Chemistry Rules!'06 |
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How Far? How Fast? - Energy Changes |
An enthalpy change is defined as an energy change at constant pressure and a stated temperature (generally 1 atm and 298 K (r.t.p.)).
Enthalpy changes are given the symbol D H, with the D (delta) meaning change and H standing for enthalpy.
(2) Enthalpy change of Formation :The process when one mole of a compound is formed from its elements in their standard states (i.e. states at r.t.p.).
The enthalpy change of formation is given the symbol D Hf (or sometimes DfH).
N.B.: Because only one mole of a compound is to be made, equations may need to be balanced by using fractions e.g. 1/2.
Exemplar equation for the formation of ethane-
2C(s) + 3H2(g) ![]() |
Try balancing this equation for the formation of propan-1-ol,
and this equation for the formation of the amino acid glycine,
N.B.: Because the definition of formation requires elements to be in their standard states, the DHf value for all elements is 0 (zero).
(3) Enthalpy change of Combustion :The process when one mole of a compound is completely burnt in excess oxygen.
The enthalpy change of combustion is given the symbol D Hc (or sometimes DcH).
N.B.: Because only one mole of a compound is to be combusted, equations may need to be balanced by using fractions e.g. 1/2.
Exemplar equation for the combustion of methane -
CH4(g) + 2O2(g) ![]() |
Try balancing this equation for the combustion of butan-1-ol,
The process where one mole of gaseous atoms is formed from an element in its standard state or when one mole of a compound is turned into its constituent atoms.
The enthalpy change of atomisation is given the symbol D Hat (or sometimes DatH).
Na(s) ![]() |
Cl2(g) ![]() |
CH4(g) ![]() |
Hess' Law states that the value of the energy change for a reaction is independent of the route used to calculate the energy change.
So in the diagram below, the energy change for reactants A and B turning into products C and D is a constant, even if the route involves going through reactions A, B, C, D, E, F and G,
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written by Dr Richard Clarkson : © Saturday, 1 November 1997
updated : Sunday, 30th October, 2005
mail to: chemistryrules@rjclarkson.demon.co.uk
created with the aid of ChemWindow®5.1
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