Daniel Hu
HEAT OF COMBUSTION PRAC REPORT Introduction:
• Alcohols burn to produce CO2 and H2O • The amount of heat released is dependent on the bonds being broken and reformed and is the heat of combustion • In this experiment, you will measure the amount of energy produced when three different alcohols burn, and determine their molar heats of combustion • Combustion is a chemical reaction in which oxygen is used and energy is released • Heats of combustion is the amount of heat released during a combustion reaction • Alkanols are a homologous group of carbon compounds containing the hydroxyl (OH) grouping, e.g. methanol, ethanol, propanol. They have the general formula CnH2n+1OH • This experiment involves the combustion of a measured mass of an alkanol to heat a measured mass of water through a measured temperature rise • It is assumed that the heat released by the combustion reaction will be absorbed by the water. The heat of combustion can then be calculated using the equation:
• Reference values for change in heat (Kj/Mol) are 726 methanol, 1367 1-propanol, 2021 2- propanol, 2676 butanol
Aim:
To design and perform an experiment to determine the heat of combustion of the three alkanols provided
Hypothesis:
1. Butanol will release the most energy during combustion. 2. The amount of energy absorbed by the water is equal to the amount of energy released by the fuel combusting
Materials:
• Copper/aluminium can • 100ml distilled water • 3 Spirit burners containing fuels (ethanol, butanol, propan-1-ol OR methanol) • Tripod • Retort stand • Clamp • Ring • Stirring rod • Electronic balance Daniel Hu
• Thermometer (0-100 degrees Celsius accurate to 0.1 degrees Celsius) • 250ml measuring cylinder
Justification of Materials:
• Heatproof mats weren’t used as they will absorb heat, too much of the heat energy • 3-5 spirit burners were used for each alcohol so that these spirit burners can be shared amongst students so they don’t have to be emptied and refilled, reducing the amount of alcohol used and preventing contamination • Electronic balance was used to measure the mass of water and the change in mass of spirit burner • Thermometer/data logger was used to measure the change in temperature of the water • Glass/stirring rod was used to evenly distribute the heat across the water
Risk Assessment:
Compound/material Hazard Precaution Reason for Precaution Methanol Methanol is toxic by all Minimise contact with Prevents harm routes of exposure, if burner, lid was used to and potential ingested causes prevent vaporisation blindness permanent blindness, highly flammable Ethanol Highly flammable, Wear eye protection, Prevents harm slightly toxic if lid was used to prevent and potential ingested vaporisation blindness Propan-1-ol Highly flammable, Wear eye protection, Prevents harm toxic if ingested or lid was used to prevent and potential inhaled vaporisation blindness Spirit burner Glass can crack if Don’t burn for an cooled down quickly extended period of time, only burn for 2-3 minutes Do not add fuel to a lit May result in the spirit burner entire body of fuel combusting and producing an uncontrolled flame
Do not blow out the Blows volatile burner to extinguish liquid which is the flame combustible, Fuels Fuels are volatile and Use a lid to prevent Prevents quite combustible vaporisation (seal combustion containers tightly) reactions, can be Review fire accident very dangerous procedures before commencing, make sure fire extinguisher is close by Daniel Hu
Work in a well- ventilated laboratory • Fuels used in this experiment have a low flash point- combustible, as a result amount of energy needed to vaporise is really high • Alkanols burn with a less luminous flame than hydrocarbons • Must ensure there is no flame when fuel is added to a burner • Extinguish the flame by replacing the cap on the burner. Do not blow out the flame
Variables:
Independent- Type of alkanol
Dependent- The heat of combustion of the alkanols
Controlled variables- same distance of tip of flame to bottom of the can for each spirit burner, same amount of water,
Diagram of Set-up
Method:
