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AQA GCSE · Question 06.2 · Chemical changes
The voltage produced by a chemical cell depends on the concentration of the electrolyte solution.
Plan an experiment to investigate how the voltage produced by a chemical cell varies with the concentration of the electrolyte solution.
The following substances are available:
• the metal electrodes in Table 3
• 1.0 mol/dm³ sodium chloride solution
• pure water.
<br/><br/>
<table border="1" cellpadding="12" cellspacing="0" style="border-collapse: collapse; font-family: Arial, sans-serif; font-size: 15px; border: 2px solid black;">
<caption style="font-weight: bold; font-size: 18px; margin-bottom: 10px;">Table 3</caption>
<tr>
<th style="border: 1px solid black; padding: 12px; text-align: left; font-weight: bold;">Electrode A</th>
<th style="border: 1px solid black; padding: 12px; text-align: center; font-weight: bold;">Symbol of metal</th>
<th style="border: 1px solid black; padding: 12px; text-align: center; font-weight: bold;">Voltage in volts</th>
</tr>
<tr>
<td style="border: 1px solid black; padding: 12px; font-weight: bold;">Copper</td>
<td style="border: 1px solid black; padding: 12px; text-align: center;">Cu</td>
<td style="border: 1px solid black; padding: 12px; text-align: center;">−0.59</td>
</tr>
<tr>
<td style="border: 1px solid black; padding: 12px; font-weight: bold;">Magnesium</td>
<td style="border: 1px solid black; padding: 12px; text-align: center;">Mg</td>
<td style="border: 1px solid black; padding: 12px; text-align: center;">2.12</td>
</tr>
<tr>
<td style="border: 1px solid black; padding: 12px; font-weight: bold;">Nickel</td>
<td style="border: 1px solid black; padding: 12px; text-align: center;">Ni</td>
<td style="border: 1px solid black; padding: 12px; text-align: center;">0.00</td>
</tr>
<tr>
<td style="border: 1px solid black; padding: 12px; font-weight: bold;">Silver</td>
<td style="border: 1px solid black; padding: 12px; text-align: center;">Ag</td>
<td style="border: 1px solid black; padding: 12px; text-align: center;">−1.05</td>
</tr>
<tr>
<td style="border: 1px solid black; padding: 12px; font-weight: bold;">Zinc</td>
<td style="border: 1px solid black; padding: 12px; text-align: center;">Zn</td>
<td style="border: 1px solid black; padding: 12px; text-align: center;">0.51</td>
</tr>
</table>
<br/><br/>
The voltage produced by a chemical cell depends on the concentration of the electrolyte solution.
Plan an experiment to investigate how the voltage produced by a chemical cell varies with the concentration of the electrolyte solution.
The following substances are available:
• the metal electrodes in Table 3
• 1.0 mol/dm³ sodium chloride solution
• pure water.
<br/><br/>
<table border="1" cellpadding="12" cellspacing="0" style="border-collapse: collapse; font-family: Arial, sans-serif; font-size: 15px; border: 2px solid black;"> <caption style="font-weight: bold; font-size: 18px; margin-bottom: 10px;">Table 3</caption> <tr> <th style="border: 1px solid black; padding: 12px; text-align: left; font-weight: bold;">Electrode A</th> <th style="border: 1px solid black; padding: 12px; text-align: center; font-weight: bold;">Symbol of metal</th> <th style="border: 1px solid black; padding: 12px; text-align: center; font-weight: bold;">Voltage in volts</th> </tr> <tr> <td style="border: 1px solid black; padding: 12px; font-weight: bold;">Copper</td> <td style="border: 1px solid black; padding: 12px; text-align: center;">Cu</td> <td style="border: 1px solid black; padding: 12px; text-align: center;">−0.59</td> </tr> <tr> <td style="border: 1px solid black; padding: 12px; font-weight: bold;">Magnesium</td> <td style="border: 1px solid black; padding: 12px; text-align: center;">Mg</td> <td style="border: 1px solid black; padding: 12px; text-align: center;">2.12</td> </tr> <tr> <td style="border: 1px solid black; padding: 12px; font-weight: bold;">Nickel</td> <td style="border: 1px solid black; padding: 12px; text-align: center;">Ni</td> <td style="border: 1px solid black; padding: 12px; text-align: center;">0.00</td> </tr> <tr> <td style="border: 1px solid black; padding: 12px; font-weight: bold;">Silver</td> <td style="border: 1px solid black; padding: 12px; text-align: center;">Ag</td> <td style="border: 1px solid black; padding: 12px; text-align: center;">−1.05</td> </tr> <tr> <td style="border: 1px solid black; padding: 12px; font-weight: bold;">Zinc</td> <td style="border: 1px solid black; padding: 12px; text-align: center;">Zn</td> <td style="border: 1px solid black; padding: 12px; text-align: center;">0.51</td> </tr> </table> <br/><br/>How to approach this question
A good plan should include:
1. **Independent Variable:** What are you changing? (Concentration of the electrolyte). How will you change it? (Diluting the stock solution with pure water).
