4 questions, one for each idea where we can. Answer them, then see which ideas to fix.
Question 1 of 4
A student wants to determine whether the dissolving of a solid in water is endothermic or exothermic. Which measurements must the student make?
Answer and reasoning
AThe temperature of the solution after the solid has dissolved and nothing else A student who thinks a single temperature shows the energy change picks this. Without the starting temperature, the student cannot tell whether the solution warmed or cooled.
BThe time taken for the solid to dissolve completely in the water A student who links fast processes with releasing energy picks this. The speed of dissolving does not show the direction of energy transfer; the temperature change does.
CThe mass of the cup and its contents before and after the solid dissolves A student who thinks energy changes show up as mass changes picks this. The mass stays the same whether energy is released or absorbed; only the temperature change shows the direction.
DThe temperature of the water before the solid is added and after it has dissolvedCorrect The direction of energy transfer is shown by the change in temperature of the water. A fall means energy passed from the water into the dissolving process (endothermic); a rise means the reverse. Both temperatures are needed to find that change.
Working Energy transfer is shown by the temperature change of the water (the surroundings): ΔT = T(final) − T(initial). A negative ΔT means energy flowed from the water into the dissolving process (endothermic); a positive ΔT means the reverse (exothermic). So the temperature before the solid is added and the temperature after it has dissolved are both needed; mass, time or a single reading cannot give the direction of the transfer.
Which of the following processes is endothermic for the substance that changes?
Answer and reasoning
ALiquid water evaporating from the surface of wet skinCorrect Evaporating separates water molecules that attract each other, which requires energy, so the water absorbs energy from the skin; that is why the skin cools.
BMolten candle wax becoming solid in a cool mold A student who thinks forming attractions requires energy picks this. As the wax solidifies, attractions between its molecules form and energy is released, so solidifying is exothermic.
CWarm water cooling down inside a refrigerator A student who thinks the refrigerator puts cold into the water picks this. The water cools because energy is transferred out of it, so for the water the process is exothermic.
DSteam condensing on the surface of a cold window A student who judges the process by the cold window rather than by the steam picks this. As steam condenses, attractions form between water molecules and energy is released to the window, so condensing is exothermic.
Zinc metal reacts with hydrochloric acid in a flask connected to a gas syringe: Zn(s) + 2 HCl(aq) → ZnCl₂(aq) + H₂(g). As the reaction occurs, the solution becomes warmer and the H₂(g) produced pushes the syringe plunger outward. Taking the reacting species as the system, which statement correctly describes the energy changes?
Answer and reasoning
AThe system gains energy, as shown by the rise in temperature of the solution A student who reads the temperature of the solution as the energy of the system picks this. The water of the solution is part of the surroundings; it warms because the reacting system transfers energy to it.
BThe system loses energy, which the reaction uses up instead of passing it to the surroundings A student who thinks energy can be used up picks this. Energy lost by the system is not destroyed: it is gained by the surroundings, which is why the solution warms.
CThe system loses energy; the surroundings gain it as heat and as work on the plungerCorrect The reacting species (the system) lose energy. The surroundings gain that energy as heat, which warms the solution and the flask, and as work, because the gas produced pushes the plunger outward against the atmosphere.
DThe system loses energy only as heat; pushing the plunger out does not transfer energy A student who thinks energy leaves a system only as heat picks this. Pushing the plunger outward against the atmosphere is work done by the system, which is also a transfer of energy to the surroundings.
The particulate diagrams represent a small part of a sample of sodium chloride and water before and after some of the NaCl(s) dissolves. Which statement correctly describes the interactions shown changing as the NaCl dissolves?
Answer and reasoning
AAttractions between ions are overcome, while the water molecules simply fill the gaps between them A student who treats the water as a passive background picks this. For water molecules to surround the ions, some attractions between water molecules must be overcome as well.
BThe solid melts: the ions separate and spread out without forming any new attractions A student who thinks dissolving is a kind of melting picks this. The separated ions are surrounded by water molecules oriented toward them, which shows that new ion–water attractions have formed.
CBonds inside NaCl molecules break, and the freed ions are then attracted to water molecules A student who thinks solid sodium chloride is made of NaCl molecules picks this. The solid is a lattice of ions, each attracted to all its oppositely charged neighbors; there are no NaCl molecules to break.
DIon–ion attractions and water–water attractions are overcome, and ion–water attractions formCorrect Before, each ion is attracted to its oppositely charged neighbors in the solid and the water molecules are attracted to one another. After, the ions are apart and each is surrounded by water molecules oriented toward it (O toward Na⁺, H toward Cl⁻): ion–ion and water–water attractions have been overcome, and ion–water attractions have formed.
In preparation: 0 of 4 sections compiled and reviewed. The rest show key terms and common mistakes from our question bank until they are.
6.1.A.1 Temperature change as evidence of an energy change Fix
Temperature change as evidence of an energy change
A change in temperature shows that energy has been transferred. When a process in solution warms the water, energy has been transferred from the process to the water; when it cools the water, energy has been transferred from the water to the process.
