Combustion stoichiometry
Experimentally verify the stoichiometry of paraffin combustion by calculating the ratio of O₂ consumed to CO₂ produced.
Activity Summary
The student measures O₂ and CO₂ changes during candle combustion in a sealed chamber and calculates the experimental O₂/CO₂ ratio.
Introduction
The combustion of paraffin (C₂₅H₅₂) in oxygen follows a balanced chemical equation: C₂₅H₅₂ + 38 O₂ → 25 CO₂ + 26 H₂O. The numbers 1, 38, 25, and 26 are the stoichiometric coefficients — they tell us the exact proportions in which the reactants combine and products form. By measuring how much O₂ is consumed and how much CO₂ is produced during a candle burning in a sealed chamber, you can experimentally determine this ratio and compare it with the theoretical prediction.
Learning Objectives
- Calculate the O₂ consumed and CO₂ produced
- Determine the experimental stoichiometric ratio
- Compare with the theoretical ratio from the balanced equation
- Understand atom conservation in chemical reactions
Instruments and sensors
Scientific instruments
- O₂ concentration meter
- CO₂ concentration meter
Sensors
- O₂ sensor
- SCD40 sensor
Required Materials
- - Smartphone or tablet with FizziQ Connect - SCD40 and O₂ sensors - M5 Stack module and multiport hub - Sealed chamber - Tea light candles - Note: the protocol remains adaptable to any comparable gas concentration data acquisition tool.
Experimental Protocol
Set up the experiment: O₂ and SCD40 sensors mounted in the sealed chamber via the multiport hub, connected to the M5 Stack module.
Before any combustion, note the initial reference concentrations in ambient air: C_ini(O₂) in % and C_ini(CO₂) in ppm.
Fix a tea light candle to the bottom of the chamber. Set the measurement interval to 250 ms on the M5 Stack.
Start recording (REC), light the candle, and quickly close the chamber lid.
Let the combustion proceed until the candle goes out. Wait 1-2 more minutes for concentrations to stabilize.
Stop the recording (STOP). Note the final concentrations: C_fin(O₂) in % and C_fin(CO₂) in ppm.
Calculate the variations: ΔC(O₂) = C_fin(O₂) - C_ini(O₂) (negative, O₂ decreases) and ΔC(CO₂) = C_fin(CO₂) - C_ini(CO₂) (positive, CO₂ increases).
Calculate the experimental ratio: n_exp = -ΔC(O₂) × 10⁴ / ΔC(CO₂). The factor 10⁴ converts % to ppm for consistent units.
Open the chamber, ventilate, replace with a fresh candle, and repeat. Perform 6 to 10 trials total for statistical reliability.
Calculate the mean n_exp after removing any outliers. Compare with the theoretical value n_att = 38/25 = 1.52.
Expected Results
Typical observed values: O₂ initial ≈ 20.9%, O₂ final ≈ 15-16%, giving ΔC(O₂) ≈ -5%. CO₂ initial ≈ 400-800 ppm, CO₂ final ≈ 25,000 ppm, giving ΔC(CO₂) ≈ 24,000-25,000 ppm. The experimental ratio n_exp is typically 1.6-2.2, somewhat higher than the theoretical 1.52 due to incomplete combustion.
Scientific Questions
- What products are formed during incomplete combustion?
- Why does the candle go out before all the O₂ is consumed?
- How does atom conservation determine the stoichiometric coefficients?
- What happens to the water produced by the combustion?
- Why do you need multiple trials to get a reliable result?
Scientific Background
The complete combustion equation for paraffin is: **C₂₅H₅₂ + 38 O₂ → 25 CO₂ + 26 H₂O**. It is **balanced**: the same number of atoms of each element appears on both sides.
The stoichiometric coefficients (1, 38, 25, 26) indicate the **molecular proportions** of the reaction. They mean that combustion of one molecule of paraffin requires 38 molecules of O₂ and produces 25 molecules of CO₂.
The theoretical ratio **n_att = 38/25 = 1.52** represents the number of O₂ molecules consumed per CO₂ molecule produced. This ratio is a direct consequence of atom conservation.
To calculate the experimental ratio, units must be converted. O₂ is measured in % and CO₂ in ppm. Since **1% = 10,000 ppm**, the formula is: **n_exp = -ΔC(O₂) × 10,000 / ΔC(CO₂)**.
In practice, the experimental ratio is often **greater than 1.52**. This means more O₂ is consumed than predicted relative to the CO₂ produced. Incomplete combustion consumes O₂ to produce CO (which the CO₂ sensor does not detect).
In the candle flame, only the blue base corresponds to complete combustion. The luminous yellow zone contains unburned carbon particles (soot) that glow from heat but have not fully reacted with O₂.
Atom conservation is the fundamental principle for balancing chemical equations. During a chemical transformation, atoms are neither created nor destroyed — they are merely rearranged into new molecules.
Performing multiple trials (6 to 10) and calculating a mean value reduces the influence of random errors. Removing outliers (values that deviate significantly from the others) improves the precision of the result.
Extensions
- Use a longer or shorter burning time and check if the ratio changes
- Test with different fuels (ethanol lamp, butane lighter) and compare the ratios
- Investigate the effect of initial O₂ concentration by flushing the chamber with O₂-enriched air
- Calculate the mass of paraffin consumed from the O₂ data and compare with the actual mass loss
Frequently Asked Questions
How do I convert between % and ppm?
1% = 10,000 ppm. So 5% O₂ change = 50,000 ppm change.
The candle goes out very quickly.
The chamber may be too small or not well sealed. A larger chamber gives a longer burn time and more data.
My ratio is much higher than 1.52.
This indicates significant incomplete combustion. Ensure the chamber has enough initial O₂ and the candle is not too large relative to the chamber volume.
Why perform 6-10 trials?
Each trial has random variations. Averaging multiple trials reduces uncertainty and gives a more reliable result.
Detailed Description
The student performs multiple candle combustions in a sealed chamber with FizziQ Connect O₂ and SCD40 sensors. For each trial, they record initial and final O₂ and CO₂ concentrations, calculate the variations, and determine the experimental O₂/CO₂ ratio to compare with the theoretical stoichiometric ratio. The FizziQ Connect app displays the real-time O₂ and CO₂ concentration curves from the M5 Stack module and lets the student read the initial and final values needed for each trial's ratio calculation.
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