1 question, one for each idea where we can. Answer them, then see which ideas to fix.
Question 1 of 1
An energy diagram for a process shows the final state at a higher energy than the initial state. Which process could the diagram represent?
Answer and reasoning
ASteam condensing to form liquid water on a cold mirror A student who thinks forming attractions requires energy picks this. As steam condenses, attractions form between water molecules and energy is released, so the liquid is lower in energy than the steam.
BA candle burning after being lit with a match A student who thinks a reaction that needs a flame to start is endothermic picks this. The match supplies only the activation energy; burning wax releases energy, so its products are lower in energy than the reactants.
CIce melting to form liquid water on a warm dayCorrect Melting separates water molecules that attract one another in the solid, which requires energy; the liquid is higher in energy than the ice, so melting is endothermic.
DIron rusting inside a hand warmer as it gets hot A student who thinks a process that heats its surroundings has gained energy picks this. The hand warmer gets hot because the rusting iron transfers energy to it, so the products are lower in energy than the reactants.
In preparation: 0 of 1 sections compiled and reviewed. The rest show key terms and common mistakes from our question bank until they are.
6.2.A.1 Energy diagram Fix
Energy diagram
A diagram that shows the energy of a system before and after a physical or chemical process (and sometimes at stages in between, such as a transition state or separated particles), so that the energy change and whether the process is endothermic or exothermic can be read from it.
Energy diagram of an exothermic process
The products (or final state) are drawn lower in energy than the reactants (or initial state): the system loses energy to the surroundings, and the energy change, final minus initial, is negative.
Energy diagram of an endothermic process
The products (or final state) are drawn higher in energy than the reactants (or initial state): the system gains energy from the surroundings, and the energy change, final minus initial, is positive.
Energy change read from a diagram
The energy of the products (final state) minus the energy of the reactants (initial state). It is set by the two ends of the diagram, not by any maximum or intermediate level between them.
Activation energy on an energy diagram
The rise from the reactants to the maximum of the curve. It shows the energy that colliding particles need for the reaction to occur, which is why some exothermic reactions need a spark or heating to start; it is separate from the energy change of the reaction.
Energy diagram for dissolving
Dissolving can be drawn in stages: separating solute particles from one another and solvent particles from one another raises the energy; forming solute–solvent attractions lowers it. The final level relative to the starting level shows whether dissolving is exothermic or endothermic.
Students often think Energy that is released is a positive amount given to the surroundings, so the energy change of a process that releases energy is written as a positive number. In fact No. The energy change is the energy of the system's final state minus its initial state. When energy is released the system ends lower in energy, so the change is negative; when energy is absorbed it is positive.
Students often think The energy change of a reaction is the rise from the reactants up to the peak of its energy diagram, so a diagram that starts by going up shows a reaction that absorbs energy. In fact No. The rise from the reactants to the maximum is the activation energy. The energy change of the reaction is the difference between the products and the reactants.
4 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 4
An energy diagram for a hypothetical reaction shows the reactants at 50 kJ/mol, a single maximum at 110 kJ/mol and the products at 20 kJ/mol. What is the energy change for the reaction?
Answer and reasoning
A−30 kJ/molCorrect The energy change is the energy of the products minus that of the reactants: 20 − 50 = −30 kJ/mol. The products are lower in energy, so the reaction is exothermic and the change is negative.
B+30 kJ/mol A student who writes energy that is released as a positive change picks this. The system ends 30 kJ/mol lower in energy, so its energy change is −30 kJ/mol.
C+60 kJ/mol A student who reads the rise to the maximum as the energy change picks this: 110 − 50 = 60 kJ/mol. That rise is the activation energy; the energy change compares the products with the reactants.
D−90 kJ/mol A student who takes the energy change as the drop from the maximum to the products picks this: 20 − 110 = −90 kJ/mol. Part of that drop only returns the 60 kJ/mol absorbed on the way up; the net change is 20 − 50 = −30 kJ/mol.
