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This question is about the rate of the reaction between hydrochloric acid and calcium carbonate - AQA - GCSE Chemistry - Question 8 - 2020 - Paper 2
Question 8
This question is about the rate of the reaction between hydrochloric acid and calcium carbonate. A student investigated the effect of changing the size of calcium c... show full transcript
Worked Solution & Example Answer:This question is about the rate of the reaction between hydrochloric acid and calcium carbonate - AQA - GCSE Chemistry - Question 8 - 2020 - Paper 2
Step 1
Complete Figure 4.
Answer
Plot the data from Table 4 onto Figure 4, marking the time in seconds on the x-axis and the number of moles of gas on the y-axis. For the points, you will plot (0, 0.0000), (30, 0.0011), (60, 0.0020), (90, 0.0028), (120, 0.0034), (150, 0.0038), and (180, 0.0040). Draw a line of best fit that represents the trend of the plotted data.
Step 2
Determine the mean rate of reaction for small calcium carbonate lumps between 20 seconds and 105 seconds.
Answer
To find the mean rate of reaction, we first identify the number of moles of gas produced at 20 seconds and 105 seconds from Figure 4. If it is 0.0038 for 105 seconds and we approximate 0.0020 for 20 seconds:
Mean rate of reaction = ( \frac{0.0038 - 0.0020}{105 - 20} = \frac{0.0018}{85} = 0.000021176 ) moles per second.
Rounding, this gives a mean rate of approximately 2.1 x 10⁻⁵ moles/s.
Step 3
How do the student’s results show that this conclusion is correct?
Answer
The data indicates that smaller lumps of calcium carbonate produce a larger volume of gas in the same time frame compared to larger lumps. Therefore, smaller lumps provide a greater surface area for the reaction with hydrochloric acid, resulting in a faster reaction rate.
Step 4
Calculate the surface area to volume ratio of the cube in Figure 5.
Answer
For the cube shown in Figure 5:
- Surface area = 6 * (0.5 cm * 0.5 cm) = 1.5 cm²
- Volume = (0.5 cm * 0.5 cm * 0.5 cm) = 0.125 cm³
Thus, the surface area to volume ratio is:
( \frac{1.5 , \text{cm}^2}{0.125 , \text{cm}^3} = 12 : 1 )
Step 5
Describe how the surface area to volume ratio of this larger cube differs from that of the cube in Figure 5.
Answer
A larger cube of calcium carbonate with sides of 5 cm will have a substantially lower surface area to volume ratio compared to the smaller cube from Figure 5. Specifically, while the smaller cube has a ratio of 12:1, the larger cube's surface area will increase proportionally but the volume will increase much more significantly, resulting in a ratio that is approximately 1:10. This illustrates that as the size of the cube increases, the efficiency of the reaction decreases due to reduced surface area relative to volume.