Extended experimental investigation

Transcription

1 Chemistry 2007 Sample assessment instrument and indicative responses Extended experimental investigation This sample is intended to inform the design of assessment instruments in the senior phase of learning. It highlights the qualities of student work and the match to the syllabus standards. Criteria assessed Knowledge and conceptual understanding Investigative processes Evaluating and concluding Assessment instrument The response presented in this sample is in response to an assessment task. The model presented here represents one method by which evidence can be presented for the extended experimental investigation by the student. This is an extended experimental investigation on the topic of energy. A range of fuels was provided and students have a choice of fuels to use for the investigation. The length of the investigation is four weeks. A summary of the task provided by the school is presented below. The task sheet has not been included. Refer to the syllabus for requirements. Maintain a journal throughout the investigation. Carry out research on the topic. Formulate your hypothesis for the investigation. Decide on a method for the investigation. Order materials and fill in the Materials Requisition form and Risk Assessment Form. Begin the investigation. Record all observations. Refine your experiment as required. Gather, record and process valid data. Write a scientific report under the headings provided. The report headings are: results, discussion and conclusion. The discussion is to include an evaluation and recommendations. Section of the syllabus requires that teachers must implement strategies to ensure authentication of student work. Journal extracts have been included with the scientific report to show where the descriptors of the standards have been demonstrated throughout the investigation. The annotations and notes help to explain the journal entry. References have not been shown in this sample. The task is part of an eightweek unit of work on energy.

2 Instrument-specific criteria and standards Indicative responses have been matched to instrument-specific criteria and standards; those which best describe the work in this sample are shown below. For more information about the syllabus dimensions and standards descriptors, see Knowledge and conceptual understanding Investigative processes Evaluating and concluding Standard A The student work has the following characteristics: comparison and explanation of complex concepts, processes and phenomena linking and application of algorithms, concepts, principles, theories and schema to find solutions in complex and challenging situations The student work has the following characteristics: formulation of justified significant questions/hypotheses which inform effective and efficient design, refinement and management of investigations assessment of risk, safe selection and adaptation of equipment, and appropriate application of technology to gather, record and process valid data systematic analysis of primary and secondary data to identify relationships between patterns, trends, errors and anomalies The student work has the following characteristics: analysis and evaluation of complex scientific interrelationships exploration of scenarios and possible outcomes with justification of conclusions/ recommendations discriminating selection, use and presentation of scientific data and ideas to make meaning accessible to intended audiences through innovative use of range of formats Standard C The student work has the following characteristics: explanation of simple processes and phenomena application of algorithms, principles, theories and schema to find solutions in simple situations The student work has the following characteristics: formulation of questions and hypotheses to select and manage investigation assessment of risk, safe selection of equipment, and appropriate application of technology to gather and record data analysis of primary and secondary data to identify obvious patterns, trends, errors and anomalies The student work has the following characteristics: description of scientific interrelationships description of scenarios and possible outcomes with statements of conclusion/ recommendation selection, use and presentation of scientific data and ideas to make meaning accessible in range of formats 2 Chemistry 2007 Sample assessment and indicative responses

3 Indicative response Standard A Written scientific report The aim of this investigation is to see the relationship between the number of carbon atoms in a straight chain molecule and the heat of combustion that is released. Comments discriminating selection, use and presentation of scientific data and ideas to make meaning accessible to intended audiences through innovative use of a table RESULTS Alkanol Molar mass Average H (kj/mol) to heat 200g of water by 15 º C Average H (kj/mol) to heat 200g of water by 30 º C Methanol Ethanol Propanol Butanol Pentanol Hexanol The primary data has been summarised into one table. 1-Heptanol Table 5 Heats of combustion released when heating 200g of water by 15 º C and 30 º C discriminating selection, use and presentation of scientific data and ideas to make meaning accessible to intended audiences through innovative use of a graph Graph 1 Heats of combustion values Queensland Studies Authority February

4 discriminating selection, use and presentation of scientific data and ideas to make meaning accessible to intended audiences through innovative use of a graph Graph 2 Comparison of heats of combustion systematic analysis of primary data to identify relationships between patterns, trends, errors and anomalies linking and application of concepts and principles and schema to find solutions in complex and challenging situations DISCUSSION Table 5 summarises the results from the primary data in the journal. Graph 1 indicates there is a linear relationship between molecular weight and heats of combustion. Graph 2 compares the heats of combustion of the two sets of primary data, the values from the literature and the values calculated from the bond dissociation energy. Graph 2 compares the heat of combustion values from the bond dissociation energy calculations, the literature values and the experimental values when 200g of water is heated by 15 0 C and 30 0 C. The bond dissociation values do not take into account the hydrogen bonding and the literature values are done under different conditions as stated. The experimental values for all trials undertaken are going to be lower than the other two values because of the methodology involved. There will always be some heat loss to the surroundings in a simple set up. Even though the amount of fuel burnt is different in each case, the heat of combustion of a fuel should theoretically be the same. These results are comparative, within the efficiency of the apparatus. There appears to be no real difference in results for the different trials. 4 Chemistry 2007 Sample assessment and indicative responses

