Measuring the enthalpy of combustion of Ethanol

Aim

The aim of the experiment is to measure the enthalpy of combustion of ethanol.

Hypothesis

The balanced equation for the combustion of ethanol is

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C2H6O(l ) + 3O2(g ) → 2CO2(g ) + 3H2O( l )

The enthalpy of combustion of ethanol can be found using three different methods:

- According to the IB Chemistry Data Booklet, the enthalpy of combustion of ethanol

under standard conditions is -1371 KJ mol-1.

- Bond enthalpies i.e. Bonds broken – Bonds formed

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4(412) + 348 + 360 + 463+ 3(496)[ ] − 4(743) + 6(463)[ ]

= 4307 − 5750

= −1443 Kj mol-1

- Enthalpy of formation i.e.

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ΔH Products − ΔH Reactants

= 2 × ΔH CO2 + 3 × ΔH H2O− ΔH C2H6O

= 2 × (−394) + 3× (−286) − (−278)

= −1368 Kj mol-1

Naturally, the design of the experiment cannot produce a result anywhere close to the actual

value of the enthalpy of combustion because much heat will be lost to the surroundings and to

the apparatus. Nevertheless, we hope to find a result within 50% of the expected value.

Also, since metal is a better heat conductor than glass, we expect the calculated enthalpy of

combustion of ethanol to be higher in experiment 2 (metallic beaker) than in experiment 1

(glass beaker).

Apparatus

The apparatus consisted of a spirit burner containing ethanol, a glass and a metallic beaker,

and a thermometer. A lid and a shielding screen were also used as insulators.

The mass of ethanol in the spirit burner and the temperature of water were measured before

and after the experiment. In theory, the heat provided by the combustion of the ethanol should

be transferred to the water and cause the temperature rise. The quantity of heat transferred to

the water is found by using the specific heat capacity formula

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Q = mcΔT . The experiment

was carried out twice, once using the glass water beaker and then using the metallic water

beaker.

It is important to know that the liquid ethanol was first poured into a smaller glass beaker so

as to measure its mass. This smaller glass beaker is called “Mass Beaker” in the Raw Data

section of the report. The ethanol was then transferred to the spirit burner for combustion. At

the end of the experiment, the leftover ethanol in the spirit burner was transferred back to the

“mass Beaker” to measure its final weight.

Spirit Burner

Water beaker

Thermometer

Raw data

The raw data from the experiment is illustrated in the tables bellow. Note that number 1 refers

to the first experiment using the glass water beaker and number 2 refers to the second

experiment using the metallic beaker. The “Mass Beaker” is the small beaker in which ethanol

was transferred in order to measure its mass.

Additional information:

- The glass water beaker contained 200ml

- The metal water beaker contained 100ml

Processed Data

In order to calculate the enthalpy of combustion of ethanol, we need to know how much

ethanol was combusted, and what change in temperature this has caused.

- The mass of ethanol poured into the spirit burner in exp.1 is:

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88.510 − 51.622 = 36.888 .

- The mass of ethanol combusted during exp.1 is:

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36.888 − 74.424 − 51.505[ ] =13.969g, or 0.3032 moles.

- Energy transferred into the 200ml glass beaker:

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ΔT = 37 − 8 = 29°C

Q = mcΔT

Q = 200 × 4.1813 × 29

Q = 24251.54J

Mass Beaker 1 ±0.001g

Mass Beaker 2 ±0.001g

Empty initial 51.505 51.581

Filled 88.510 99.289

Empty after transfer 51.622 51.644

FIlled at end exp. 74.424 88.274

Water Beaker 1

Water Beaker 2

Temp Initial ±0.5° 8 11

Temp Final ±0.5° 37 40

- Assuming that all heat is transferred to the water, the calculated enthalpy of

combustion of ethanol is:

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Energy delivered

Number of moles=

24251.54 J

0.3032 mol.= 80 kJ mol-1

Error calculations

The electronic balance used during the experiment read to the nearest 0.001g. The volume of

the 200ml and 100ml class B beakers was known with 2.5% accuracy and the thermometer

read to the nearest 0.5°C.

- The percentage error on the number of moles of ethanol combusted is:

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δmm

=0.001× 5

13.969= 0.0358%

- The percentage error on the energy transferred to the water beaker is:

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δEE

=δV

V+δT

T

⎡ ⎣ ⎢

⎤ ⎦ ⎥

δE

E= 0.025 +

1

29

⎡ ⎣ ⎢

⎤ ⎦ ⎥

δE

E= 5.95%

- The error on the calculated enthalpy of combustion of ethanol is:

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δH = 5.95% + 0.0358%[ ] × 80000

δH = ±5000 kJ

Similar calculations are made for experiment 2. The outcome of the processed data section is

summarized in the table bellow.

