AP® Chemistry review sheet from Aim for Five (aimforfive.com/chem/units/8/8-8)
Unit 8 · Topic 8.8
8.8 Properties of Buffers
A buffer is a solution with large amounts of both a weak acid and its conjugate base. Added base is used up by the weak acid and added acid is used up by the conjugate base, so the pH changes only slightly.
Key terms
- buffer
- conjugate acid-base pair
- weak acid
- conjugate base
What a buffer is made of
A buffer contains a conjugate acid–base pair, with substantial amounts of both. Examples: acetic acid with sodium acetate (CH₃COOH / CH₃COO⁻), or ammonia with ammonium chloride (NH₄⁺ / NH₃). Blood is buffered mainly by carbonic acid and hydrogen carbonate (H₂CO₃ / HCO₃⁻).
You can make a buffer two ways: mix a weak acid with a salt of its conjugate base, or start with a weak acid and add strong base until only part of the acid has reacted (topic 8.4). Adding strong acid to a weak base until only part of the base has reacted works the same way.
How a buffer resists pH change
When strong base is added, the OH⁻ reacts with the weak acid: CH₃COOH(aq) + OH⁻(aq) → CH₃COO⁻(aq) + H₂O(l). The strong base is replaced by a weak base, so the pH barely rises.
When strong acid is added, the H₃O⁺ reacts with the conjugate base: CH₃COO⁻(aq) + H₃O⁺(aq) → CH₃COOH(aq) + H₂O(l). The strong acid is replaced by a weak acid, so the pH barely falls.
Both reactions go essentially to completion, because they involve a strong acid or base. Without the buffer, the same small addition to pure water would change the pH by several units.
What is not a buffer
- A strong acid with its conjugate base, like HCl with NaCl. Cl⁻ is too weak a base to react with added acid.
- A weak acid alone. It has almost no conjugate base to absorb added acid.
- A weak acid exactly neutralized by a strong base. Only the conjugate base is left.
- A weak base mixed with excess strong acid. No weak base remains.
What you need to know for the exam
You should be able to identify a buffer, write the net ionic equation for its reaction with added acid or base, and explain why the pH changes only a little. You won't be asked to calculate exactly how much the pH changes after acid or base is added to a buffer.
Buffers in your body
Your blood stays between about pH 7.35 and 7.45, even though your cells constantly release acids. The main buffer is carbonic acid with hydrogen carbonate: added H₃O⁺ is used up by HCO₃⁻, and added OH⁻ is used up by H₂CO₃. Proteins and phosphate ions also act as buffers inside cells.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Spotting buffers
Which of these mixtures (each in 1.0 L of solution) is a buffer? (a) 0.10 mol HCl + 0.10 mol NaCl; (b) 0.10 mol NH₃ + 0.10 mol NH₄Cl; (c) 0.20 mol CH₃COOH + 0.10 mol NaOH; (d) 0.10 mol CH₃COOH + 0.10 mol NaOH.
Show the solutionHide the solution
- Step 1: (a) HCl is a strong acid; Cl⁻ is not a usable base. Not a buffer.
- Step 2: (b) NH₃ is a weak base and NH₄⁺ is its conjugate acid, both in large amounts. Buffer.
- Step 3: (c) React first: 0.10 mol OH⁻ converts 0.10 mol CH₃COOH to CH₃COO⁻, leaving 0.10 mol of each. Buffer.
- Step 4: (d) The acid is completely converted to 0.10 mol CH₃COO⁻ with no CH₃COOH left. Not a buffer; it's a weakly basic solution.
Answer: (b) and (c) are buffers; (a) and (d) are not.
- Example 2
Trap: writing the buffer's reaction
A buffer contains NH₃ and NH₄Cl. Write the net ionic equation for what happens when a small amount of HCl is added, and when a small amount of NaOH is added. Explain why the pH changes only slightly.
Show the solutionHide the solution
- Step 1: Added HCl supplies H₃O⁺. The base in the buffer, NH₃, reacts with it: NH₃(aq) + H₃O⁺(aq) → NH₄⁺(aq) + H₂O(l).
- Step 2: Added NaOH supplies OH⁻. The acid in the buffer, NH₄⁺, reacts with it: NH₄⁺(aq) + OH⁻(aq) → NH₃(aq) + H₂O(l).
- Step 3: A common trap is writing Cl⁻ or Na⁺ as the reacting species. They're spectator ions.
- Step 4: Each reaction swaps a strong acid or base for a weak one, and the ratio [NH₃]/[NH₄⁺] changes only a little, so the pH changes only a little.
Answer: With HCl: NH₃ + H₃O⁺ → NH₄⁺ + H₂O. With NaOH: NH₄⁺ + OH⁻ → NH₃ + H₂O. Strong acid/base is converted to a weak one, so the pH barely changes.
Common mistakes
- Calling a strong acid plus its salt (like HCl/NaCl) a buffer.
- Writing buffer reactions with spectator ions, or as equilibria when they actually go to completion.
- Saying a buffer keeps the pH perfectly constant. The pH does change, just very little.
- Missing buffers that form by partly neutralizing a weak acid with a strong base.
On the exam
- Explaining how a buffer works almost always requires the net ionic equation for the reaction with added acid or base. Write it with the correct buffer component.
- Expect 'which mixture would make a buffer?' questions; react strong species first, then look for both members of a conjugate pair.
Connected topics
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Check yourself
4 questions on 8.8 Properties of Buffers. Pick an answer to see if you got it, and why.
A buffer contains NH₃ and NH₄Cl. Which equation shows what happens when a small amount of HCl is added to the buffer?
A buffer contains NaH₂PO₄ and Na₂HPO₄. Which equation shows what happens when a small amount of NaOH is added to the buffer?
Which of the following pairs, dissolved in water in roughly equal amounts, would make a buffer?
Each mixture below is dissolved in enough water to make 1.0 L of solution. Which one is a buffer?
0 of 4 answered