Chem EPI

Identify and classify any errors that arose in your experiment. What affect did these have on the data you obtained?

- One major error was the systematic error of heat being lost to the environment. Systematic errors are errors that affect the accuracy of a measurement, causing all readings to differ by a consistent value each time. As a calorimeter was not used, a significant amount of heat that was produced by the burning of the fuel was absorbed by the beaker, instead of heating up the water, and is thus not accounted for. Similarly, heat may have been lost to the surroundings as the flame was not shielded. Thus, the processed results were consistently much lower than the 'true' value each time, making them very inaccurate due to this systematic error. To account for this error, we could have used a calorimeter, so that the calibration factor will factor in the heat lost to the calorimeter and shielded the flame to reduce heat loss to surroundings. - Another major potential systematic error was that of incomplete combustion occurring. Systematic errors are errors that affect the accuracy of a measurement, causing all readings to differ by a consistent value each time. Soot was observed on the bottom of the beaker, a clear indication of incomplete combustion. Due to this, the total amount of energy released per gram fuel burnt is decreased, and thus more fuel is burnt to cause the same temperature rise, meaning that our results would have been inaccurate. This could have been accounted for by oxygenating the device to ensure excess oxygen for complete combustion to occur. - Another error was the personal error of the beaker rising in temperature each time after being heated. A personal error includes mistakes or miscalculations that can be eliminated through performing the experiment correctly. This meant that the starting temperature increased for each test as there was inadequate time for the beaker to return to room temperature. However, this was accounted for by measuring the temperature change specific to that test. - Similarly, another personal error was that there were remnants of water in the beaker after each trial. A personal error includes mistakes or miscalculations that can be eliminated through performing the experiment correctly. This meant that when the scale was "zeroed" to account for the weight of the beaker so we could measure 100g of water, the little droplets of water from the previous experiment were not accounted for, meaning that the amount of water in the beaker was slightly greater than what we measured out. This would have caused decreased precision in our results but could have been accounted for by drying the beaker after each experiment. - Another error was the personal error of misreading the thermometer, this would have decreased the precision of the experiment as each time the spirit burner may have been extinguished slightly too early or too late, meaning that different amounts of fuel were burnt. This could be accounted for through the use of a digital thermometer.

Was the data you gathered valid? Why/why not.

A measurement is 'valid' if it measures what it claims to be measuring, and if the measurements that have been made have only been affected by a single independent variable. Therefore, the data obtained was valid, as we tested, as intended, the effect of changing fuels on the enthalpy change of combustion, and we ensured only one independent variable was present; the type of fuel combusted.

Is the data you obtained accurate? Why/why not.

A measurement is considered accurate if it is judged to be close to the 'true' value of the quantity being measured. Therefore, the data obtained was very inaccurate. Although the results follow the correct trend, when compared to the 'true' value of enthalpy changes in the data booklet, it is clear that they are all of significantly less value than they should be (for example -951.58 kJ/mol compared to the true value of -2021 kJ/mol). This significant disparity is majorly due to the high percentage of heat being produced by the fuel, being lost to the surroundings, and the beaker. As in our calculations we assumed that the amount of energy calculated by the temperature change of the water was indicative of the amount of energy released, we were not accounting for this significant heat loss, and received a much lower value than the 'true' value. To account for this error, we could have used a calorimeter, so that the calibration factor will factor in the heat lost to the calorimeter and shielded the flame to reduce heat loss to surroundings.

What is validity?

A measurement is valid if it measures what it claims to be measuring and if the measurements that have been made are affected by a single independent variable only.

What improvements could be made to your experimental design? How would these suggested improvements affect the outcome of your experiment?

By using a digital thermometer, the error of misreading the thermometer would be eliminated and thus, our results would be much more precise. By accounting for heat loss by channelling the flame towards the beaker and using a calorimeter, the value for energy absorbed by the water would be a much more accurate indicator of the energy released by the fuel, and thus the processed results for enthalpy change would be far more accurate, closer to the 'true' value. By providing excess oxygen to the spirit burner, we can ensure complete combustion occurs, decreasing the error of incomplete combustion, producing a far more accurate result. By drying the beaker after each trial, the error of remnants of water in the beaker will be accounted for and will thus increase the precision of the experiment. By letting the beaker cool down to room temperature after each experiment, the error of the beaker increasing in temperature after each experiment will be accounted for and will thus increase the precision of the experiment. By repeating the experiment even more times (100 times), the reliability of the experiment will be drastically increased.

What is experimental error?

Error is the difference between the measured value and the true value of what is being measured.

What is a personal error?

Personal errors include mistakes or miscalculations. They can be eliminated by performing the experiment again correctly next time, and do not form part of an analysis of uncertainties.

What is precision?

Precision refers to how closely measurement values agree with each other. A set of precise measurements will have very little spread about their mean value.

Was the data you gathered precise? Why/why not.

Precision refers to how closely measurement values agree with each other. A set of precise measurements will have very little spread about their mean value. The raw data that was gathered was overall quite precise, many values showed small disparities of 0.040grams such as propanol's 3 trials (0.386g, 0.386g and 0.335g), and others showed slightly larger disparities of 0.100grams such as methanol's 3 trials (0.571g, 0.659g and 0.546g). These disparities were most likely due to the manual judgement of when the temperature has risen by 10 degrees. This means that the spirit burner may be extinguished slightly earlier or later than when the temperature has perfectly risen by 10 degrees, causing very minor disparities in the amount of fuel burnt to raise the temperature by 10 degrees. These disparities were an example of a random error, and we addressed such an issue by calculating the mean of the 3 trials to get an average mass of fuel combusted.

What is a random error?

Random errors affect the precision of a measurement and are always present in measurements (except for 'counting' measurements). Random errors can be reduced by repeating measurements or refining method.

What is reliability?

Reliability refers to the degree of consistency between independent results obtained with an identical method and test material, under identical conditions.

Was the data you gathered reliable? Why/why not.

Reliability refers to the degree of consistency between independent results obtained with an identical method and test material, under identical conditions. The reliability of the experiment was upheld by repeating each test 3 times. In doing so, by repeating the measurements and calculating our final data using the mean result, the effect of random errors is reduced. Ultimately, our experiment was reliable, however the reliability could be improved by doing the test even more times.

What is reproducibility?

Reproducibility refers to the closeness of agreement between independent results obtained with the same method on identical test material but under different conditions

What is a systematic error?

Systematic errors are errors that affect the accuracy of a measurement. They cause readings to differ from the true value by a consistent amount each time.

What trend or relationship did you observe? Why?

The hypothesis was supported in that as the length of the carbon chain increased, so too did the enthalpy change per gram of fuel combusted. This is due to the increased amount of both intramolecular and intermolecular bonds in the fuels with a longer carbon chain, storing more energy, which then releases more energy when the fuel is burnt.

What is experimental uncertainty?

Uncertainty is a quantification of the doubt associated with the measurement result. They cannot be eliminated by repeating the experiment. Includes systematic and random errors.

Did you test what you planned to?

Yes, and ultimately, despite the inaccuracy of our results, we still revealed the correct underlying trend that as carbon chain increases, so too does enthalpy change of combustion.

What is accuracy?

a measurement value is considered to be accurate if it is judged to be close to the 'true' value of the quantity being measured.