Unit 25A Forensic Fire investigation, this assignment covers the criteria from a Pass to Distinction - My assignment was awarded Distinction, evidence can be provided if required... Any queries please do not hesitate to contact :)
Unit 25 Assignment A: Explore the chemistry of combustion and methods for extinction and heat
transfer - Investigating the chemistry of Fire.
For Pass:
Heat of combustion
Energy released as heat when one mole of a compound/ element undergoes complete combustion
with oxygen under standard conditions. (Units KJ mol-1)
Complete combustion
Complete combustion happens when there is plenty of oxygen, it produces water and carbon
dioxide as by-products. It also produces high amount of energy and creates a clear blue flame
without soot. Overall, this makes combustion of alkanes a highly efficient process.
The general equation for this is:
Alkane + Oxygen → Carbon dioxide + Water
For example, Propane + Oxygen → Carbon dioxide + Water
Equation for the above: C3H8(g) + 5O2(g) → 3CO2(g) + 4H2O(g)
Incomplete combustion
Incomplete combustion is the opposite of complete. This is where there is a limited amount of
oxygen. Incomplete oxidation products will form carbon monoxide (air pollutant) (CO) and Carbon
(C). Overall, it will be a mixture of water, carbon dioxide and soot form. This can be identified by a
smoky yellow flame.
Hydrocarbon + Oxygen → Carbon monoxide + Carbon + water
For example, propane + oxygen → carbon + water
Equation for the above: C3H8 + 2O2 → 3C + 4H2O
Flame extinction
Hypothesis: As we increase the volume of oxygen the tealight will burn for longer due to oxygen
being present in the beaker.
Null hypothesis: There is no corelation between the volume of the flask and the amount of oxygen
captured.
Variables
Independent variable – Volume of the flask.
, Dependent variable – Time taken for the flame to extinguish.
Control variable – Same type of tealight.
Risk assessment
Hazard Risk Precaution
Long Hair/ lanyards Get in contact with the flame or get Hair should be tied back, dangling
caught on glassware. accessories e.g., lanyards should be taken
off.
Lighter/ candle Skin burns/ blisters Avoid putting fingers close to the lighter or
candle – PPE such as gloves can also be
used however, not required due to the
practical being categorised as a low hazard.
Broken glass Cuts/ scratches Gently handle delicate glassware when
using, if broken the class supervisor should
be aware to then assist and deal with the
hazard caused.
Bags/ stuff in the way Slips/trips Ensure that your workstation is clear to
avoid unsafe lab practises.
Method
1. Measure the volume of different glass beakers using water as accurately as possible.
2. Record the values of measured volume of flasks.
3. Dry the beakers to ensure no excess water is left.
4. Plasticine is then used to cover the spouts on the beakers to prevent the heat escaping.
5. Use a heat proof mat and place the candle on top ready to light.
6. Light candle and place the first beaker on top.
7. Keep repeating until all different beakers have been used whilst recording the results with
three repeats.
Photos - K.Kaur – 23/02/2022
Results
, Volume of Repeat 1 Repeat 2 Repeat 3 Average in
flask/beakers Seconds (S)
ml
470ml 22.84 21.10 21.61 21.85
400ml 18.00 19.41 15.31 17.57
380 ml 18.01 12.27 12.38 14.22
300 ml 14.20 13.27 12.98 13.48
120ml 02.80 03.67 03.57 03.35
Overall, from this practical we can conclude, that there is a positive corelation between the volume
and time taken due to a higher quantity of oxygen being present in the beaker as it has a bigger
surface area. We can, therefore, infer that oxygen supports the burning because of oxidation, as the
gases are being released by a fuel when heated up. Heat energy is produced during combustion due
to the reaction being exothermic and releases enough heat to make the fire self-perpetuating.
Furthermore, a fuel is known to be a flammable material due to it beginning the process of
combustion. When a fuel is heated past its critical temperature it enters a gas phase where it
releases vapour pressure and ignites the air which assists combustion.
For example, there is evidence to be interpreted from the graph above. The first volume of the flask
being 120ml and the candle being extinguished within 3.35 seconds, this can be compared to the
highest volume of the flask being 470ml and the time being 21.85 seconds, it conveys clear
differences between the two variables and therefore, we can accept the hypothesis as it has been
proven and reject the Null hypothesis.
3. Perform the demonstrations to show heat transfer.
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