heat of combustion prac report
TRANSCRIPT
Daniel Hu
HEAT OF COMBUSTION PRAC REPORT Introduction:
• Alcohols burn to produce CO2 and H2O
• The amount of heat released is dependent on the bonds being broken and reformed and is
the heat of combustion
• In this experiment, you will measure the amount of energy produced when three different
alcohols burn, and determine their molar heats of combustion
• Combustion is a chemical reaction in which oxygen is used and energy is released
• Heats of combustion is the amount of heat released during a combustion reaction
• Alkanols are a homologous group of carbon compounds containing the hydroxyl (OH)
grouping, e.g. methanol, ethanol, propanol. They have the general formula CnH2n+1OH
• This experiment involves the combustion of a measured mass of an alkanol to heat a
measured mass of water through a measured temperature rise
• It is assumed that the heat released by the combustion reaction will be absorbed by the
water. The heat of combustion can then be calculated using the equation:
• Reference values for change in heat (Kj/Mol) are 726 methanol, 1367 1-propanol, 2021 2-
propanol, 2676 butanol
Aim:
To design and perform an experiment to determine the heat of combustion of the three alkanols
provided
Hypothesis:
1. Butanol will release the most energy during combustion.
2. The amount of energy absorbed by the water is equal to the amount of energy released by
the fuel combusting
Materials:
• Copper/aluminium can
• 100ml distilled water
• 3 Spirit burners containing fuels (ethanol, butanol, propan-1-ol OR methanol)
• Tripod
• Retort stand
• Clamp
• Ring
• Stirring rod
• Electronic balance
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• Thermometer (0-100 degrees Celsius accurate to 0.1 degrees Celsius)
• 250ml measuring cylinder
Justification of Materials:
• Heatproof mats weren’t used as they will absorb heat, too much of the heat energy
• 3-5 spirit burners were used for each alcohol so that these spirit burners can be shared
amongst students so they don’t have to be emptied and refilled, reducing the amount of
alcohol used and preventing contamination
• Electronic balance was used to measure the mass of water and the change in mass of spirit
burner
• Thermometer/data logger was used to measure the change in temperature of the water
• Glass/stirring rod was used to evenly distribute the heat across the water
Risk Assessment:
Compound/material Hazard Precaution Reason for Precaution
Methanol Methanol is toxic by all routes of exposure, if ingested causes permanent blindness, highly flammable
Minimise contact with burner, lid was used to prevent vaporisation
Prevents harm and potential blindness
Ethanol Highly flammable, slightly toxic if ingested
Wear eye protection, lid was used to prevent vaporisation
Prevents harm and potential blindness
Propan-1-ol Highly flammable, toxic if ingested or inhaled
Wear eye protection, lid was used to prevent vaporisation
Prevents harm and potential blindness
Spirit burner Glass can crack if cooled down quickly Do not add fuel to a lit spirit burner Do not blow out the burner to extinguish the flame
Don’t burn for an extended period of time, only burn for 2-3 minutes
May result in the entire body of fuel combusting and producing an uncontrolled flame Blows volatile liquid which is combustible,
Fuels Fuels are volatile and quite combustible
Use a lid to prevent vaporisation (seal containers tightly) Review fire accident procedures before commencing, make sure fire extinguisher is close by
Prevents combustion reactions, can be very dangerous
Daniel Hu
Work in a well-ventilated laboratory
• Fuels used in this experiment have a low flash point- combustible, as a result amount of
energy needed to vaporise is really high
• Alkanols burn with a less luminous flame than hydrocarbons
• Must ensure there is no flame when fuel is added to a burner
• Extinguish the flame by replacing the cap on the burner. Do not blow out the flame
Variables:
Independent- Type of alkanol
Dependent- The heat of combustion of the alkanols
Controlled variables- same distance of tip of flame to bottom of the can for each spirit burner, same
amount of water,
Diagram of Set-up
Method:
1. Set up the experiment/apparatus as shown in the diagram
2. Light the first spirit burner
3. Adjust the height of the can with the clamp so that the tip of the flame will just touch the
can when lit
4. Using a measuring cylinder, add 200ml of cold water to the can. Place a thermometer in the
water and record its initial temperature
5. Weigh the initial mass of the spirit burner with the cap with its liquid contents as accurately
as possible, and record the mass in the results table
6. Light the wick and stir the water gently with the stirring rod to ensure uniform heat. Monitor
the temperature and observe the flame. The thermometer should be kept halfway in the
water
7. When the temperature has risen by about 10 degrees Celsius, replace the cap on the spirit
burner to extinguish the flame. (never blow out flame)
