ATAR Notes: Forum
HSC Stuff => HSC Science Stuff => HSC Subjects + Help => HSC Chemistry => Topic started by: wjk1999921 on November 12, 2017, 08:34:11 pm
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Hi guys,
Im having trouble trying to find the molar heat of combustion (KJ Mole-1) and the heat of combustion (KJg-1) of alcoholes from just the molar mass. If anyone can explain to me the step/process on how to work it out, it wouls be great！
Thank you! :)
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Hey,
Molar heat of combustion is defined as the amount of energy released as heat when one mole of a substance (e.g. alkanol) is combusted to form products at standard states (25degrees, 100kPa). It is usually written as a positive value.
Heat of combustion is similar to change in enthalpy, which we can work out from this equation △H = -mC△T (where m is the mass of water, c is the specific heat capacity, (Specific heat capacity of H2O = 4.18 J k-1g-1) , △T is the change in temperature (final temperature - initial temperature) ). Heat of combustion is generally written as a negative value.
△H simply tells us the amount of heat released by fuel which has been absorbed by the water, assuming that there is no heat loss to environment (IF THERE IS: the question becomes a little trickier)
Now, assuming no heat loss, △H will equal to the heat released by fuel. This is how you find the heat of combustion when given the mass of water.
To find molar heat of combustion, you will need to divide the heat of combustion value by the number of moles of fuel/alcholol burnt. (to find the number of moles, you do n = m/mm (where m is mass, mm is molar mass). Hence, you will get the molar heat of combustion.
I have attached an example of how you would approach these types of calculation questions.
Hopefully this helps!!
D.H.