Unit 7 · Topic 7.1 · about 30 minutes · 7 parts · 12 scored checks, 1 of them multiple choice, plus 2 from earlier lessons
AP Chemistry 7.1: Introduction to Equilibrium
Unit 7: 7 to 9% of the multiple-choice section
What you'll be able to do
- Explain how a reversible change in a closed system reaches equilibrium, and why a system at equilibrium looks unchanged while its forward and reverse processes continue.
Key vocabulary
| Term | What it means | The exam trap |
|---|---|---|
| Reversible process | A change that can run in both directions, so that its products can change back into its reactants. | A double arrow shows that a change can run both ways, but it does not say how much of each substance is present. |
| Closed system | A system in which no matter enters or leaves, so the substances stay together and can react in both directions. | Decide whether anything can enter or leave a container before you say anything about the state of its contents. |
| Equilibrium | The state of a closed system in which the forward and reverse processes continue at equal rates, so the concentrations or partial pressures stay constant. | The word suggests stillness or balance, so read what is held constant and what continues before you picture it. |
| Forward and reverse reactions | The forward reaction turns the reactants on the left of the equation into the products on the right; the reverse reaction does the opposite. | Which direction is called forward depends only on how the equation is written, so read the equation before you label either one. |
Tap to open the picture full sizePart 1 Think first A sealed bottle of water
Commit to a guess first. Being wrong now makes the right idea stick better.
Predict first
A student half fills a clear bottle with water, screws the cap on tight, and leaves it on a shelf. For the first day the water level drops a little and a fog appears on the inside of the bottle. After a week the water level has not changed, and the bottle looks the same every day. What is going on at the surface of the water?
In short. A reversible change in a closed system reaches equilibrium when the forward and reverse processes run at equal rates, so the concentrations or partial pressures stop changing even though both processes continue.
- Equilibrium is dynamic: nothing seems to change, but the forward and reverse processes never stop.
- At equilibrium reactants and products are both present, and their concentrations or partial pressures are constant, not necessarily equal.
- Graphs of concentration, partial pressure or rate against time level off, or meet, when equilibrium is reached.
Part 2 Learn it From a reversible change to equilibrium
2a. Reversible changes in a closed system
Many changes can run in either direction. Liquid water evaporates, and water vapor condenses. A salt dissolves in water, and its ions can come back out as a solid, which is precipitation. A gas can be absorbed by a liquid and later desorbed. Chemical changes can be reversible too: in an acid-base reaction a proton moves from one particle to another and can move back, and in a redox reaction electrons move and can move back.
A reversible reaction is written with a double arrow:
N₂O₄(g) ⇌ 2 NO₂(g)
Colorless N₂O₄ breaking into brown NO₂ is the forward reaction, and NO₂ joining into N₂O₄ is the reverse reaction. Which one is forward depends only on how the equation is written.
For a reversible change to settle, the substances have to stay together, so the system must be closed: no matter enters or leaves. In a sealed flask the vapor stays where it can condense again. In an open dish the vapor drifts away.
Quick check · Match
Match each change to the process that reverses it.
2b. What you can observe at equilibrium
Seal colorless N₂O₄ alone in a flask at constant temperature. At first the gas is colorless. Brown NO₂ forms and the color deepens. After a while the color stops changing. The system has reached equilibrium.
At equilibrium:
- No changes can be observed. The color, the pressure and the temperature stay the same.
- Reactants and products are both present. The flask holds N₂O₄ and NO₂ together.
- The concentrations are constant. For a gas, the partial pressure of each species is constant too.
The constant values are whatever the system settles at. They depend on the reaction, and later topics show how.
Quick check · Select all
A sealed flask that began with only colorless N₂O₄(g) has shown the same brown color for an hour, at constant temperature. N₂O₄(g) ⇌ 2 NO₂(g). Which two statements describe the gas in the flask now? Select two.
Select 2 answers, then check.
