AP Chemistry Quiz: Introduction To Entropy

What this quiz covers

This quiz focuses on Introduction To Entropy, giving you a quick way to practice the rules, question types, and explanations that matter most for AP Chemistry.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

Question 1

1. Entropy decreases because boiling requires energy input.
2. Entropy remains approximately constant because nitrogen remains N$_2$.
3. Entropy increases because gas particles are more dispersed than liquid particles. (correct answer)
4. Entropy remains approximately constant because the pressure is constant at 1 atm.
5. Entropy decreases because the temperature increases during warming.

Explanation: This question tests entropy during boiling at constant pressure. Boiling N₂(l) to N₂(g) disperses molecules from liquid to gas, increasing entropy due to greater freedom. The open container and 1 atm facilitate the phase change. Warming does not decrease entropy here. A tempting distractor is choice B, which claims entropy remains constant because it's still N₂, but this misconceives that unchanged formula prevents entropy changes, disregarding phase differences. In boiling, highlight the entropy gain from gaseous expansion.

Question 2

1. Entropy decreases because the perfume concentration decreases.
2. Entropy increases because the molecules become more dispersed throughout the room. (correct answer)
3. Entropy remains approximately constant because the temperature of the room is constant.
4. Entropy decreases because diffusion is exothermic.
5. Entropy remains approximately constant because no new substances form.

Explanation: This question assesses entropy changes due to diffusion of molecules in a gas phase. Opening the perfume container allows its molecules to spread from a concentrated area to disperse throughout the room air, increasing their randomness and accessible microstates. This diffusion process results in an entropy increase for the perfume molecules as the system. The room's constant temperature supports the spontaneous nature of this dispersal. A tempting distractor is choice C, which suggests entropy remains constant because room temperature is constant, but this misconceives that isothermal conditions prevent entropy increases, overlooking diffusion's role in enhancing disorder. In diffusion scenarios, think about how spreading molecules over larger volumes inherently increases entropy.

Question 3

1. Entropy remains approximately constant because total pressure is unchanged.
2. Entropy decreases because mixing reduces the partial pressure of each gas.
3. Entropy increases because mixing increases the number of accessible microstates. (correct answer)
4. Entropy remains approximately constant because N$_2$ and O$_2$ are both diatomic.
5. Entropy decreases because the process is not a chemical reaction.

Explanation: This question probes entropy changes upon mixing ideal gases. Removing the partition allows N₂ and O₂ to mix uniformly, increasing the dispersal of each gas throughout the entire volume and creating more possible arrangements. This mixing process, known as entropy of mixing, results in an overall entropy increase for the system at constant temperature and pressure. The gases do not react, so the change is purely physical. A tempting distractor is choice A, which claims entropy remains constant because total pressure is unchanged, but this misconceives that constant total pressure implies no entropy change, disregarding the role of mixing in increasing disorder. For gas mixing, consider how combining components increases configurational entropy through greater particle arrangements.

Question 4

1. Entropy increases because freezing releases heat to the surroundings.
2. Entropy decreases because particles become more ordered in the solid. (correct answer)
3. Entropy remains approximately constant because the chemical formula does not change.
4. Entropy increases because the temperature decreases.
5. Entropy remains approximately constant because pressure is constant.

Explanation: This question examines entropy variations during freezing, a liquid-to-solid phase change. Cooling ethanol to its freezing point and solidifying it organizes the molecules into a rigid crystal lattice, reducing their freedom of motion and dispersal. This transition to a more ordered state decreases the entropy of the system. The constant pressure of 1 atm does not alter the fundamental entropy decrease associated with freezing. A tempting distractor is choice C, which suggests entropy remains constant because the chemical formula is unchanged, but this misconceives that phase changes do not affect entropy, ignoring the order difference between liquids and solids. When analyzing phase changes, compare the molecular disorder in the initial and final states to determine entropy direction.

