Comparing The Enthalpy Change Of Combustion Of Different Alcohols :: GCSE Chemistry Coursework Investigation

Comparing The Enthalpy Change Of Combustion Of Different Alcohols The aim of my experiment is to investigate the enthalpy of combustion of a range of alcohols. The standard enthalpy of combustion is the enthalpy change that occurs when 1 mole of a fuel is burned completely in oxygen under standard conditions – 1 atmosphere pressure and 298K. All combustion reactions are exothermic which is why I am expecting all the values for the enthalpy change of combustion to always be negative. Prediction ========== I predict that the greater the number of carbon atoms there are in the alcohol molecule, the greater the enthalpy change of combustion it will have. This is because in a chemical reaction energy is needed in order to break bonds and energy is released when bonds are made. If I were to look at these two values; the energy required to break the bonds of the reactants and the energy released when the bonds of the product are made then I am able to work out the enthalpy change of combustion of the alcohol. In order to do this we need to know the bong enthalpies of the bonds in the process. The quantity of energy needed to break a particular bond in a molecule is called the bond enthalpy. Below is a table containing the bond enthalpies of the bonds that occur in the combustion of an alcohol. Bonds We use the average bond enthalpies as the exact value of a bond enthalpy depends on the particular compound in which the bond is found. GRAPH DH1 By looking at the equation for the reaction that occurs when an alcohol burns, we realise that the reaction involves both breaking bonds and making new ones. From the equation of the process we can tell how many of the different number of bonds are broken and made and so work out the enthalpy change of combustion. We can work out the enthalpy of combustion of methanol by using the bond enthalpy values: CH3OH + 1.5O2 CO2 + 2H2O We can use the enthalpy cycle above to work out the value for the enthalpy change of combustion of methanol, represented by DH1. The calculation is done as shown below: DH2 = enthalpy change when bonds are broken = 3 Â ´ E(C-H) + 1 Â ´ E(C-O) + 1 Â ´ E(O-H) + 1.5 Â ´ E(O=O) = 3(413) + 358 + 464 + 1.5(498) = 2808 kJ mol-1 DH3 = enthalpy change when bonds are made