Lesson focus: neutralisationA reaction between an acid and a base that makes a salt and water. consumes acidic and basic species to form products, but a neutral pH requires suitable amounts and conditions.
Write neutralisation equations, interpret pH change and explain why excessMore than enough, so some is left over after the reaction. reagent affects the final solution.
Build on earlier learning
Indicators and pHIndicators and pH now track a reaction between acid and base.
Practical
Neutralisation
- Add alkali 1 cm³ at a time to 10 cm³ of acid with universal indicator.
- Stir and record the pH after each addition.
- Stop when the indicator is green (pH 7).
Record: A graph of volume of alkali added against pH.
Safety: Wear eye protection.
Neutralising an acid
Add sodium hydroxide solution to 10 cm³ of hydrochloric acid with universal indicator. Watch the colour and the pH.
Controls
You will be successful when you can…
- State the neutralisation pattern.
- Name salts from the acid.
- Explain pH movement.
- Recognise excess acid or alkali.
Words that carry the science
- neutralisation
- Reaction in which an acid reacts with a base.
- salt
- An ionic compound formed when acid hydrogen is replaced by a metal or ammonium ion.
- endpoint
- Observed point at which an indicator changes as intended.
- excess
- More reactant present than can be consumed by the other reactant.
- titration
- A measured addition method used to find reacting volumes.
Acid and base form salt and water
01Acid and base form salt and water
The general pattern is acid + base → salt + water. With an alkali, hydrogen ions and hydroxide ions combine to form water.
Hydrochloric acid forms chlorides, sulfuric acid sulfates and nitric acid nitrates. The positive ion comes from the base.
02pH moves toward and beyond neutral
Adding alkali to acid raises pH as acidic species are consumed. If alkali continues after the acid is consumed, the mixture becomes alkaline.
neutralisationA reaction between an acid and a base that makes a salt and water. does not automatically mean the final measured solution is pH 7: reagent amounts, acid/base strength, temperature and salt behaviour matter.
03Indicators reveal an endpoint
Universal indicator demonstrates the broad pH path but is not ideal for precise titrationA careful method for finding exactly how much acid is needed to cancel out an alkali. because it changes through many colours.
Add reagent slowly near the endpointThe clear sign you agree in advance that tells you when to stop the timer or the test, such as a cross disappearing from view., mix thoroughly and use clean apparatus. Never neutralise a spill on a person with another corrosive reagent; follow the laboratory emergency procedure and rinse as instructed.
Name the neutralisation products
Sulfuric acid reacts with potassium hydroxide.
- Use acid + base → salt + water.
- Potassium hydroxide supplies potassium for the salt.
- Sulfuric acid supplies the sulfate name.
- Write sulfuric acid + potassium hydroxide → potassium sulfate + water.
How to make the reasoning visible: The salt name combines the positive ion from the base and acid-derived ending.
pH change
Interpret illustrative readings as alkali is added to acid.
| Added alkali / cm³ | 0 | 8 | 10 | 12 |
|---|---|---|---|---|
| pH | 1.5 | 3.0 | 7.0 | 11.2 |
| indicator | red | orange | green | purple |
| acid status | excess | excess | consumed | consumed |
| alkali status | none | consumed | consumed | excess |
Where is the approximate neutral endpoint?
Why does pH rise sharply near the endpoint?
Check your understanding
Answer from memory, then check against the suggested answer.
Neutralisation products?
Hydrochloric acid salt ending?
What does excess mean?
Why can too much alkali make a neutralisation mixture hazardous?
Why might an equivalence-point solution not be exactly pH 7?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
Sodium hydroxide solution is added slowly to hydrochloric acid until it is in excess.
Explain why the pH rises as sodium hydroxide is added.
(3)Describe the pH when exactly enough is added, and when it is in excess.
(2)Total for this question: 5 marks
Command word: Follow the changing particle balance through the whole addition.