Sketch acid–alkali titration curves
Sketch and compare pH curves for strong and weak acids titrated with strong and weak alkalis.
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Equilibrium constants apply at a fixed temperature; include only gaseous or aqueous species in the relevant expressions. For acid–base work, distinguish strength from concentration. Written practice is self-checked
Before you begin
- Balance equations, interpret reversible arrows and use concentration in mol dm⁻³.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which quantities are plotted on a titration curve?
What you will learn
- Sketch pH against titrant volume
- Compare strong/weak acid and alkali curve shapes
- Identify equivalence region and its relative pH
Label the axes and recognise the steep region
A titration curve plots pH vertically against volume of added titrant horizontally. Mark the initial pH, gradual region, steep change near equivalence and the pH after excess titrant is added.
For a strong acid–strong alkali titration, the equivalence point is near pH 7 in the stated reference. Weak-acid/strong-alkali equivalence is above 7; strong-acid/weak-alkali equivalence is below 7.
Strength changes the initial region and jump
A weak acid titrated by a strong base has a buffer region and an equivalence point above 7. A strong acid titrated by a weak base has an equivalence point below 7 and a smaller steep region.
A weak acid–weak base titration often has no sharp vertical region. Curves should be qualitative and use the right direction: pH rises when alkali is added to acid, and falls when acid is added to alkali.
Pause and explain
What is the approximate equivalence pH for a strong acid–strong alkali titration in the stated reference?
Worked example
Describe how a strong acid–strong base curve changes as NaOH is added to acid.
Show the worked solution
- The initial pH is low.
- pH rises slowly at first, then sharply near equivalence.
- The equivalence pH is near 7; after equivalence pH is above 7.
Answer The curve rises from low pH, has a steep near-vertical region around equivalence pH 7, then levels in excess alkali.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Sketch in words the pH curve for a weak acid titrated with a strong alkali.
Give me a hint
Start acidic, include the buffer region and place equivalence above neutral.
Compare my reasoning
- Label pH and volume of strong alkali.
- Begin at a moderately acidic pH and rise gradually through a buffer region.
- Show a steep region with equivalence above pH 7, then level in excess alkali.
The curve rises from an acidic start, has a buffer region, a steep rise around an equivalence point above 7, then levels at high pH.
Look for these in your work
- Axes correct
- Buffer region shown
- Equivalence above 7
Compare equivalence points for strong acid–weak base and weak acid–strong base titrations.
Give me a hint
Use the nature of the salt solution at equivalence.
Compare my reasoning
- Strong acid–weak base equivalence lies below 7.
- Weak acid–strong base equivalence lies above 7.
- The conjugate ion from the weak reactant changes the equivalence solution's pH.
Strong acid–weak base equivalence is below 7; weak acid–strong base equivalence is above 7.
Look for these in your work
- Both cases ordered correctly
- Reason linked to weak conjugate species
- pH reference used
What qualitative feature distinguishes a weak acid–weak base curve from a strong acid–strong base curve?
Give me a hint
Compare the size and sharpness of the pH change near equivalence.
Compare my reasoning
- The weak–weak system has a smaller, less sharp change.
- It may not have a steep vertical region.
- That can make indicator endpoint selection less reliable.
A weak acid–weak base curve has no sharp vertical region in many cases, so the equivalence change is less distinct than for a strong–strong titration.
Look for these in your work
- Weak–weak identified
- Steep region distinguished
- Endpoint implication stated
Common mistakes
- Reversing the curve direction
- Putting every equivalence point at pH 7
- Giving weak–weak a sharp vertical step
- Forgetting to label volume of titrant
What can you explain now?
Which quantities are plotted on a titration curve?
Compare with the explanation
pH against volume of titrant
Volume is added along the x-axis and pH is measured vertically.
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Make it stick
How do acid and alkali strength affect the initial pH, steep region and equivalence point?
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