1. Set up the experiment/apparatus as shown in the diagram 2. Light the first spirit burner 3. Adjust the height of the can with the clamp so that the tip of the flame will just touch the can when lit 4. Using a measuring cylinder, add 200ml of cold water to the can. Place a thermometer in the water and record its initial temperature 5. Weigh the initial mass of the spirit burner with the cap with its liquid contents as accurately as possible, and record the mass in the results table 6. Light the wick and stir the water gently with the stirring rod to ensure uniform heat. Monitor the temperature and observe the flame. The thermometer should be kept halfway in the water 7. When the temperature has risen by about 10 degrees Celsius, replace the cap on the spirit burner to extinguish the flame. (never blow out flame) 8. Accurately record the maximum temperature of the water Daniel Hu
9. Re- weigh the spirit burner with the cap with its liquid contents as accurately as possible, and record its final mass in the results table 10. Examine the bottom of the can for soot accumulation (check for incomplete combustion). Remove soot before using the other alcohols
Results:
Ethanol Propanol Butanol Volume of beaker of 102 100 99.32 water (ml) Mass of water being 102 100 99.32 heated (g) Initial mass of spirit 207.53 209.48 189.12 burner + alkanol (g) Final mass of spirit 206.42 207.71 187.90 burner + alkanol (g) Mass of alkanol burnt 1.11 1.77 1.22 (initial mass- final mass) (g) Initial temperature of 22.5 35 27 water (before heating) Final temperature of 42 56 52 water Rise in water 19.5 21 25 temperature (final- initial) Observations made As you go from ethanol to propanol, the colour of the flame becomes (colour of flame, any more yellow, as there is an increased amount of soot. As a result, the soot present, soot longer the chain, the more likely to incompletely combust, as increasing deposition) number of carbons requires more O2
Sample Calculation using an accurate result:
Ethanol was burned in a spirit burner and used to heat a container of water. The volume of water heated was 100ml. The mass of the spirit burner and ethanol was 240.0 g before burning and 239.1g after burning. The temperature of the water rose from 22.0 to 36.2 degrees Celsius
Hints Sample Calculation Mass of alkanol burned (g) Initial mass of spirit burner + 240.0 – 239.1 = 0.9 alkanol – final mass of spirit burner + alkanol Initial temperature Temp before heating starts 22.0 Final Temperature Max temp water reaches 36.2 due to heating Rise in water temperature Final – Initial 14.2 Volume of water heated 100ml 100ml Mass of water heated 1ml weighs 1g 100g Heat released by burning fuel (enthalpy of reaction) (J)
Daniel Hu
Enthalpy of reaction (KJ) Heat released by burning 1g of 0.9g of fuel has released 5935.6 J of heat energy by burning fuel (heat of combustion) (Kj/g) 1g of fuel will release 5935.6J/0.9 = 6595.1 J heat Heat of combustion is 6.60 Kj/g Molar mass of fuel (g/Mol) Formula is Molar mass = 2 x 12.01 + 6 x 1.008 + 16 = 46.068 g Molar heat of combustion 1 gram of ethanol will release 6.60 Kj heat (Kj/Mol) 46.068 g of ethanol will release 6.60 x 46.068 Heat released by burning the = 303.6917 Kj molar mass of fuel = 3.04 x 10^2 Kj Molar heat of combustion of ethanol = -3.04 x 10^2 Kj/Mol *Note, you can also get molar heat of combustion by dividing change in enthalpy (Kj) over number of moles of fuel used in experiment
Expected results:
Graph (theoretical values)
Graph the heat released by burning 1 gram of fuel for each of the three alkanols
Heat of combustion vs Type of Alkanol 3000
2500
2000
1500
1000
500 Heat of combustion (Kj/g)
0 Ethanol Propan-1-ol Butanol Type of alkanol
Daniel Hu
Graph the molar heat of combustion against molar mass of the alkanols
Molar Heat of combustion vs Molar mass of fuel 3000
2500
2000
1500
1000
500 Heat of combustion (Kj/mol) 0 30 35 40 45 50 55 60 65 70 Molar mass (g/mol)
Result Analysis:
There is a steady increase in the theoretical molar heat of combustion as molecular mass increases because molar heat of combustion is a measure of the change in enthalpy of a combustion. The larger the number of bonds broken in the reactants, the greater the change in enthalpy.