2. **Dependent Variable:** What are you measuring? (Voltage). What equipment will you use? (Voltmeter).
3. **Control Variables:** What factors must you keep the same to ensure a fair test? (e.g., the pair of metals used, volume of solution, temperature).
4. **Method:** A clear, step-by-step procedure.
5. **Range:** Mention making several different concentrations.
6. **Reliability:** You could mention repeating measurements and calculating a mean, but this is less critical than the core method.
Full Answer
**Method:**
1. Choose two different metal electrodes from the list, for example, zinc and copper. Keep this pair of metals the same throughout the experiment.
2. Set up the chemical cell by placing the two chosen electrodes into a beaker containing 1.0 mol/dm³ sodium chloride solution, ensuring they do not touch.
3. Connect the electrodes to a voltmeter and record the voltage produced.
4. Create a range of different concentrations of sodium chloride solution by diluting the 1.0 mol/dm³ stock solution with pure water. For example, make 0.8, 0.6, 0.4, and 0.2 mol/dm³ solutions using a measuring cylinder for measuring volumes.
5. For each concentration, empty and rinse the beaker, then add the new solution. Place the same two electrodes back into the solution.
6. Measure and record the voltage for each concentration.
7. Control variables: use the same two metals, the same volume of electrolyte, and the same temperature for each test.
**Analysis:**
Plot a graph of voltage (y-axis) against concentration (x-axis) to observe the relationship.
This question requires a clear, logical experimental plan. A good answer will be structured and consider all the key aspects of a scientific investigation.
**Objective:** To investigate the effect of electrolyte concentration on the voltage of a chemical cell.
**Variables:**
* **Independent Variable:** Concentration of sodium chloride solution.
* **Dependent Variable:** Voltage produced by the cell (measured in Volts).
* **Control Variables:** The pair of metals used as electrodes, the volume of the electrolyte solution, the temperature of the solution, the surface area of the electrodes submerged.
**Method:**
1. Select two different metals from Table 3 to use as electrodes, for example, Zinc (Zn) and Copper (Cu), as they produce a reasonable voltage. These two metals must be used for the entire experiment.
2. Prepare a series of sodium chloride solutions of different concentrations. This can be done by diluting the 1.0 mol/dm³ stock solution with measured volumes of pure water. For example, to make a 0.5 mol/dm³ solution, mix equal volumes of the 1.0 mol/dm³ solution and pure water. Create at least 4 other concentrations (e.g., 0.8, 0.6, 0.4, 0.2 mol/dm³).
3. Set up the apparatus as shown in Figure 6. Pour a fixed volume (e.g., 50 cm³) of the 1.0 mol/dm³ NaCl solution into a beaker.
4. Clean the surfaces of the zinc and copper electrodes with sandpaper to remove any oxide layer.
5. Place the electrodes into the solution, making sure they do not touch.
6. Connect the electrodes to a voltmeter. Record the steady voltage reading.
7. Remove the electrodes, rinse and dry them. Empty the beaker, rinse it with the next solution to be tested.
8. Repeat steps 3-7 for each of the prepared concentrations of NaCl solution.
9. Record all results in a suitable table with columns for Concentration (mol/dm³) and Voltage (V).
10. The results can be used to plot a graph of Voltage against Concentration to show the relationship.
Common mistakes
✗ Forgetting to state the control variables.
✗ Changing the metals instead of the concentration.
✗ Not explaining how to make the different concentrations.
✗ A vague or illogical method.
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