AP® Chemistry review sheet from Aim for Five (aimforfive.com/chem/units/7/7-1)
Unit 7 · Topic 7.1
7.1 Introduction to Equilibrium
Many physical and chemical processes can run in both directions. In a closed system, a reversible process settles into dynamic equilibrium: the forward and reverse processes keep happening at equal rates, so the amounts of everything stop changing even though particles never stop reacting.
Key terms
- reversible reaction
- dynamic equilibrium
- forward reaction
- reverse reaction
- concentration vs. time graph
Reversible processes are everywhere
A reversible process can go forward or backward. We write it with a double arrow, ⇌. Physical examples: water evaporating and condensing in a sealed bottle, a salt dissolving and re-forming a solid in a saturated solution, and gas molecules sticking to and leaving a solid surface.
Chemical examples: acid–base reactions, where protons move back and forth (CH₃COOH + H₂O ⇌ CH₃COO⁻ + H₃O⁺), and redox reactions, where electrons move back and forth. Most of Units 7, 8 and 9 build on this idea.
How equilibrium is reached
Picture a sealed flask that starts with only colorless N₂O₄ gas. Some N₂O₄ molecules split into brown NO₂, so the flask starts turning brown. At first only the forward reaction can happen, because there's no NO₂ yet.
As NO₂ builds up, pairs of NO₂ molecules start combining back into N₂O₄. The forward rate falls (less N₂O₄ left) and the reverse rate rises (more NO₂ around). Eventually the two rates become equal. From then on, N₂O₄ is made exactly as fast as it's used up, so the brown color stops changing. That is equilibrium.
Dynamic, not stopped
At equilibrium nothing you can observe changes: color, pressure, concentration and temperature all hold steady. But at the particle level, both reactions keep going at the same rate. That's why it's called dynamic equilibrium.
Evidence: if you add a little solid salt made with a radioactive isotope to a saturated solution of the same salt, radioactive ions soon show up in the solution, even though the total amount dissolved doesn't change. Particles keep dissolving and re-forming the solid.
What equilibrium does not mean
- It does not mean equal concentrations of reactants and products. It means each concentration is constant.
- It does not mean the reaction has stopped. Forward and reverse rates are equal, not zero.
- It does not mean only products remain. At equilibrium, all reactants and products are present together.
- It requires a closed system. If a product escapes (say, a gas leaves an open beaker), equilibrium can't be reached.
Reading equilibrium graphs
On a graph of concentration (or partial pressure) against time, reactant curves fall and product curves rise, then all of them level off at the same moment. The flat part is equilibrium. The flat values don't have to be equal to each other.
The sizes of the changes follow the coefficients. For N₂O₄ ⇌ 2NO₂, the NO₂ curve rises twice as far as the N₂O₄ curve falls.
On a graph of rate against time, the forward rate starts high and falls, the reverse rate starts at zero and rises, and they meet and stay equal once equilibrium is reached.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Interpreting a concentration–time graph
A flask starts with 0.100 M N₂O₄ and no NO₂. On a graph of concentration against time, one curve falls from 0.100 M to 0.070 M and is flat after 60 s; the other rises from 0 to 0.060 M and is flat after 60 s. (a) Which curve is NO₂? (b) When is equilibrium reached? (c) Are the concentrations equal at equilibrium?
Show the solutionHide the solution
- Step 1: (a) NO₂ starts at zero and is produced, so the rising curve is NO₂. Check with coefficients: N₂O₄ fell by 0.030 M, and each N₂O₄ makes 2 NO₂, so NO₂ should rise by 0.060 M. It does.
- Step 2: (b) Equilibrium is reached at about 60 s, when both curves become flat. After that, the forward and reverse rates are equal.
- Step 3: (c) No. At equilibrium [N₂O₄] = 0.070 M and [NO₂] = 0.060 M. Equilibrium means constant concentrations, not equal ones.
Answer: (a) The rising curve; (b) at about 60 s; (c) no: 0.070 M N₂O₄ and 0.060 M NO₂.
- Example 2
Trap: has the reaction stopped?
A student says: 'Once the color of the N₂O₄/NO₂ flask stops changing, the reaction has stopped.' Explain what is wrong with this statement.
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- Step 1: A constant color means the concentrations are constant. It does not mean no reaction is happening.
- Step 2: At equilibrium, N₂O₄ molecules are still splitting into NO₂ and NO₂ molecules are still combining into N₂O₄.
- Step 3: These two processes happen at the same rate, so they cancel out and there's no net change. The equilibrium is dynamic.
Answer: The reaction hasn't stopped; the forward and reverse reactions continue at equal rates, so concentrations (and color) stay constant.
Common mistakes
- Thinking equilibrium means equal amounts of reactants and products.
- Saying the reactions stop at equilibrium. They continue at equal rates.
- Drawing concentration curves that level off at different times. All species reach constant values at the same moment.
- Ignoring coefficients when matching curves: for A ⇌ 2B, B changes twice as much as A.
On the exam
- Expect concentration–time or rate–time graphs. Identify the point where equilibrium is reached and justify it with 'concentrations are constant because forward and reverse rates are equal'.
- Free-response explanations should use the phrase 'rates of the forward and reverse reactions are equal', not just 'it's balanced'.
Connected topics
Videos
Check yourself
4 questions on 7.1 Introduction to Equilibrium. Pick an answer to see if you got it, and why.
A small amount of solid NaCl made with radioactive sodium-24 is added to a beaker of saturated NaCl solution that is in contact with undissolved solid. The temperature stays constant. After some time, the solution is radioactive, but the concentration of Na⁺ in solution has not changed. Which of the following best explains these observations?
Some liquid water is placed in an evacuated flask, which is then sealed and kept at constant temperature. The pressure inside rises for a while and then stays constant, with liquid still present. Which of the following best describes the flask once the pressure is constant?
| Time (s) | [N₂O₄] (M) | [NO₂] (M) |
|---|---|---|
| 0 | 0.100 | 0.000 |
| 10 | 0.070 | 0.060 |
| 20 | 0.055 | 0.090 |
| 30 | 0.050 | 0.100 |
| 40 | 0.050 | 0.100 |
| 50 | 0.050 | 0.100 |
Hypothetical data. Concentrations in a sealed flask for the reaction N₂O₄(g) ⇌ 2NO₂(g) at constant temperature
Which of the following is the earliest time in the table at which the system is at equilibrium?
Which of the following best describes the system at 40 s?
0 of 4 answered