# Reaction - Magnesium & Hydrochloric Acid ## Overview - This practical investigates how changing the **concentration of hydrochloric acid** affects the rate of reaction with magnesium ribbon - Temperature is kept constant throughout - The reaction produces hydrogen gas, which can be measured to monitor reaction rate

Hydrogen gas is too low in density for mass change measurements to work reliably on a lab balance

## Measuring Gas Volume - Two main methods can measure the volume of hydrogen produced: - **Displacement of water** into an inverted measuring cylinder - **Gas syringe** connected to the reaction vessel - The size of the gas syringe must be carefully chosen - Calculate expected gas volume beforehand - Ensure the syringe can hold the full volume produced - Too small = incomplete data; too large = poor resolution ## Equipment Setup

Check the apparatus is gas-tight before starting by assembling without reactants and gently pushing the plunger—you should feel resistance

- Conical flask (Erlenmeyer flask) for the reaction - Gas syringe mounted on a stand with boss and clamp - Stopwatch for timing - Measuring cylinders for preparing acid dilutions - Distilled water for dilutions ## Method ### Preparing Acid Concentrations - Start with a stock solution (typically $2.0 \text{ mol dm}^{-3}$ hydrochloric acid) - Prepare a range of concentrations using **serial dilution** | Volume of HCl / cm³ | Volume of water / cm³ | Total volume / cm³ | |---------------------|----------------------|--------------------| | 40 | 0 | 40 | | 35 | 5 | 40 | | 30 | 10 | 40 | | 25 | 15 | 40 | | 20 | 20 | 40 |

Always keep total volume constant when diluting to ensure valid comparisons

### Running the Experiment 1. Measure the required volume of acid (e.g. $40 \text{ cm}^3$) using a measuring cylinder 2. Pour the acid into the conical flask **first** - This allows quick addition of magnesium to start timing accurately 3. Connect the gas syringe loosely to the flask 4. Add the magnesium ribbon quickly, secure the connection, and **start the stopwatch immediately** 5. Swirl the mixture gently to ensure good contact 6. Record the volume of gas collected every 10 seconds 7. Stop when gas production ceases (the volume plateaus) 8. Repeat for at least four more concentrations ## Practical Tips ### Before Starting - Ensure the plunger is **fully inserted** before beginning - Otherwise you'll have a systematic volume error throughout - Clean magnesium ribbon with sandpaper if it looks dull or oxidized - A shiny surface reacts more predictably ### During the Reaction - Make sure the plunger is secure and won't pop out if gas volume exceeds $100 \text{ cm}^3$ - If the reaction is too fast to record accurately, use a more dilute acid - If too slow, increase the concentration or temperature

Starting concentration should not exceed $2.0 \text{ mol dm}^{-3}$ to keep the reaction at a measurable rate

## Specimen Results Here's a typical set of results for three different acid concentrations: | Time / s | Volume of gas / cm³ (2.0 mol dm⁻³) | Volume of gas / cm³ (1.0 mol dm⁻³) | Volume of gas / cm³ (0.5 mol dm⁻³) | |----------|-----------------------------------|-----------------------------------|-----------------------------------| | 0 | 0 | 0 | 0 | | 10 | 10 | 6 | 3 | | 20 | 20 | 11 | 6 | | 30 | 28 | 15 | 9 | | 40 | 34 | 19 | 12 | | 50 | 40 | 21 | 14 | | 60 | 44 | 22 | 15 | The graph below shows how gas volume changes over time for each concentration:

## Graphing the Results - Plot **all three concentrations** on the same graph - X-axis: Time / seconds - Y-axis: Volume of gas / cm³ - Each concentration produces a curve that: - Starts steep (fast initial rate) - Gradually levels off (reaction slowing) - Plateaus when reaction is complete - Higher concentrations produce: - Steeper initial gradients - Higher final gas volumes (if magnesium is limiting) - OR the same final volume reached faster (if acid is limiting) ## Analysis ### Finding the Initial Rate

Initial rate is the gradient of the tangent drawn from the origin $(0,0)$ at the start of the reaction

**Steps:** 1. Draw a curve of best fit through each set of data points 2. From the origin, draw a **tangent** to the curve - The tangent should just touch the curve at one point near the start 3. Make the tangent line as long as possible for accuracy 4. Calculate the gradient: $$\text{Gradient} = \frac{\Delta y}{\Delta x} = \frac{\text{change in volume}}{\text{change in time}}$$ **Example calculation** for $2.0 \text{ mol dm}^{-3}$ acid: $$\text{Rate} = \frac{40}{38} = 1.05 \text{ cm}^3 \text{ s}^{-1}$$

Longer tangents reduce percentage error—extend them as far as the graph allows without losing contact with the curve

### Comparing Rates - Repeat the gradient calculation for each concentration - Compare the rates to see how concentration affects reaction speed - You should observe that **doubling concentration approximately doubles the initial rate** - This indicates a first-order relationship with respect to HCl ## Key Skills Developed - **Using a gas syringe** to collect and measure gas volumes accurately - **Preparing serial dilutions** to vary concentration systematically - **Timing reactions** using a stopwatch with appropriate intervals - **Plotting multiple curves** on one graph for comparison - **Drawing tangents** accurately to determine initial rates from gradients - **Controlling variables** (temperature, mass of magnesium, total volume) ## Common Issues and Solutions | Problem | Cause | Solution | |---------|-------|----------| | Gas escapes before timing starts | Poor seal | Check connections are tight before adding magnesium | | Plunger sticks or jams | Friction or misalignment | Lubricate with water or check alignment | | Reaction too fast to measure | Concentration too high | Dilute acid further or reduce temperature | | No gas produced | Magnesium heavily oxidized | Clean with sandpaper or use fresh ribbon | | Final volumes differ unexpectedly | Inconsistent magnesium mass | Weigh magnesium precisely each time |