<< Chapter < Page Chapter >> Page >

Standard enthalpy of formation

A standard enthalpy of formation Δ H f ° is an enthalpy change for a reaction in which exactly 1 mole of a pure substance is formed from free elements in their most stable states under standard state conditions. These values are especially useful for computing or predicting enthalpy changes for chemical reactions that are impractical or dangerous to carry out, or for processes for which it is difficult to make measurements. If we have values for the appropriate standard enthalpies of formation, we can determine the enthalpy change for any reaction, which we will practice in the next section on Hess’s law.

The standard enthalpy of formation of CO 2 ( g ) is −393.5 kJ/mol. This is the enthalpy change for the exothermic reaction:

C ( s ) + O 2 ( g ) CO 2 ( g ) Δ H f ° = Δ H 298 ° = −393.5 kJ

starting with the reactants at a pressure of 1 atm and 25 °C (with the carbon present as graphite, the most stable form of carbon under these conditions) and ending with one mole of CO 2 , also at 1 atm and 25 °C. For nitrogen dioxide, NO 2 ( g ), Δ H f ° is 33.2 kJ/mol. This is the enthalpy change for the reaction:

1 2 N 2 ( g ) + O 2 ( g ) NO 2 ( g ) Δ H f ° = Δ H 298 ° = +33.2 kJ

A reaction equation with 1 2 mole of N 2 and 1 mole of O 2 is correct in this case because the standard enthalpy of formation always refers to 1 mole of product, NO 2 ( g ).

You will find a table of standard enthalpies of formation of many common substances in Appendix G . These values indicate that formation reactions range from highly exothermic (such as −2984 kJ/mol for the formation of P 4 O 10 ) to strongly endothermic (such as +226.7 kJ/mol for the formation of acetylene, C 2 H 2 ). By definition, the standard enthalpy of formation of an element in its most stable form is equal to zero under standard conditions, which is 1 atm for gases and 1 M for solutions.

Evaluating an enthalpy of formation

Ozone, O 3 ( g ), forms from oxygen, O 2 ( g ), by an endothermic process. Ultraviolet radiation is the source of the energy that drives this reaction in the upper atmosphere. Assuming that both the reactants and products of the reaction are in their standard states, determine the standard enthalpy of formation, Δ H f ° of ozone from the following information:

3 O 2 ( g ) 2 O 3 ( g ) Δ H 298 ° = +286 kJ

Solution

Δ H f ° is the enthalpy change for the formation of one mole of a substance in its standard state from the elements in their standard states. Thus, Δ H f ° for O 3 ( g ) is the enthalpy change for the reaction:

3 2 O 2 ( g ) O 3 ( g )

For the formation of 2 mol of O 3 ( g ), Δ H 298 ° = +286 kJ. This ratio, ( 286 kJ 2 mol O 3 ) , can be used as a conversion factor to find the heat produced when 1 mole of O 3 ( g ) is formed, which is the enthalpy of formation for O 3 ( g ):

Δ H ° for 1 mole of O 3 ( g ) = 1 mol O 3 × 286 kJ 2 mol O 3 = 143 kJ

Therefore, Δ H f ° [ O 3 ( g ) ] = +143 kJ/mol .

Check your learning

Hydrogen gas, H 2 , reacts explosively with gaseous chlorine, Cl 2 , to form hydrogen chloride, HCl( g ). What is the enthalpy change for the reaction of 1 mole of H 2 ( g ) with 1 mole of Cl 2 ( g ) if both the reactants and products are at standard state conditions? The standard enthalpy of formation of HCl( g ) is −92.3 kJ/mol.

Answer:

For the reaction H 2 ( g ) + Cl 2 ( g ) 2 HCl ( g ) Δ H 298 ° = −184.6 kJ

Got questions? Get instant answers now!

Questions & Answers

what is the meaninh of link
Aisha Reply
Yes wish to join am a science ⚗ student
Emmanuel Reply
What is acid
Emmanuel
is agroup of substance that relaise hydrogen ion in the water solution.
mengistu
thanks
Aisha
An acid is a proton donor or an electron pair acceptor
amanda
how can someone understand chemistry vividly
Mercy Reply
Maybe by reading proofs or practical work and application in modern world.
Abdelkarim
what is isotopes
Samuel Reply
whats de shape of water
Amara Reply
water has no shape because it's liquid
Wil
water is a shapeless, odourless, colourless and tasteless substance that only takes the shape of its container.
mikefred
i think they're referring to the molecular shape?
It has no shape but takes the shape of the container
kpadonu
Yeah I think so too and it's a bent shape
amanda
what isthe maening pkw
Wilson Reply
what is the meaning of STP
Laura
introduction to chemistry for beginner
Lansana Reply
hi
noble
through out human history
marwan Reply
Yes?
Esther
hello
noble
yes
Bol
what
noble
is euglena a unicellular organ
Agio Reply
is euglena a unicellular organism
Agio
how is hydrogen can be heated
Buwembo Reply
what is difference between atom and molecule
Aqeela Reply
Atom is the smallest part of matter; it consists of equal number of protons and electrons. It may have neutrons. A molecule is a compound made of atoms covalently bonded.
Abdelkarim
does amoeba has structure
Mercy Reply
what is the effect of green house on the earth 🌎
kolawole Reply
what is the effect of green house on the earth 🌎
kolawole
what's an atom?
Davy Reply
it's the smallest unit of Matter
Orsine
atom
Eden
smallest part of an element
lydia
also, depending on its (atom's) structure, that is the amount of protons and neutrons and electrons, is the determining factors of what element it is.
Richard
is a smallast particals of an element
Buwembo
it is the smallest part of an element that can take part in a chemical reaction
Ayub
is the smallest part of an element
Jonathan
An atom is the smallest indivisible part of a matter
kpadonu
Oy kl konsa test hay or kitna hay?
Faisal Reply

Get Jobilize Job Search Mobile App in your pocket Now!

Get it on Google Play




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?

Ask