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CHEM1001 Lecture 26 Worksheet

Fuels and Combustion

1. Heat of Combustion of a Model Fuel

Combustion refers to a reaction of afuel with an oxidizeror oxidant. When we think of

fuels we most commonly think of organic materials, especially hydrocarbons, and the most

common oxidizer for terrestrial applications is oxygen, O2(g). Lets take ethanol as a

model fuel, and calculate its enthalpy of combustion exactly using heats of formation, and

also approximately using bond enthalpies.

(a) First you need to balance the equation for the reaction, whose products are CO2 and

H2O (all in the gas phase).

C2H5OH(l) + O2(g) CO2(g) + H2O(g).

To calculate how many water molecules will be produced by this combustion reaction,

balance the hydrogens.

To calculate how many carbon dioxide molecules will be produces, balance the carbons.

Finally, work out how many O2 molecules are needed to account for all the O atoms

produced in H2Oand CO2. Dont forget to deduct the O already present in ethanol.

(b) Calculate the enthalpy of combustion using the formula for enthalpies of formation of

the correct number of moles of products and reactants,

Hrxn = Hf(products) - Hf(reactants), and the data table below.

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(c)Estimate the enthalpy of combustion using average bond enthalpies (shown below).

You do this by identifying the number and type of every bond to be broken in order to

(hypothetically) fully dissociate or atomise the correct number of moles of ethanol and O2

reactants. The do the same to form all the necessary bonds for the correct number of moles

of water and CO2 products.

Suggest reasons why the estimated answer (c) does not agree with the exact calculation (b).

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