Exp.1 Exp.2

Temp. Change 29±1°C 29±1°C

Moles ethanol combusted

0.3032 ±0.0001

0.23910 ±0.00001

Energy transfered to

water

24000 ±1000J

12100 ±700J

Enthalpy of combustion

80000 ±5000 J/mol

50000 ±3000J/mol

Evaluation

The results of the experiment suggest that the enthalpy of combustion of ethanol lies between

50 and 80 kJ mol-1, which is less than 6% of the hypothetical value of 1368 kJ mol -1.

Moreover, it appears that the metallic beaker produced a smaller enthalpy of combustion than

the glass beaker, which again contradicts with the hypothesis. The outcomes of the

experiment are therefore extremely small and erroneous and this implies a critical flaw in the

design.

In order to discus the possible limitations and flaws of the experiment, it is important to note

what has been done initially to improve it.

It was clear that the main limitation was the loss of heat to the surroundings. This was caused

by the beaker radiating heat through its walls, or by the flame creating air convection currents.

In order to reduce the heat radiation of the beaker, we cooled down the water to about 10°C.

The theory was that if the final temperature was about 33°C then, any heat lost from the water

to the surroundings would be cancelled out by the heat transferred from the surroundings to

the water. This reasoning is based on the assumption that the water spends as much time

above as bellow room temperature, and that heat transfer in both directions is comparable. On

the other hand, it was difficult to stop air convection currents from rising around and on the

sides of the beaker. We were only able to place a plastic shield around the whole apparatus so

as to stop any transverse wind that would displace the flame from underneath the beaker.

Consequently, most of the heat was lost between the flame and the beaker because of the layer

of air separating them. Air is a poor heat conductor so it is logical that little energy is

transferred through it. Also, most heat rises around the beaker relatively fast meaning it stays

in contact with the beaker for a short lapse of time only. This could explain why the metallic

beaker produced a smaller value for the enthalpy of combustion since it was smaller and

narrower, and hot air could easily flow around it.

The most effective way to improve the heat transfer between the flame and the water is to

change the design of the apparatus. All the heated air and combustion gases should be

captured in a metal tube going through the water in the container. This way, the heat in the air

is nearly all transferred to the water.

The second greatest source of error was the spirit burner used to combust the ethanol. The

spirit burner contained a piece string that allowed the fuel to rise from the bottom to the flame

by capillarity. The problem is that when the ethanol is transferred from the spirit burner to the

“mass beaker” after the combustion, a large amount of fuel is trapped in the piece of string.

To prevent this limitation, the ethanol was transferred to the “mass beaker” and the flame was

left burning until all the ethanol contained in the string was combusted. This combustion,

however, was incomplete and slow since only a limited quantity of fuel was available. A

possible improvement would involve measuring the mass of ethanol directly, without any

transfer from one container to another. The spirit burner is placed on a balance and the mass

before and after combustion is measured. This way, any volume of ethanol trapped in the

string is taken into account.

Another limitation of the spirit burner is the type of flame it produces. The yellow/orange

flame is a clear sign of the incomplete combustion of the ethanol producing soot and

particulates, which again decrease the efficiency of the combustion. Perhaps the easiest way

Spirit Burner

Metal tube

Water

to improve this limitation is to add excess oxygen. This should result in a blue flame that

would transfer most of the energy contained in the fuel.

Finally, a small percentage of the missing heat could be absorbed by the different pieces of

apparatus such as the beaker, the heating mat or even the thermometer. Moreover, it should be

noted that the ethanol used during the experiment was not 100% pure meaning some of the

mass was water needing to be boiled off. Nevertheless these limitations are insignificant

compared to the quantity of heat lost during the transfer between the flame, the air, and the

water.

Conclusion

All the outcomes of the experiment are in complete contradiction with the hypothesis and the

expected results. The measured enthalpy of combustion of ethanol is only 6% of its actual

value and the metallic beaker was less effective in absorbing the heat from the flame. These

results suggested a critical flaw in the design of the experiment and allowed the analysis of

the limitations and possible improvements. It appears that a change in the design would

greatly improve the efficiency of the transfer of heat from the flame to the water, which was

the greatest source of error.

Perhaps the most successful outcome of this experiment is that we were able to demonstrate

that air is a relatively good heat insulator and that convection currents carry much of the heat

energy transferred.