Question 5

1. Entropy remains approximately constant because the temperature stays at the boiling point during the phase change.
2. Entropy remains approximately constant because the pressure is held constant throughout.
3. Entropy decreases because boiling requires energy input, which creates order.
4. Entropy decreases because intermolecular forces are overcome and particles separate.
5. Entropy increases because the particles move more freely in the gas phase than in the liquid phase. (correct answer)

Explanation: This question tests the understanding of entropy changes during vaporization at constant pressure. The stimulus describes liquid ethanol being heated to boil and become vapor, transitioning from liquid to gas phase. Entropy increases because in the gas phase, ethanol molecules have greater freedom of motion, higher kinetic energy, and more accessible microstates than in the more constrained liquid phase. This is supported by the positive ΔS_vap values for substances, reflecting the disorder increase upon overcoming intermolecular forces. A tempting distractor is choice C, which incorrectly claims entropy remains constant because temperature is constant at the boiling point, misconstruing isothermal conditions with no change in microstates and ignoring the phase transition's effect. For entropy evaluations in phase changes, assess how breaking intermolecular interactions allows for greater particle independence in the vapor phase.

Question 6

1. $\Delta S$ decreases because ions in solution are more ordered than a crystal.
2. $\Delta S$ increases because the ions disperse throughout the solvent. (correct answer)
3. $\Delta S$ remains approximately constant because the mass of solute is unchanged.
4. $\Delta S$ decreases because dissolving must absorb heat to break the lattice.
5. $\Delta S$ increases because the temperature of the solution increases.

Explanation: This question tests understanding of entropy changes during ionic dissolution. When crystalline KBr dissolves, the K+ and Br- ions transition from fixed positions in a highly ordered crystal lattice to freely moving hydrated ions dispersed throughout the solution. This dramatic increase in the freedom of movement and the number of possible arrangements for both the ions and the water molecules that hydrate them results in a positive ΔS. A common misconception is that ΔS decreases because ions in solution are more ordered than in a crystal (choice A), but the opposite is true - dissolved ions have much more freedom than those in a crystal lattice. For ionic dissolution, entropy typically increases due to increased particle dispersal.

Question 7

1. The entropy remains approximately constant because the temperature stays at $-78\,^{\circ}\mathrm{C}$.
2. The entropy increases because a gas has more accessible microstates than a solid. (correct answer)
3. The entropy decreases because the solid CO$_2$ is denser than the gas.
4. The entropy decreases because the CO$_2$ absorbs heat from the surroundings.
5. The entropy remains approximately constant because the chemical identity of CO$_2$ does not change.

Explanation: This question tests the understanding of entropy changes in sublimation from solid to gas. As solid CO2 sublimes into gas at constant temperature, the molecules move from a highly ordered lattice to a dispersed gaseous state. This phase change significantly increases the number of accessible microstates due to greater positional freedom. The process is endothermic, but entropy is governed by the increase in disorder. A tempting distractor is choice A, which claims entropy remains constant because temperature is constant, but this misconception disregards the profound impact of phase transitions on microstates. When assessing sublimation or deposition, focus on the entropy increase associated with transitioning to less constrained phases like gases.

Question 8

1. The entropy increases because the solution becomes less concentrated as solid forms.
2. The entropy decreases because forming an ordered crystal reduces the number of accessible microstates. (correct answer)
3. The entropy remains approximately constant because the total amount of KNO$_3$ in the beaker is unchanged.
4. The entropy increases because cooling always increases entropy by lowering kinetic energy.
5. The entropy remains approximately constant because crystallization is a physical change, not a chemical reaction.

Explanation: This question tests the understanding of entropy changes during crystallization from solution. Cooling the saturated KNO3 solution causes dissolved ions to form an ordered solid crystal, reducing the dispersion of KNO3 particles. This transition to a more structured state decreases the number of accessible microstates, lowering entropy. The remaining solution is less concentrated, but the ordering in the solid dominates. A tempting distractor is choice D, which claims entropy increases due to lower concentration, but this misconception overlooks the entropy decrease from forming an ordered solid. In crystallization processes, evaluate entropy by comparing the disorder in solution versus the order in the crystalline product.

