Core practical - Energy changes (AQA GCSE Chemistry): Revision Notes
Core practical - Energy changes
What is this practical about?
This experiment shows how chemical reactions release or absorb energy. When acids react with metals, metal carbonates, or alkalis, they cause temperature changes. Different amounts of acid will create different temperature rises.
This type of energy change experiment is fundamental to understanding how chemical reactions can be measured and compared using simple temperature measurements.
Aim of the experiment
To investigate how temperature changes when different concentrations of acid react with zinc.
Equipment you need
- Eye protection - for safety
- Dilute hydrochloric acid - the reactant being tested
- Zinc powder - reacts with the acid
- Glass beaker - to hold equipment
- Polystyrene cup - special container that keeps heat in
- Thermometer - to measure temperature changes
- Measuring cylinder - to measure acid volumes accurately
Why use a polystyrene cup?
Polystyrene is a poor conductor of heat. This means most of the heat energy from the reaction stays inside the container instead of escaping. This gives more accurate temperature measurements and prevents heat loss to the surroundings.
Method - step by step
Worked Example: Complete Experimental Procedure
Step 1: Set up the apparatus - Put the polystyrene cup inside the glass beaker with the thermometer ready
Step 2: Measure 50 cm³ of dilute hydrochloric acid - Use the measuring cylinder for accuracy
Step 3: Add acid to the polystyrene cup - Record the starting temperature
Step 4: Add excess zinc powder - Make sure there's more than enough zinc to react with all the acid
Step 5: Stir vigorously - The zinc must be mixed well to react completely
Step 6: Watch the temperature - Keep stirring until the temperature stops rising
Step 7: Record the maximum temperature - This shows how much energy was released
Step 8: Repeat with different concentrations - Try mixtures like:
- 40 cm³ acid + 10 cm³ water
- 30 cm³ acid + 20 cm³ water
- 20 cm³ acid + 30 cm³ water
- 10 cm³ acid + 40 cm³ water
What the results show
The sample results demonstrate a clear pattern that shows the relationship between acid concentration and energy release:
Worked Example: Results Analysis
Temperature Rise Data:
- 10 cm³ acid: Temperature rise of 5.1°C
- 20 cm³ acid: Temperature rise of 10.0°C
- 30 cm³ acid: Temperature rise of 14.9°C
- 40 cm³ acid: Temperature rise of 20.1°C
- 50 cm³ acid: Temperature rise of 25.0°C
Key pattern observed: More acid present = Higher temperature rise
Why this happens
When zinc reacts with hydrochloric acid, it's an exothermic reaction. This means it releases heat energy. The more acid available to react, the more zinc dissolves, and the more heat energy gets released.
The total volume stays the same (50 cm³) each time, so we're heating the same amount of liquid. This makes the comparison fair and ensures that the temperature changes are only due to the different concentrations of acid.
The total volume stays the same (50 cm³) each time, so we're heating the same amount of liquid. This makes the comparison fair.
Critical Safety Requirements
- Always wear eye protection - acid splashes can cause serious eye damage
- The zinc powder must be stirred well to react completely
- Use excess zinc so all the acid reacts completely
- Handle hot solutions carefully - the beaker may become warm during the reaction
Key Points to Remember:
- Exothermic reactions release heat and cause temperature to rise
- Polystyrene cups are used because they're poor heat conductors
- More concentrated acid produces bigger temperature rises
- Excess zinc ensures all the acid reacts completely
- This practical shows how energy changes can be measured and compared