Entropy has units of energy divided by temperature, such as joules per kelvin.
Correct Answer
True
True. Entropy changes are defined by ΔS = qrev/T, heat divided by absolute temperature, so entropy is reported in J/K, or J/(mol·K) for molar quantities.
Category: Entropy and Microstates
Question 2
If a process is spontaneous in one direction, its reverse is also spontaneous under the same conditions.
Correct Answer
False
False. When a process is spontaneous in one direction, its reverse is nonspontaneous under the same conditions. For example, at room temperature ice melts on its own, but water does not freeze on its own.
Category: Spontaneity
Question 3
Because entropy is a state function, ΔS for a change depends only on the initial and final states of the system.
Correct Answer
True
True. Like enthalpy and free energy, entropy is a state function, so its change is the same whatever path connects the two states. This is why tabulated standard entropies can be combined to find ΔS° for a reaction.
Category: Entropy and Microstates
Question 4
A gas that expands into an evacuated container has fewer microstates available after the expansion than before it.
Correct Answer
False
False. A larger volume gives the gas particles more possible positions, so the number of microstates increases, and so does the entropy. This is why a gas spreads into a vacuum spontaneously.
Category: Entropy and Microstates
Question 5
Heating a substance increases its entropy.
Correct Answer
True
True. Raising the temperature increases the average kinetic energy of the particles and spreads them over more energy levels, increasing the number of available microstates.
Category: Predicting Entropy Changes
Question 6
Mixing two different ideal gases at constant temperature and pressure decreases entropy.
Correct Answer
False
False. Mixing lets each gas spread through the whole volume, which increases the number of available microstates. The entropy increases, which is why gases mix spontaneously.
Category: Predicting Entropy Changes
Question 7
For a given amount of heat transferred, the entropy change of the surroundings is larger in magnitude when the surroundings are at a lower temperature.
Correct Answer
True
True. The entropy change of the surroundings equals q/T, so the same quantity of heat has a larger effect on entropy at low temperature than at high temperature.
Category: Second Law
Question 8
A spontaneous process must increase the entropy of the system.
Correct Answer
False
False. The second law requires the entropy of the universe, not the system, to increase. Water freezing below 0 °C is spontaneous even though the system's entropy decreases, because the heat released raises the entropy of the surroundings by more.
Category: Second Law
Question 9
The third law of thermodynamics provides a reference point that allows absolute entropies of substances to be measured and tabulated.
Correct Answer
True
True. Because a perfect crystal at 0 K has zero entropy, the entropy of a substance at any other temperature can be found by adding up the entropy gained as it is heated from absolute zero.
Category: Third Law and Standard Entropy
Question 10
The standard entropy S° of an element in its standard state is zero, just as its standard enthalpy of formation is zero.
Correct Answer
False
False. Standard entropies are absolute values measured from the zero of the third law, and every substance at 298 K, including elements, has a positive S°. Only formation quantities such as ΔH°f and ΔG°f are defined as zero for elements in their standard states.
Category: Third Law and Standard Entropy
Question 11
The standard free energy of formation of an element in its standard state is zero.
Correct Answer
True
True. Forming an element in its standard state from itself involves no change, so ΔG°f is zero by definition for substances such as O2(g), N2(g) and C(graphite).
Category: Free Energy Calculations
Question 12
A reaction with a positive ΔH and a negative ΔS becomes spontaneous at high temperatures.
Correct Answer
False
False. With ΔH positive and ΔS negative, both terms of ΔG = ΔH - TΔS are positive, so ΔG is positive at every temperature and the reaction is never spontaneous as written.
Category: Temperature and Spontaneity
Question 13
The free energy change of a spontaneous process represents the maximum useful work the process can perform, an amount reached only under reversible conditions.
Correct Answer
True
True. ΔG is the energy that is free, or available, to do useful work. Real processes are not reversible, so they always deliver less work than this maximum and lose some energy as heat.
Category: Free Energy Calculations
Question 14
If ΔG° for a reaction is positive, no products at all form when the reactants are mixed under standard conditions.
Correct Answer
False
False. A positive ΔG° means K is less than 1, so reactants are favored at equilibrium, but some products still form until the mixture reaches equilibrium.
Category: Free Energy and Equilibrium
Question 15
At the normal boiling point of a liquid, ΔG for converting the liquid to vapor at 1 atm is zero.
Correct Answer
True
True. At the normal boiling point, liquid and vapor at 1 atm are in equilibrium, and a system at equilibrium has ΔG = 0. Above this temperature vaporization at 1 atm is spontaneous, and below it condensation is.
Category: Temperature and Spontaneity
Question 16
The value of ΔG° for a reaction changes as the reaction mixture moves toward equilibrium.
Correct Answer
False
False. ΔG° refers to fixed standard conditions and is constant at a given temperature. It is ΔG, which depends on the reaction quotient Q, that changes as the reaction proceeds and reaches zero at equilibrium.
Category: Free Energy and Equilibrium
Question 17
When ΔH is negative and ΔS is positive, ΔG is negative no matter what the temperature is.
Correct Answer
True
True. With ΔH negative and ΔS positive, both ΔH and -TΔS are negative, so ΔG = ΔH - TΔS is negative at every temperature.
Category: Temperature and Spontaneity
Question 18
Raising the temperature makes the TΔS term less important in determining the sign of ΔG.
Correct Answer
False
False. The entropy term is multiplied by T, so it grows in importance as temperature rises. At high temperatures the sign of ΔS tends to control the sign of ΔG.
Category: Temperature and Spontaneity
Question 19
Ice melting at 10 °C and 1 atm is a spontaneous process even though it absorbs heat.
Correct Answer
True
True. Melting is endothermic, but its positive ΔS makes the TΔS term larger than ΔH above 0 °C, so ΔG is negative. An endothermic process can be spontaneous when the entropy gain is large enough.
Category: Spontaneity
Question 20
The Boltzmann equation relates the entropy of a system to the number of molecules it contains.
Correct Answer
False
False. The Boltzmann equation, S = k ln W, relates entropy to W, the number of microstates, meaning the distinct arrangements of position and energy available to the system.
Category: Entropy and Microstates