5 Experimental values are below the literature values which could be due to the methodology involved. A linear relationship is seen between the length of the carbon chain and the heat of combustion released. The values differ from the theoretical values. This may have been due to any of several factors: comparison and explanation of complex concepts, processes and phenomena Incomplete combustion may have occurred Some evaporation may have occurred and the exact volume may not have been 200ml Errors in reading the thermometer, measuring the exact volume of water and measuring the mass of the fuel The apparatus (burner, stand, mat, glass beaker) may not have cooled down to room temperature after each experiment. The whole investigation and gathering of data was extended over several days The readings may have been done by different people All of the trials may not have been done in sequence One of the main factors in this investigation is the inability to tell if all of the fuel burnt has been converted into carbon dioxide and water as shown in Equation 2. With such small amounts of fuel being used it is probable that all is burnt, so for the purposes of this discussion it is assumed that complete combustion has occurred. Originally it was proposed to use a solid fuel i.e hexdecanol but the first trial on this substance produced soot and it could not be placed in the burner. It was decided not to continue with this and use only liquids. The products of the reactions of complete combustion of alkanols are carbon dioxide and water. The equation for methanol to release 726 kj of energy is shown below: CH 3 OH (l) + 3/2 O 2 (g) CO 2 (g) + 2H 2 O (l) H = -726 kj/mo l (Eqn 2) Figure 1 Equation of the complete combustion of methanol. If all of the fuel is not burnt during the reaction, incomplete combustion occurs. The products are carbon monoxide and water. This will be impossible to ascertain in this experiment The exact amount of heat released is then unknown. It is assumed that complete combustion occurs in this investigation. The incomplete combustion of methanol releasing an unknown amount of energy is shown below: CH 3 OH (l) + O 2 (g) CO (g) + 2H 2 O (l) H = - kj/mol (Eqn 3) Figure 2 Equation of the incomplete combustion of methanol. Alcohols are polar, because of the O-H bonds, allowing alcohol molecules to attract each other through hydrogen bonds. Since oxygen atoms are much more electronegative than hydrogen atoms, the O-H bond is especially polar. The partially-negatively charged oxygen atom on one alcohol molecule is strongly attracted to the partially positively charged hydrogen atom on another alcohol molecule; this strong attraction results in stronger intermolecular forces between alcohol molecules. The hydrogen bonding between ethanol molecules is shown in Figure 4. These polar bonds contribute to the substantial dipole moments. The bond between the dipoles is a hydrogen bond. It is typically 197pm, depending on bond strength, temperature and pressure. The O-H bond in alkanols is 0.96 Å. This distance is sufficiently small that some hydrogen bonds may have a significant amount of covalent or shared electron character. Queensland Studies Authority February

6 analysis and evaluation of complex scientific interrelationships Figure 3 Hydrogen bonding in ethanol The literature values used in Table 1 (see page 5 extract of journal) for comparison are calculated from combustion experiments by the National Bureau of Standards USA under the following conditions; the sample is burnt in a bomb of constant volume in pure oxygen at an initial pressure of 30 atm and at 25 0 C the number of grams of sample burnt is equal to three times the volume of the bomb in litres the number of grams of water placed in the bomb before combustion is equal to three times the volume of the bomb in litres. Reactants and products are in their standard states at one atmosphere pressure ( files/jpcrd6.pdf). These conditions are impossible to copy in an ordinary laboratory but the literature values are very accurate. Several factors are involved when considering the values in Table 1 (see page 5 extract of journal): The values calculated from bond dissociation energies do not consider other energies between the molecules such as hydrogen bonding and van der Waal s forces The molecules are different shapes and sizes and stability The alcohols are polar molecules as shown in Figure 3. Oxygen has an electronegativity of 3.44, higher than both carbon 2.55 and hydrogen This makes the oxygen atom very negative and the whole molecule very polar. The journal entry on page 12 depicts the structure of the substances used. It can be seen that in going from methanol to propanol, the shape changes from linear to zig-zag. Molecules with three or more carbon atoms are this shape. As the molecules become longer they have a greater capacity to link around each other and more hydrogen bonding results as the molecules become closer. 6 Chemistry 2007 Sample assessment and indicative responses