8. Accurately record the maximum temperature of the water
Daniel Hu
9. Re- weigh the spirit burner with the cap with its liquid contents as accurately as possible,
and record its final mass in the results table
10. Examine the bottom of the can for soot accumulation (check for incomplete combustion).
Remove soot before using the other alcohols
Results:
Ethanol Propanol Butanol
Volume of beaker of water (ml)
102 100 99.32
Mass of water being heated (g)
102 100 99.32
Initial mass of spirit burner + alkanol (g)
207.53 209.48 189.12
Final mass of spirit burner + alkanol (g)
206.42 207.71 187.90
Mass of alkanol burnt (initial mass- final mass) (g)
1.11 1.77 1.22
Initial temperature of water (before heating)
22.5 35 27
Final temperature of water
42 56 52
Rise in water temperature (final-initial)
19.5 21 25
Observations made (colour of flame, any soot present, soot deposition)
As you go from ethanol to propanol, the colour of the flame becomes more yellow, as there is an increased amount of soot. As a result, the longer the chain, the more likely to incompletely combust, as increasing number of carbons requires more O2
Sample Calculation using an accurate result:
Ethanol was burned in a spirit burner and used to heat a container of water. The volume of water
heated was 100ml. The mass of the spirit burner and ethanol was 240.0 g before burning and 239.1g
after burning. The temperature of the water rose from 22.0 to 36.2 degrees Celsius
Hints Sample Calculation
Mass of alkanol burned (g) Initial mass of spirit burner + alkanol – final mass of spirit burner + alkanol
240.0 – 239.1 = 0.9
Initial temperature Temp before heating starts 22.0
Final Temperature Max temp water reaches due to heating
36.2
Rise in water temperature Final – Initial 14.2
Volume of water heated 100ml 100ml
Mass of water heated 1ml weighs 1g 100g
Heat released by burning fuel (enthalpy of reaction) (J)
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Enthalpy of reaction (KJ)
Heat released by burning 1g of fuel (heat of combustion) (Kj/g)
0.9g of fuel has released 5935.6 J of heat energy by burning 1g of fuel will release 5935.6J/0.9 = 6595.1 J heat Heat of combustion is 6.60 Kj/g
Molar mass of fuel (g/Mol) Formula is Molar mass = 2 x 12.01 + 6 x 1.008 + 16 = 46.068 g
Molar heat of combustion (Kj/Mol) Heat released by burning the molar mass of fuel
1 gram of ethanol will release 6.60 Kj heat 46.068 g of ethanol will release 6.60 x 46.068 = 303.6917 Kj = 3.04 x 10^2 Kj Molar heat of combustion of ethanol = -3.04 x 10^2 Kj/Mol
*Note, you can also get molar heat of combustion by dividing change in enthalpy (Kj) over number of
moles of fuel used in experiment
Expected results:
Graph (theoretical values)
Graph the heat released by burning 1 gram of fuel for each of the three alkanols
0
500
1000
1500
2000
2500
3000
Ethanol Propan-1-ol Butanol
Hea
t o
f co
mb
ust
ion
(K
j/g)
Type of alkanol
Heat of combustion vs Type of Alkanol
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Graph the molar heat of combustion against molar mass of the alkanols
Result Analysis:
There is a steady increase in the theoretical molar heat of combustion as molecular mass increases
because molar heat of combustion is a measure of the change in enthalpy of a combustion. The
larger the number of bonds broken in the reactants, the greater the change in enthalpy.
However, the experimental data deviated from the theoretical data as incomplete combustion
occurred (observation of soot and a yellow flame), thus releasing less energy whereby energy is
absorbed by the surroundings.
