What's the difference between 'enthalpy of combustion' vs 'heat of combustion'? What sign does each one use? Why does the book exclusively use negative values for heat of combustion while the databook uses positive values?
Heat of combustion = positive. Sources:
Heat of combustion may be defined as the heat energy released when a
specified amount of a substance burns completely in oxygen and is, therefore, reported as a positive value,
indicating a magnitude. Enthalpy of combustion, ΔH, for the substances in this table would be reported as
negative values, indicating the exothermic nature of the combustion reaction.
This can be quite confusing; the terms 'heat' and 'enthalpy' are sometimes used interchangeably when they really shouldn't.
Heat is a form by which energy is transferred; energy is released as heat when combustion occurs. Hence, the heat of combustion is positive.
However, enthalpy (which is delta H) is a measure of the chemical energy present in a chemical. The energy that is released as heat was originally chemical energy (H). Hence, in combustion, chemical energy is lost, and H decreases.
So, the heat of combustion is positive (we use the symbol q to denote heat), the enthalpy (H) of combustion is negative.
Energy released means heat of combustion= positive?
https://chem.libretexts.org/Textbook_Maps/Introductory_Chemistry_Textbook_Maps/Map%3A_Introductory_Chemistry_(CK-12)/17%3A_Thermochemistry/17.14%3A_Heat_of_Combustion values are all positive
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Heat of combustion = negative. Sources:
link= http://www.chemicalforums.com/index.php?PHPSESSID=m9t0aopffcvu5d2qtp8te8b3b7&topic=76354.msg277861#msg277861heat of combustion is a negative number, the more negative it is the more heat is released.
The heat of combustion of a fuel is defined as the enthalpy change that occurs when a specified amount (e.g. 1 g, 1 L, 1 mol) of the fuel burns completely in oxygen. It is usually measured at conditions of 298 K and 100 kPa, which means that the water produced should be shown in the liquid state. The heat of combustion can be given the symbol ∆Hc
.
The heats of combustion for some common elements and compounds present in fuels are listed in Table 2.3.1. Heat energy is released during combustion, so ∆Hc
always has a negative value.
i.e.: textbook: "heat of combustion = delta H. Heat energy is released makes delta H negative so heat of combustion is negative since delta H=heat of combustion.
link= https://lavelle.chem.ucla.edu/forum/viewtopic.php?t=9281] The heat of combustion will always be negative since the reaction will be giving off heat as it breaks the chemical bonds. The heat of formation should always be positive since it will be taking in heat in order to produce the chemical bonds. They can be related to one another by using the sum of the heat of formation for the products and the sum of the heat of formation for the reactants and taking the difference (Hess's law) will give you the heat of combustion.
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Other sources:
link=http://www.ausetute.com.au/heatcomb.html[the heat of combustion (enthalpy of combustion, ΔHc)
implies heat of combustion = enthalpy of combustion.
Wikipedia:
https://en.wikipedia.org/w/index.php?title=Enthalpy_of_combustion&redirect=no The link to 'enthalpy of combustion' automatically redirects you to the page for 'heat of combustion'. Implying they're identical terms. Heat of combustion values positive here.
https://chemistry.stackexchange.com/questions/7138/whats-the-difference-between-enthalpy-of-combustion-and-heat-of-combustion another fellow confused person.
I've been researching this for hours upon hours. Contradictions everywhere. I can find dozens more links to where people claim heat of combustion is negative/positive, but these are enough to show there's an inconsistency.
What's the correct answer?
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