1 question, one for each idea where we can. Answer them, then see which ideas to fix.
Question 1 of 1
A reaction occurs by a two-step mechanism. Its energy profile has two maxima with an energy minimum between them. What does that minimum represent?
Answer and reasoning
ASpecies that are formed in the first step and used up in the secondCorrect The minimum between the two maxima is a reaction intermediate: it is a product of the first elementary step and a reactant in the second, so it is present only while the reaction is occurring.
BSpecies with bonds partly broken and partly formed, at their least stable A student who mixes up transition states and intermediates picks this. Partly broken and partly formed bonds describe a transition state, which sits at a maximum; the minimum between the maxima is the intermediate.
CSpecies that are present at the start and that are re-formed at the end A student who treats any species missing from the overall equation as a catalyst picks this. A catalyst is present at the start and the end; the species at this minimum is formed in step 1 and used up in step 2, so it is absent at both ends.
DThe moment in the reaction at which half of the reactants are products A student who reads the reaction coordinate as a time axis picks this. The horizontal axis shows the progress of the rearrangement of atoms, not time, so a point on it does not mark a moment or a fraction converted.
In preparation: 0 of 1 sections compiled and reviewed. The rest show key terms and common mistakes from our question bank until they are.
5.10.A.1 Energy profile for a multistep reaction Fix
Energy profile for a multistep reaction
A graph of energy along the reaction coordinate for a reaction that occurs by a mechanism of two or more elementary steps. It is built step by step from the energetics of each elementary reaction: each step contributes one maximum (its transition state), the minima between maxima are intermediates, and the curve starts at the reactants and ends at the products.
Elementary step in an energy profile
One elementary reaction of the mechanism. In the profile it runs from the energy minimum of the species that react in that step, over one maximum, to the minimum of the species it forms, so the number of maxima equals the number of steps.
Transition state
The highest-energy arrangement of atoms along the path of one elementary step, with bonds partly broken and partly formed. It sits at a maximum of the profile and is not a species that can be collected.
Reaction intermediate in an energy profile
A species formed in one elementary step and used up in a later step. In the profile it sits at an energy minimum between two maxima; it is present only while the reaction is occurring and does not appear in the overall equation.
Activation energy of a step
The energy difference between the transition state of an elementary step and the species that react in that step. For a step after the first, it is measured from the minimum just before that step's maximum (for example from an intermediate), not from the starting reactants.
Overall energy change of a multistep reaction
The energy of the products minus the energy of the reactants, read from the two ends of the profile. It is negative when the products are lower in energy than the reactants and positive when they are higher; a different set of steps between the same reactants and products ends at the same energy difference.
Rate-determining step in a profile
The slowest elementary step, which limits the rate of the overall reaction. In profiles used at AP level it is identified as the step with the largest activation energy; it need not be the first step.
Reaction coordinate
The horizontal axis of an energy profile: the progress of the rearrangement of atoms from reactants, through transition states and any intermediates, to products. It is not a time axis, so the width of a hump does not show how long a step takes.
Students often think The species at a minimum between two humps of a profile is a transition state (activated complex), the least stable arrangement in the reaction. In fact No. A transition state is at a maximum, where bonds are partly broken and partly formed. A minimum between two maxima is a reaction intermediate: a species formed in one step and used up in the next.
Students often think The horizontal axis of an energy profile is time, so a point partway along it marks a moment in the reaction, such as when half of the reactants have become products. In fact No. The reaction coordinate shows how far the rearrangement of atoms has progressed, not time. A point along it does not tell how long the reaction has run or what fraction of the reactants has been converted.
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
A hypothetical reaction, A + B → C + D, occurs by a two-step mechanism. Step 1, A + B → X, has an activation energy of 40 kJ/mol and an energy change of +20 kJ/mol. Step 2, X → C + D, has an activation energy of 80 kJ/mol and an energy change of −60 kJ/mol. Which of the numbered graphs shown best represents the energy profile for the reaction?
Answer and reasoning
AGraph 1 A student who thinks an energy profile shows only one transition state, because the intermediate X cancels out of the overall equation, picks this single hump, with the larger activation energy, 80 kJ/mol, as its barrier. The profile follows the mechanism: two steps give two maxima, with X as a minimum between them.
BGraph 2 A student who reads a positive energy change as energy given out picks this. Step 1's +20 kJ/mol means X is 20 kJ/mol above the reactants, not below; with the signs reversed, X falls to −20, the second maximum is at 60 and the products end at +40 kJ/mol, making the reaction absorb energy overall.
CGraph 3Correct Starting at 0, step 1 rises 40 kJ/mol to its maximum and ends at the intermediate X, +20 kJ/mol. Step 2 is measured from X: its maximum is 20 + 80 = 100 kJ/mol and the products are at 20 − 60 = −40 kJ/mol. Only this graph shows maxima at 40 and 100, a minimum at 20 and products at −40.
