Calculate Heat of Combustion: Grams, Heat Capacity, Temp Change

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Discussion Overview

The discussion revolves around calculating the heat of combustion using parameters such as grams of fuel, heat capacity, and temperature change. Participants explore both theoretical and practical aspects of combustion calculations, including the influence of temperature and the energy involved in bond formation and breaking.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Homework-related

Main Points Raised

  • One participant inquires about the formula for calculating heat of combustion based on grams, heat capacity, and temperature change, and asks for clarification on the heat combustion per kJ/mol.
  • Another participant suggests writing a balanced chemical equation for the combustion process and emphasizes the importance of accounting for the energy required to break bonds in the reactants and the energy released when forming products.
  • A participant expresses a preference for a straightforward formula rather than a detailed equation, questioning the rationale behind using heat capacity and temperature difference divided by grams converted to moles.
  • One participant argues that the temperature difference should not affect the energy output in combustion, citing that burning in liquid oxygen yields the same energy regardless of temperature, although they acknowledge that density and energy loss in non-condensing systems may introduce temperature effects.
  • A specific example is presented involving the combustion of naphthalene in a calorimeter, with a request for guidance on the calculation of heat of combustion in kJ/mol.
  • Another participant clarifies that the heat of combustion is independent of the actual temperature, indicating that the temperature change of the calorimeter reflects the energy released, and emphasizes the need to calculate the moles of fuel used to determine joules per mole.

Areas of Agreement / Disagreement

Participants express differing views on the relevance of temperature change in combustion calculations, with some arguing it is not significant while others suggest it may have implications under certain conditions. The discussion remains unresolved regarding the best approach to calculate heat of combustion.

Contextual Notes

Participants mention various factors that could influence the calculations, such as the state of the fuel, density considerations, and energy losses in specific systems, but these factors are not fully explored or agreed upon.

HeartSoul132
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What's the formula for calculating this, given: grams, heat capacity of substance, and temperature change?

How about heat combustion per kJ/Mol?

Thanks
 
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You would write a balanced equation for
Fuel + oxygen -> product (typically water + CO2)

Then account for all the energy needed to break the bonds in the original fuel and oxygen plus the energy released when bonds form in the product.
For real world energy values the water formed is usually steam so you have to account for the heating and vapourisation energy needed.

Or you can lookup the heat of combustion/gram for lots of different fuels on the web or in databooks
 
Last edited:
Thanks for the reply.
Don't need an equation, according to solution; just a plug and chug formula. It seems like it is heat capacity * temp diff ALL divided by the grams converted to mols -- wondering why?
 
For simply burning the temperature difference shouldn't come into it - if you burn something in liquid oxygen (-200degC) you get just the same energy out!

There is a temperature effect if you have to take into account the density of the fuel (as in liquified gas, or jet fuel at high altitudes) or if you have a non-condensing boiler where some energy is lost as hot steam.
 
A .922 sample of napthalene (C10H8) is burned in a calorimeter that has a heat capacity of .944 kJ/K. The temperature of the calorimeter rose from 15.73 C to 19.75 C. Calculate the heat of combustion for this chemical in kJ/mol.

I'm sure there's an equation, wondering how?
 
The heat of combustion doesn't depend on the actual temperature - the temperature change of the calorimeter is telling you how much energy was given off.
You get the point - if the fuel burned and heated the calorimeter from 115.73 C to 119.75 C you would get the same result.

You need to work out how many moles of fuel you used, and how much energy the calorimter received that gives you the joules/mole for the fuel.
 

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