The bonds that are formed in an exothermic reaction can be of two types. The first could be ionic, where a metal is produced. Ionic bonding involves electrons transferring from one atom to the other consequently leaving an electrostatic force between them. The other form of bonding is covalent where atoms share electrons to complete their outer orbit. An example being Methane where four hydrogen atoms each share an electron with a carbon atom.
The variables that must be controlled are:
- Mass of alcohol
- Mass of water
- Amount of wick on burner
- Type of alcohol
- Height of can above flame
- Type of can
- Time of burning
The alcohols used in this experiment are:
Investigating the combustion of alcohols
Stirring the water means that there is uniform temperature in the can, and monitoring the temperature rise to ensure uniform heating. The can is copper as copper is a good conductor of heat, so more is transferred to the water. The height of the can above the flame is also a factor, so this needs too be kept constant (3 cm from the can.)
I decided to burn the alcohol for three minutes (in one minute sections) to make sure that enough energy had been transferred for an accurate experiment. The can was kept the same as different cans have different conduction properties, the heat is transferred from the flame by vibrating air particles in the flame, caused by the exothermic reaction, which then cause the molecules in the can to vibrate, and so the molecules in the water vibrate.
To keep this a fair test we have to bear certain aspects in mind. The can the water is contained in must be the same shape because if it is not the flame may have more surface area of where to heat the water. The alcohol must be weighed accurately with scales that weigh up to, least, one decimal point. During weighing the spirit lamp must be covered to avoid and evaporation of the alcohol. The alcohol has to be weighed accurately before and after the experiment. The alcohol has to be blown out immediately when the water temperature has been raised 30 degrees; it must be covered after the experiment to avoid evaporation. The thermometer must be swirled around the water before a reading can be taken, this insures that you are measuring the temperature of the whole water not just the bottom of the can. The shape of the spirit lamp must stay the same and so must the wick length. If all this is done we can ensure that we will get an accurate reading.
- Record initial mass of alcohol plus burner, this is so that at the end of the experiment the alcohol and burner can be measured to get the results.
- Measure out 100cm3 of water in to the copper can using measuring cylinder.
- Stir and record initial temperature of water, so you can calculate the amount of temperature rise later.
- Start clock and light burner at the same time, so the timing of the experiment is exactly right.
- Place can so flame is 3 cm from the bottom of the it,
- Stir water, so the temperature is spread among all of the water in the can.
- Burn for three minutes, and then extinguish flame by putting the wick protector over the top of the wick.
- Record final temperature of water to find out the amount of increase in temperature.
- Record final mass of burner and alcohol to get all of the results and to see the correlation between mass of alcohol burnt and temperature of the water.
I feel that my results do not show what I would have expected. For some reason the alcohols with the highest amount of carbon bonds don’t have the highest amount of energy given off. My hypothesis was incorrect in the end but I feel it was not because of a misjudged prediction. My reasons for this are below.
I believe my results were very inaccurate. Below I have a list of reasons of why my results were so inaccurate:
- Different groups took the different results so different methods may have been used.
- Energy given off through sound and light.
- Heat conducted and convected away through the air.
- Radiation of heat out into the atmosphere.
- The size of the wick.
- Not all of the water was the same temperature.
- The flame size changed due to the type of alcohol; hence it was a different distance away from the can each time.
- The fact that the can gets hot.
- The metal clamp transferred heat way.
- Heat may have been taken away through gusts of wind.
- The amount of energy you give the alcohol originally.
- The availability of alcohol for the wick to burn, if not enough then the wick would burn not the alcohol, which would give an inaccurate result.
- Evaporation of water so there will be less water to heat, making the water hotter.
The equipment that I used in this experiment was very inaccurate because heat is a bad way of transferring energy without any loss of it. Any molecule will conduct heat; radiation happens and can be reduced but not completely halted. I feel that the most limiting factor of the experiment is the convection of air and to a lesser extent, of water. Also during the experiment, some of the water will have evaporated, thus the water mass/temp reading will be altered.
Another error is that of incomplete combustion. Complete combustion occurs if there are lots of oxygen atoms available when the fuel burns, then you get carbon dioxide (carbons atoms bond with two oxygen atoms). If there is a limited supply of oxygen then you get carbon monoxide (each carbon atom can only bond with one oxygen atom). This is when incomplete combustion has occurred. This is so because the carbon monoxide could react some more to make carbon dioxide. Since heat is given out when bonds form, less energy is given out by incomplete combustion. So this is why it affects the outcome of the experiment. To make this problem irrelevant I could have made sure a constant supply of oxygen was present in the experiment.
Graph of averages: