Continuous monitoring tracks a physical or chemical variable continuously throughout the course of a single reaction run. This guide focuses on measuring carbon dioxide gas evolution from the reaction between marble chips (\(\text{CaCO}_3\)) and hydrochloric acid (\(\text{HCl}\)), using a gas syringe setup.
🔑 Core Specification Link
This practical supports 3.1.5 Kinetics, explicitly addressing the determination of rate equations, reaction order, and rate constants (\(k\)) using graphical methods.
Reaction Equation
\[ \text{CaCO}_3(\text{s}) + 2\text{HCl}(\text{aq}) \rightarrow \text{CaCl}_2(\text{aq}) + \text{CO}_2(\text{g}) + \text{H}_2\text{O}(\text{l}) \]The rate of reaction is monitored by collecting and measuring the volume of carbon dioxide gas evolved over time.
Aim
To measure the volume of carbon dioxide gas evolved over time at different concentrations of hydrochloric acid, and to determine the order of reaction with respect to hydrochloric acid.
Equipment List
- Conical flask (250 cm³)
- Gas syringe (100 cm³)
- Delivery tube fitted with a rubber bung
- Marble chips (\(\text{CaCO}_3\) - large granules, used in excess)
- Dilute hydrochloric acid (\(1.0\text{ mol dm}^{-3}\) and \(0.5\text{ mol dm}^{-3}\))
- Stopwatch
- Retort stand, boss, and clamps
- Electronic balance (precision to 0.01 g)
- Measuring cylinders
Experimental Method
- Measure exactly 50 cm³ of 1.0 mol dm⁻¹ hydrochloric acid and pour it into the 250 cm³ conical flask.
- Clamp the gas syringe horizontally in a stand and connect its inlet to the delivery tube.
- Weigh out approximately 5.00 g of large marble chips (an excess, ensuring hydrochloric acid is the limiting reactant).
- Add the marble chips to the flask, immediately insert the rubber bung tightly, and start the stopwatch.
- Record the volume of gas collected in the syringe every 10 seconds for the first 2 minutes, and then every 30 seconds until the reaction stops.
- Thoroughly clean and dry the flask. Repeat steps 1 to 5 using 50 cm³ of 0.50 mol dm⁻¹ hydrochloric acid. Keep all other variables (temperature, mass, and surface area of marble chips) constant.
- Plot a graph of gas volume (y-axis) against time (x-axis) for both acid concentrations.
Safety & Risk Assessment
| Hazard | Risk | Precaution |
|---|---|---|
| 1.0 mol dm⁻¹ Hydrochloric acid | Skin and eye irritation. | Wear safety goggles and lab coat. Wash spills off skin immediately. |
| Splattering of acid | Acid droplets splashing into eyes during the addition of marble chips. | Wear eye protection. Add marble chips carefully down the neck of the flask. |
| Carbon dioxide build-up | Over-pressurisation causing syringe plunger to shoot out or connections to pop. | Do not exceed the maximum volume limit of the gas syringe. Do not clamp the plunger. |
Results & Molar Order Processing
From the volume-time curves, the initial rate (rate at \(t = 0\)) is determined by drawing a tangent line at the origin and calculating its gradient:
\[ \text{Initial Rate} = \frac{\Delta \text{Volume}}{\Delta \text{Time}} \quad (\text{cm}^3\text{ s}^{-1}) \]- For \(1.00\text{ mol dm}^{-3}\text{ HCl}\): Initial Rate = \(1.80\text{ cm}^3\text{ s}^{-1}\)
- For \(0.50\text{ mol dm}^{-3}\text{ HCl}\): Initial Rate = \(0.90\text{ cm}^3\text{ s}^{-1}\)
Step 1: Determine the reaction order
Compare the change in concentration to the change in initial rate:
\[ \text{Concentration ratio} = \frac{1.00}{0.50} = 2.0 \quad (\text{doubled}) \] \[ \text{Rate ratio} = \frac{1.80}{0.90} = 2.0 \quad (\text{doubled}) \]Because doubling the concentration of \(\text{HCl}\) exactly doubles the initial rate of gas evolution, the reaction is first order with respect to \(\text{HCl}\).
Step 2: Write the rate equation
\[ \text{Rate} = k[\text{HCl}]^1 \]Step 3: Calculate the rate constant (\(k\))
\[ k = \frac{\text{Rate}}{[\text{HCl}]} = \frac{1.80\text{ cm}^3\text{ s}^{-1}}{1.00\text{ mol dm}^{-3}} = 1.80\text{ cm}^3\text{ mol}^{-1}\text{ dm}^3\text{ s}^{-1} \]The rate constant is 1.80 dm³ mol⁻¹ s⁻¹ (using relative rate volumetric units).
Sources of Error & Improvements
| Error Source | Classification | Consequence & Mitigation |
|---|---|---|
| Gas escape during bung insertion | Systematic | Some carbon dioxide gas escapes between adding the solid marble chips and sealing the bung, reducing the recorded gas volume at early time intervals. Mitigation: Use a two-necked flask or addition funnel, allowing chips to be dropped without opening the bung. |
| Friction in the gas syringe | Random | Syringe plungers can stick due to friction, causing sudden, jumpy movements and inaccurate readings. Mitigation: Ensure the syringe is dry and clean, and rotate the plunger slightly to minimise friction. |
| Variable marble chip surface area | Random | Marble chips vary in size and shape, changing the exposed surface area and affecting reaction kinetics. Mitigation: Use uniform marble chips or ground calcium carbonate of a consistent mesh size. |
Common Exam Questions
1. Suggest a chemical method that would allow you to measure the rate of this reaction continuously without using a gas syringe.
Place the conical flask on an electronic balance plugged with cotton wool. Monitor the mass loss of the flask at regular intervals. As carbon dioxide gas escapes through the cotton wool, the mass decreases. The rate of mass loss is proportional to the rate of reaction.
2. Explain why cotton wool is placed in the neck of the flask if mass loss is monitored, rather than sealing the flask.
Cotton wool allows carbon dioxide gas to escape freely, preventing pressure build-up, while preventing any acid spray or aerosol from splashing out, which would cause an overestimate of mass loss.
3. How does a concentration-time graph for a zero-order reactant differ from a first-order reactant?
For a zero-order reactant, the plot of concentration against time is a straight line with a constant gradient because rate is independent of concentration. For a first-order reactant, the curve is exponential with a constant half-life (\(t_{1/2}\)).
CPAC Skills Assessed
- CPAC 2: Design, set up, and apply gas collection apparatus to measure rate variables.
- CPAC 3: Safely sets up gas collection glassware.
- CPAC 4: Records gas volume data accurately at strict time intervals.
When drawing tangents in exams to find gradients, extend your tangent line as far as possible across the graph axes. This reduces the relative error when reading the coordinates (\(\Delta y\) and \(\Delta x\)) to calculate the gradient.