The table below shows results from a hand-warmer test. Iron powder in each sealed sachet reacts with oxygen in air. Fig. 1 is a diagram of the sachet before...
Question 1 Report
The table below shows results from a hand-warmer test. Iron powder in each sealed sachet reacts with oxygen in air. Fig. 1 is a diagram of the sachet before it is opened.
| time after opening / min | surface temperature / degrees C |
|---|---|
| 0 | 19 |
| 10 | 42 |
| 25 | 51 |
(a) Describe the trend in temperature shown in Table 1. [1]
(b) Explain why a hand-warmer feels warm during this reaction. [2]
(c) Name one variable, other than time, that should be kept the same when comparing two sachets. [1]
During a chemistry lesson, a student compares the energy changes when different salts dissolve. Equal masses of each salt are added to 50 cm3 of water in identical foam cups. Table 1 gives the results.
(a) Use Table 1 to identify the salt that gives the greatest temperature decrease. [1]
(b) Calculate the temperature change when potassium chloride dissolves. [1]
(c) Explain why equal volumes of water are needed. [2]
(d) Describe one improvement that would reduce heat transfer between each cup and the air. [1]
| salt | initial temperature / degrees C | final temperature / degrees C |
|---|---|---|
| sodium chloride | 20 | 19 |
| potassium chloride | 20 | 16 |
| calcium chloride | 20 | 29 |
Answer Details
Iron oxidation in a hand-warmer is exothermic. In dissolving experiments, equal water volumes are needed for a fair comparison of temperature changes.
- Hand-warmer (a) The temperature increases or rises with time. [1]
- Hand-warmer (b) Oxidation of iron is exothermic and releases energy. Thermal energy is transferred from the warmer to the hand, so it feels warm. [2]
- Hand-warmer (c) One suitable control variable is the mass of iron powder, sachet size, starting temperature, or exposure to air. [1]
- Salt dissolving (a) Potassium chloride gives the greatest temperature decrease, from \(20\ ^\circ\mathrm{C}\) to \(16\ ^\circ\mathrm{C}\). [1]
- Salt dissolving (b) \[16 - 20 = -4\ ^\circ\mathrm{C}\] This is a decrease of \(4\ ^\circ\mathrm{C}\). [1]
- Salt dissolving (c) Equal water volumes control a variable and make the comparison fair. Different volumes would warm up or cool down by different amounts for the same energy transfer. [2]
- Salt dissolving (d) Use a lid or a thicker insulated cup to reduce heat transfer between the cup and air. [1]
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