Scientific Principles & Theory
Scientific Background: Metals react with acids to form a salt and hydrogen gas:
The rate of hydrogen gas production and the temperature rise of the reaction are direct indications of the reactivity of the metal. Metals below hydrogen in the reactivity series (such as copper) will not react with dilute acids.
Experimental Variables
Independent Variable
The type of metal used (magnesium, zinc, iron, copper).
Dependent Variable
The rate of reaction (measured by bubble production / temperature rise).
Control Variables
Concentration of acid (1.0 mol/dm³), volume of acid (10 cm³), surface area/mass of metal.
⚠️ Lab Risk Assessment
| Hazard | Associated Risk | Control Measure |
|---|---|---|
| Dilute acids (hydrochloric and sulfuric) | Skin/eye irritation | Wear safety goggles; wash skin immediately if contact occurs. |
| Hydrogen gas produced | Flammable and explosive | Keep away from open flames unless carrying out a controlled pop test. |
Apparatus & Procedure
Required Apparatus
- Four test tubes and a test tube rack
- Measuring cylinder (10 cm³)
- Thermometer (0 to 50 °C)
- Emery paper (to clean metal surfaces)
- Metal strips or granules (magnesium, zinc, iron, copper)
- 1.0 mol/dm³ hydrochloric acid
- 1.0 mol/dm³ sulfuric acid
- Wooden splints
Step-by-Step Procedure
- Use emery paper to clean the surfaces of the metal strips (magnesium, zinc, iron, copper) to remove any oxide layers.
- Use a measuring cylinder to add exactly 10 cm³ of 1.0 mol/dm³ hydrochloric acid to four separate test tubes.
- Place a thermometer in the first test tube and record the initial temperature of the acid.
- Add a strip of magnesium ribbon to the first test tube. Start a stopclock immediately.
- Observe the rate of effervescence (fizzing) and record qualitative observations (e.g. rapid fizzing, slow bubbles, no change).
- Monitor the thermometer and record the maximum temperature reached during the reaction.
- Perform the squeaky pop test by collecting gas in an inverted tube and introducing a lit wooden splint.
- Repeat the procedure (steps 3 to 7) for zinc, iron, and copper, ensuring you clean each metal and keep the acid volume and concentration constant.
- Repeat the entire experiment using 1.0 mol/dm³ sulfuric acid instead of hydrochloric acid.
Fig 1. Laboratory experimental setup for Core Practical 2.21.
Sample Data & Calculations
This representative dataset illustrates the values typically obtained when carrying out this experiment in the laboratory:
| Metal tested | Initial Temp (°C) | Maximum Temp (°C) | Temp Rise (°C) | Effervescence Rate Description |
|---|---|---|---|---|
| Magnesium (Mg) | 20.5 | 38.0 | 17.5 | Vigorous effervescence; metal dissolved rapidly |
| Zinc (Zn) | 20.5 | 25.2 | 4.7 | Moderate effervescence; continuous bubbles |
| Iron (Fe) | 20.5 | 22.1 | 1.6 | Slow, steady bubble formation |
| Copper (Cu) | 20.5 | 20.5 | 0.0 | No bubbles; no visible reaction |
Data Processing & Analysis
- Temp Rise = Maximum Temp - Initial Temp
- Mg Temp Rise = 38.0 °C - 20.5 °C = 17.5 °C
- Reactivity Order deduced: Mg > Zn > Fe > Cu
Conclusion & Evaluation
Chemical Explanation: Saturated solutions are heavily dependent on temperature. Heating shifts solubility limits, allowing more solute to form coordinate bonds or ion-dipole interactions with solvent molecules. When cooled, the reverse process happens and solute precipitates out.
Experimental Error Analysis
| Error Type & Source | Effect on Final Result | Mitigation Strategy |
|---|---|---|
| Random Error Heat loss from the test tube to the surroundings |
The measured temperature rise is lower than it should be, particularly for slower reactions. | Perform the reaction in an insulated container, such as a polystyrene cup inserted in a beaker. |
| Systematic Error Surface oxide layer on the metals |
The reaction takes time to start because the acid must first dissolve the protective oxide layer, slowing the initial rate. | Thoroughly clean all metal pieces with emery paper right before adding them to the acid. |
Exam Practice
A student is given four unlabelled metal samples: magnesium, zinc, iron, and copper. Plan an investigation to identify each metal by comparing their reactions with dilute hydrochloric acid, detailing variables and control measures.
View Model Answer & Mark Scheme
Model Answer (6/6 Marks):
- Preparation: Clean all metal strips with emery paper to remove surface oxide layers. Measure 10 cm³ of 1.0 mol/dm³ HCl into four test tubes.
- Execution: Add a piece of metal to a test tube, start a timer, and place a thermometer inside.
- Observations: Note the rate of bubble production and record the initial and maximum temperature to find the temperature rise.
- Identification:
- Copper: No fizzing and no temperature rise because it is below hydrogen in reactivity.
- Magnesium: Vigorous, rapid fizzing and the largest temperature rise (approx. 15-20 °C) because it is the most reactive.
- Zinc: Moderate fizzing and a small temperature rise (approx. 4-5 °C).
- Iron: Very slow bubble formation and a tiny temperature rise (approx. 1-2 °C).
- Control Measures: Keep the volume and concentration of acid, initial temperature, and surface area of the metals constant.
Examiner Tip:
Be sure to state the control variables explicitly: concentration and volume of acid must be kept constant to ensure a fair comparison of reactivity.