2c. Equilibrium is dynamic
No observable change does not mean nothing is happening. The equilibrium state is dynamic: N₂O₄ molecules keep breaking into NO₂, and NO₂ molecules keep joining into N₂O₄. At equilibrium the forward and reverse reactions go on at equal rates, so every N₂O₄ that is used up is matched by another that forms, and the amounts stay the same. The result is no net observable change.
Picture two rooms joined by a doorway. If people cross in each direction at the same rate, the number in each room stays the same, although people are always moving. The head count is constant; the crossing never stops.
The same is true of the sealed bottle of water: molecules leave the liquid and return to it at equal rates, so the water level holds still.
This is why the state is called a dynamic equilibrium. If the molecules really stopped, there would be no reverse reaction to balance the forward one.
Quick check · Decide and justify
The brown color of the sealed flask has not changed for an hour. Have the forward and reverse reactions stopped?
Which reason supports your decision?
2d. Graphs of a system reaching equilibrium
Graphs show how a system gets to equilibrium. Start with only N₂O₄ in a closed flask.
Concentration (or partial pressure) against time. The concentration of N₂O₄ falls and the concentration of NO₂ rises. Each changes fastest at the start, and each curve bends over and levels off. From the time every curve is level, the system is at equilibrium. The curves change in the ratio of the balanced equation: for each N₂O₄ used up, two NO₂ form.
Rate against time. The forward rate is greatest at the start, when N₂O₄ is plentiful, and it falls as N₂O₄ is used up. The reverse rate starts at zero, because there is no NO₂ yet, and it rises as NO₂ builds up. The two curves move toward each other. At equilibrium they run together at one height, and that height is above zero.
To read a graph, find where every curve has gone level, and ask what each curve is doing before that.
Quick check · Sketch the graph
A flask holds only colorless N₂O₄ at the start, at constant temperature. N₂O₄(g) ⇌ 2 NO₂(g). The system is close to equilibrium by the last time shown, but still creeping toward it. Sketch the concentration of NO₂ against time. The first point is placed for you; drag the others so the graph shows how the concentration of NO₂ changes on the way to equilibrium.
One shape that fits
Drag a point up or down, or tap it and use the arrow keys. Only the shape is checked.
Part 3 Watch it done How much hydrogen is left?
Predict first, then watch each step, then finish the calculation yourself.
A student holds a rigid flask at constant temperature. It starts with N₂ at 1.00 atm and H₂ at 2.00 atm and no NH₃, and the reaction N₂(g) + 3 H₂(g) ⇌ 2 NH₃(g) begins. The partial pressure of N₂ is read every 10 s, from 0 to 60 s: 1.00, 0.80, 0.70, 0.66, 0.64, 0.64 and 0.64 atm. The partial pressure of NH₃ over the same times is 0.00, 0.40, 0.60, 0.68, 0.72, 0.72 and 0.72 atm. These numbers are a teaching example. When has the system reached equilibrium, and what is the partial pressure of H₂ there?
Predict first
Before you open the first step, predict: once the partial pressures stop changing, what will the partial pressure of NH₃ be?
Step 1 · Find when the readings stop changing
N₂ reads 0.64 atm at 40 s, 50 s and 60 s, and NH₃ reads 0.72 atm at the same three times. At 30 s the readings (0.66 atm and 0.68 atm) were still moving. So the system is at equilibrium from 40 s, to the precision of the readings.
Why this step?
Why can the reaction still be going on after 40 s when the readings no longer change?
At equilibrium the forward and reverse reactions continue at equal rates. N₂ and H₂ are still being used up and NH₃ is still breaking apart, but each is formed as fast as it is used, so the partial pressures stay constant.
Step 2 · Find how much N₂ reacted
N₂ fell from 1.00 atm to 0.64 atm, so 1.00 − 0.64 = 0.36 atm of N₂ reacted. In a rigid flask at constant temperature, partial pressures change in the same ratio as the amounts of gas, so the coefficients of the balanced equation scale them.
