AQA A-Level Chemistry 7405
Atomic Structure
Amount of Substance
Bonding
Energetics and Thermodynamics
Kinetics
Chemical Equilibria
Redox and Electrochemistry
Acids and Bases
Periodicity and Group Chemistry
Period 3 Elements and Their Oxides
Transition Metals
Introduction to Organic Chemistry
Hydrocarbons
Halogenoalkanes, Alcohols and Organic Analysis
Carbonyls, Carboxylic Acids and Derivatives
Aromatic Chemistry and Amines
Polymers, Amino Acids, Proteins and DNA
Organic Synthesis, NMR and Chromatography
Acids and Bases
Indicators
- 3 things to know
- Papers 1 and 3
Overview
How an indicator works, and choosing one whose range falls on the vertical section of the curve.
What you need to know
Answer each one before you reveal it.
How does an indicator function during a titration?
What determines whether an indicator is suitable for a particular acid-base titration?
Questions from this subtopic
Each one with the model answer underneath.
- 1
What is an acid-base indicator in terms of its chemical nature?
Model answer
A weak acid (or weak base) that has different colours in its protonated and deprotonated forms
- 2
How does an indicator function during a titration?
Model answer
The indicator changes colour when the pH passes through its colour change range, signalling the end point
- 3
Why must the pH range over which an indicator changes colour match the vertical section of a pH curve?
Model answer
So that the colour change occurs sharply at the equivalence point rather than gradually over a wide pH range
- 4
What determines whether an indicator is suitable for a particular acid-base titration?
Model answer
The indicator's colour change range must fall within the steep vertical section of the pH curve for that titration
- 5
For a strong acid-strong base titration, what type of indicator would be appropriate and why?
Model answer
Any indicator with a colour change range between approximately pH 3 and pH 11, because the vertical section spans this entire range
- 6
For a weak acid-strong base titration, which indicator would be suitable?
Model answer
Phenolphthalein, because the equivalence point pH is above 7 and phenolphthalein changes colour in the appropriate range (approximately pH 8-10)
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