Investigating the energy released from burning different alcohols.

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Peter Cui GCSE Chemistry

Investigating the energy released from burning different alcohols

Alcohols burn and react with oxygen forming water and carbon dioxide; the equations below show the reactions between oxygen and 8 alcohols from methanol to ethanol.

                         

For these reactions to start, energy is required to break the existing bonds between the atoms of molecules of the reactants. For example in methanol, C-H bonds, C-O bond and O-H bonds and O=O double bonds need to be broken, energy is required to break these bonds as well as others in the reaction. However when new bonds are formed between atoms at the end of the reaction: C=O double bonds as well as O-H bonds are formed and as they are formed they release energy. As the energy released from the new bonds formed is more than the energy required to break the old bonds, the reaction is exothermic, overall it release energy into the surroundings.

        The aim of this investigation is to find out how the energy released from the different alcohols differs.

Hypothesis

My hypothesis is based upon the theoretical values of the different heats of combustion of the alcohols. Each will be different as different number of bonds will be broken and different number of bonds will be made, although in each case all of the bonds broken and made will be the same for each alcohol.

Below is a graph drawn from the values in the table above showing the relationship between the different alcohols and the heats of combustion for each alcohol.

From the graph we can see that the relationship between the different alcohols and their heats of combustion is a straight line on the graph. Therefore the according to the graph the heats of combustion should be directly proportional to the alcohol, and as the heats of combustion are going up as the alcohol molecule gets bigger, we can also say that they have a positive correlation.

        As the size of the molecules of the alcohols increase, more energy is needed to break the bonds of the alcohols. However, even more energy is released from the reactions as even more C=O and H-O bonds are formed, as the energy released from the reactions is not directly proportional to the activation energy. In fact as the activation energy increases, the energy given out from the reaction should increase even more as there is a greater increase in the number of carbon dioxide and water molecules formed, therefore the heats of combustion should also increase as the size of the molecules increase. More energy is released from the formation of C=O double bonds, as it is a double bond. In fact, in theory the formation of C=O double bonds releases 803 kJ/Mol. For the bonds broken, the bond that requires the most energy to break it is the O=O double bond, which requires 497 kJ/Mol. The energy given out from the formation of a C=O double bond is almost twice as much as the maximum energy requirement for any bond. Hence the energy given out from the reaction should be higher than the activation energy.

        As the molecules of the alcohols increase in size, the activation energy will increase also as there’re more bonds to break. But in addition there will be more bonds formed from the reaction. Though increase in the number of bonds broken will be the same as the increase in the number of new bonds formed, the bond energies of the new bonds are higher than the bond energies of the bonds broken therefore. The increase in the energy given out in the reaction will be more than the increase in the activation energy.

        Therefore I hypothesise the heats of combustion will have a positive, directly proportional correlation. And the shape of the graph drawn from the results obtained from the investigation should be similar to the shape of the graph displaying the relationship between the theoretical heats of combustion and the alcohols.

Planning

        I plan to use an alcohol burner to heat up 100g of water until it rises 10oC in temperature. I will measure the mass of the alcohol burner before and after the combustion, from these values I will calculate how much of the alcohol was used to raise the temperature of 100g of water by 10oC, and from that I can calculate the heat of combustion of the alcohol.

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The heats of combustion from the investigation can be calculated by the formula:

Energy given out (J) = specific heat capacity x mass of water x temperature change

As the specific heat capacity is 4.2 J/kg/oC, and the mass of water will be 100g and the temperature change will be 10oC. Using the formula the energy given out will always be 4200J or 4.2kJ. Using this formula:  

Heat of combustion (kJ/mol) = 4.2/ mass of alcohol used x relative molecular mass

The heat of combustion per mole of the alcohol can be calculated using the formula above.

Apparatus

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