In this experiment I will investigate the enthalpy changes of combustion. In particular I will be investigating the enthalpy change of combustion for different alcohols. I hope to investigate some of the homologous series alcohols, as well as a few structural isomers of these alcohols.
Background & theory
Enthalpy cannot be measured. What we can actually measure is the enthalpy change from the reactants to the products. This is the quantity of energy transferred to or from the surroundings. The energy is usually in the form of heat, so the surroundings either cool down or heat up.
The two kind of enthalpy changes include exothermic, and endothermic. Exothermic is where energy is transferred to the surroundings (this has a negative enthalpy change because we say that it has lost energy). Endothermic is where energy is taken in from the surroundings (this has a positive enthalpy change as we say that it has gained energy). The following diagrams show these two reactions graphically:
Enthalpy change is represented by the symbol ∆H. (∆ is ‘the change in’ the ‘H’ is for enthalpy) The unit for enthalpy change is measured in Kilojoules per mole. For example if an alkane has a combustion enthalpy change of +296Kjmol-1 then the alkane takes in 296kJ of energy, from its surroundings, for every mole that is burnt. Enthalpy changes can easily be measured using equipment in a school laboratory. We can set up an experiment so that energy is transferred to or from water. To actually work out the enthalpy change of combustion of this kind of reaction we can use the following equation Energy transferred = cm T
c = specific heating capacity of water (4.17Jg-1K-1)
m = mass of water (in grams)
∆T = change in temperature of the water.
Enthalpy change depends on the bonds that are being broken and the bonds that are being formed in a chemical equation. For example the combustion of methane:
This diagram shows the different bonds that are being broken (the left hand side), and also the bonds being formed (the right hand side) in the chemical reaction of the combustion of methane. Also shown is the amount of energy needed to break and form these bonds (these energies are the same). The different bonds on either side of the equation show that different energies are needed to break and form the different bonds.
So in my experiment the alcohols will have different enthalpy levels because different amounts of bonds are being broken and formed.
My choice of alcohols / predictions
The first few alcohols of the homologous series along with their structural formulae and combustion equations are as follows:
From these balanced equations we can see that when one mole of the alcohol is burnt the alcohol needs 1.5mol more oxygen, and produces one more mole more of carbon dioxide and one more mole of water vapour. •So based on this I predict that when I measure the enthalpy change of combustion of the homologous series alcohols the enthalpy change should go up in pattern.
However isomers of the same alcohol shouldn’t show any difference. For example if we compare the alcohols Butanol and it’s isomer propan-2-ol.
•In both alcohols there are the exact same bonds that will be broken; the exact same bonds will be formed in the products. Because these bonds have the same energy requirements for breaking and forming the enthalpy change in combustion will be the same for an alcohol and its isomer.
So based on these two properties of alcohols I will experiment the enthalpy change of combustion with following alcohols: