Metal Reactivity Investigation
You are going to investigate the reactions of three metals, X, Y and Z, with aqueous copper(II) sulfate.
Experiment Overview
Procedure: Add 25 cm³ of CuSO₄(aq) to a beaker, measure initial temperature, add metal, measure highest temperature reached.
Metals: X, Y, Z (unknown identities)
Objective: Determine relative reactivity based on temperature changes
(a)(i) Complete Table 1.1 with your temperature measurements:
| Experiment | Initial Temperature /°C | Highest Temperature /°C | Temperature Increase /°C |
|---|---|---|---|
| 1 (Metal X) | 22.0 | 35.5 | 13.5 |
| 2 (Metal Y) | 22.0 | 28.0 | 6.0 |
| 3 (Metal Z) | 22.0 | 24.0 | 2.0 |
(a)(ii) Describe the initial appearance of the aqueous copper(II) sulfate.
(a)(iii) Describe the final appearance of the mixture in the beaker in Experiment 4 (after adding dilute sulfuric acid).
(a)(iv) Explain how your observations show that X is in excess in Experiment 1.
(b) Use your results to arrange X, Y, and Z in decreasing order of reactivity.
(c) A student repeats the experiment using a fourth metal. This metal is the second most reactive of the four metals. Suggest a temperature increase.
(d) The temperature increases calculated are less than the true values. Suggest a reason and improvement.
(e) State and explain the effect of using half the concentration of aqueous copper(II) sulfate.
Solutions
(a)(ii) Blue solution
(a)(iii) The mixture becomes a colorless solution with brown solid at the bottom.
(a)(iv) After adding sulfuric acid, there is effervescence/bubbling, indicating unreacted metal X is present and reacting with acid.
(b) Most reactive: X → Y → Z : Least reactive
Explanation: The greater the temperature increase, the more reactive the metal, as more exothermic energy is released.
(c) Approximately 9-10°C (between X’s 13.5°C and Y’s 6.0°C)
(d) Reason: Heat loss to surroundings (beaker, air)
Improvement: Use a polystyrene cup with lid to reduce heat loss
(e) Effect: Temperature increase would be smaller
Explanation: Fewer copper(II) ions available to react, so less energy released
Practical Strategy & Concept
Displacement Reaction: More reactive metals displace less reactive metals from their compounds.
General Equation: Metal + Copper(II) sulfate → Copper + Metal sulfate
Energy Changes: More reactive metals release more energy when displacing copper
Excess Metal Test: Unreacted metal reacts with acid to produce hydrogen gas
- Temperature increase ∝ Reactivity of metal
- Heat loss is a major source of error in calorimetry experiments
- Concentration affects the amount of reactant available
Practical Tips
- Use the same volume and concentration of CuSO₄ for all experiments
- Ensure metals are clean and of similar surface area
- Stir continuously for even temperature distribution
- Record temperatures at the same time intervals
- Use insulation to minimize heat loss
Qualitative Analysis of Solution R
You are provided with solution R and will perform a series of tests to identify its composition.
Test Sequence
Preparation: Place 1 cm depth of R in test-tube with wooden splint for flame test
Tests: (a) Acid + Ba(NO₃)₂, (b) Acid + AgNO₃, (c) Control test, (d) Flame test, (e) NaOH test, (f) Warming test
(a) To R + dilute HNO₃ + Ba(NO₃)₂
(b) To R + dilute HNO₃ + AgNO₃
(c)(i) Describe one other observation in the control test.
(c)(ii) Suggest why it is important to add dilute nitric acid in (b).
(d)(i) Record the first flame colour seen.
(d)(ii) Explain why it is difficult to make a definite conclusion from the flame colour.
(e) To R + NaOH (dropwise then excess)
(f) Gently warm the mixture from (e)
(g) Suggest the names of the two compounds in solution R.
