Scientific Principles & Theory
Scientific Background: The rate of a chemical reaction is the change in concentration of reactants or products per unit time. According to collision theory, a reaction occurs when reacting particles collide with sufficient energy (equal to or greater than the activation energy) and in the correct orientation.
The reaction between marble chips (calcium carbonate) and hydrochloric acid produces carbon dioxide gas:
The rate can be monitored by measuring the volume of CO₂ gas produced over time using a gas syringe, or by measuring the mass loss of the reaction flask on a balance as the gas escapes.
Experimental Variables
Independent Variable
The surface area of marble chips (large, small, powder) OR concentration of hydrochloric acid.
Dependent Variable
The rate of reaction (measured as volume of gas produced or mass lost per second).
Control Variables
Mass of marble chips (5.0 g), volume of acid (50 cm³), temperature of the acid.
⚠️ Lab Risk Assessment
| Hazard | Associated Risk | Control Measure |
|---|---|---|
| Dilute hydrochloric acid | Skin/eye irritation | Wear safety goggles; wash skin immediately if contact occurs. |
| Carbon dioxide gas buildup | Pressure buildup and glass shattering | Ensure the gas syringe moves smoothly and the delivery tube is not blocked. |
Apparatus & Procedure
Required Apparatus
- Conical flask (100 cm³)
- Cotton wool plug
- Digital balance (0.01 g resolution)
- Stopclock
- Measuring cylinder (50 cm³)
- Large marble chips, small marble chips, and calcium carbonate powder
- 1.0 mol/dm³ and 2.0 mol/dm³ hydrochloric acid
- Gas syringe and delivery tube (alternative setup)
Step-by-Step Procedure
- Measure 50 cm³ of 1.0 mol/dm³ hydrochloric acid using a measuring cylinder and pour it into a conical flask.
- Place the conical flask on a digital balance and tare (zero) the balance.
- Weigh exactly 5.0 g of large marble chips.
- Add the marble chips to the acid in the flask, immediately place a loose plug of cotton wool in the neck of the flask, and start the stopclock.
- Record the mass reading on the balance every 30 seconds for 5 minutes. The mass will decrease as carbon dioxide gas escapes.
- Repeat the experiment using 5.0 g of small marble chips, keeping the acid volume, concentration, and temperature constant.
- Repeat the experiment using 5.0 g of calcium carbonate powder.
- Alternatively, connect the flask to a gas syringe and record the volume of gas collected every 10 seconds.
- Plot a graph of mass loss (or gas volume) against time for each run.
Fig 1. Laboratory experimental setup for Core Practical 3.15.
Sample Data & Calculations
This representative dataset illustrates the values typically obtained when carrying out this experiment in the laboratory:
| Time (s) | Mass Loss - Large Chips (g) | Mass Loss - Small Chips (g) | Mass Loss - Powder (g) |
|---|---|---|---|
| 0 | 0.00 | 0.00 | 0.00 |
| 30 | 0.08 | 0.18 | 0.45 |
| 60 | 0.15 | 0.32 | 0.72 |
| 90 | 0.22 | 0.44 | 0.80 |
| 120 | 0.28 | 0.54 | 0.80 |
| 150 | 0.34 | 0.62 | 0.80 |
Data Processing & Analysis
- Rate of reaction at a specific time = Gradient of tangent to the curve at that point.
- Initial Rate (Powder) = 0.72 g / 60 s = 0.012 g/s
- Initial Rate (Small Chips) = 0.32 g / 60 s = 0.0053 g/s
- Explanation: Powder has a larger surface area to volume ratio, exposing more particles to collisions. This results in a higher frequency of successful collisions.
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 |
|---|---|---|
| Systematic Error Acid spray escaping from the flask |
The mass loss appears larger than it should be because liquid is leaving the flask along with the gas, leading to an overestimation of rate. | Place a loose plug of cotton wool in the neck of the flask. This lets the gas escape but traps any escaping liquid droplets. |
| Random Error Delay in starting the stopclock and placing the cotton wool plug |
Some carbon dioxide gas escapes before the balance and timer are started, causing a loss of initial data points. | Perform the addition quickly, or use a divided flask (one containing acid, one containing solid) that can be tipped to start the reaction without opening. |
Exam Practice
A student wants to investigate the effect of changing the concentration of hydrochloric acid on the rate of reaction between marble chips and acid. Plan an experiment to measure this, listing variables and explaining results using particle collision theory.
View Model Answer & Mark Scheme
Model Answer (6/6 Marks):
- Variables: The independent variable is the concentration of hydrochloric acid (e.g. 0.5, 1.0, 2.0 mol/dm³). The dependent variable is the volume of gas produced per second. Control variables include the temperature, the volume of acid (50 cm³), and the mass (5.0 g) and size (surface area) of the marble chips.
- Method: Measure 50 cm³ of 0.5 mol/dm³ HCl and place it in a conical flask. Add 5.0 g of small marble chips.
- Gas Collection: Immediately insert a bung connected to a gas syringe and start a timer.
- Data Recording: Record the volume of gas in the syringe every 10 seconds for 2 minutes.
- Repetition: Repeat the experiment using 1.0 mol/dm³ acid, and then 2.0 mol/dm³ acid, keeping the same mass and size of marble chips.
- Collision Theory Explanation: Higher acid concentration means there are more acid particles per unit volume. This increases the frequency of collisions between acid particles and marble chips, leading to a higher frequency of successful collisions and a faster rate of reaction.
Examiner Tip:
Always specify the exact concentrations and volumes of acid you would use, and use key collision theory phrases like 'number of particles per unit volume' and 'frequency of successful collisions'.