Equations:
Theoretical Values
Discussion:
➢ Possible sources of errors
- Mass loss, energy loss due to escape of water vapour
0
500
1000
1500
2000
2500
3000
30 35 40 45 50 55 60 65 70
Hea
t o
f co
mb
ust
ion
(K
j/m
ol)
Molar mass (g/mol)
Molar Heat of combustion vs Molar mass of fuel
Daniel Hu
o To address this error, a conical flask can be used or a proper calorimeter
o Use a better insulated calorimeter or wind break to minimise the heat loss
- Incomplete combustion (as seen by the presence of black soot on the bottom of the can)
will release less energy and the reaction will take longer
o Ensure the tip of the flame touches the bottom of the can so as to minimise heat
loss directly
o Use a copper can in order to contain as much of the heat as possible
o Enclose the experiment within a covering of aluminium foil in order to minimise
the loss of heat into the surroundings
o Position the burner from the beaker at the same distance for each fuel
o Introduce extra oxygen values
o Will affect validity
- Failure to keep the thermometer in the middle of the can
o Results in inaccurate measurements
- Heat absorption by the copper can, would not necessarily have been completely
transferred to the water
- Vaporisation
o Use a conical flask to minimise evaporation
➢ Validity
- The design of the experiment allowed me to mostly determine and compare the heats of
combustion of different alkanols but at the cost of a large number of errors and
deviation from the theoretical values. This is an invalid method for determining heats of
combustion- as it did not give me obtain accurate and reliable measurements. I could
not really compare the heats of combustion effectively- making accurate and reliable
comparisons between them
- The experiment is not valid as considerable heat is lost to the surroundings. Incomplete
combustion occurred for all three fuels and thus produced a lower molar heat of
combustion as compared to the true value
- If we assume that a fixed proportion of heat is lost in each run, then the calculated
results will produce a consistent pattern for analysis. However, the pattern was not very
consistent throughout
- The experiment also assumes that the extent of combustion is the same for all alkanols
and that is not true as can be seen by the flame colours.
- Additionally, the experiment assumed that all energy given off by the flame was
absorbed by the water However, the surrounding atmosphere and equipment absorbed
a large portion
➢ Improvements to Validity of results
- Surround the apparatus by draught guards/shields to minimise heat loss (limits how
much air absorbs heat energy)
- Account for the heat absorption by the calorimeter (beaker) in the calculations
- Use a better calorimeter
- Stir the water constantly to uniformly distribute the heat
- Conduct the experiment in an oxygen rich environment
➢ Reliability
- Reliability can be improved by performing more runs for each alkanol, excluding outliers
and averaging results
- Using more water, allow more fuel to be burned and a larger change in temperature will
reduce percentage errors
Daniel Hu
➢ Accuracy
- Experimental values are inaccurate, explanation was explained in result analysis
- The fuels did not burn cleanly, incomplete combustion occurred
- Not all the heat released went into heating the water
- Highest water temperature may not have recorded correctly
- Balance was not sensitive enough to measure the small change in mass of the fuel
IMPROVEMENTS TO ACCURACY
- When measuring water volume, take the reading with the line at eye level to reduce
parallax error.
- A 0-50 degrees’ Celsius thermometer will be more accurate than a 0-100 degrees Celsius
one as the scale divisions are smaller.
- Use a calibrated scale and thermometer.
- Use a better calorimeter with less heat loss
Validity and accuracy can also be determined from the graph of heat of combustion vs molar mass.
The trend line would be invalid because of extrapolation; you are working outside the data range.
Trend line would also be inaccurate as line of best fit deviates from actual line due to incomplete
combustion.
Conclusion:
We successfully measured the heat of combustion of a series of alkanols. Butanol released the most
heat per gram and per mole burned. An increase in molar mass corresponds to an increase in molar
heat of combustion (directly proportional relationship) The amount of energy absorbed by water
was not equal to the amount of energy released by the combusting fuel
Remember to state the heats of combustion determined experimentally (shown as a negative value
since combustion is an exothermic reaction)
Compare experimental values to the accepted values
Compare the experimental heats of combustion for the three fuels