Alkane & Alkene Practical
The cyclohexane contains only single bonded carbon atoms and does not react to the bromine water because the alkane is saturated. Therefore no reaction took place, hence the solution stayed red in colour.
The cyclohexene contains a reactive double bond that is high in electron density. It reacts with the bromine water. Therefore the reaction causes the decolourising of the solution.
Cyclohexane and cyclohexene were chosen because they are identical except that cyclo hexane has a double bond and cyclohexane doesn't have one. They are both liquids so they are easy to handle and the experiment was able to be conducted (some alkanes/alkenes like methane are gases at room temperature and you would not be able to easily react them with the bromine water).
Bromine was chosen because it will react with a double bonded hydrocarbon, and not a saturated hydrocarbon. It also changes colour when it reacts with the cycloalkene.
Molar Heat of Combustion Practical
To obtain accurate values of the heat of combustion, a bomb calorimeter is used. It allows the heat to be kept inside, and water to absorb the heat from all around it.
An accurate figure for the heat of combustion cannot be achieved in the school laboratory using a rudimentary calorimeter because there is heat loss to the environment from the flame and also the complete combustion cannot be achieved in air that only contains about 20% of oxygen. A calorimeter can be set up to get an idea of how this figure could be arrived at.
Potential hazards may include the the naked flame of the spirit burner and the fuel itself, wear safety goggles to protect the eyes.
I hope that helps.