Step 3 · Check the ratio with NH₃
The equation makes 2 NH₃ for each N₂, so NH₃ should reach 2 × 0.36 = 0.72 atm. The table says 0.72 atm at equilibrium, so the coefficients do give the changes. Use the same idea for H₂, whose coefficient is 3.
Your step · Calculate
Finish it. The H₂ started at 2.00 atm, and N₂ fell by 0.36 atm. What is the partial pressure of H₂ at equilibrium, in atmospheres? Give your answer to two decimal places.
atm
All three gases changed in the ratio of the coefficients, and all three went level together. Flat curves show that there is no net change. They do not show that the reaction has stopped.
Part 4 Try it A sealed bottle of club soda
A bottle of club soda holds water with CO₂ dissolved in it and a little CO₂ gas above the liquid: CO₂(g) ⇌ CO₂(aq). Gas dissolving is the forward process, and dissolved CO₂ leaving the liquid is the reverse process.
Try it · Spot the error
A student explains a sealed bottle of club soda, where CO₂ is dissolved in the liquid and some CO₂ gas sits above it. Tap the line with the mistake.
Now go back to the flask of N₂O₄ and think about how the system gets from the first moment to equilibrium.
Try it · Put it in order
Only N₂O₄ is placed in a sealed flask at constant temperature. Put what happens in order, from the moment the gas is sealed in.
Part 5 Explain it Liquid bromine in a sealed flask
On the exam this is handwritten, so answer on paper first. Nothing you type here leaves this page. Then check your answer against the model and each scoring point.
Free response is 50% of the exam score. This one is shaped like a short-answer question, which is worth 4 points.
Free-response practice · Short-answer style
A student puts a little liquid bromine, Br₂(l), in a flask, seals it, and reads the partial pressure of Br₂ vapor at constant temperature. The readings at 0, 10, 20, 30, 40, 50 and 60 s are 0.00, 0.12, 0.19, 0.22, 0.23, 0.23 and 0.23 atm. Liquid bromine is still visible at 60 s. These numbers are a teaching example.
(a) Identify the first time shown at which the system has reached equilibrium, and the evidence for it.
(b) Explain why the partial pressure of the vapor rises at first.
(c) Explain, in terms of the rates of evaporation and condensation, why the partial pressure stops changing while liquid bromine remains.
(d) Represent the data with a graph of partial pressure against time. Draw it on paper, then compare it with the model graph.
Model answer
(a) 40 s. The partial pressure is 0.23 atm at 40 s, 50 s and 60 s, so it has stopped changing.
(b) At the start there is no vapor, so liquid bromine evaporates and nothing condenses. The vapor builds up, and the partial pressure rises.
(c) As vapor builds up, more of it returns to the liquid, so the rate of condensation rises while the rate of evaporation stays about the same. At equilibrium the two rates are equal. Bromine molecules still leave and return to the liquid, but there is no net change, so the partial pressure is constant and liquid bromine remains.
(d) The graph starts at 0.00 atm at 0 s, rises steeply, bends over, and runs level at 0.23 atm from 40 s to 60 s.
Model graph for (d)
Partial pressure of Br₂ vapor against time
Score your own answer. This is a self-check, not a grade. Tick each point your answer makes.
Part 6 Check yourself A flask of N₂O₄ and NO₂
All three case questions use the table.
A student seals pure N₂O₄ in a flask at constant temperature and follows the reaction N₂O₄(g) ⇌ 2 NO₂(g). The table gives the concentrations and the rates of the forward and reverse reactions. These numbers are a teaching example.
| Time (s) | [N₂O₄] (M) | [NO₂] (M) | Forward rate (× 10⁻⁴ M/s) | Reverse rate (× 10⁻⁴ M/s) |
|---|---|---|---|---|
| 0 | 0.100 | 0.000 | 20.0 | 0.00 |
| 10 | 0.070 | 0.060 | 14.0 | 2.00 |
| 20 | 0.054 | 0.092 | 10.8 | 4.70 |
| 30 | 0.046 | 0.108 | 9.20 | 6.48 |
| 40 | 0.042 | 0.116 | 8.40 | 7.48 |
| 50 | 0.040 | 0.120 | 8.00 | 8.00 |
| 60 | 0.040 | 0.120 | 8.00 | 8.00 |
Case question 1 · Multiple choice
Using the table: which statement describes the flask at 60 s?
