δ S ꝋ system = Σ S ꝋ products − Σ S ꝋ reactants where Σ denotes the sum of standard entropies, accounting for stoichiometry.

Why does the entropy change of the surroundings depend on temperature?

Transferring a given amount of energy to cool surroundings produces a greater entropy increase than transferring the same energy to hot surroundings. The relationship is ΔS surr = −ΔH/ T , so a lower T gives a larger ΔS surr .

The entropy change of the surroundings is calculated using: Δ S ꝋ surr = ..........

Δ S ꝋ surr = −ΔH / *T * where Δ H is the enthalpy change (in J mol -1 ) and T is the absolute temperature in K.

The formation of NaCl from its elements has Δ S ꝋ sys = −90.1 J K -1 mol -1 and Δ S ꝋ surr = +1379 J K -1 mol -1 . Calculate Δ S ꝋ total and explain why the reaction is spontaneous.

Δ S ꝋ total = −90.1 + 1379 = +1289 J K -1 mol -1 The reaction is spontaneous because Δ S ꝋ total is positive . The large exothermic energy release increases the entropy of the surroundings, outweighing the decrease in system entropy.

True or False? When calculating Δ S ꝋ system , the standard entropy of an element in its standard state is taken as zero.

False. Unlike standard enthalpy of formation, standard entropy values for elements are not zero and must be looked up in data books when calculating Δ S ꝋ system .

Calculate Δ S ꝋ system for 2Mg (s) + O 2 (g) → 2MgO (s), given S ꝋ [Mg(s)] = 32.60, S ꝋ [O 2 (g)] = 205.0, S ꝋ [MgO(s)] = 38.20 J K -1 mol -1 .

Δ S ꝋ system = (2 × 38.20) − (2 × 32.60 + 205.0) = 76.40 − 270.2 = −193.8 J K -1 mol -1 The negative value reflects a decrease in disorder as a solid is formed.

What is Gibbs free energy ?

Gibbs free energy is a thermodynamic quantity that combines enthalpy and entropy to determine whether a reaction is feasible. The Gibbs equation is: ΔG ꝋ = ΔH reaction ꝋ − TΔS system ꝋ A reaction is feasible when ΔG ꝋ ≤ 0.

What is the condition for a reaction to be feasible ?

A reaction is feasible when ΔG ꝋ is equal to or less than zero (ΔG ꝋ ≤ 0). This means the combined effect of enthalpy and entropy changes makes the reaction thermodynamically spontaneous.

Complete the Gibbs equation: ΔG ꝋ = ..........

ΔG ꝋ = ΔH reaction ꝋ − TΔS system ꝋ Note: ΔS system ꝋ must be converted from J K -1 mol -1 to kJ K -1 mol -1 by dividing by 1000 before substituting.

An exothermic reaction has a positive ΔS system ꝋ . Is it feasible at all temperatures?

Yes — it is always feasible at all temperatures. Both the ΔH ꝋ term (negative) and the −TΔS system ꝋ term (negative) make ΔG ꝋ negative regardless of temperature.

True or False? A more negative lattice enthalpy indicates a more stable ionic lattice.

True. Both the ΔH ꝋ term (positive) and the −TΔS system ꝋ term (positive) make ΔG ꝋ positive at all temperatures, so the reaction is never feasible.

An endothermic reaction has a positive ΔS system ꝋ . At what temperatures does it become feasible?

It becomes feasible at high temperatures . At high T, the −TΔS system ꝋ term becomes sufficiently negative to overcome the positive ΔH ꝋ , giving a negative ΔG ꝋ .

How do you calculate the temperature at which a reaction becomes feasible ?

Set ΔG ꝋ = 0 and rearrange: ΔG ꝋ = ΔH ꝋ − TΔS ꝋ = 0 T = ΔH ꝋ / ΔS ꝋ Remember to convert ΔS from J K -1 mol -1 to kJ K -1 mol -1 first.

What is thermodynamic stability ?

Thermodynamic stability refers to whether a reaction is feasible under given conditions. A substance is thermodynamically stable if ΔG for its decomposition is positive (reaction is not feasible).

What is kinetic stability ?

Kinetic stability refers to how fast a reaction occurs. A reaction that is thermodynamically feasible may still be kinetically stable if the activation energy (E a ) is too high for it to proceed at a measurable rate.

True or False? Lattice enthalpy can be measured directly in a lab.

False. A reaction can be thermodynamically feasible (ΔG ≤ 0) yet kinetically stable if the activation energy is high. Methane combustion is an example — it is feasible but requires ignition.