Students often think Cold is something that can be given out or absorbed, so an endothermic process cools its surroundings by releasing cold into them. In fact No. Cold is not a substance and is not transferred. When something cools, energy has been transferred out of it; in an endothermic process the surroundings cool because energy passes from them to the system.
Students often think Energy always leaks out of a container into the room, whatever the temperatures, so a poorly insulated container always loses energy. In fact No. Energy flows from the warmer body to the cooler one. If a solution is colder than the room, energy flows into it from the room; if it is warmer, energy flows out of it.
6.1.A.2 Exothermic process Fix
Exothermic process
A process in which the energy of the system decreases and energy is transferred to the surroundings, for example condensing, freezing, the cooling of a substance or a combustion reaction.
Endothermic process
A process in which the energy of the system increases and energy is transferred from the surroundings, for example melting, boiling or evaporating, the heating of a substance, or the dissolving of ammonium nitrate in water.
Phase change at constant temperature
While a pure substance melts or boils it absorbs energy and while it freezes or condenses it releases energy, even though its temperature stays constant; the energy changes the arrangement and attractions of the particles rather than their average kinetic energy.
Students often think Energy is needed to form attractions or bonds between particles, so processes in which attractions form, such as freezing or forming ion–water attractions, absorb energy. In fact No. Forming attractions or bonds between particles releases energy; separating particles that attract each other requires energy. That is why freezing and condensing release energy, and why forming ion–water attractions releases energy during dissolving.
Students often think If the temperature of a substance stays the same, no energy is being transferred to or from it, because heat and temperature are the same thing. In fact No. During a phase change, such as melting or freezing, a pure substance absorbs or releases energy while its temperature stays constant. Temperature and energy are different quantities: the energy changes the attractions between particles, not their average kinetic energy.
6.1.A.3 System and surroundings Fix
System and surroundings
The system is the part being studied, for a reaction the reacting species; the surroundings are everything else, including the water of a solution, the container and the air. Energy lost by the system is gained by the surroundings, and energy gained by the system comes from the surroundings.
Heat transfer and work
The two ways in which energy passes between a system and its surroundings: heat transfer, driven by a temperature difference, and work, for example a gas produced by a reaction pushing back a piston or the atmosphere.
Energy change of a reaction
When a reaction occurs, the energy of the reacting system decreases (exothermic), increases (endothermic) or stays the same; the energy lost or gained by the system equals the energy gained or lost by the surroundings.
Students often think The temperature of the solution measures the energy of the system, so a falling temperature means the system has lost energy and a rising temperature means it has gained energy. In fact No. The thermometer measures the water and the container, which are part of the surroundings. A falling temperature shows that energy has been transferred from the surroundings to the system, so the system has gained energy and the process is endothermic; a rising temperature shows the reverse.
Students often think Energy can be used up, so some of the energy lost by a reacting system, or by a substance that freezes, disappears rather than going to the surroundings. In fact No. Energy is not destroyed. Energy lost by the system is gained by the surroundings, as heat or work, and energy gained by the system comes from the surroundings.
6.1.A.4 Energy of solution formation Fix
Energy of solution formation
Dissolving requires energy to overcome the attractions between solute particles and between solvent particles, and releases energy as solute–solvent attractions form. If the attractions formed are stronger overall, dissolving is exothermic; if they are weaker overall, it is endothermic.
Students often think When an ionic solid dissolves, only the attractions between the ions are overcome; the water just surrounds the ions and its own attractions are unchanged. In fact Yes. To make room for the ions, some attractions between water molecules must be overcome, which requires energy, as well as the attractions between the ions. Ion–water attractions then form.
Students often think Dissolving is a kind of melting: the solid's particles are pulled apart and spread out, and no new attractions form. In fact No. Melting separates particles of one pure substance by heating it to its melting point. Dissolving separates solute particles because they become surrounded by, and attracted to, solvent particles; new solute–solvent attractions form.
5 more questions. Every wrong answer here is a real mistake students make, and you see why it is wrong as soon as you answer.
Question 1 of 5
A student adds solid ammonium nitrate, NH₄NO₃, to water in an insulated cup at 30 s and stirs until the solid has dissolved. The graph shows the temperature of the contents of the cup. Which claim about the dissolving of NH₄NO₃, with its justification, is correct?
Answer and reasoning
AExothermic: the temperature fell, which shows that the dissolving process lost energy A student who reads the temperature of the solution as the energy of the system picks this. The thermometer measures the water, part of the surroundings; its temperature fell because the dissolving process took energy from it.
BEndothermic: the water cooled, so energy passed from the water to the dissolving solidCorrect The temperature of the water (the surroundings) fell from 22.0 °C to 17.5 °C. A temperature change shows an energy transfer: energy passed from the water to the dissolving NH₄NO₃ (the system), so the process is endothermic.
CExothermic: the dissolving solid gave out cold, which lowered the temperature of the water A student who thinks of cold as something that can be given out picks this. Cold is not transferred; the water cooled because energy was transferred out of it, into the dissolving solid.
DEndothermic: energy was used up in forming the new attractions between the ions and water A student who thinks forming attractions requires energy picks this. Forming ion–water attractions releases energy; the process is endothermic because separating the ions (and water molecules) requires more energy than that.
