Entropy (S) is a measure of the disorder or randomness in a system — the variety of ways particles and their energy can be arranged. Higher disorder = higher entropy.

True or False? Entropy increases when a solid melts to form a liquid.

True. In the liquid state, particles are more randomly arranged with greater freedom of movement than in the solid state, so entropy increases.

Why does the thermal decomposition of CaCO 3 lead to an increase in entropy?

The reaction produces CO 2 gas from a single solid reactant:\n\n CaCO 3 (s) → CaO (s) + CO 2 (g)\n\nGas molecules have far greater freedom of movement than solid particles, and the number of product particles increases — both effects raise disorder, increasing entropy.

True or False? For a gas-phase reaction, entropy increases when the total moles of gaseous products exceed the total moles of gaseous reactants.

True. More moles of gas means more particles with independent translational freedom, giving a greater number of possible arrangements and therefore higher entropy.

Of the four states — solid, liquid, aqueous, gas — rank them in order of increasing entropy. Explain the trend.

Solid < Liquid < Aqueous < Gas\n\nSolids have particles fixed in a lattice (low disorder). Liquids allow freer movement. Aqueous species are dispersed through solvent. Gases have the highest freedom of movement, giving the greatest number of possible arrangements.

Entropy (S) is a measure of the .......... or .......... in a system. When a system becomes more disordered, its entropy .......... , signaling increased energetic stability.

Entropy (S) is a measure of the disorder or randomness in a system. When a system becomes more disordered, its entropy increases , signaling increased energetic stability.

Define standard entropy change (Δ S °) .

The standard entropy change (Δ S °) is the difference in total entropy between the products and reactants under standard conditions . Δ S °(reaction) = ΣS°(products) − ΣS°(reactants)

Why must stoichiometric coefficients be applied when calculating Δ S ° for a reaction?

Δ S ° is an extensive property — it scales with the amount of substance. Each standard molar entropy value (S°) must be multiplied by its stoichiometric coefficient to reflect the actual moles of each substance consumed or produced in the balanced equation.

For the reaction 2Mg (s) + O 2 (g) → 2MgO (s), what sign would you predict for Δ S ° and why?

Δ S ° is negative. The reaction converts a gaseous reactant ( O 2 ) and two solids into two solid products only. Removing the gas phase drastically reduces the number of possible particle arrangements, so disorder — and entropy — decreases.

For the reaction 2Mg (s) + O 2 (g) → 2MgO (s), what sign would you predict for Δ S ° and why?

Δ S ° is negative. The reaction converts a gaseous reactant (O 2 ) and two solids into two solid products only. Removing the gas phase drastically reduces the number of possible particle arrangements, so disorder — and entropy — decreases.

True or False? Entropy values are measured in J K -1 mol -1 , not kJ mol -1 , because entropy changes are much smaller in magnitude than enthalpy changes.

True. Entropy changes are far smaller than enthalpy changes in magnitude, so joules (rather than kilojoules) are used as the standard unit for entropy.

The standard entropy change of a reaction is calculated using: Δ S °(reaction) = ΣS°( .......... ) − ΣS°( .......... ). Unlike enthalpy of formation, the standard entropy of a pure element is .......... zero.

The standard entropy change of a reaction is calculated using: Δ S °(reaction) = ΣS°( products ) − ΣS°( reactants ). Unlike enthalpy of formation, the standard entropy of a pure element is not zero.

AP®ChemistryCollege BoardFlashcardsUnit 9: Thermodynamics & ElectrochemistryEntropy

Entropy (College Board AP® Chemistry): Flashcards

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FrontIntroduction to Entropy
Define entropy.

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Cards in this collection (12)

Define entropy.
Entropy (S) is a measure of the disorder or randomness in a system — the variety of ways particles and their energy can be arranged.
Higher disorder = higher entropy.
True or False?
Entropy increases when a solid melts to form a liquid.
True.
In the liquid state, particles are more randomly arranged with greater freedom of movement than in the solid state, so entropy increases.
Why does the thermal decomposition of CaCO₃ lead to an increase in entropy?
The reaction produces CO₂ gas from a single solid reactant:\n\nCaCO₃ (s) → CaO (s) + CO₂ (g)\n\nGas molecules have far greater freedom of movement than solid particles, and the number of product particles increases — both effects raise disorder, increasing entropy.
True or False?
For a gas-phase reaction, entropy increases when the total moles of gaseous products exceed the total moles of gaseous reactants.
True.
More moles of gas means more particles with independent translational freedom, giving a greater number of possible arrangements and therefore higher entropy.
Of the four states — solid, liquid, aqueous, gas — rank them in order of increasing entropy. Explain the trend.
Solid < Liquid < Aqueous < Gas\n\nSolids have particles fixed in a lattice (low disorder). Liquids allow freer movement. Aqueous species are dispersed through solvent. Gases have the highest freedom of movement, giving the greatest number of possible arrangements.
Entropy (S) is a measure of the .......... or .......... in a system. When a system becomes more disordered, its entropy .........., signaling increased energetic stability.
Entropy (S) is a measure of the disorder or randomness in a system. When a system becomes more disordered, its entropy increases, signaling increased energetic stability.
Define standard entropy change (ΔS°).
The standard entropy change (ΔS°) is the difference in total entropy between the products and reactants under standard conditions.
ΔS°(reaction) = ΣS°(products) − ΣS°(reactants)
Why must stoichiometric coefficients be applied when calculating ΔS° for a reaction?
ΔS° is an extensive property — it scales with the amount of substance. Each standard molar entropy value (S°) must be multiplied by its stoichiometric coefficient to reflect the actual moles of each substance consumed or produced in the balanced equation.
For the reaction 2Mg (s) + O₂ (g) → 2MgO (s), what sign would you predict for ΔS° and why?
ΔS° is negative.
The reaction converts a gaseous reactant (O₂) and two solids into two solid products only. Removing the gas phase drastically reduces the number of possible particle arrangements, so disorder — and entropy — decreases.
For the reaction 2Mg (s) + O₂ (g) → 2MgO (s), what sign would you predict for ΔS° and why?
ΔS° is negative.
The reaction converts a gaseous reactant (O₂) and two solids into two solid products only. Removing the gas phase drastically reduces the number of possible particle arrangements, so disorder — and entropy — decreases.
True or False?
Entropy values are measured in J K^-1 mol^-1, not kJ mol^-1, because entropy changes are much smaller in magnitude than enthalpy changes.
True.
Entropy changes are far smaller than enthalpy changes in magnitude, so joules (rather than kilojoules) are used as the standard unit for entropy.
The standard entropy change of a reaction is calculated using: ΔS°(reaction) = ΣS°(.......... ) − ΣS°(.......... ). Unlike enthalpy of formation, the standard entropy of a pure element is .......... zero.
The standard entropy change of a reaction is calculated using: ΔS°(reaction) = ΣS°(products) − ΣS°(reactants). Unlike enthalpy of formation, the standard entropy of a pure element is not zero.

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