a carbon chain length of eight so it has twice as many carbons as butanol. So I can see if the enthalpy of combustion is proportional to the number of carbons in the alcohol. I would expect the enthalpy value of octanol to be roughly twice that of butanol.
Variables
The variables in this investigation are:
- Volume of water used
- Height of cooper can above flame
- Type of can
- Temperature the water is heated to
- Draft
- Type of alcohol
Fair Test
To make this investigation a fair test all of the variables except the one I am investigating, the type of alcohol need to be kept constant. I will always use 100 cm3 of water for each experiment. I will use the same size copper can for each experiment and I will wipe of the bottom of the can after each experiment. This is because carbon builds up on the bottom of the can and carbon is an insulator. This means the carbon will stop heat reaching the water so it is important it is removed. I will measure the distance between the can and spirit burner and ensure it is always 7 cm. I will also need to make sure I always measure it from the same place. Each time the experiment is done the water will be vented to 50oC from room temperature. A draught my blow the fame and heat away from the tin so I will make a draft excluder out of card to put around the burner. I will also ensure the lids are on the alcohol burners right up until they are lit as the alcohol may evaporate and make the masses inaccurate.
Using calculations I will calculate the enthalpies of composition for my chosen alcohols. Then I will do experiments to find the enthalpies of combustion from my chosen alcohol. I will compare the two results to help me reach my conclusion.
Equipment List
Methanol, ethanol, propanol, butanol, octanol
Alcohol burners (5)
Lamp stand
Clamp
Heatproof mat
Copper can
Thermometer
Card windshield
Measuring cylinder
Scales
Method
The apparatus was set up as shown in the diagram with the draft shield around the. 100 cm3 of water was measured using the measuring cylinder and poured into the copper can. The initial mass of the alcohol burner will be recorded and the initial temperature of the water will be recorded. The distance between the copper can and the burner was measured and made to be 7cm. When this has been done the burner can be lit. When the temperature of the water reaches 50oC the flame on the burner will be extinguished. The burner will be weighed and the mass will be recorded.
I decided that I would burn the alcohol until the water reached 50oC so that enough energy has been transferred for an accurate experiment. The heat from the flame heats up the copper can, which conducts the heat and energy to the water. Copper is a good conductor which is why a copper can is used, most of the heat the copper can gets will be transferred. The higher the enthalpy of combustion of the alcohol the faster the water will be heated. This is because the more energy that’s released the more energy can be transferred to the water. The energy causes the water molecules to vibrate producing heat, which makes the water warmer. So the more energy given out the less time the water takes to heat. So from the amount of alcohol burned to heat water to 50oC the enthalpy of combustion can be calculated. 100cm3 of water was used because it is an amount that is easy to measure accurately and the more accuracy there is the more accurate the results will be.
I think that this plan is likely to provide precise results because the alcohols will be weighed to two decimal places and the thermometer is accurate to one degree C. so the temperature can be estimated to half a degree, which is reasonably precise. The experiment being repeated improves the accuracy.
Risk Assessment
To make this experiment safe the following precautions will be taken:
- Goggles will be worn at all times.
- The experiment will be done standing up
- Heatproof mats will be put under the alcohol burners.
- Handle hot equipment with care.
- Avoid spillages because the alcohols are flammable.
- Keep alcohols I am not using away from naked flames.
This experiment is fairly safe and there is little risk if these guidelines are followed,
Prediction
I predict that the longer the carbon chains in the alcohol the larger the enthalpy change or combustion will be for that reaction. This means more energy is given out by alcohols with the most carbons.
With alcohols with small carbon chains only a small amount of energy is need to break the bonds because there are fewer of them. This means that less bond breaking energy is needed so there is a smaller amount of energy used to break the bonds than if there was a bigger carbon chain. However this also means that less energy is given out to make bonds so they give out less energy than alcohols with longer carbon chains. This can be shown with energy level diagrams.
Diagram 1 is an energy level diagram for the combustion of an alcohol with a longer carbon chain. There is more energy needed to break bonds but also more energy is given out to make bonds because there are more atoms to be bonded.
The calculated enthalpy of combustion for methanol is – 658 kj/mol.
The calculated enthalpy of combustion for Octanol is – 4984 kj/mol.
With the combustion of Octanol more energy is given out than with Methanol because the enthalpy is lower which means more heat is given out.
I predict that the difference between the enthalpies of combustion of the alcohols will be approximately 618 kj/mol. Because that is the difference between the enthalpies of combustion I have calculated for these alcohols with carbon chains where one carbon and two hydrogen atoms have been added the enthalpy will give out an extra 618 kj/mol.
I also think that alcohols enthalpies of combustion will be roughly proportional to each other. Butanol has four carbons in its chain and Octanol has eight.
So Octanol should give out roughly twice as much energy.
Octanols enthalpy calculated was –4984 and Butanols was –2512. If we make these positive to show the energy given out half of ectanol is 2492 which is close to 2512 so they should be similar.
Bibliography
Chemistry for you
Salter’s Chemical Ideas
www.bbc.co.uk/bitesize/chemistry
