Reactions of Acids (AQA GCSE Chemistry): Revision Notes
Core practical - Titration
What is this practical about?
This practical helps you learn how to find the exact volume of alkali needed to neutralise a known volume of acid. It's a key skill you'll need to master for your GCSE chemistry exam.
Aim of the experiment
The main goal is to find out how much alkali (sodium hydroxide solution) you need to exactly neutralise 25cm³ of dilute hydrochloric acid.
Equipment you'll need
You'll use several important pieces of equipment, each with a specific purpose:
- Burette - holds the alkali solution and lets you add it drop by drop
- Pipette - measures exactly 25.0cm³ of acid
- Conical flask - contains the acid during the experiment
- Indicator - shows when the neutralisation is complete
- White tile - goes under the flask to see colour changes clearly
Safety first
Critical Safety Requirements:
- Wear eye protection - acids and alkalis can damage your eyes
- Handle chemicals carefully - both acid and alkali solutions can be corrosive
- Use a safety filler with the pipette - never suck solutions by mouth
These safety measures are not optional - they're essential for preventing serious injury.
Step-by-step method
Follow these steps carefully to get accurate results:
1. Prepare your equipment Rinse the burette with sodium hydroxide solution, then fill it completely. Make sure there are no air bubbles in the tip.
Rinsing the burette with the solution you're using prevents dilution that would affect your results. Air bubbles can cause inaccurate volume readings.
2. Measure the acid Use a pipette to measure exactly 25.0cm³ of dilute hydrochloric acid. Put this into a clean conical flask.
3. Add indicator Add a few drops of phenolphthalein indicator to the acid. This will be colourless in the acidic solution.
4. Record starting volume Note the volume reading on the burette at the start.
5. Add alkali carefully Add the sodium hydroxide from the burette, swirling the flask constantly. When you get close to the end point, add the alkali drop by drop.
Swirling ensures thorough mixing and helps you spot the colour change more easily. The "drop by drop" technique near the end point is crucial for accuracy.
6. Spot the end point Stop adding alkali when the indicator just changes colour and stays changed. With phenolphthalein, this means the solution turns from colourless to pink.
Recording your results
Reading the burette
- Read volumes to 2 decimal places
- The final digit should always be 0 or 5
- Read from the bottom of the meniscus (curved surface)
Getting concordant results
- Repeat the experiment several times
- Look for results that are within 0.10cm³ of each other - these are called concordant results
- Calculate the average of your concordant results only
Worked Example: Calculating Average Volume
If your concordant results were 24.65cm³ and 24.75cm³:
Only use concordant results (within 0.10cm³) in your average calculation.
Why these techniques matter
Adding alkali drop by drop near the end point This gives you much more accurate results. If you add too much alkali too quickly, you'll overshoot the end point.
The end point occurs very quickly - just one extra drop can take you past the neutralisation point, making your results inaccurate.
Using a white tile This makes it much easier to see the colour change clearly, especially the faint pink colour of phenolphthalein.
Reading the meniscus correctly The curved surface of the liquid can make readings inaccurate if you don't read from the right point.
Rinsing equipment This ensures you're not diluting your solutions with water or contaminating them with other chemicals.
Common exam questions
You might be asked to explain why certain procedures are used:
Why use a pipette instead of a measuring cylinder? A pipette measures volumes much more accurately than a measuring cylinder.
Why is phenolphthalein used rather than universal indicator? Phenolphthalein gives a sharp, clear colour change at the end point. Universal indicator shows gradual colour changes that make it harder to spot the exact end point.
Key Points to Remember:
- The aim is to find the volume of alkali needed to neutralise 25cm³ of acid
- Add the alkali drop by drop near the end point for accuracy
- Concordant results are within 0.10cm³ of each other
- Always rinse your equipment with the solutions you're using
- Safety equipment is essential when working with acids and alkalis
- Read burette measurements to 2 decimal places from the bottom of the meniscus