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Enthalpy changes are typically tabulated for reactions in which both the reactants and products are at the same conditions. A standard state    is a commonly accepted set of conditions used as a reference point for the determination of properties under other different conditions. For chemists, the IUPAC standard state refers to materials under a pressure of 1 bar and solutions at 1 M, and does not specify a temperature. Many thermochemical tables list values with a standard state of 1 atm. Because the Δ H of a reaction changes very little with such small changes in pressure (1 bar = 0.987 atm), Δ H values (except for the most precisely measured values) are essentially the same under both sets of standard conditions. We will include a superscripted “o” in the enthalpy change symbol to designate standard state. Since the usual (but not technically standard) temperature is 298.15 K, we will use a subscripted “298” to designate this temperature. Thus, the symbol ( Δ H 298 ° ) is used to indicate an enthalpy change for a process occurring under these conditions. (The symbol Δ H is used to indicate an enthalpy change for a reaction occurring under nonstandard conditions.)

The enthalpy changes for many types of chemical and physical processes are available in the reference literature, including those for combustion reactions, phase transitions, and formation reactions. As we discuss these quantities, it is important to pay attention to the extensive nature of enthalpy and enthalpy changes. Since the enthalpy change for a given reaction is proportional to the amounts of substances involved, it may be reported on that basis (i.e., as the Δ H for specific amounts of reactants). However, we often find it more useful to divide one extensive property (Δ H ) by another (amount of substance), and report a per-amount intensive value of Δ H , often “normalized” to a per-mole basis. (Note that this is similar to determining the intensive property specific heat from the extensive property heat capacity, as seen previously.)

Enthalpy of combustion

Standard enthalpy of combustion ( Δ H C ° ) is the enthalpy change when 1 mole of a substance burns (combines vigorously with oxygen) under standard state conditions; it is sometimes called “heat of combustion.” For example, the enthalpy of combustion of ethanol, −1366.8 kJ/mol, is the amount of heat produced when one mole of ethanol undergoes complete combustion at 25 °C and 1 atmosphere pressure, yielding products also at 25 °C and 1 atm.

C 2 H 5 OH ( l ) + 3 O 2 ( g ) 2 CO 2 + 3 H 2 O ( l ) Δ H 298 ° = −1366.8 kJ

Enthalpies of combustion for many substances have been measured; a few of these are listed in [link] . Many readily available substances with large enthalpies of combustion are used as fuels, including hydrogen, carbon (as coal or charcoal), and hydrocarbons (compounds containing only hydrogen and carbon), such as methane, propane, and the major components of gasoline.

Standard Molar Enthalpies of Combustion
Substance Combustion Reaction Enthalpy of Combustion, Δ H c ° ( kJ mol at 25 °C )
carbon C ( s ) + O 2 ( g ) CO 2 ( g ) −393.5
hydrogen H 2 ( g ) + 1 2 O 2 ( g ) H 2 O ( l ) −285.8
magnesium Mg ( s ) + 1 2 O 2 ( g ) MgO ( s ) −601.6
sulfur S ( s ) + O 2 ( g ) SO 2 ( g ) −296.8
carbon monoxide CO ( g ) + 1 2 O 2 ( g ) CO 2 ( g ) −283.0
methane CH 4 ( g ) + 2 O 2 ( g ) CO 2 ( g ) + 2 H 2 O ( l ) −890.8
acetylene C 2 H 2 ( g ) + 5 2 O 2 ( g ) 2 CO 2 ( g ) + H 2 O ( l ) −1301.1
ethanol C 2 H 5 OH ( l ) + 3 O 2 ( g ) 2CO 2 ( g ) + 3 H 2 O ( l ) −1366.8
methanol CH 3 OH ( l ) + 3 2 O 2 ( g ) CO 2 ( g ) + 2 H 2 O ( l ) −726.1
isooctane C 8 H 18 ( l ) + 25 2 O 2 ( g ) 8 CO 2 ( g ) + 9 H 2 O ( l ) −5461

Questions & Answers

Modern Electronic Theory
Levi Reply
find the volume of oxygen produced from the electrolysis of acidified water of a current of a 2A was passed through the electrolysis of acidified water for 2 hours
Ngwu Reply
A group of atoms that are connected by chemical bounds
Valerie Reply
molecule
NARCOS_
the atoms are found in the shell, it is called atomic bond.
Andrew
what is molecules
Kelly Reply
what atoms
Simon
Moecules are groups of atoms
Adewole
What is collision theory
Isah Reply
hello guys am new here
Andrew Reply
what is the si unit for energy
enow Reply
joules
udoh
what is Charles law
Ogbu
how do mass of solvent be gram or kilogram ?
Yussuf Reply
by converting
Andrew
its a force a gas exerts on the wall of the container
udoh Reply
compressed it is
Andrew
what is gas pressure
Sandra Reply
the pressure is the amount of Gas that is compressed in the container. in other words because of the gas that is compressed in the container there is pressure on it.
Andrew
write the electron configuration for probium
Magano Reply
what is an accuracy
amadi Reply
what is it
Nhagn
accuracy has to do with speed
Emmanuel
accuracy is the movement of an object in a particular direction without any obstructions.
Andrew
what is titration curve
johnson Reply
a titration curve is the separation of a solvent in a curved direction.
Andrew
explain gaslaws
Felix Reply
explain accuracy
amadi
gas law states that the amount of molecule in the container is inversely proportional to the pressure.
Andrew
what is the functional group in this compound:CH3COCH2CH2-O-CH2
Obafemi Reply
this belongs to the Akane family. after going through this, I noticed the structural formula you gave was wrong. The answer to this is C5H10 heptane.
Andrew

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Source:  OpenStax, Chemistry. OpenStax CNX. May 20, 2015 Download for free at http://legacy.cnx.org/content/col11760/1.9
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