Table 1 gives measurements from a student’s investigation of citric acid and sodium hydrogencarbonate in water. This reaction is used in fizzy tablet demons...
Question 1 Report
Table 1 gives measurements from a student’s investigation of citric acid and sodium hydrogencarbonate in water. This reaction is used in fizzy tablet demonstrations. The student uses the apparatus shown in Fig. 1 to reduce heat transfer.
| measurement | value |
|---|---|
| initial temperature / degrees C | 22.5 |
| lowest temperature / degrees C | 17.0 |
| mass of solid / g | 5.0 |
(a) Calculate the temperature decrease. [1]
(b) Name the gas made in this reaction. [1]
(c) Explain why the reaction mixture becomes cooler. [2]
(d) Complete this statement: energy is transferred from the surroundings to the ........ . [1]
Table 1 shows a student’s results when adding equal masses of four metals to dilute copper chloride solution. Fig. 1 shows one test tube after potassium has reacted. The student wore eye protection because potassium is very reactive.
| metal | highest temperature / degrees C |
|---|---|
| magnesium | 31 |
| zinc | 25 |
| gold | 20 |
(a) Name the metal in Table 1 that gave no temperature rise. [1]
(b) Use the table to identify the metal giving the most exothermic reaction. [1]
(c) Explain why the test tubes should start at the same temperature. [2]
Answer Details
A temperature decrease indicates an endothermic reaction, whereas a higher temperature rise in a controlled comparison indicates a more exothermic reaction.
- Citric acid and sodium hydrogencarbonate (a) \[22.5 - 17.0 = 5.5\ ^\circ\mathrm{C}\] The temperature decreases by \(5.5\ ^\circ\mathrm{C}\). [1]
- Citric acid and sodium hydrogencarbonate (b) The gas is carbon dioxide. [1]
- Citric acid and sodium hydrogencarbonate (c) The reaction is endothermic, so it requires energy. It absorbs energy from the water and the cup, making the mixture cooler. [2]
- Citric acid and sodium hydrogencarbonate (d) Energy is transferred from the surroundings to the reaction mixture, or reacting chemicals. [1]
- Metals and copper chloride (a) Gold gave no temperature rise. [1]
- Metals and copper chloride (b) Magnesium gave the most exothermic reaction because it reached the highest temperature, \(31\ ^\circ\mathrm{C}\). [1]
- Metals and copper chloride (c) The test tubes must start at the same temperature as a control variable, making the comparison fair. Differences in temperature rise can then be attributed to the metal used. [2]
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