Working Energy change = E(products) − E(reactants) = 20 kJ/mol − 50 kJ/mol = −30 kJ/mol (exothermic: the products are lower). Distractors: sign reversed for energy released, +30 kJ/mol; rise to the peak, 110 − 50 = +60 kJ/mol; drop from the peak to the products, 20 − 110 = −90 kJ/mol.
The energy diagram shown represents the dissolving of a hypothetical molecular solid in water in two stages. Based on the diagram, what is the energy change for the dissolving process?
Answer and reasoning
A+45 kJ/mol A student who thinks dissolving only involves separating particles picks this, the rise from 0 to 45 kJ/mol. The attractions that form between solute and water then release 60 kJ/mol, so the net change is −15 kJ/mol.
B−60 kJ/mol A student who thinks the energy change of dissolving is only the energy released as solute–water attractions form picks this, the drop from 45 to −15 kJ/mol. Separating the particles first absorbs 45 kJ/mol, so the net change is −15 kJ/mol.
C+15 kJ/mol A student who writes energy that is released as a positive change picks this. The solution ends 15 kJ/mol below the starting level, so the energy change is −15 kJ/mol.
D−15 kJ/molCorrect The energy change is the final level minus the starting level: the solution is at −15 kJ/mol and the solid + water at 0, so the change is −15 kJ/mol. Separating the particles absorbs 45 kJ/mol, and forming solute–water attractions releases 60 kJ/mol.
A mixture of methane and oxygen does not react at room temperature, but once a spark is applied the mixture burns: CH₄(g) + 2 O₂(g) → CO₂(g) + 2 H₂O(g). The energy diagram for the reaction is shown. Which claim about the reaction, with its justification, is correct?
Answer and reasoning
AEndothermic: energy from a spark must be supplied before the reaction can start A student who thinks a reaction that needs a spark to start is endothermic picks this. The spark supplies the activation energy; the products are lower in energy than the reactants, so the reaction releases energy overall.
BExothermic: the products are lower in energy than the reactants, as the diagram showsCorrect On the diagram, CO₂ + 2 H₂O ends well below CH₄ + 2 O₂, so the system loses energy and the reaction is exothermic. The spark only supplies the activation energy, the rise to the maximum, that the first colliding molecules need.
CExothermic: energy is released as the bonds in the CH₄ and O₂ molecules break apart A student who thinks breaking bonds releases energy picks this. Breaking the bonds in CH₄ and O₂ requires energy, and energy is released as the bonds in CO₂ and H₂O form; the diagram shows that the products end lower in energy overall.
DEndothermic: the reactants have to absorb energy to rise to the diagram's peak A student who reads the rise to the peak as the energy change picks this. That rise is the activation energy; the energy change compares the products with the reactants, and the products are lower.
An energy diagram for the dissolving of a hypothetical solid, Z, in water shows the dissolved state, Z(aq), at a higher energy than Z(s) and water. A student will dissolve Z in water at room temperature in an insulated cup. Which prediction is consistent with the diagram?
Answer and reasoning
AThe temperature of the water will rise as Z dissolves A student who thinks a system that gains energy makes its surroundings hotter picks this. The energy Z(aq) gains comes from the water, so the water cools.
BThe temperature of the water will not change as Z dissolves A student who thinks physical changes such as dissolving involve no energy change picks this. The diagram shows Z(aq) higher in energy than Z(s) and water, so energy is absorbed from the water.
CZ will dissolve only if the water is heated throughout A student who thinks an endothermic process can occur only while it is heated picks this. The energy can come from the water itself, which cools as Z dissolves at room temperature.
DThe temperature of the water will decrease as Z dissolvesCorrect The diagram shows that dissolving Z is endothermic: the system gains energy, and in an insulated cup that energy comes from the water, so the temperature of the water falls.
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
