Effect of an Increase in Molar Mass on Enthalpy Change
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An investigation to determine the effect of an increase in molar mass on the enthalpy change of combustion of fuels
Molar mass (type) of alcohol.
The following variable will be observed and measured:
* Mass of the alcohol used.
The following variables will need to be controlled:
* Mass of water, the mass of water will be measured using a measuring cylinder.
* Amount of wick on burner, the amount of wick on the burner will be measured using a standard ruler and kept constant as it affects the amount of alcohol burnt.
* Height of beaker above flame, the beaker will be set up so that the base just touches the flame.
* Type of beaker, the same beaker will be used and marked, as the density and size of the beaker affect the amount of heat energy transferred to the water.
* Agitation of the water, the water will need to be stirred in every experiment as to prevent any anomalous results.
* Temperature change is held constant, measured with a thermometer reading from -10oC to 110oC, with an uncertainty of ±0.5oC
1. Measure 100cm3 of water in the measuring cylinder.
2. Pour the water into the 250cm3 beaker and record its temperature.
3. Choose a spirit burner.
Record the name of the fuel, and the mass of the whole burner (including the lid and fuel inside).
4. Clamp the beaker, and set it up so that the spirit burner will fit comfortably under it.
5. Light the 6mm wick of the spirit burner, and put it under the 250 cm3 beaker.
6. Stir the water gently with the thermometer, and watch the temperature.
When it has increased by 20°C, put the lid on the spirit burner to put the flame out.
7. Record the new mass of the whole burner (including the lid and fuel inside).
8. Using fresh water each time, repeat the experiment at least twice with the same fuel.
9. Repeat all for different fuels.
Quantitative raw data:
Mass Before/g (±0.01)
Mass After/g (±0.01)
Observations during Experiment:
All reactions were exothermic as the beaker and the surrounding began to warm up.
It burnt with a short dim orangey yellow flame. The base of the beaker was partly covered with soot. Small bubbles formed at the base of the beaker.
It burnt with a pale orangey yellow flame. The base of the beaker was slightly darkened by the formation of soot. Small bubbles formed at the base of the beaker.
It burnt with a thin bright orangey yellow flame. The base of the beaker was again slightly darkened by the formation of soot. Small bubbles formed at the base of the beaker.
It burnt with a narrow and long yellowish orange flame. The base of the beaker was considerably darkened by the formation of soot. Small bubbles formed at the base of the beaker.
It burnt with a narrow and long yellowish orange flame. The base of the beaker was completely darkened by the formation of soot. Small bubbles formed at the base of the beaker.
It burnt with a narrow and long distinct yellow flame. The base of the beaker was fully obscured by the formation of soot. Small bubbles formed at the base of the beaker.
The heat that is released in the combustion of an alcohol is absorbed by the liquid. The temperature change of the liquid is then related to the heat of combustion of the alcohol (heat released in a reaction (combustion in this case) = heat gained by the substance). I can work out the heat energy absorbed by the liquid using the formula:
Heat Energy transferred (Q) = mâˆ™câˆ™âˆ†T
c = specific heating capacity of water (4.18 Jg-1K-1)
m = mass of water (in grams)
âˆ†T = change in temperature of the water.
And find the enthalpy (heat) change of combustion per fraction of a mole of the alcohol.
Mass of water (m) = 100g (±0.5), Change in Temperature (âˆ†T) = 20.00°C (±0.10)
Therefore, Heat Energy Transferred (Q) = mâˆ™câˆ™âˆ†T (degree of uncertainty)
= 100 âˆ™ 4.18 âˆ™ 20 (±0.5 + ±0.10) = 8360 J (±0.6) = 8.36 kJ (±0.6)
This is the same for every reaction as the mass of water remains constant.
From here on, I can calculate the enthalpy change per fraction of a mole of the substance as it combusts to form its products:
Alcohol + Oxygen → Carbon Dioxide + Water
Mass of Methanol used = 1.52g (±0.02)
Number of moles (N) = M/RMM (percentage degree of uncertainty)
= 1.52g / 32.04g (±2.00% + ±0%) = 0.0474 mol (±2.00%)
M = Mass of the Alcohol used to heat the amount of water
RMM = Relative Molar Mass of the Alcohol obtained from the data book, so the percentage uncertainty is ±0%
âˆ™âˆ™âˆ™ Enthalpy change of Combustion (âˆ†Hc) = Q/N (percentage degree of uncertainty) = 8.36 kJ / 0.0474 mol (±7.17% + ±2.00%) = 176.22 (±9.17%)
All calculations are done similarly.
Average Initial mass/g (±0.01)
Average Final mass/g (±0.01)
(M) Mass used/g (±0.02)
(âˆ†T) Temp change/°C (±0.10)
(Q) Heat Energy Transferred/kJ (±0.12)
(N) Number of Moles used/mol (±2.00%)
(âˆ†Hc) Enthalpy change of Combustion (±9.17%) /kJ mol-1
From the table it is evident that the molar mass of an alcohol increases the amount of heat energy it dispenses per fraction of a mole. Also, the graph shows that there is a positive correlation between molar mass and enthalpy change of combustion for alcohols. What could explain the relation is that as the molar mass increases there is an increase in the number of available carbon atoms to combine with oxygen and release energy. Therefore, an increase in molar mass will have an incremental effect on the enthalpy change of combustion.
âˆ†Hc Value (LV)
Experimental âˆ†Hc Value (EV)
(EV – LV)/LV*100
Table depicting the percentage error of the experimental value from the actual value available in the data book
Average Literature error: 74.22%
The results are not consistent with the literature values, all of the reactants did not completely combust due to the lack of oxygen, leading to the formation of soot and carbon monoxide, which means that the heat output is less than it would have been if all of the carbon burnt, since the calculations are based on the mass of the un-burnt carbon, the calculated value is less than the literature value. The beaker would not have transferred all the heat across; some would have been lost in heating the beaker as well as the clamp and stand, this would have caused the value to be less than the actual value. Other possible sources of error could be by slight differences in the values of the fixed variables, like the mass of water not being exactly 100g, due to incorrect reading of the measuring cylinder caused by a parallax (when the scale is read at an angle to the eye, as the light is refracted through the glass, the reading appears to be at a different position). The same error could have been also made in the reading of the thermometer, causing there to be wrong temperature readings. There were some anomalies when reading the graph as two values were almost identical. It could have been due to the amount of wick on the burner as it would not have been exactly the same (6mm) on each burner as this was difficult to measure. This would have caused differences in the amount of alcohol burnt. The flame was not always just touching the beaker, as this again was difficult to measure accurately, and would have caused differences in the amount of heat given off as the temperature of the flame is different at different heights. Also, the thermometer was not in the same place at each temperature recording, as even though the water was stirred, there would be differences in the temperature of the water at different depths.
Errors mentioned in conducting Experiment
Possible corrections that could be made
Incomplete Combustion of reactants
Placing the fuel in an airtight chamber and controlling the flow of air through valves as to make the correct stoichiometric balance ratio of carbon and oxygen.
Heat loss to the surrounding
Insulation of the beaker, boss, clamp and stand by wrapping them with polystyrene. Also, Preventing any draught from carrying the heat energy away by placing a board to shield it.
Position of eye at all volumetric vessels must be at the same level as the meniscus.
Transfer of heat energy to the liquid
A calorimeter made of a better heat conducting material, perhaps something strong and lightweight like aluminium.
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