Unit 5 check
Unit 5 check: Kinetics
11 questions from 11 lessons in Unit 5. Answer each one, and if you miss, follow the link back to where it is taught, then try again. Your score counts first tries.
Question 1 · from 5.1 · Calculate
The concentration of a reactant falls from 0.800 M to 0.560 M in 40.0 s. What is the average rate at which the reactant is used? Give your answer in units of 10⁻³ M/s, to three significant figures.
Question 2 · from 5.2 · Calculate
A reaction has the rate law rate = k[A][B]². At a constant temperature, [A] is doubled and [B] is halved. By what factor does the rate change? Give your answer to three significant figures.
Question 3 · from 5.3 · Calculate
A reaction is first order in A. [A] falls from 0.800 M to 0.200 M in 2.00 min. What is the rate constant, in min⁻¹? Give your answer to three significant figures.
Question 4 · from 5.4 · Decide and justify
Teaching example: for the overall reaction X + 2Y → Z, doubling [Y] while [X] is held constant does not change the rate, and doubling [X] doubles the rate. A student writes rate = k[X][Y]² from the equation. Is the student’s rate law right for this reaction?
Which reason supports your decision?
Setup for question 5
Use the collision model on situations the lesson has not shown.
Question 5 · from 5.5 · Rank it
Four gas-phase reactions are run at the same temperature. They differ only in activation energy. Rank them by the fraction of collisions that have at least the activation energy.
Largest fraction first
Give each one a place, 1 for first. Give two the same place if they are equal.
Your ranking: none yet
Setup for question 6
Draw a profile of your own, then compare four reactions by their barriers. These numbers are a teaching example.
Question 6 · from 5.6 · Rank it
Four elementary reactions have these energies, in kJ/mol. Reaction A: reactants 20, transition state 80, products 0. Reaction B: reactants 40, transition state 85, products 10. Reaction C: reactants 10, transition state 90, products 60. Reaction D: reactants 30, transition state 75, products 70. Rank them by the activation energy of the forward reaction.
Largest activation energy first
Give each one a place, 1 for first. Give two the same place if they are equal.
Your ranking: none yet
Setup for question 7
Nitrogen oxides help turn sulfur dioxide into sulfur trioxide, a step toward sulfuric acid. The table gives a simplified mechanism. Step 1 happens twice for every time step 2 happens once. These steps are a teaching example.
| Step | Equation |
|---|---|
| 1 | NO₂ + SO₂ → NO + SO₃ |
| 2 | 2 NO + O₂ → 2 NO₂ |
Question 7 · from 5.7 · Multiple choice
Using the table: which is the overall equation for this mechanism?
Question 8 · from 5.8 · Put it in order
Put the steps for testing a mechanism that has a slow first step against measured data in order.
Question 9 · from 5.9 · Fill the statement
Complete the description of the pre-equilibrium approximation.
For a fast, reversible first step A + B ⇌ I, the forward rate k₁[A][B] and the reverse rate k₋₁[] are once the step is in balance. Solving for the gives [I] = (k₁/k₋₁)[A][B], which replaces it in the slow step's rate law.
Setup for question 10
A student is given the energies along the profile of a three-step reaction. These numbers are a teaching example.
| Point on the profile | Energy (kJ/mol) |
|---|---|
| Reactants | 20.0 |
| Transition state 1 | 75.0 |
| First intermediate | 40.0 |
| Transition state 2 | 60.0 |
| Second intermediate | −5.0 |
| Transition state 3 | 110. |
| Products | −30.0 |
Question 10 · from 5.10 · Calculate
Using the table: what is the overall energy change of the reaction, in kJ/mol? Include its sign, and give your answer to three significant figures.
Question 11 · from 5.11 · Match
Match each kind of catalyst to how it speeds up a reaction.
0 of 11 answered.