<< Chapter < Page Chapter >> Page >

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

what us atomic of molecule
Imhologhomhe Reply
chemical formula for water
Muhammad Reply
what is elemental
Maryam Reply
what are the properties of pressure
How can water be turned to gas
what's a periodic table
Okiemute Reply
how does carbon catenate?
obuke Reply
condition in cracking from Diesel to petrol
Brient Reply
hey I don't understand anything in chemistry so I was wondering if you could help me
Ruth Reply
i also
I also
condition for cracking diesel to form kerosene
can you tell me
please let me know
what is periodic law
rotimi Reply
periodic law state that the physical and chemical properties of an element is the periodic function of their atomic number
how is valency calculated
Ashley Reply
How is velency calculated
Hi am Isaac, The number of electrons within the outer shell of the element determine its valency . To calculate the valency of an element(or molecule, for that matter), there are multiple methods. ... The valency of an atom is equal to the number of electrons in the outer shell if that number is fou
what is the oxidation number of this compound fecl2,fecl3,fe2o3
Asmau Reply
bonds formed in an endothermic reaction are weaker than the reactants but y r these compound stable at higher temperatures
zille Reply
what is a disproportionation reaction
Ogor Reply
name the force that exist in cao
folarin Reply
what is chemistry
Richard Reply
chemistry is one of the three main branches of pure science the other two being physics and biology. chemistry deals with the composition properties and uses of matter. it probes into the principle governing the changes that matter undergo.
Chemistry is a branch of science that deals with atom & constitution of substances & d changes they undergo
chemistry is a group of science deals with mater
Hy! I'm Abdul gafar chemistry is a branch of science dat deals with an atom and change they undergo as a result of alterations in d constitution of there molecules
so from dis concept rust x an atom
quame Reply

Get the best Chemistry course in your pocket!

Source:  OpenStax, Chemistry. OpenStax CNX. May 20, 2015 Download for free at http://legacy.cnx.org/content/col11760/1.9
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Chemistry' conversation and receive update notifications?