Chemistry Rules!'06

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How Far? How Fast? - Energy Changes

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Energy Changes - Definitions

(1) Enthalpy change :

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,

C(s) + H2(g) + O2(g) -----> CH3CH2CH2OH(l)

and this equation for the formation of the amino acid glycine,

C(s) + H2(g) + O2(g) + N2(g) -----> H2NCH2CO2H(s)

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,

CH3(CH2)4OH(l) + O2(g) -----> CO2(g) + H2O(l)

(4) Enthalpy change of Atomisation :

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 -

Na(s) -----> Na(g)
Cl2(g) -----> 2Cl(g)
CH4(g) -----> C(g) + 4H(g)

(5) Hess's Law :

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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Calculations

(1) Enthalpy change of combustion from DHf data :

(2) Enthalpy change of formation from DHc data :

(3) Enthalpy change for a reaction from DHf data :

(4) Enthalpy change for a reaction from DHc data :

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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 ChemWindow5.1


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