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2.
4.
3 Gibbs free energy of standard mole generation
Similar to the concept of standard heat of formation, if the standard Gibbs free energy of the designated element of each element in the standard state is set to 0, the standard Gibbs free energy of various substances can be defined
.
Chemical thermodynamics stipulates that the change in Gibbs free energy of 1 mol of a substance generated from the specified elemental substance of each element in the standard state at a certain temperature is called the standard moles of the substance generated Gibbs free energy at this temperature, referred to as the standard generation Free energy or free energy of formation is represented by the symbol △ r G m Θ , and the unit is kJ·mol -1
.
For example, reaction
C(graphite)+O 2 (g)=CO 2 (g) △ r G m Θ = -394.
4kJ·mol -1
Then the standard mole of gaseous CO 2 generates Gibbs free energy △ f Gm Θ = -394.
4kJ·mol -1
.
Similar to the standard heat of formation, the standard Gibbs free energy of formation △ f Gm Θ must also be a relative value, that is, the relative value of the specified elemental Gibbs free energy is 0
.
2.
4.
4 Calculation of standard Gibbs free energy change
1.
Obtain the Gibbs free energy from the standard Moore
Look up the table to obtain the standard moles of Gibbs free energy data for various substances, and use the standard moles to generate Gibbs free energy to calculate the reaction △ r G m Θ
[Example 2-9] Find the △ r G m Θ of the reaction ZnSO 4 (s) = ZnO (s) + SO 3 (g) at 298K .
A known
2.
Obtained by the Gibbs-Helmholtz formula
Definition by Gibbs Free Energy
G=H-TS
Gibbs-Helmholtz formula
△ r G m Θ =△ r H m Θ -T△ r S m Θ
The Gibbs free energy change △ r G m Θ of the reaction can be obtained from the melting change and entropy change of the reaction
.
According to the Gibbs-Helmholtz formula, temperature has a greater influence on the Gibbs free energy change of the reaction
[Example 2-10] The following thermodynamic data are known
Find the reaction 2NO 2 (g) = N 2 O 4 (g) at 318K and 338K △ r G m Θ
Reason
Get
by
Temperature has little effect on reaction enthalpy change and reaction entropy change, so the Gibbs-Helmholtz formula can be used to calculate △ r G m Θ at other temperatures
.
by
The Gibbs-Helmholtz formula combines the effects of △ r H m Θ and △ r S m Θ on the reaction direction.
According to the signs of the △ r H m Θ and △ r S m Θ boxes, △ r G m can be predicted The symbol of Θ can further predict the trend of the reaction
The above table shows that if △ r H m Θ product and △ r S m Θ have the same sign, the reaction direction is related to temperature; if △ r H m Θ product and △ r S m Θ have different signs, the reaction direction has nothing to do with temperature
.
As the temperature changes, the direction of the reaction may be reversed, that is, the sign of the Gibbs free energy of the reaction changes
.
According to the Gibbs-Helmholtz formula, the reaction reversal temperature can be obtained, that is, the temperature at which the Gibbs free energy of the reaction is zero
by