The question
Name: ______________________ Date: ____________ Zinc and copper(II) ions in a coffee-cup calorimeter (10 points)
Graph
Temperature of the solution
A student places 50.0 mL of 0.200 M CuSO₄(aq) in a coffee-cup calorimeter (two nested foam cups with a lid). At 45 s she adds 1.00 g of zinc powder and stirs. The reaction is Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s). Assume the solution has a mass of 50.0 g and a specific heat of 4.18 J/(g·°C), and that the calorimeter absorbs a negligible amount of energy. Molar mass of Zn: 65.38 g/mol.
Data table
| Time (s) | Solution |
|---|---|
| 0 | 22.1 |
| 30 | 22.1 |
| 60 | 28.6 |
| 90 | 31.4 |
| 120 | 32.2 |
| 150 | 32.3 |
| 180 | 32.1 |
| 240 | 31.8 |
| 300 | 31.5 |
(a) Identify the limiting reactant. Justify your answer with a calculation. [1 point]
(b) Using the data, calculate the energy, in joules, absorbed by the solution. [1 point]
(c) Calculate ΔH for the reaction, in kJ per mole of Cu²⁺ reacted. Include the correct sign. [2 points]
(d) After 150 s the temperature slowly falls. Explain why the student uses the temperature at 150 s, not the temperature at 300 s, as the final temperature. [1 point]
(e) A second student does the experiment in a single uncovered foam cup. Will that student's calculated value of ΔH be more negative, less negative, or the same as the first student's? Justify your answer. [2 points]
(f) The experiment is repeated with 100.0 mL of 0.200 M CuSO₄(aq) and the same 1.00 g of zinc. Will the temperature rise be greater than, less than, or equal to 10.2 °C? Justify your answer with a calculation or with reasoning about the limiting reactant and the mass of solution. [2 points]
(g) The reaction Cu(s) + 2 Ag⁺(aq) → Cu²⁺(aq) + 2 Ag(s) has ΔH = −146.4 kJ/mol. Using this and your answer to part (c), calculate ΔH for Zn(s) + 2 Ag⁺(aq) → Zn²⁺(aq) + 2 Ag(s). [1 point]