Hess’s Law

It is states that total enthalpy change that occur in physical & chemical process is the same whether the process takes place in 1, 2 or several steps.

Numerical- Enthalpy of a solution of an hydrous BaCL₂ & BaCL₂.2H₂O or -20.6 & 8.8 KJ per mole respectively. Calculate the enthalpy of hydration of BaCL₂ to BaCL₂.2H₂O  

ΔH = ΔH₁ + ΔH₂

ΔH - ΔH₂ = ΔH₁

-20.6 – 8.8 = ΔH₁

-29.4 J/mol = ΔH₁

Application of Hess’s law

When one mole of substance is formed from its constituent elements, enthalpy change is enthalpy formation

Calculation of enthalpy of formation-

The enthalpy of formation of hydrocarbon & carbon monoxide or many other compound calculated by using Hess’ law.

Spontaneous Process and  Spontaneity

Spontaneous Process are those process which takes place by itself without the assistance of external agency in respective of its rate is known as spontaneous process.

Examples of spontaneous Process

(1) Evaporation of water from ponds

(2) Evaporation of Nail Polish Remover (Acetone)

(3) Burning of coal

(4) Flow of water down the hill

(5) Decomposition of calcium carbonate (CaCo₃), in heating (1100 K heat provide) [require initiation]

(6) Diffusion of gases

Spontaneous Process are of two types

(1) Those require initiation. e.g.- Burning of Coal, Burning of candle Burning of methane gas.

(2) Those not require initiation. e.g.- Evaporation of water from pond

Flow of heat from hot body to cold body

Dissolution of Nacl in water

Flow of water down the hill

A process which neither takes place itself nor occur on initiation is known as a non-spontaneous Process.

For example- (1) Flow of water up a hill

(2) Dissolving clay in water

(3) Flow of heat from cold body to hot body.

Driving force for the Spontaneity of the Process

The force which is responsible for the spontaneity of a process is known as driving force.

(1). Energy factor:- Tendency towards minimum energy. It is a common observation that all the system in the universe tend to acquire a state of minimum energy. So that they have man stability.

For example- Heat flow from hot body to cold body.

Burning of coal

    H₂ + ½ O₂ à H₂O + Heat

But, it is observed that there are many process in which the energy absorption takes place, yet, the processes spontaneous.

For example- H₂O (L) → H₂O (g)

NH₄CL(s) + Heat→ NH₄⁺ (aq) + Cl^-(aq)   

(2). Tendency towards maximum randomness:- A process is said to be spontaneous if the randomness goes on increasing.

H₂O (L) → H₂O (g)

H₂O (S) → H₂O (l)

 

But, it is found that there are many process in which the randomness goes on decreasing, Yet the process is spontaneous.

For example- (1) Condensation of water vapour

(2) Snowfall

Hence, randomness factor is not only the sole criteria for the spontaneity of the process. But, out of these two, which one have larger magnitude is responsible for the spontaneity of the process