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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) CH3CH3(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) CO2(g) + 2H2O(l)|
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).
Exemplar equations -
|CH4(g) C(g) + 4H(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,
written by Dr Richard Clarkson : © Saturday, 1 November 1997
updated : Sunday, 30th October, 2005
mail to: firstname.lastname@example.org
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