A sample of liquid water at 0 °C is placed in a freezer. While the water freezes, its temperature stays at 0 °C. How does the energy of the water change while it freezes?
Answer and reasoning
AIt decreases, because energy is transferred from the water to the colder freezerCorrect Freezing is exothermic: as the molecules settle into the solid and attractions form, energy is released and transferred to the colder surroundings. The temperature stays constant, but the energy of the water decreases.
BIt stays the same, because the temperature of the water does not change A student who equates heat with temperature picks this. During a phase change the temperature stays constant while energy is transferred; freezing water releases energy to the freezer.
CIt increases, because energy is needed to form the attractions that hold the ice together A student who thinks forming attractions requires energy picks this. Forming attractions between water molecules releases energy, which is why freezing is exothermic.
DIt decreases, because part of the water's energy is destroyed as the ice forms A student who thinks energy can be destroyed picks this. The energy of the water does decrease, but the energy is transferred to the freezer, not destroyed.
When a hypothetical ionic solid, MX, dissolves in water in an insulated cup, the temperature of the solution decreases. Which statement about the interactions involved is consistent with this observation?
Answer and reasoning
AThe attractions formed are weaker overall than those overcome, so dissolving absorbs energyCorrect The water cooled, so the dissolving process absorbed energy overall. Overcoming ion–ion and water–water attractions absorbs energy; forming ion–water attractions releases energy. Net absorption means the attractions formed are weaker overall than those overcome.
BThe attractions formed are stronger overall, as the falling temperature shows the system lost energy A student who reads the temperature of the solution as the energy of the system picks this. The falling temperature shows that the dissolving process took energy from the water, so the attractions formed are weaker overall.
CThe attractions formed are stronger overall, because forming attractions absorbs energy from the water A student who thinks forming attractions requires energy picks this. Forming ion–water attractions releases energy; the process absorbs energy overall only because the attractions overcome are stronger.
DThe attractions formed and those overcome are equal, because dissolving is only a physical change A student who thinks physical changes involve no energy change picks this. Dissolving changes the attractions between particles, and the temperature change shows that energy was absorbed overall.
A student dissolves ammonium nitrate, NH₄NO₃, in water at room temperature in an insulated cup and records the decrease in temperature. The student then repeats the experiment with the same masses of NH₄NO₃ and water, starting at room temperature as before, in an uninsulated glass beaker. How will the measured temperature decrease in the beaker compare with that in the insulated cup?
Answer and reasoning
AIt will be smaller, because cold escapes from the solution through the glass into the room A student who thinks of cold as something that is transferred picks this. The decrease is smaller, but because energy flows into the colder solution from the room; cold does not flow anywhere.
BIt will be larger, because energy escapes from the solution through the glass to the room A student who thinks energy always leaks out of a container picks this. Energy flows from warmer to cooler, and the solution is colder than the room, so energy flows in, making the decrease smaller.
CIt will be smaller, because energy flows into the cold solution from the warmer roomCorrect Once the solution becomes colder than the room, energy flows from the room into it. The uninsulated beaker lets more energy in, partly offsetting the cooling caused by the dissolving, so the measured decrease is smaller.
DIt will be the same, because the energy absorbed depends only on the amount of NH₄NO₃ A student who thinks the measured temperature change depends only on the process picks this. The energy absorbed by dissolving is the same, but energy entering from the room also changes the temperature that is measured.
A sample of a pure hypothetical substance, initially a solid, is heated at a constant pressure. The graph shows the temperature of the sample as a function of the total energy added. Based on the graph, how much energy does the sample absorb as it melts?
Answer and reasoning
A6.0 kJ A student who reads the energy for a section from the value at its end picks this. The axis shows the total energy added since the start, so the energy for melting is 6.0 − 2.0 = 4.0 kJ.
B4.0 kJCorrect The sample melts on the first horizontal section, at 20 °C, which runs from 2.0 kJ to 6.0 kJ of energy added. It absorbs 6.0 − 2.0 = 4.0 kJ while melting, although its temperature does not change.
C0.0 kJ A student who thinks no energy is absorbed when the temperature does not change picks this. The sample keeps absorbing energy along the horizontal section; that energy melts it rather than raising its temperature.
D2.0 kJ A student who thinks the solid melts as it warms toward its melting point picks this, taking the 2.0 kJ needed to warm the solid from −10 °C to 20 °C. Melting happens at 20 °C, on the horizontal section from 2.0 to 6.0 kJ.
Working Melting occurs on the first horizontal section, at 20 °C, from 2.0 kJ to 6.0 kJ of energy added. Energy absorbed = 6.0 kJ − 2.0 kJ = 4.0 kJ. Distractors: value at the end of the section, 6.0 kJ; no temperature change taken as no energy, 0.0 kJ; energy to warm the solid to its melting point, 2.0 kJ − 0 kJ = 2.0 kJ.
Compiled from the AP Chemistry Course and Exam Description (effective Fall 2024) and our question bank · Specialist review in progress. How these pages are made · Free, no account
