Question #16100

Provide symbols (where appropriate) and definitions for the following terms:
(a) Enthalpy change
(b) Entropy change
(c) Free-energy change
(d) Spontaneity process
(e) Non-spontaneous process

Expert's answer

(a) An enthalpy change, ΔH, describes the change in enthalpy observed in the

constituents of a thermodynamic system when undergoing a transformation or

chemical reaction. It is the difference between the enthalpy after the process

has completed, i.e. the enthalpy of the products, and the initial enthalpy of

the system, i.e. the reactants.

(b) The entropy change of a system may

be defined as the integral of all the terms involving heat exchanged (q) divided

by the absolute temperature (T) during each infinitesimally small change of the

process carried out reversibly at constant temperature.

ΔS = Sfinal –

Sinitial = qrev/T

If heat is absorbed, then ΔS = +ve and if heat is evolved,

then ΔS = –ve

(c) The free energy change of a chemical process under

standard state conditions, ΔG, can be determined four different ways:

1. If

we know the standard free energy changes of formation, ΔG(f), of each species in

a change we can determine the standard state free energy change, ΔG, for the

change using the following equation:

ΔG = ΣΔG(products) -

ΣΔG(reactants)

2. If we know the enthalpy change, ΔH, and the entropy change,

ΔS, for a chemical process, we can determine the standard state free energy

change, ΔG, for the process using the following equation:

ΔG = ΔH - TΔS

3.

If we know the equilibrium constant, Keq, for a chemical change (or if we can

determine the equilibrium constant), we can calculate the standard state free

energy change, Go, for the reaction using the equation:

ΔG = - R * T *

lnKeq

4. If we know the standard state cell potential, Eo, for an

electrochemical cell (or if we can determine the standard state cell potential),

we can calculate the standard state free energy change, Go, for the cell

reaction using the equation:

ΔG = - n * F * Eo

(d) A spontaneous process

is a process that occurs in a system without any input of energy from the

surroundings.

(e) A nonspontaneous process is an unnatural process that

after it is started will not continue to happen without outside intervention. A

nonspontaneous process only happens when outside action introduces energy to

drive the process.

constituents of a thermodynamic system when undergoing a transformation or

chemical reaction. It is the difference between the enthalpy after the process

has completed, i.e. the enthalpy of the products, and the initial enthalpy of

the system, i.e. the reactants.

(b) The entropy change of a system may

be defined as the integral of all the terms involving heat exchanged (q) divided

by the absolute temperature (T) during each infinitesimally small change of the

process carried out reversibly at constant temperature.

ΔS = Sfinal –

Sinitial = qrev/T

If heat is absorbed, then ΔS = +ve and if heat is evolved,

then ΔS = –ve

(c) The free energy change of a chemical process under

standard state conditions, ΔG, can be determined four different ways:

1. If

we know the standard free energy changes of formation, ΔG(f), of each species in

a change we can determine the standard state free energy change, ΔG, for the

change using the following equation:

ΔG = ΣΔG(products) -

ΣΔG(reactants)

2. If we know the enthalpy change, ΔH, and the entropy change,

ΔS, for a chemical process, we can determine the standard state free energy

change, ΔG, for the process using the following equation:

ΔG = ΔH - TΔS

3.

If we know the equilibrium constant, Keq, for a chemical change (or if we can

determine the equilibrium constant), we can calculate the standard state free

energy change, Go, for the reaction using the equation:

ΔG = - R * T *

lnKeq

4. If we know the standard state cell potential, Eo, for an

electrochemical cell (or if we can determine the standard state cell potential),

we can calculate the standard state free energy change, Go, for the cell

reaction using the equation:

ΔG = - n * F * Eo

(d) A spontaneous process

is a process that occurs in a system without any input of energy from the

surroundings.

(e) A nonspontaneous process is an unnatural process that

after it is started will not continue to happen without outside intervention. A

nonspontaneous process only happens when outside action introduces energy to

drive the process.

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