DGraph 4 A student who measures every activation energy from the starting reactants picks this, placing the second maximum at 0 + 80 = 80 kJ/mol. Step 2 starts from X, at +20 kJ/mol, so its maximum is at 100 kJ/mol.
Working Start the reactants at 0. Step 1: maximum at 0 + 40 = 40 kJ/mol; intermediate X at 0 + 20 = +20 kJ/mol. Step 2: maximum at 20 + 80 = 100 kJ/mol; products at 20 − 60 = −40 kJ/mol. Key: maxima at 40 and 100, minimum at 20, products at −40. Distractors: one hump from 0 to 80 (the larger activation energy taken as the single barrier) and down to −40 (intermediate left out); signs of the energy changes reversed, X at −20, second maximum at −20 + 80 = 60, products at −20 + 60 = +40; second barrier measured from the reactants, maximum at 0 + 80 = 80, products at −40.
The energy profile shown represents a hypothetical reaction that occurs by a two-step mechanism. Based on the profile, what is the activation energy of the second step?
Answer and reasoning
A70 kJ/mol A student who measures every activation energy from the starting reactants picks this: 90 − 20 = 70 kJ/mol. The second step starts from the intermediate at 60 kJ/mol, so its barrier is 30 kJ/mol.
B90 kJ/mol A student who takes the energy of the transition state, read from the axis, as the activation energy picks this. Activation energy is a difference: 90 kJ/mol minus the 60 kJ/mol of the intermediate that reacts in the second step.
C80 kJ/mol A student who measures the barrier from the maximum down to the products picks this: 90 − 10 = 80 kJ/mol. That drop belongs to the reverse direction; the forward barrier is the rise from the intermediate, 30 kJ/mol.
D30 kJ/molCorrect The second step starts from the intermediate at the minimum, 60 kJ/mol, and passes over the second maximum, 90 kJ/mol. Its activation energy is 90 − 60 = 30 kJ/mol.
Working Second step: from the intermediate (minimum at 60 kJ/mol) to the second maximum (90 kJ/mol). Ea(step 2) = 90 − 60 = 30 kJ/mol. Distractors: measured from the reactants, 90 − 20 = 70 kJ/mol; the maximum read from the axis, 90 kJ/mol; measured down to the products, 90 − 10 = 80 kJ/mol.
A hypothetical reaction occurs by a three-step mechanism. Relative to the reactants, the energies of the species along the reaction coordinate are: first transition state, +30 kJ/mol; first intermediate, +20 kJ/mol; second transition state, +105 kJ/mol; second intermediate, −10 kJ/mol; third transition state, +40 kJ/mol; products, −70 kJ/mol. Which step is the rate-determining step, and why?
Answer and reasoning
AStep 1, because the first step of a mechanism sets the rate of the reaction A student who thinks the first step always controls the rate picks this. Step 1 has the smallest barrier, 30 kJ/mol, so it is fast; the slowest step is step 2, with an 85 kJ/mol barrier.
BStep 2, because its activation energy is the largest of the three stepsCorrect The activation energies are: step 1, 30 − 0 = 30 kJ/mol; step 2, 105 − 20 = 85 kJ/mol; step 3, 40 − (−10) = 50 kJ/mol. Step 2 has the largest barrier, and its transition state is also the highest point of the profile, so it is the slowest step.
CStep 1, because its products are higher in energy than its reactants A student who thinks steps that absorb energy are slow picks this. Step 1 does end higher in energy (+20 kJ/mol), but its rate depends on its barrier, only 30 kJ/mol; step 2's barrier, 85 kJ/mol, is the largest.
DStep 3, because it has the largest energy change of the three steps A student who thinks a larger energy change goes with a larger barrier picks this. Step 3 has the largest energy change (−70 − (−10) = −60 kJ/mol), but its barrier is 50 kJ/mol, smaller than step 2's 85 kJ/mol.
A student draws energy profiles for two proposed mechanisms for the same exothermic overall reaction. Mechanism I is a single elementary step. Mechanism II has two elementary steps, and each step has a lower activation energy than the single step of mechanism I. How does the magnitude of the overall energy change shown in the profile for mechanism II compare with that shown for mechanism I?
Answer and reasoning
AThe same, because both start at the same reactants and end at the same productsCorrect The overall energy change is the energy of the products minus that of the reactants. Both mechanisms convert the same reactants into the same products, so both profiles begin and end at the same energies, whatever lies between.
BSmaller for II, because some of the energy remains stored in the intermediate A student who thinks energy stays stored in the intermediate picks this. The intermediate is used up in the second step, so at the end only the products are present, at the same energy as for mechanism I.
CLarger for I, because its single step has the larger activation energy of the two A student who links a higher barrier with a larger energy change picks this. The barrier height and the energy change are separate: the higher barrier of mechanism I does not move its products to a different energy.
DLarger for II, because the faster mechanism releases more energy to the surroundings A student who ties the speed of a reaction to the energy it releases picks this. Lower barriers make mechanism II faster, but the energy released depends only on the energies of the reactants and products.
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
