Fig. 1 shows a recycling trial at a construction-skills centre. Magnesium trimmings from old window frames are added to dilute hydrochloric acid. Before one...
Question 1 Report
Fig. 1 shows a recycling trial at a construction-skills centre. Magnesium trimmings from old window frames are added to dilute hydrochloric acid. Before one trial, the student rubs the metal with abrasive paper. A second, similar piece is used with its dull surface unchanged. Gas from each reaction is collected in a measuring cylinder over water.
Table 1 gives the volume of gas collected. The gas is hydrogen. The magnesium pieces had the same mass and were placed in the same volume and concentration of acid.
| Time / min | Hydrogen from polished magnesium / cm3 | Hydrogen from unpolished magnesium / cm3 |
|---|---|---|
| 0 | 0 | 0 |
| 1 | 18 | 4 |
| 2 | 31 | 11 |
| 3 | 40 | 18 |
(a) Describe the difference between the results for the two magnesium pieces. [2]
(b) Give two reasons why rubbing the magnesium changes the rate of reaction. [2]
(c) Use chemical symbols to write the balanced equation for magnesium reacting with hydrochloric acid. [2]
(d) Calculate the mean rate of hydrogen production by the polished magnesium during the first 2 minutes. Give your answer in cm3 per minute. [2]
(e) Give one safety precaution for this reaction. [1]
Answer Details
(a) Polished magnesium produces more hydrogen at every stated time [1]. It reacts faster, reaching 40 cm3 in 3 minutes, whereas unpolished magnesium produces only 18 cm3 [1].
(b) Abrasive paper removes the magnesium oxide coating [1], exposing fresh magnesium to the acid [1]. It also gives a larger effective surface area, so more successful acid-metal collisions occur.
(c) \[\mathrm{Mg+2HCl\rightarrow MgCl_2+H_2}\] [2]
(d) In the first 2 minutes, 31 cm3 of hydrogen is produced [1].
\[\text{mean rate}=\frac{31\text{ cm}^3}{2\text{ min}}=15.5\text{ cm}^3\text{ min}^{-1}\] [1]
(e) Wear eye protection [1]. Keeping hydrogen away from flames is another valid precaution because hydrogen is flammable.
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