AS unit how far how fast energy changes page

Chemistry Rules!^'06
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How Far? How Fast? - Energy Changes

Definitions

Enthalpy Change

Of formation
Of combustion
Of atomisation

Hess's Law

Calculations

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 H_f (or sometimes D_fH).

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) + 3H_2(g)

CH₃CH_3(g)

Try balancing this equation for the formation of propan-1-ol,

C_(s)	+	H_2(g)	+	O_2(g)		CH₃CH₂CH₂OH_(l)

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

C_(s)	+	H_2(g)	+	O_2(g)	+	N_2(g)		H₂NCH₂CO₂H_(s)

N.B.: Because the definition of formation requires elements to be in their standard states, the DH_f 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 H_c (or sometimes D_cH).

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 -

CH_4(g) + 2O_2(g)

CO_2(g) + 2H₂O_(l)

Try balancing this equation for the combustion of butan-1-ol,

CH₃(CH₂)₄OH_(l)	+	O_2(g)		CO_2(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 H_at (or sometimes D_atH).

Exemplar equations -

Na_(s)

Na_(g)

Cl_2(g)

2Cl_(g)

CH_4(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,