Required practical - Energy changes (AQA GCSE Chemistry Combined Science): Revision Notes
📚Revision Notes
Required practical - Energy changes
What this practical is about
This practical shows how energy changes happen during chemical reactions. When acids react with metals, metal carbonates, or alkalis, they release or absorb energy. This causes the temperature of the mixture to change.
Different concentrations of acid will give different temperature changes. The stronger the acid concentration, the bigger the temperature rise.
Note
This practical demonstrates the relationship between chemical reactions and thermal energy transfer, helping you understand how concentration affects reaction intensity.
Aim of the practical
To investigate how temperature changes when different concentrations of acid react with zinc metal.
Equipment you need
- Eye protection (safety glasses)
- Dilute hydrochloric acid
- Zinc powder
- Measuring cylinder
- Glass beaker
- Polystyrene cup
- Thermometer
Note
Safety First! Always wear eye protection when working with acids and metal powders. Work in a well-ventilated area and follow your teacher's safety instructions.
Why we use a polystyrene cup
A polystyrene cup is used because polystyrene is a poor conductor of heat. This means most of the heat energy released by the reaction stays inside the container. This gives us more accurate temperature measurements.
Important
Thermal Insulation is Key! The polystyrene cup acts as thermal insulation, preventing heat loss to the surroundings. This ensures that the temperature change we measure accurately reflects the energy released by the chemical reaction itself.
Method - step by step
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Set up the apparatus with the thermometer in the polystyrene cup inside a glass beaker for support.
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Measure 50 cm³ of dilute hydrochloric acid using the measuring cylinder.
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Add the acid to the polystyrene cup and record the starting temperature.
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Add all the zinc powder quickly. The zinc must be in excess (more than needed) so all the acid reacts.
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Stir vigorously with the thermometer until the temperature stops rising.
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Record the maximum temperature reached.
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Repeat the experiment using different acid-water mixtures:
- 40 cm³ acid + 10 cm³ water
- 30 cm³ acid + 20 cm³ water
- 20 cm³ acid + 30 cm³ water
- 10 cm³ acid + 40 cm³ water
Important
Critical Steps for Accuracy:
- Add zinc powder quickly to ensure all reactions start simultaneously
- Stir vigorously to maximise contact between reactants
- Wait for temperature to stabilise before recording maximum value
What the results show
The results show that more concentrated acid gives a higher temperature rise.
Example
Example Results - Temperature Rise vs Acid Concentration
| Acid Volume | Water Volume | Total Volume | Temperature Rise |
|---|---|---|---|
| 50 cm³ | 0 cm³ | 50 cm³ | 25.0°C |
| 40 cm³ | 10 cm³ | 50 cm³ | 20.1°C |
| 30 cm³ | 20 cm³ | 50 cm³ | 14.9°C |
| 20 cm³ | 30 cm³ | 50 cm³ | 10.0°C |
| 10 cm³ | 40 cm³ | 50 cm³ | 5.1°C |
Pattern observed: As acid concentration decreases, temperature rise decreases proportionally.
Key points to understand
This is an exothermic reaction - it releases heat energy and causes the temperature to increase.
The total volume of acid + water is always 50 cm³. This means we heat the same volume of solution each time, making it a fair test.
Note
Controlled Variables: By keeping the total volume constant at 50 cm³, we ensure that the only variable changing is the acid concentration. This makes our comparison valid and reliable.
The zinc powder must be stirred well to make sure it reacts completely with the acid.
Most chemical reactions are exothermic (release heat). Some reactions are endothermic (absorb heat and cause temperature to decrease).
Important
Understanding Energy Changes:
- Exothermic reactions: Release energy → Temperature increases → Feel warm
- Endothermic reactions: Absorb energy → Temperature decreases → Feel cold
The acid-metal reaction you're studying is exothermic, which is why the temperature rises during the practical.
Remember!
Summary
Key Points to Remember:
- Exothermic reactions release energy and increase temperature
- Higher concentration of acid = higher temperature rise
- Polystyrene cups keep heat in because they're poor heat conductors
- Always use excess zinc so all the acid reacts
- Stir well to get complete reaction and accurate results