The enthalpy change of combustion for the alcohol homologous series.

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I am going to investigate the enthalpy change of combustion for the alcohol homologous series. I will investigate how alcohols with increasing number of carbons affect the enthalpy change when an alcohol goes under combustion.

The energy that alcohols release when being used is called the enthalpy change of combustion. This is defined as

"Standard enthalpy change of combustion, is the enthalpy change when one mole of a substance burns completely in oxygen under standard conditions (298K, 100 KPa and 1 Molar) with all products in their standard state" (Collins advanced Chemistry)

Enthalpy is the total energy content of the reacting materials. Enthalpy is given the symbol H. the only was to measure enthalpy is the measure the change in energy before and after as energy cannot be measured directly.

The symbol in the notation used for standard enthalpy change under combustion means that the combustion has taken place under standard conditions, s.t.p. Therefore the statement DHq =-286 kJ mol-1 means the enthalpy change of the combustion of hydrogen when one mole of hydrogen is completely burnt in oxygen under standard conditions with reactants and products in their standard states at that temperature.

CH4 (g)+2O2 (g) CO2 (g)+ 2H2O(l)

DHq= -890 kJ mol-1 meaning that the stored reactants methane and oxygen are higher than the stored energy of the products of the reaction, carbon dioxide and water and the difference in energy is released to the surroundings when the methane and oxygen react. The reaction of methane can be displayed in the form of an energy level diagram. The enthalpy change is negative because the reactants loose energy, therefore all exothermic reactions will have a negative value for their enthalpy change. Energy is not actually made in this reaction, it is only transferred from one state to another, and some energy in this reaction is lost in the atmosphere.

This can be demonstrated with an energy level diagram which shows that this reaction is an exothermic reaction because energy is lost in the surroundings from the reactants. All of the reactions for the combustion of alcohols will be exothermic and will therefore have negative values.

The formulas that will be needed in this experiment to calculate the energy transferred are E=mc delta t. where m= the mass of the water, c= specific heat capacity of water and delta t is the change in temperature. There will also be a further 2 sets of data as well as my experimental results. These will be my theoretical and calculated results. The theoretical results will be obtained by adding the bond energies together for each alcohol and the calculated will be obtained from the Nuffield Science Data book which is the experiment under standard conditions.

The bond enthalpy is the energy required to break a bond between two molecules in a gaseous molecule. The energy to break a bond is referred to as the energy required to break one mole of bonds. This is defined as:

"Bond Enthalpy, E, is the energy required to break one mole of bonds of the same type in gaseous molecules under standard conditions (298 K, 100 kPa)" (Collins Advanced Chemistry)

This reaction for the combustion of alcohol is an exothermic reaction, this meaning that energy is released into the surroundings and that bonds are made.

The 1st law of thermodynamics is that "energy cannot be made, only transferred"

For calculating my theoretical values I will split up the equation for bonds broken and bonds made. The bond breaking process is an endothermic process as we a re putting in energy to break the electrostatic attraction in the bond. Also as they are endothermic reaction they are positive reactions. Bond energies are also average values meaning that the actual enthalpy value varies depending on the type of molecule it is bonded with. Different molecules will exert different strengths for the actual bond. In making bonds the same energy is released in that of in making...
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