Unit 9 Practice: Easy
Applications of Thermodynamics · 20 questions. Try each question before revealing the answer and worked explanation.
Question 1
What does entropy measure?
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Answer: The dispersal of energy and the number of possible microscopic arrangements of a system.
Explanation: Greater numbers of possible microstates generally correspond to greater entropy.
Question 2
Which state of matter generally has the greatest entropy for the same substance?
A. Solid B. Liquid C. Gas D. All are equal
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Answer: C. Gas
Explanation: Gas particles have much greater freedom of movement and many more possible arrangements than particles in liquids or solids.
Question 3
What is the sign of ΔS for melting?
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Answer: Positive
Explanation: A liquid has more possible particle arrangements than a solid, so entropy increases.
Question 4
What is the sign of ΔS for condensation?
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Answer: Negative
Explanation: Gas particles become much more restricted when they form a liquid, decreasing entropy.
Question 5
Predict the sign of ΔS for:
CaCO₃(s) → CaO(s) + CO₂(g)
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Answer: Positive
Explanation: A gas is produced from solid reactants, greatly increasing the number of accessible particle arrangements.
Question 6
Write the equation for standard reaction entropy.
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Answer:
ΔS°rxn = ΣnS°products − ΣnS°reactants
Explanation: Multiply each standard molar entropy by its coefficient before subtracting reactants from products.
Question 7
Is the standard molar entropy of O₂(g) equal to zero at 298 K?
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Answer: No
Explanation: Standard molar entropy is not automatically zero for elements. The zero reference for entropy is associated with a perfect crystal at 0 K.
Question 8
Write the Gibbs free-energy equation.
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Answer:
ΔG = ΔH − TΔS
Question 9
What does ΔG < 0 mean?
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Answer: The forward process is thermodynamically favored.
Explanation: A negative Gibbs free-energy change indicates the process has a thermodynamic driving force in the forward direction under those conditions.
Question 10
What does ΔG > 0 mean?
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Answer: The forward process is thermodynamically unfavored.
Explanation: The reverse direction is thermodynamically favored under those conditions.
Question 11
What is ΔG at equilibrium?
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Answer: 0
Explanation: At equilibrium, there is no net thermodynamic driving force in either direction.
Question 12
A reaction has ΔH < 0 and ΔS > 0. At what temperatures is it favored?
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Answer: All temperatures
Explanation:
ΔG = ΔH − TΔS
The ΔH term is negative and −TΔS is also negative, so both favor ΔG < 0.
Question 13
A reaction has ΔH > 0 and ΔS < 0. At what temperatures is it favored?
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Answer: None
Explanation: Both terms cause ΔG to be positive.
Question 14
What equation connects standard Gibbs free energy and equilibrium?
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Answer:
ΔG° = −RT ln K
Question 15
If K > 1, what is the sign of ΔG°?
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Answer: Negative
Explanation: If K > 1, ln K is positive. The negative sign in the equation makes ΔG° negative.
Question 16
Where does oxidation occur in an electrochemical cell?
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Answer: Anode
Remember: AN OX
Question 17
Where does reduction occur?
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Answer: Cathode
Remember: RED CAT
Question 18
In what direction do electrons travel through the external wire?
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Answer:
Anode → cathode
Explanation: Electrons are produced during oxidation and consumed during reduction.
Question 19
What type of cell uses a favored chemical reaction to produce electricity?
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Answer: Galvanic (voltaic) cell
Question 20
What is Faraday's constant approximately?
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Answer:
96,485 C/mol e⁻
Explanation: One mole of electrons carries approximately 96,485 coulombs of charge.