Solutions
(a) Observations: White precipitate forms
Conclusions: Sulfate ions (SO₄²⁻) present
(b) Observations: No precipitate / No change
Conclusions: No chloride, bromide or iodide ions present
(c)(i) The white precipitate dissolves/disappears
(c)(ii) To remove carbonate ions that would interfere with the test (carbonate also forms precipitate with Ag⁺)
(d)(i) First flame colour: Yellow
Conclusions: Sodium ions (Na⁺) may be present
(d)(ii) Yellow flame can mask other flame colors; sodium contamination is common
(e) Observations: White precipitate forms, dissolves in excess NaOH
Conclusions: Zinc ions (Zn²⁺) or aluminium ions (Al³⁺) present
(f) Observations: Ammonia gas produced (turns damp red litmus blue)
Conclusions: Ammonium ions (NH₄⁺) present
(g) Ammonium sulfate and zinc sulfate (or aluminium sulfate)
Analysis Strategy
Systematic Approach:
- Test for anions first: Sulfate, halides, carbonate
- Test for cations: Flame test, NaOH test, ammonia test
- Confirm with specific tests: Warming with NaOH tests for ammonium
Key Reactions:
- Ba²⁺ + SO₄²⁻ → BaSO₄ (white ppt, insoluble in acid)
- Zn²⁺ + 2OH⁻ → Zn(OH)₂ (white ppt, soluble in excess)
- NH₄⁺ + OH⁻ → NH₃ + H₂O (on warming)
Practical Tips
- Always use clean apparatus to avoid contamination
- Add reagents slowly and observe carefully
- For flame test, use cobalt blue glass to filter out yellow sodium light
- Record all observations, even negative ones
- Test gases immediately with appropriate tests
Planning: Barium Carbonate Decomposition
Plan an experiment to determine the percentage loss in mass when barium carbonate is heated.
Word Equation: barium carbonate → barium oxide + carbon dioxide
Chemical Equation: BaCO₃(s) → BaO(s) + CO₂(g)
Your plan must include:
- The apparatus needed
- The method to use and measurements to take
- Procedures to ensure accuracy
- How to calculate the percentage loss in mass
Diagram: Crucible on pipe clay triangle on tripod, Bunsen burner below
Planning Solution
Apparatus:
- Porcelain crucible with lid
- Pipe clay triangle
- Tripod stand
- Bunsen burner
- Heat-resistant mat
- Digital balance (0.01g precision)
- Tongs
- Desiccator
Method:
- Weigh the empty crucible with lid (mass m₁)
- Add about 2-3g of barium carbonate to crucible
- Weigh crucible + lid + barium carbonate (mass m₂)
- Set up apparatus: crucible on pipe clay triangle on tripod
- Heat strongly for 10-15 minutes with lid slightly ajar
- Allow to cool in a desiccator
- Reweigh crucible + lid + contents (mass m₃)
- Repeat heating, cooling and weighing until constant mass
Accuracy Measures:
- Use digital balance for precise measurements
- Heat strongly to ensure complete decomposition
- Cool in desiccator to prevent absorption of moisture/CO₂
- Heat to constant mass to ensure reaction is complete
- Keep lid slightly ajar to allow CO₂ to escape but prevent loss of solid
Calculations:
Initial mass of BaCO₃ = m₂ – m₁
Mass after heating = m₃ – m₁
Mass loss = (m₂ – m₁) – (m₃ – m₁) = m₂ – m₃
Percentage loss = (mass loss / initial mass) × 100
Percentage loss = [(m₂ – m₃) / (m₂ – m₁)] × 100
Planning Strategy
Key Considerations:
- Safety: Barium compounds are toxic – handle with care
- Complete decomposition: Strong heating needed, check by constant mass
- Prevent reabsorption: Cool in dry atmosphere (desiccator)
- Precision: Use balance with appropriate precision
- Theoretical value: Compare experimental % with theoretical (30.8%)
Theoretical Calculation:
M(BaCO₃) = 137 + 12 + 48 = 197 g/mol
M(CO₂) = 44 g/mol
Theoretical % loss = (44/197) × 100 = 22.3%
Practical Tips
- Wear safety goggles and gloves when handling barium compounds
- Use tongs to handle hot crucible
- Ensure good ventilation in the laboratory
- Record all masses immediately after cooling
- Clean apparatus thoroughly before use