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Entropy (Edexcel International A Level (IAL) Chemistry): Flashcards

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Define entropy
Entropy (S) is the number of possible arrangements of particles and their energy in a system — a measure of disorder or chaos. Units: J K^-1 mol^-1.
What happens to the entropy of a system when it becomes more disordered?
Entropy increases. A more disordered system has more possible arrangements of particles and energy, making it energetically more stable.
Why does enthalpy change alone not explain why endothermic reactions occur?
In endothermic reactions, products are at a higher energy state than reactants, so a decrease in enthalpy cannot be the driving force. The driving force is an increase in entropy, which makes the system energetically more stable.
Entropy (S) is related to the number of possible arrangements (W) by:
S = ..........
S = k ln W
where k is the Boltzmann constant (1.38 × 10^-23 J K^-1). As W increases, entropy increases.
True or False?
When ice melts to form liquid water, the entropy of the system increases.
True.
In the liquid state, water molecules are arranged more randomly than in the fixed lattice of ice, so there are more possible arrangements and entropy increases.
What is a quantum of energy, and how does it relate to entropy?
A quantum is a discrete packet of energy. The more quanta and the more particles in a system, the greater the number of ways energy can be shared — so entropy increases.
Five molecules of bromine gas are confined to one gas jar. When a divider is removed, they spread into two jars. How many possible arrangements are there, and what does this show?
W = 2⁵ = 32 possible arrangements. The molecules spread spontaneously because the number of possible arrangements — and therefore entropy — increases.
True or False?
The entropy of an element in its standard state is zero.
False.
Unlike standard enthalpy of formation, the entropy of elements in their standard state is not zero. Entropy values for elements can be found in data books.
What happens to entropy when a solid melts?
Entropy increases. The regular lattice of the solid becomes a more randomly arranged liquid, so disorder and the number of possible arrangements both increase.
What happens to entropy when a liquid boils?
Entropy increases significantly. Gas particles are far apart and move freely, giving a far greater number of possible arrangements than in the liquid state.
True or False?
The thermal decomposition of CaCO₃ (s) → CaO (s) + CO₂ (g) results in a decrease in entropy.
False.
Entropy increases. A gas molecule (CO₂) is produced, which is far more disordered than the solid reactant, increasing the number of possible arrangements.
Explain why dissolving ammonium nitrate in water is an endothermic process that is still spontaneous.
Dissolving NH₄NO₃ results in a large increase in entropy as ions become free to move in solution, changing from a fixed lattice. The increase in disorder drives the process despite the endothermic enthalpy change.
In the reaction 2NH₄Cl (s) + Ba(OH)₂.8H₂O (s) → BaCl₂.2H₂O (s) + 2NH₃ (g) + H₂O (l), what is the sign of the entropy change and why?
The entropy change is positive (ΔS > 0). Two solids react to produce a gas (NH₃) and a liquid, greatly increasing disorder.
True or False?
When a gas condenses to a liquid, entropy increases.
False.
Entropy decreases when a gas condenses. Particles become more ordered and there are fewer possible arrangements of the particles.
Why is it difficult to predict whether dissolving an ionic solid in water is endothermic or exothermic?
Two competing processes occur: bond-breaking between ions (increases disorder, takes in energy) and bond-making between ions and solvent (decreases disorder, releases energy). The net outcome depends on the balance of these processes.
Give two changes of state that result in a decrease in entropy.
1. Condensation (gas → liquid)
2. Freezing (liquid → solid)
In both, particles become more ordered and the number of possible arrangements decreases.
The total entropy change for a reaction is given by:
ΔS^ꝋ_total = .......... + ..........
ΔS^ꝋ_total = ΔS^ꝋ_sys + ΔS^ꝋ_surr
where sys = system and surr = surroundings.
What are the units of standard entropy and why is entropy measured in joules rather than kilojoules?