7 Figure 4 Bond lengths in propanol. The bond lengths between atoms shown in Table 2 (see page 6 extract from journal) and in propanol in Figure 4 contribute to the overall shape of the molecule. As the molecular weight increases the density increases. This means the molecules are packed closer together. When combustion occurs some heat will be needed to break these attractive forces between the molecules. exploration of scenarios and possible outcomes with justification of conclusions/ recommendations Graph 3 Molecular weight vs relative polarity Graph 3 shows that as the molecular weight increases the relative polarity of the molecule decreases. This occurs because the carbon chain becomes longer and each molecule has only one C-OH group at the end. If the experiment was repeated, the methodology could be improved by having a more efficient calorimeter e.g. some form of bomb calorimeter to retain as much heat as possible. If the same spirit burner was used, then the cover would need to be made of more heat proof material. A larger amount of fuel could be combusted as long as the fuel did not boil. Boiling would require heat and so results would be inaccurate. CONCLUSION A clear trend is seen. As the molecular weight increases so does the heat of combustion released. The stated hypothesis was as the carbon chain increases in an alkanol, the heat of combustion released will increase, increasing from one in methanol to four in 1-butanol. The experimental values show there is a linear relationship between the molecular weight and the heat of combustion released. Queensland Studies Authority February

8 JOURNAL EXTRACTS Page 4 Extract The student has started initial planning of the investigation and considered topics related to fuels Initial idea or question to answer Page 4 Extract Initial question student now has to research to see if this question can be justified The student is linking concepts to the research question The structure of compounds is researched to see the nature of branching The student researched definitions relevant to the topic Page 5 Extract The student researched the heat of combustion values for the compounds from different sources 8 Chemistry 2007 Sample assessment and indicative responses

9 Page 5 Extract These values will be compared with experimental and bond energy calculation values discriminating selection, use and presentation of scientific data and ideas to make meaning accessible to intended audiences through innovative use of a table This represents the most recent data on heats of combustion The student decided that using compounds that were branched would lead to indistinguishable results. Research was also conducted on boiling points but this was disregarded as an option. Page 6 Extract These values show the energy stored within the molecule Linking and application of concepts, principles, theories and schema to find solutions in complex and challenging situations The values were used to calculate the bond dissociation energies of the compounds Queensland Studies Authority February

10 linking and application of algorithms and concepts to find solutions in a complex and challenging situation Page 7 Extract Calculating the heat of combustion from bond dissociation energy values. All calculations are based on this method The student understands the term exothermic and energy diagrams The stored chemical energy for methanol is 653kJ Page 7 extract shows that the bond dissociation energy calculations for the branched materials were the same. The student has used secondary data in the decision making process for the design of the investigation formulation of justified significant questions/hypoth eses which inform effective and efficient design, refinement and management of investigation Hypothesis/question has been modified due to research Page 10 Extract 10 Chemistry 2007 Sample assessment and indicative responses

11 Page 12 Extract The student identified the 3 dimensional shapes of each of the compounds linking and application of concepts, principles, theories and schema to find solutions in a complex and challenging situation Methanol CH 3OH 1-pentanol C 5H 11OH Ethanol C 2H 5OH 1-hexanol C 6H 13OH 1-propanol C 3H 7OH 1-butanol (n-butanol) C 4H 9OH 1-heptanol C 7H 15OH Page 10 Extract Several calorimetry methods were compared. The student identified the placement of the thermometer, the support of the thermometer, heat loss to the surroundings and sufficient fuel in the burner as issues in the methodology This basic set up was considered to be too inefficient to gather accurate and valid comparison and explanation of complex concepts, processes and phenomena This bomb calorimeter is a specialised piece of equipment but has features that could be incorporated into a set-up. Queensland Studies Authority February

12 The efficiency of the apparatus was tested and found to be 51.21%. An initial trial showed the efficiency at 49.93%. The student considered this too low. Page 13 Extract Application of the algorithm q = m x C x T Mathematical check was done to see how much fuel was needed to raise the temperature of 100g of water by 20 0 C. This was g. Refinement of the cover to minimise heat loss effective and efficient design, refinement and management of the investigation The student now has a planned course of action for the investigation The student has addressed the issues associated with the equipment 12 Chemistry 2007 Sample assessment and indicative responses

13 Page 12 Extract The apparatus has a small inlet to ensure that there is sufficient oxygen for combustion The apparatus is designed to ensure as much as possible of the heat released goes into heating the water effective and efficient design, refinement and management of the investigation A description of the experiment is provided assessment of risk, safe selection and adaptation of equipment Queensland Studies Authority February

14 Pages Extracts Four pages of primary data was collected Only three examples of primary data have been shown. Data from three trials was collected each time. appropriate application of technology to gather, record and process valid data Values considered out of range were disregarded 14 Chemistry 2007 Sample assessment and indicative responses

