Chemical Thermodynamics

Spontaneous and non-spontaneous process

A natural phenomena which proceed without any external assistance is called spontaneous and tendency of a process to occur naturally is called spontaneity.

A process which occurs by the help of external assistance is called non-spontaneous process or un-natural process.

Examples in spontaneous rxn

Dissolution of salt in water. Flow from high altitude to lower. Rusting of iron when exposed to moist air. Heat flow from hot body to cold body. Rxn bet’n HCl & NaOH to give NaCl & H₂O etc.

Characteristics of spontaneous process

1. It is uni-directional process. It takes place in one direction.
2. Time has no role whether the rxn is fast or slow.
3. It will be spontaneous unless it reaches to equilibrium.

Entropy (S)

The thermodynamic parameter which measures the degree of disorderliness or randomness of the molecules of system is called Entropy. It is denoted by S.

It is express in terms of its change & given by:

It’s unit is J K⁻¹.

where,

• ΔS = change in entropy
• Q = Heat supply
• T = Temperature in kelvin scale

Entropy is state function therefore it’s value depends on initial & final state of a system.

Entropy of Rxn [ΔS_r]

The amount of entropy change when certain molar quantity of reactant are converted into product at given condition temperature & pressure. It is denoted by ΔS_r & given by:

where,

• ΔS_r = Entropy change of rxn
• ΔS_p = Entropy of product
• ΔS_r = Entropy of reactant

Types of Entropy of Physical Transformation

i) Entropy of Fusion [ΔS_fus]

The amount of entropy change when one mole of solid change into liquid at its melting point is called Entropy of Fusion. It is denoted by ΔS_fus & given by:

where,

• ΔS_fus = Entropy of Fusion
• ΔH_fus = Latent heat of Fusion
• T_m = melting point

ii) Entropy of vapourization [ΔS_vap]

The amount of entropy change when one mole of liquid change into vapour at its boiling point is called entropy of vapourization. It is denoted by ΔS_vap & given by:

where,

• ΔH_vap = Latent heat of Vapourization
• T_b = Boiling point

iii) Entropy of transition [ΔS_tran]

The amount of entropy change when any one allotrops of the substance change into another allotropic of same substance is called Entropy of transition. It is denoted by ΔS_tran & given by:

where,

• ΔH_tran = latent heat of transition
• T_t = transition temp

iv) Entropy of sublimation [ΔS_sub]

The amount of entropy change when 1 mole of solid change into vapour at its sublimation point is called Entropy of sublimation. It is denoted by ΔS_sub & given by:

where,

• ΔH_sub = Latent heat of sublimation
• T_sub = sublimation temp

Molar entropy change (ΔS_m)

The amount of entropy change by 1 mole of substance is called molar entropy change. Denoted by ΔS_m. It’s unit is J K⁻¹ mol⁻¹.

Effect of temp on entropy

Entropy is directly proportional to temp. With increase in temp increases K.E., velocity of molecule as well as arrangement of molecule or disorderness or randomness. Hence entropy also increases.

Physical significance

Entropy is the degree of randomness or disorderness. It is also used to measure the unavailable energy of system which is not used to do useful work.

The order of entropy in gas, liquid and solid is:

Features of entropy

• It is state function and extensive property.
• It is expressed in terms of its change i.e. ΔS.
• Entropy change in entropy is zero for cyclic process and at equilibrium.
• For a spontaneous process there always increase in entropy i.e. ΔS > 0 or ΔS = +ve.

Entropy and Spontaneity

• If ΔS_total > 0 i.e. ΔS_total = +ve then the process is spontaneous.
• If ΔS_total < 0 i.e. ΔS_total = −ve then the process is non-spontaneous.
• If ΔS_total = 0 then the process is cyclic or at an eqm.

Second law of thermodynamics

It states that, the entropy of universe [system + surrounding] continuously increase but total energy of universe remain.

Explanation of Second law of thermodynamics

Consider a spontaneous flow of heat from a system higher temperature to lower temperature i.e. surrounding. Temperature of system and temperature of surrounding are T_sys and T_surr respectively.

