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AQA GCSE · Question 04.1 · Energy changes
A student investigated the energy change of the reaction between zinc and copper sulfate solution. The method and results are shown.
Draw two lines of best fit on Figure 3. The lines should cross.
A student investigated the energy change of the reaction between zinc and copper sulfate solution. The method and results are shown.
Draw two lines of best fit on Figure 3. The lines should cross.
How to approach this question
1. Identify the two distinct trends in the data. The first few points show the temperature increasing as more zinc is added. The later points show the temperature staying roughly constant.
2. Use a ruler to draw a straight line that passes as close as possible to the points in the first trend (the rising section).
3. Use a ruler to draw a second straight line that passes as close as possible to the points in the second trend (the flat section).
4. Extend both lines until they intersect.
Full Answer
One straight line of best fit should be drawn through the first four points (0.00 to 0.60 g), showing a positive correlation. A second, near-horizontal straight line of best fit should be drawn through the last three points (0.80 to 1.20 g). The lines should be extended to cross each other.
The graph shows two phases of the reaction.
1. **Rising section (0.00 g to ~0.7 g of zinc):** As more zinc is added, more reaction occurs, releasing more heat and causing the temperature to rise. The temperature increase is proportional to the mass of zinc added. A straight line of best fit should be drawn through these points. Zinc is the limiting reactant here.
2. **Plateau section (~0.7 g onwards):** At a certain point, all the copper sulfate in the solution has reacted. Adding more zinc will not cause any further reaction or temperature increase. The temperature remains constant (or may slightly decrease due to heat loss). A second, near-horizontal line of best fit should be drawn through these points. The copper sulfate is the limiting reactant here.
The two lines are extended to cross. The intersection point represents the exact point where the reaction is complete, and the maximum temperature rise is achieved.
Common mistakes
✗ Joining the dots instead of drawing lines of best fit.
✗ Forcing one single curve through all the points.
✗ Drawing lines that do not extend to cross each other.
Practice the full AQA GCSE Chemistry Higher Tier Paper 1
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