Question 9

1. The entropy decreases because dissolving ions creates strong ion–dipole attractions that make the system more ordered.
2. The entropy increases because mixing produces a more dispersed distribution of solute particles. (correct answer)
3. The entropy remains approximately constant because the temperature does not change appreciably.
4. The entropy decreases because the concentration of NaCl decreases upon dilution.
5. The entropy remains approximately constant because the total volume doubles.

Explanation: This question tests the concept of entropy changes upon mixing or dilution of solutions. Pouring 1.0 M NaCl into pure water doubles the volume, dispersing the NaCl ions more widely and creating a more uniform mixture. This increased dispersion enhances the number of possible arrangements of solute particles, increasing the entropy of the system. The temperature remains constant, but the mixing effect drives the entropy change. A tempting distractor is choice A, which suggests entropy decreases due to ion-dipole attractions creating order, but this misconception overemphasizes interactions while ignoring the overall increase in dispersion. To analyze entropy in solutions, consider how dilution or mixing promotes greater particle distribution and randomness.

Question 10

1. The entropy remains approximately constant because the phase does not change.
2. The entropy increases because increasing temperature increases the number of accessible vibrational microstates. (correct answer)
3. The entropy decreases because heating increases order in a solid lattice.
4. The entropy remains approximately constant because the hot plate provides heat at constant pressure.
5. The entropy decreases because copper has a fixed composition and fixed molar mass.

Explanation: This question tests the concept of entropy changes with temperature in solids. Heating copper from 25°C to 80°C increases atomic vibrational energy, expanding accessible microstates. This temperature rise enhances entropy without phase change. The solid remains structured, but thermal motion increases disorder. A tempting distractor is choice C, which suggests entropy remains constant because no phase change occurs, but this misconception ignores temperature's role in vibrational entropy. When heating materials, recognize that higher temperatures generally increase entropy through greater energy distribution.

Question 11

1. The entropy remains approximately constant because the syringe is sealed and the number of moles is constant.
2. The entropy decreases because the pressure increases and higher pressure always lowers entropy.
3. The entropy decreases because the volume decreases and fewer spatial microstates are available. (correct answer)
4. The entropy remains approximately constant because compression is a mechanical change, not a chemical change.
5. The entropy increases because the temperature increases and higher temperature increases accessible microstates.

Explanation: This question tests the understanding of entropy changes in irreversible gas compression. Rapid compression halves the volume, limiting spatial microstates, while the temperature increase is less than in a reversible process due to irreversibility. Overall, the volume reduction dominates, causing entropy to decrease for the gas. The syringe is sealed, and the process is mechanical. A tempting distractor is choice A, which claims entropy increases solely due to temperature rise, but this misconception underestimates the volume effect in irreversible paths. For compression processes, calculate or compare state function changes considering path irreversibility.

Question 12

1. Entropy decreases because the average molar mass of the gas mixture increases.
2. Entropy increases because mixing increases the number of accessible microstates. (correct answer)
3. Entropy remains approximately constant because the total pressure is unchanged.
4. Entropy decreases because collisions between different gases reduce randomness.
5. Entropy remains approximately constant because no chemical reaction occurs.

Explanation: This question tests the understanding of entropy changes due to the mixing of ideal gases. The stimulus involves helium and argon gases, initially separated, mixing spontaneously after removing a partition at constant temperature and pressure. Entropy increases because the mixing allows molecules of each gas to occupy the entire volume, increasing the number of possible arrangements and accessible microstates. This follows the entropy of mixing formula ΔS = -nR (x1 ln x1 + x2 ln x2), which is positive for non-identical gases. A tempting distractor is choice A, which falsely claims entropy decreases because the average molar mass increases, confusing mass with the disorder introduced by mixing different species. For predicting entropy in gas mixing, evaluate how the intermingling of different particles enhances randomness regardless of other unchanged parameters like pressure.

