To find out difference in the heat of combustion for different types of alcohols. Hypothesis The higher the number of carbon atoms in an alcohol is, the higher the energy for the heat of combustion. Alcohol is a homologous series, a series of organic compounds with similar formula and chemical properties, and increase in molecular size and mass. When the equations for combustion of these alcohols are listed in the order of increasing number of carbon atoms, Methanol 1 CH3OH(l) + 3/2 O2(g) ? 1 CO2(g) + 2 H2O(l) Ethanol 1 C2H5OH(l) + 3 O2(g) ? 2 CO2(g) + 3 H2O(l)
Propanol 1 C3H7OH(l) + 9/2 O2(g) ? 3 CO2(g) + 4 H2O(l) Butanol 1 C4H9OH(l) + 6 O2(g) ? 4 CO2(g) + 5 H2O(l) Pentanol 1 C5H11OH(l) + 15/2 O2(g) ? 5 CO2(g) + 6 H2O(l) The number of CO2 molecule increases in a linear fashion, as well as H2O. In the formation of carbon dioxide, C=O bonds are formed, releasing 1486 kJ per mol[i] in each molecule. As the coefficient for carbon dioxide is the same as the number of carbon atoms in the alcohol, the coefficient increases linearly in the increase in the homologous series of alcohol. The energy for combustion increases in the order of methanol ? thanol ? propanol ? butanol ? pentanol (and that in linear fashion) because the increase in the number of molecules of carbon dioxide means that more energy must be released for the formation of the C=O bond. Equipment 5 spirit lamps for methanol, ethanol, propanol, butanol, and pentanol. Water 100ml beakers Thermometer Matches Stop watch Retort stand Ceramic tile Method and Variables Pour 50ml of water into a beaker. Place the beaker in the clamp connected to the retort stand on a ceramic tile. Measure and record the mass of lamp and the temperature of the water.
Heat the beaker for 1 minute. Measure and record the final mass of lamp and the temperature of water. Repeat step 1-5 for 3 more times. Repeat the experiment with different types of alcohols. Type of alcohol will be changed each time as independent variable, whereas for the dependent variable the heat of combustion will be measured. Other variables that should be kept under constant condition will be the height from the lamp to the beaker, equipments (thermometer, beakers, cylinder, etc. ), room temperature, etc. Results Table 1.
Table to Show the Initial and Final Mass of Lamp and the Temperature of Water Before and After the Combustion | | |Trial 2 | |Trial 3 | |Trial 4 | |Trial 2 | |Trial 3 | Trial 4 |? H Heat of Combustion (KJmol-1) |146. 67 |270. 9 |440. 59 |648. 57 |836. 00 | | |Percentage Uncertainty (%) |20. 51 |29. 47 |21. 79 |22. 84 |30. 09 | | |Absolute Uncertainty (KJmol-1) |±30. 08 |±79. 68 |±96. 00 |±148. 13 |±251. 55 | | Table 3. The Literature Values of Heat of Combustion for Different Types of Alcohol[ii] Alcohol |Methanol |Ethanol |Propanol |Butanol |Pentanol | |Literature Value of Heat of Combustion (KJmol-1) |-715 |-1371 |-2010 |-2673 |(no data) | | [pic] [pic] [pic] [pic] Qualitative Data: After boiling for a while, the bottom of the beakers that was directly exposed to the heat started to get covered by soot.
The amount of the soot seemed to be more when alcohol with more carbons was used, but the actual mass or volume of soot was not measured. Data Processing The Heat of Combustion can be calculated by using the equation: . For methanol for example (from the first trial): Then the uncertainty must be calculated by adding up the relative uncertainties. The specific heat capacity of water (4. 18) and the molar mass of the alcohol do not have uncertainty because they are literature values. The mass of water (50g) must have uncertainty of . The temperature has uncertainty of (31°C±1°C) – (23°C±1°C) = 8°C±2°C therefore the relative uncertainty is .
The mass of alcohol lamp has uncertainty of (149. 76g±0. 01g)-(149. 50±0. 01g) = 0. 26g±0. 02g therefore the relative uncertainty is . When all relative uncertainties are added up(1+25+7. 69=33. 69(%)), it is multiplied to the total value to get the range of uncertainties: Thus the Heat of Combustion for methanol is: For ethanol(from the first trial): The specific heat capacity of water (4. 18) and the molar mass of the alcohol do not have uncertainty because they are literature values. The mass of water (50g) must have uncertainty of . The temperature has uncertainty of (39°C±1°C) – (24°C±1°C) = 15°C±2°C herefore the relative uncertainty is . The mass of alcohol lamp has uncertainty of (149. 71g±0. 01g)-(149. 30±0. 01g) = 0. 41g±0. 02g therefore the relative uncertainty is .
When all relative uncertainties are added up(1+13. 33+4. 88=19. 21(%)), it is multiplied to the total value to get the range of uncertainties: Thus the Heat of Combustion for methanol is: Justification of Uncertainties In the process of measuring the values given in the experiment, a thermometer with scale of 2°C, a cylinder with scale of 1ml, and a scale showing 2 digits after decimal were used. An uncertainty value of ±1°C in the measurement of temperature, ±0. g of in the measurement of amount of water (A cylinder was used to measure 40g of water. Although the cylinder measures in ml, 1 ml corresponds with 1g, thus the exchange in the measurement is possible. ), and ±0. 01g in the measurement of change in the mass of alcohol should be considered. These uncertainties are chosen because they are half the scale shown on the equipment, allowing ±half the measurement of error to be accepted. Conclusion and evaluation The enthalpy of combustion did increase in linear fashion as the number of carbon atoms in alcohol increased (graph 1-4).
This shows that the hypothesis was correct. The linearity of the graph indicates the relationships between each type of alcohol in the homologous series. Because CH2 is added to each alcohol as the homologous series progress, more energy is released for the formation of the C=O bond. However, the values of Heat of Combustion gathered from the experiment are lower than the literature value. In order to get values closer to the literature value, the duration of heating could be extended so that the change in the temperature increases.
With current method, the value for change in temperature of water is too small that it may be not providing enough data for values closer to the literature value. When the change in the temperature of water increases, the ? T in the equation also increases, meaning that the value of Heat of Combustion will increase as well. The values are multiplied by -1 in order to account for the exothermal nature or the reactions therefore the graph should be increasing, which is what is shown in the graphs using the data from the experiment.
There were no distinguishable errors but minor errors the experiment that resulted in gaps between the values from the experiment and the literature values (The range of error is relatively high). Random errors are uncertainties of measurements and systematic errors are from method. The relative uncertainty is high. In order to reduce the gap and improve the experiment, either random error can be corrected as much as possible or systematic errors can be fixed, although it is the systematic error that is responsible for most of the errors. Other than those errors, all other controlled variables were under control.
For random errors, for example, different thermometer with smaller scales can be used to reduce the risk of having ±1°C. Also thermometers using mercury is more precise than the ones using alcohol. For systematic error, parts of method must be changed to make the procedure easier to get more accurate results. For instance, when heating the water in the beaker, the upper part of the beaker is exposed to the air, which, in other words, mean that the increase in the temperature that must be measure may be affected by the loss of heat from the water to the air from the top of the beaker where it is not covered.
In order to fix this problem the beaker could be insulated. Also, the main reason for the high percentage of the relative uncertainty is the thermometer that had scales of 2°C leading to an uncertainty value of ±1°C, which is comparatively high. In fact, most errors derive from systematic errors rather than from random errors thus in order to make improvement more efficient, it is preferred to work on the systematic error.