Standard entropy has units of J K^-1 mol^-1. Entropy changes are an order of magnitude smaller than enthalpy changes, so joules are used rather than kilojoules.
The standard entropy change of the system is calculated using:
ΔS^ꝋ_system = .......... − ..........
ΔS^ꝋ_system = ΣS^ꝋ_products − ΣS^ꝋ_reactants
where Σ denotes the sum of standard entropies, accounting for stoichiometry.
Why does the entropy change of the surroundings depend on temperature?
Transferring a given amount of energy to cool surroundings produces a greater entropy increase than transferring the same energy to hot surroundings. The relationship is ΔS_surr = −ΔH/T, so a lower T gives a larger ΔS_surr.
The entropy change of the surroundings is calculated using:
ΔS^ꝋ_surr = ..........
ΔS^ꝋ_surr = −ΔH / *T*
where ΔH is the enthalpy change (in J mol^-1) and T is the absolute temperature in K.
The formation of NaCl from its elements has ΔS^ꝋ_sys = −90.1 J K^-1 mol^-1 and ΔS^ꝋ_surr = +1379 J K^-1 mol^-1. Calculate ΔS^ꝋ_total and explain why the reaction is spontaneous.
ΔS^ꝋ_total = −90.1 + 1379 = +1289 J K^-1 mol^-1
The reaction is spontaneous because ΔS^ꝋ_total is positive. The large exothermic energy release increases the entropy of the surroundings, outweighing the decrease in system entropy.
True or False?
When calculating ΔS^ꝋ_system, the standard entropy of an element in its standard state is taken as zero.
False.
Unlike standard enthalpy of formation, standard entropy values for elements are not zero and must be looked up in data books when calculating ΔS^ꝋ_system.
Calculate ΔS^ꝋ_system for 2Mg (s) + O₂ (g) → 2MgO (s), given S^ꝋ[Mg(s)] = 32.60, S^ꝋ[O₂(g)] = 205.0, S^ꝋ[MgO(s)] = 38.20 J K^-1 mol^-1.
ΔS^ꝋ_system = (2 × 38.20) − (2 × 32.60 + 205.0)
= 76.40 − 270.2 = −193.8 J K^-1 mol^-1
The negative value reflects a decrease in disorder as a solid is formed.
What is Gibbs free energy?
Gibbs free energy is a thermodynamic quantity that combines enthalpy and entropy to determine whether a reaction is feasible.
The Gibbs equation is: ΔG^ꝋ = ΔH_reaction^ꝋ − TΔS_system^ꝋ
A reaction is feasible when ΔG^ꝋ ≤ 0.
What is the condition for a reaction to be feasible?
A reaction is feasible when ΔG^ꝋ is equal to or less than zero (ΔG^ꝋ ≤ 0).
This means the combined effect of enthalpy and entropy changes makes the reaction thermodynamically spontaneous.
Complete the Gibbs equation:
ΔG^ꝋ = ..........
ΔG^ꝋ = ΔH_reaction^ꝋ − TΔS_system^ꝋ
Note: ΔS_system^ꝋ must be converted from J K^-1 mol^-1 to kJ K^-1 mol^-1 by dividing by 1000 before substituting.
An exothermic reaction has a positive ΔS_system^ꝋ. Is it feasible at all temperatures?
Yes — it is always feasible at all temperatures.
Both the ΔH^ꝋ term (negative) and the −TΔS_system^ꝋ term (negative) make ΔG^ꝋ negative regardless of temperature.
True or False?
A more negative lattice enthalpy indicates a more stable ionic lattice.
True.
Both the ΔH^ꝋ term (positive) and the −TΔS_system^ꝋ term (positive) make ΔG^ꝋ positive at all temperatures, so the reaction is never feasible.
An endothermic reaction has a positive ΔS_system^ꝋ. At what temperatures does it become feasible?
It becomes feasible at high temperatures.
At high T, the −TΔS_system^ꝋ term becomes sufficiently negative to overcome the positive ΔH^ꝋ, giving a negative ΔG^ꝋ.
How do you calculate the temperature at which a reaction becomes feasible?
Set ΔG^ꝋ = 0 and rearrange:
1. ΔG^ꝋ = ΔH^ꝋ − TΔS^ꝋ = 0
2. T = ΔH^ꝋ / ΔS^ꝋ
Remember to convert ΔS from J K^-1 mol^-1 to kJ K^-1 mol^-1 first.
What is thermodynamic stability?
Thermodynamic stability refers to whether a reaction is feasible under given conditions.
A substance is thermodynamically stable if ΔG for its decomposition is positive (reaction is not feasible).
What is kinetic stability?
Kinetic stability refers to how fast a reaction occurs.
A reaction that is thermodynamically feasible may still be kinetically stable if the activation energy (E_a) is too high for it to proceed at a measurable rate.
True or False?
Lattice enthalpy can be measured directly in a lab.
False.
A reaction can be thermodynamically feasible (ΔG ≤ 0) yet kinetically stable if the activation energy is high. Methane combustion is an example — it is feasible but requires ignition.

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