However, the experimental data deviated from the theoretical data as incomplete combustion occurred (observation of soot and a yellow flame), thus releasing less energy whereby energy is absorbed by the surroundings.
Equations:
Theoretical Values
Discussion:
➢ Possible sources of errors - Mass loss, energy loss due to escape of water vapour Daniel Hu
o To address this error, a conical flask can be used or a proper calorimeter o Use a better insulated calorimeter or wind break to minimise the heat loss - Incomplete combustion (as seen by the presence of black soot on the bottom of the can) will release less energy and the reaction will take longer o Ensure the tip of the flame touches the bottom of the can so as to minimise heat loss directly o Use a copper can in order to contain as much of the heat as possible o Enclose the experiment within a covering of aluminium foil in order to minimise the loss of heat into the surroundings o Position the burner from the beaker at the same distance for each fuel o Introduce extra oxygen values o Will affect validity - Failure to keep the thermometer in the middle of the can o Results in inaccurate measurements - Heat absorption by the copper can, would not necessarily have been completely transferred to the water - Vaporisation o Use a conical flask to minimise evaporation ➢ Validity - The design of the experiment allowed me to mostly determine and compare the heats of combustion of different alkanols but at the cost of a large number of errors and deviation from the theoretical values. This is an invalid method for determining heats of combustion- as it did not give me obtain accurate and reliable measurements. I could not really compare the heats of combustion effectively- making accurate and reliable comparisons between them - The experiment is not valid as considerable heat is lost to the surroundings. Incomplete combustion occurred for all three fuels and thus produced a lower molar heat of combustion as compared to the true value - If we assume that a fixed proportion of heat is lost in each run, then the calculated results will produce a consistent pattern for analysis. However, the pattern was not very consistent throughout - The experiment also assumes that the extent of combustion is the same for all alkanols and that is not true as can be seen by the flame colours. - Additionally, the experiment assumed that all energy given off by the flame was absorbed by the water However, the surrounding atmosphere and equipment absorbed a large portion ➢ Improvements to Validity of results - Surround the apparatus by draught guards/shields to minimise heat loss (limits how much air absorbs heat energy) - Account for the heat absorption by the calorimeter (beaker) in the calculations - Use a better calorimeter - Stir the water constantly to uniformly distribute the heat - Conduct the experiment in an oxygen rich environment ➢ Reliability - Reliability can be improved by performing more runs for each alkanol, excluding outliers and averaging results - Using more water, allow more fuel to be burned and a larger change in temperature will reduce percentage errors Daniel Hu
➢ Accuracy - Experimental values are inaccurate, explanation was explained in result analysis - The fuels did not burn cleanly, incomplete combustion occurred - Not all the heat released went into heating the water - Highest water temperature may not have recorded correctly - Balance was not sensitive enough to measure the small change in mass of the fuel
IMPROVEMENTS TO ACCURACY
- When measuring water volume, take the reading with the line at eye level to reduce parallax error. - A 0-50 degrees’ Celsius thermometer will be more accurate than a 0-100 degrees Celsius one as the scale divisions are smaller. - Use a calibrated scale and thermometer. - Use a better calorimeter with less heat loss
Validity and accuracy can also be determined from the graph of heat of combustion vs molar mass. The trend line would be invalid because of extrapolation; you are working outside the data range. Trend line would also be inaccurate as line of best fit deviates from actual line due to incomplete combustion.
Conclusion:
We successfully measured the heat of combustion of a series of alkanols. Butanol released the most heat per gram and per mole burned. An increase in molar mass corresponds to an increase in molar heat of combustion (directly proportional relationship) The amount of energy absorbed by water was not equal to the amount of energy released by the combusting fuel
Remember to state the heats of combustion determined experimentally (shown as a negative value since combustion is an exothermic reaction)
Compare experimental values to the accepted values
Compare the experimental heats of combustion for the three fuels