Part A: build a simple cell
You need: a zinc plate, a copper plate, dilute sulfuric acid (or lemon juice), a beaker, wires with clips and a voltmeter. Wear goggles. Clean the plates with sandpaper first.
- Pour the acid into the beaker.
- Dip the zinc and copper plates in, not touching.
- Clip one wire from zinc to the voltmeter's negative side and another from copper to its positive side.
- Read the voltage (about 1.0 V) and watch for bubbles on the copper plate.
What happens: zinc is more reactive, so it gives electrons: Zn → Zn²⁺ + 2e⁻ (anode, −). The electrons run through the wire. At copper, hydrogen ions take them: 2H⁺ + 2e⁻ → H₂ (cathode, +). That is chemical energy turning into electrical energy.
Part A: change the metal pair
Repeat with the same acid, the same plate size and the same gap, but change one plate:
| Pair | Voltage (about) |
|---|---|
| Zn–Cu | 1.0 V |
| Fe–Cu | 0.7 V |
| Cu–Cu | 0 V |
The further apart the two metals are in the reactivity series, the bigger the voltage. Two of the same metal give no push at all. Joining cells in series adds their voltages: two Zn–Cu cells give about 2.0 V.
Values vary a little with the acid, the plate area and how clean the plates are. Write what you actually read.
Part B: measure a rate
Rate means how fast a product forms (or a reactant is used up). We use marble chips and hydrochloric acid: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. Collect the CO₂ in a gas syringe and note the volume every 10 seconds. Plot volume against time.
- Rate = volume of gas ÷ time, for example 24 mL in 40 s = 0.6 mL/s.
- The steeper the line, the faster the rate.
- The line flattens when one reactant is used up.
Part B: the four factors
Change one factor and keep the rest equal. That is a fair test: the changed factor is the variable, everything else is controlled.
| Factor | Change | Effect on rate | Why (collision idea) |
|---|---|---|---|
| Concentration | 1 mol/L → 2 mol/L acid | Faster (about 2×) | More acid particles meet the marble each second |
| Temperature | +10 °C | Faster (often about 2×) | Particles move faster and hit harder |
| Surface area | Lumps → powder | Much faster | More marble surface is open to the acid |
| Catalyst | H₂O₂ with MnO₂ | Much faster | Gives an easier path; MnO₂ is not used up |
None of these changes the total amount of gas, only the time taken.
Recording and safety
Make a table with columns time (s) and volume (mL) for each test, and repeat a test twice to check. Keep the same mass of marble, the same volume of acid and the same flask each time. Use a water bath to hold a steady temperature. Do not heat acid on a naked flame. Wipe spills at once, wash your hands, and put used chemicals in the labelled waste bottle.
Try it: predict, then check
Before step 6, predict how many times faster the reaction runs with 2 mol/L acid at 40 °C. Using the rules (2× for acid and 2×2 = 4× for +20 °C), the answer is about 8×. Then set the sliders and compare. At home: drop one effervescent tablet whole into cold water and another crushed into the same amount of warm water. Time both and see which fizzes out first.
Key formulas and definitions
- Rate = amount of product ÷ time (for example mL of gas per second)
- Steeper line on a volume–time graph = faster rate
- Rule of thumb: rate often about doubles for every 10 °C rise
- Cells in series: V total = V of one cell × number of cells
- Zn → Zn²⁺ + 2e⁻ (anode); 2H⁺ + 2e⁻ → H₂ (cathode)
Worked examples
1. A student collects 24 mL of gas in 40 s. What is the average rate?
Rate = 24 ÷ 40 = 0.6 mL/s.
2. A test at 20 °C takes 80 s to finish. About how long would it take at 40 °C if the rate doubles for every 10 °C?
+20 °C means doubling twice, so the rate is 4× faster. Time = 80 ÷ 4 = 20 s.
3. Two Zn–Cu cells (1.0 V each) are joined in series. What is the voltage?
Voltages add in series: 2 × 1.0 = 2.0 V.
4. A student compares marble lumps with 1 mol/L acid and marble powder with 2 mol/L acid, and says powder is faster because of more surface area. Is the test fair?
No. Both size and concentration changed, so we cannot tell which caused the speed-up. Change only one factor at a time.
Common mistakes
- Changing two things at once in a rate test, so the result cannot be explained.
- Thinking a faster reaction makes more product. The total amount is set by how much reactant you start with.
- Reading the rate from the end value of the graph instead of its slope.
- Using plates that are dirty or touching in the cell, which gives a low or false reading.