Môn Hóa Lý: Chapter 7 application of TMD laws

Hess's law states that the enthalpy or heat change accompanying a chemical reaction is independent of the pathway between the initial and final states.In other words, if a chemical chang

Dr Ngo Thanh An

PHYSICAL CHEMISTRY 1

Chapter 7 – Applications of TMD laws

Hess's law states that the enthalpy or heat change accompanying a chemical reaction is independent of the pathway between the initial and final states.

In other words, if a chemical change takes place by several different routes, the overall enthalpy change is the same, regardless of the route

by which the chemical change occurs (provided the initial and final condition are the same).

Hrev = – Hirrev

The standard enthalpy of formation or standard heat of formation, H f o

of a compound is the change of enthalpy from the formation of 1 mole of the compound from its elements, with all substances in their standard states

at 101.3 kPa and 298 K

Note: is the mathematical symbol meaning “the sum of”, and m and n

are the coefficients of the substances in the chemical equation.

) reactants (

H m

) products (

H n

Standard enthalpy changes of combustion, ΔH°c

Heat of Combustion of a substance is the heat liberated when 1 mole of the substance undergoes complete combustion with oxygen at constant pressure

Application of Hess’s law

1 Hess’s law

 Standard enthalpy of formation for a pure element in its

standard state is zero (O 2 , H 2 , Al …) is zero

 Standard enthalpy of combustion for oxide with the

highest oxidation number is zero

Attention:

1 Hess’s law

1 Hess’s law

 If CP =

const:  HT2   HT1   C TP  2  T1 

2 Effect of temp on heat of rxn

For a reaction: N2 + 3 H2 = 2NH3

Heat of reaction at 25oC is –22,08 Kcal Determine HT

= f(T) and calculate heat of reaction at 1000K

(S max ): Equilibrium (reversible)

3 Criteria for determining direction of process

dU= TdS – P.dV – A’max

For a reversible process

3 Criteria for determining direction of process

• An irreversible process (spontaneously occur):

• At equilibrium: G reach minimum

dG < – A’ < 0

dG =0 và d2G > 0

G decrease

For an isothermal and isobaric process

3 Criteria for determining direction of process

• Entropy

• Enthalpy

• What ties the two together is:

Gibbs Free Energy.

What factors determine the spontaneity of a rxn?

3 Criteria for determining direction of process

• Predicts the direction of a spontaneous reaction.

• Uses properties of the system to calculate

• For a constant pressure-temperature process:

• G = 0 The reaction is at equilibrium

Gibbs free energy

3 Criteria for determining direction of process

Gibbs free energy

3 Criteria for determining direction of process

• In order to make use of Gibbs energies to predict chemical changes, we need to know the free energies

of the individual components of the reaction For this purpose we can combine the standard enthalpy of formation and the standard entropy of a substance to

get its standard free energy of formation

ΔGf° = ΔHf° – TΔSf°

Recall that the symbol ° refers to the standard state of a substance measured

under the conditions of 1 atm pressure or an efective concentration of 1 mol L     –1

and a temperature of 298K.

Gibbs free energy

3 Criteria for determining direction of process

It's not necessarily "free"

• The appellation “free energy” for G has led to so much confusion that many scientists now refer to it simply as the Gibbs energy The “free” part of the older name reflects

the steam-engine origins of thermodynamics with its interest in converting heat into

work: ΔG is the maximum amount of energy which can be “freed” from the system to

perform useful work.

It's not even "real"!

• G differs from the thermodynamic quantities H and S in another significant way: it has

no physical reality as a property of matter, whereas H and S can be related to the

quantity and distribution of energy in a collection of molecules

• The free energy is simply a useful construct that serves as a criterion for change and makes calculations easier

It's not energy!

• A much more serious difficulty with the Gibbs function, particularly in the context of

chemistry, is that although G has the units of energy (joules, or in its intensive form,

J mol –1 ), it lacks one of the most important attributes of energy in that it is not

conserved

Some remarks on the Gibbs free energy

3 Criteria for determining direction of process

The standard free-energy of reaction (G 0 ) is the

free-energy change for a reaction when it occurs under state conditions

standard-rxn

Standard free energy of

formation (G0) is the free-energy

change that occurs when 1 mole

of the compound is formed from its

elements in their standard states

f

G0 rxn = nG0f (products) - mG0f (reactants)

Standard free-energy change:

3 Criteria for determining direction of process

Standard free-energy change:

3 Criteria for determining direction of process

Factors affecting G

3 Criteria for determining direction of process

1 Gibbs – Helmholtz equation

Describing effect of temperature on G function

Integrating from upper bound T1 to lower bound T2, we have:

Applying for n mol of ideal gas: V  nRT

Chemical potential and partial molecular quantity

6 Chemical potential

The last term is chemical potential

(partial molecular quantity:

Partial molecular quantities:

Quantities G, F, S, V, H, U,… are symbolized as X

6 Chemical potential