Let the system evolves a quantity of heat spontaneously to the surrounding.

Now, system decrease the entropy which is given by:

and surrounding increases the entropy which is given by:

Total entropy of universe:

Since,

Therefore:

ΔS_univ = +ve.

Total heat energy change = Q − Q = 0.

Hence, total energy of universe is unchanged whereas the total entropy of universe remain constant which is second law of thermodynamics.

Gibbs free energy change [ΔG]

It is the thermodynamic parameter which measures amount of energy available for doing useful work at constant temp and pressure. It is denoted by G and given by:

where, H, T and S are enthalpy, temperature in kelvin scale and S is entropy respectively. H, T and S are state functions then G is also state function.

This is the relation bet’n change in Gibbs free energy both change in enthalpy and change in entropy at constant temp and pressure called Gibbs-Helmholtz eqn.

Standard Gibbs free energy of formation (ΔG°_f)

The amount of free energy change when one mole of compound is formed from its elements in given condition of temperature and pressure.

Standard Gibbs free energy change (ΔG°_r)

The amount of Gibbs free energy change when certain molar quantities of reactant are converted into product at given condition of temp and pressure. It is denoted by ΔG°_r and given by:

where, ΣΔG°_f(product) = Sum of standard free energy change of formation of product.

ΔG°_f(reactant) = sum of standard free energy change of formation of reactant.

Relation bet’n standard free energy change and equilibrium constant

From eqn:

where,

• ΔG = Gibbs free energy change
• ΔG° = Standard Gibbs free energy change
• R = Universal gas constant
• T = Temp in kelvin scale
• Q = Rxn quotient

At eqm:

ΔG = 0 and Q is replaced by K where K is eqm constant.

This is the relation between standard Gibbs energy and eqm constant.

Relation bet’n Gibbs free energy change with total entropy

Let us consider a system which release heat energy to the surrounding spontaneously then total entropy change of universe is:

When system and surrounding is carried at same T:

Gibbs free energy change and criteria for spontaneity

1. If TΔS_univ > 0 then ΔG = −ve and the process is spontaneous.
2. If TΔS_univ < 0 then ΔG = +ve and the process is non-spontaneous.
3. If TΔS_univ = 0 then ΔG = 0 and the process is in equilibrium or cyclic process.

Relation bet’n free energy change and useful work

Let w be the total work done which is the sum of useful work (net work) and mechanical (experimental work).

Gibbs-Helmholtz eqn of direct chemical change for exothermic and endothermic Rxn

We know Gibbs Helmholtz eqn:

Given,

• ΔG = Gibbs free energy
• ΔH = enthalpy change
• T = Temp in kelvin scale
• ΔS = Entropy change

1) For exothermic Rxn [ΔH = −ve]

1. If ΔS = +ve then ΔG = −ve and the process is spontaneous.
2. If ΔS = −ve then ΔG = −ve or +ve and depends on temp:
   1. If temp is low, ΔG = −ve and process is spontaneous in given direction.
   2. If temp is high, ΔG = +ve and the process is non-spontaneous in the given direction.

2) For endothermic Rxn [ΔH = +ve]

1. If ΔS = −ve then ΔG = +ve and the process is non-spontaneous.
2. If temp is low, ΔG = +ve and the process is non-spontaneous in given direction.
3. If temperature is high, ΔG = −ve and the process is spontaneous in given direction.

Hence, exothermic rxn is favoured at low temperature but endothermic rxn is favoured on high temperature.

Copper

Uses

• It is used as an electrolytic soln in electroplating.
• It is used as preservative for timber.
• It is used as weedicide in water reservoirs and swimming pool.

Write the molecular formula of green-vitriol and mohr’s salt and also write its uses

Green-vitriol / hepta-hydrated ferrous sulphate: FeSO₄·7H₂O

• It is used as reducing agent.

Mohr’s salt: [FeSO₄·(NH₄)₂SO₄·6H₂O]

• It is used as reducing agent.
• It is used in volumetric analysis for standardization of KMnO₄ soln.