It is clear from the results that the candle transferred the most energy to the water per gram of fuel burnt. Being the least expensive of the fuels also meant that it is clearly the most economical. However the final temperature of the water when heated by the candle was cool when compared to the other fuels meaning, in a practical sense at least, although it is the most economical it is not the most convenient if wishing to heat things to high temperatures. Discussion
All the fuels burnt share the same key characteristics in that they are all hydrocarbons; basically they are made up of hydrogen and carbon. They are all also stable at room temperatures (and above) and require the presence of oxygen in order for them to burn. Not only do they need oxygen but also an initial input of energy, in this case a match. This initial energy is called the activation energy. Once the initial energy has gone in to start the reaction there is a short period of endothermic activity. At this point the bonds between the hydrogen and carbon are being broken.
The oxygen then reacts with the hydrogen to make water and with the carbon to produce carbon monoxide and then again to form carbon dioxide. All the while these bonds are being formed energy is being released and the reaction is said to be exothermic. In order for a reaction to be exothermic overall, the amount of endothermic energy (energy being taken/bonds being broken) cannot be greater than the amount of exothermic energy (energy being released/bonds being formed).
The difference between the two is said to be the enthalpy change (H) and of course H can be either plus or minus depending on whether or not the reaction is overall endothermic or exothermic respectively. In terms of the fuels burnt there is clearly a release of energy, we saw the temperature of the water rise and we could see the release of energy in the form of light from the flame. Although our results were consistent and reasonably reliable there were still a number of sources of error. Were the experiment to be completed again there are a number of things I would change in order to make it a fairer test.
Firstly, whilst burning the fuels it was noticed energy was lost from around the sides of the apparatus, if we could feel this energy as heat it was not contributing to the transfer of heat to the water. The levels of loss could not be measured and could have been greater with some fuels tested than others, distorting the final results should one have experienced greater loss than another. This, of course, given the apparatus available was unavoidable. The only way around this I can see would be to have a sealed chamber between fuel and water, limiting substantially any loss of energy.