Question 13

1. Entropy decreases because the solution becomes uniformly colored, which looks more ordered.
2. Entropy increases because the dispersed particles are spread over a larger volume with more possible arrangements. (correct answer)
3. Entropy remains approximately constant because diffusion does not change temperature.
4. Entropy decreases because the particles slow down as they spread out.
5. Entropy remains approximately constant because no chemical bonds are broken or formed.

Explanation: This question tests the understanding of entropy changes due to diffusion and dispersal in solution. The stimulus involves a KMnO4 crystal dissolving and its purple color spreading uniformly through still water without stirring. Entropy increases because the KMnO4 particles disperse from a concentrated solid to a uniform distribution throughout the beaker, creating more possible arrangements and microstates. This process exemplifies spontaneous diffusion, driven by entropy increase as particles explore larger volumes. A tempting distractor is choice A, which falsely states entropy decreases because the uniform color looks more ordered, confusing visual uniformity with molecular-level disorder and failing to recognize that dispersal enhances randomness. To predict entropy in diffusion, think about how spreading particles over larger spaces multiplies the number of microscopic configurations.

Question 14

1. Entropy decreases because dilution lowers the concentration and therefore lowers entropy.
2. Entropy remains approximately constant because the temperature is 25°C throughout.
3. Entropy increases because mixing and dilution increase dispersal and the number of microstates. (correct answer)
4. Entropy decreases because stirring makes the solution more uniform and therefore more ordered.
5. Entropy remains approximately constant because the total volume is conserved when combining liquids.

Explanation: This question tests understanding of entropy changes during dilution and mixing. When pure water mixes with sucrose solution, the sucrose molecules become distributed throughout a larger volume (200 mL instead of 100 mL), and water molecules that were initially separated from sucrose now intermingle with them. This creates many more possible arrangements as sucrose molecules have more positions available and two previously separated components are now mixed. The increased dispersal of particles and mixing of different substances both contribute to increased entropy. Choice A incorrectly assumes lower concentration means lower entropy, missing that dilution increases positional disorder. Remember that both mixing different substances and diluting solutions increase entropy.

Question 15

1. Entropy decreases because the gas temperature stays constant.
2. Entropy remains approximately constant because no heat is transferred.
3. Entropy increases because the gas occupies a larger volume. (correct answer)
4. Entropy decreases because the pressure decreases during expansion.
5. Entropy remains approximately constant because the amount of gas does not change.

Explanation: This question tests understanding of entropy changes during gas expansion. When the valve opens, the neon gas expands from half the container to fill the entire container, doubling its volume while temperature remains constant. This expansion increases entropy because the gas molecules now have more spatial positions available to them, creating more possible microstates for the system. The fact that no heat transfer occurs (free expansion into vacuum) does not mean entropy is constant—entropy is a state function that depends on the system's configuration, not just heat flow. A common misconception (choice B) is thinking that entropy only changes when heat is transferred, but entropy fundamentally measures the number of accessible microstates. To determine entropy changes, always consider whether particles have more or fewer ways to arrange themselves in the final state compared to the initial state.

Question 16

1. Entropy decreases because the average molar mass of the gas mixture increases.
2. Entropy remains approximately constant because the container is insulated so no heat flows.
3. Entropy increases because mixing increases dispersal and the number of possible arrangements. (correct answer)
4. Entropy decreases because the pressure becomes uniform throughout the container.
5. Entropy remains approximately constant because the total number of moles of gas is 2.0 mol both before and after.

Explanation: This question tests understanding of entropy of mixing. When the partition is removed, N₂ and O₂ molecules mix spontaneously, with each gas expanding to fill the entire container. Initially, N₂ molecules are confined to the left and O₂ to the right, but after mixing, both types of molecules can be found anywhere in the container. This creates many more possible arrangements (microstates) because we now have distinguishable particles that can be distributed in more ways. Choice E incorrectly assumes that constant total moles means constant entropy, missing that mixing different substances always increases entropy. Remember that mixing of different gases is irreversible precisely because it increases entropy.

Question 17

1. Entropy decreases because the pressure of the gas decreases after expansion.
2. Entropy remains approximately constant because the temperature is unchanged.
3. Entropy increases because the gas occupies a larger volume and has more microstates. (correct answer)
4. Entropy decreases because no heat is added to the gas during the process.
5. Entropy remains approximately constant because argon is monatomic and has no vibrations.

Explanation: This question tests understanding of entropy changes during gas expansion. When the stopcock opens, argon gas expands from one bulb to fill both bulbs, doubling its available volume. With more space available, the gas molecules have many more possible positions and velocities they can adopt, significantly increasing the number of microstates. This expansion occurs spontaneously precisely because it increases entropy. The fact that temperature remains constant (isothermal expansion) doesn't mean entropy is constant - entropy depends on volume for gases. Choice B incorrectly assumes constant temperature means constant entropy, missing that volume changes affect entropy. Remember that any spontaneous process at constant temperature must increase entropy.

Question 18

1. Entropy remains approximately constant because the container is closed and no matter enters or leaves.
2. Entropy decreases because sublimation requires energy and endothermic processes reduce entropy.
3. Entropy increases because forming a gas phase increases dispersal and accessible microstates. (correct answer)
4. Entropy remains approximately constant because equilibrium means forward and reverse rates are equal.
5. Entropy decreases because the purple color indicates stronger intermolecular forces in the gas.

Explanation: This question tests understanding of entropy changes during sublimation. When solid iodine sublimes to form gaseous I₂, some molecules escape the ordered crystal lattice to move freely in the gas phase. In the solid, molecules are confined to fixed positions, but in the gas phase, they have translational, rotational, and vibrational freedom throughout the container volume. This dramatic increase in molecular freedom creates many more accessible microstates and increases entropy. Choice D incorrectly assumes equilibrium means no entropy change, missing that the system's entropy has increased from the initial all-solid state to the final solid-plus-gas state. The key principle is that creating a gas phase from a solid always increases system entropy.

Question 19

1. Entropy decreases because boiling requires input of heat.
2. Entropy increases because the vapor has more accessible microstates than the liquid. (correct answer)
3. Entropy remains approximately constant because the temperature is constant at the boiling point.
4. Entropy decreases because the density decreases as liquid becomes gas.
5. Entropy remains approximately constant because ethanol's chemical identity does not change.

Explanation: This question tests understanding of entropy changes during vaporization. When liquid ethanol boils to form vapor at constant temperature, the molecules escape the relatively ordered liquid phase where they experience significant intermolecular attractions and enter the gas phase with much greater freedom of motion. In the vapor phase, molecules can move independently throughout the available volume with minimal intermolecular interactions, creating vastly more accessible microstates than in the condensed liquid phase. This increase in molecular freedom and spatial distribution causes entropy to increase significantly. A common misconception (choice C) is thinking that constant temperature during phase change means constant entropy, but phase transitions involve large entropy changes precisely because molecular arrangements change dramatically. For any liquid-to-gas transition, entropy always increases because gases have far more configurational possibilities than liquids.

Question 20

1. Entropy remains approximately constant because both substances are liquids in both states.
2. Entropy increases because mixing increases dispersal and the number of possible arrangements. (correct answer)
3. Entropy decreases because dissolving bromine in hexane is exothermic for many solutes.
4. Entropy remains approximately constant because the total volume is 100 mL in both cases.
5. Entropy decreases because forming a homogeneous solution means the system is more uniform.

Explanation: This question tests understanding of entropy changes during liquid mixing. When bromine and hexane mix to form a homogeneous solution, the molecules of each substance become dispersed throughout the entire volume rather than being confined to separate layers. Initially, Br₂ molecules are only in the bromine layer and C₆H₁₄ molecules only in the hexane layer, but after mixing, both types of molecules can be found anywhere in the solution. This increase in positional possibilities creates many more microstates and increases entropy. Choice E incorrectly equates uniformity with lower entropy, missing that molecular-level mixing increases disorder. The principle to remember is that mixing of different substances always increases entropy, regardless of phase.