Case question 2 · Calculate
Using the table: by what factor is the rate of the reverse reaction at 50 s greater than at 10 s? Give your answer to three significant figures.
Case question 3 · Decide and justify
Using the table: a student says, "At 60 s nothing is happening in the flask, because the concentrations no longer change." Do the data support the claim?
Which piece of evidence decides it?
The rest stand on their own and do not use the case.
Check yourself · Fill the statement
A student stirs table salt into water until some salt stays undissolved on the bottom, then seals the jar for a week. Complete the description of the jar.
In the jar, solid salt keeps while dissolved ions keep as solid. The two processes go on at rates, so the concentration of the salt solution stays .
Check yourself · Sort it
Each card describes a container left alone for a long time. Tap a card, then the bin that says whether a reversible change inside it can reach equilibrium.
Can reach equilibrium
Cannot reach equilibrium
Part 7 Keep it Bring back earlier ideas
This lesson leans on two earlier ideas: gas particles never stop moving, and how a gas mixture's pressure splits into partial pressures. Pulling them back up is what makes them last.
From 3.5 Kinetic Molecular Theory · Sort it
Each card describes the particles in a sealed flask of gas that has sat untouched on a bench at room temperature for a week. Tap a card, then the bin it belongs in.
Fits the kinetic molecular theory
Does not fit the kinetic molecular theory
From 3.4 Ideal Gas Law · Calculate
A cylinder holds a mixture of 0.640 mol of oxygen and 1.36 mol of nitrogen at a total pressure of 3.00 atm. What is the partial pressure of the oxygen, in atmospheres? Give three significant figures.
In three lines
- A reversible change in a closed system reaches equilibrium when the forward and reverse processes run at equal rates.
- At equilibrium nothing observable changes: reactants and products are both present, and their concentrations or partial pressures are constant but not necessarily equal.
- The state is dynamic, because both processes continue; graphs of concentration, partial pressure or rate against time level off, or meet, at equilibrium.
Exam strategy for this topic
- Section I, Multiple Choice: 60 questions in 90 minutes.
- Section II, Free Response: 7 questions in 105 minutes.
- A scientific or graphing calculator is recommended for use on both sections of the exam.
On a graph of concentration or partial pressure against time, equilibrium begins where every curve is level. Describe what is happening there as both reactions continuing at equal rates with no net change, and never as the reactions stopping.
Questions students ask
What is dynamic equilibrium?
A state of a closed system in which the forward and reverse processes keep going at equal rates, so the concentrations or partial pressures stay constant.
Do the reactions stop at equilibrium?
No. Both the forward and the reverse reaction continue. Only the net change stops, because they go on at equal rates.
Are the concentrations equal at equilibrium?
Not necessarily. They are constant, which means they no longer change, but they can be very different from one another.
What is a reversible process?
A change that can run in both directions, such as evaporation and condensation, dissolution and precipitation, or the transfer of protons or electrons.
What does a concentration versus time graph look like as a system reaches equilibrium?
The concentration of each reactant and product changes fastest at the start, then levels off. Once every curve is level, the system is at equilibrium.
Course alignment, for teachers
AP Chemistry topic 7.1, Unit 7: Equilibrium.
- Skill 6.D: Provide reasoning to justify a claim using chemical principles or laws, or using mathematical justification.
This lesson maps all 4 essential knowledge statements of the topic to the part that teaches each one: 4 are checked by a scored question. The map is in the lesson file.