15 Indicative response Standard C Written scientific report The aim of this investigation is to see if the compound with the most carbon atoms will produce the most heat of combustion. Comments RESULTS Alcohol Molar mass Average H (kj/mol) Average % Efficiency methanol ethanol propanol selection, use and presentation of scientific data and ideas to make meaning accessible in the form of a table and a graph Table 1 Heats of combustion butanol Graph 1 Heats of combustion vs molecular weight Queensland Studies Authority February

16 explanation of simple processes and phenomena DISCUSSION One of the observations made during the investigation was that the flames were different colours. Some were yellow and some yellow and blue. This may be due to the fact that the fuel was not burning completely. When all the fuel burns in oxygen, complete combustion occurs. This is shown by the equation below. The processes have been CH 3 OH (l) + 3/2 O 2 (g) CO 2 (g) + 2H 2 O (l) compared Figure 1 Equation of the complete combustion of methanol. When some of the fuel does not burn it is incomplete combustion. If all the fuel does not burn, the products are carbon monoxide and water. It will be impossible to tell what is happening in this experiment. It is assumed that complete combustion occurs in this investigation. Only when all of the fuel burns can the heat of combustion be found correctly. CH 3 OH (l) + O 2 (g) CO (g) + 2H 2 O (l) Figure 2 Equation of the incomplete combustion of methanol. analysis of primary and secondary data to identify obvious patterns, trends and anomalies Table 1 shows the average heat of combustion found for each of the compounds. These values were relatively accurate considering that heat was lost to the atmosphere. The values for ethanol were lower than the other compounds. The carbon chain increases from methanol to butanol from one carbon to four carbons. Graph 1 shows that there is a linear increase in the heat of combustion released as the carbons increase. The value for ethanol has made the line slightly off straight. The value for ethanol is only 49.33% efficient as opposed to the other values which are above 50%. The second trial for methanol was very much lower than the other two trials even though more fuel was burnt i.e g of methanol as opposed to g in the first trial. The second trial was less efficient to the other trials. Only kj was released compared to and kj. In other trials when less fuel was burnt less heat of combustion was released. This anomaly could be due to heat loss to the surroundings and the overall inefficiency of the set up. The student has identified sources of error in the investigation description of scientific interrelationships The data confirms the hypothesis that as the carbon chain increases so does the heat of combustion. As the carbon chain increases so does the molecular weight. This is why heat of combustion was graphed against molecular weight. A source of error in the experiment was the heat loss to the surroundings. This was impossible to stop. Also the beaker and the equipment became hot as the experiment proceeded. So all of the heat would not have gone into the water. The readings of the thermometer may not have been done accurately. These were done on different days. 16 Chemistry 2007 Sample assessment and indicative responses

17 Alcohols are polar. This is shown in Figure 3 below. description of scenarios and possible outcomes with statements of conclusion/ recommendation Figure 3 Polarity in alcohols The OH group has a polar bond because the oxygen atom is more electronegative than the hydrogen bond. This means that the oxygen has a greater attraction for the electrons than the hydrogen. The oxygen is more negative than the hydrogen so the bond is polar. The OH can then form hydrogen bonds with other atoms. This may contribute to some of the compounds having more heat of combustion than others. If the investigation was to be repeated there are some recommendations that could improve the results. The experimental trials should be conducted in a controlled environment that has a maintained temperature as well as amount of wind. The temperature of the water at the start should be the same to ensure the change in temperature is about the same for each trial. This would keep the temperatures low so the equipment would not get too hot. This lowers the likelihood of evaporation and therefore loss of the mass of water. Further experiments could see if the amount of water heated had any effect on the heat of combustion. Ambient temperatures could be raised or lowered to see the effect. CONCLUSION The aim of the investigation was supported and the hypothesis achieved. It was found that as the chain length increased the amount of heat of combustion increased. Queensland Studies Authority February

18 JOURNAL EXTRACTS Page 1 Extract An initial question has been proposed the student has to research to see if this is feasible to investigate The student has started initial planning of the investigation and has considered topics related to fuels Initial idea or question to answer The student is linking concepts to the research question The concept of energy within the compound is researched The student has established the availability of fuels Page 2 Extract 18 Chemistry 2007 Sample assessment and indicative responses

19 Page 2 Extract The student has researches the heat of combustion values for the compounds from a source formulation of a hypothesis to select and manage investigation The set-up for the experiment is established Page 3 Extract The student has a planned course of action for the investigation assessment of risk, safe selection of equipment Queensland Studies Authority February

20 The student has considered other aspects in the management of the investigation An initial trial was conducted to see if the set up and measurements taken were suitable Initial trials establish the set-up is 69.4% efficient 20 Chemistry 2007 Sample assessment and indicative responses

21 Page 4 Extract application of algorithms to find solutions in a simple situation appropriate application of technology to gather and record data Page 6 Extract Trials were conducted heating 100g of water by 10 º C Queensland Studies Authority February