2 Chemistry -- Thermodynamics

Define enthalpy of a reaction. State and explain Hess's Law of constant heat summation.

1 Answers

The amount of heat evolved or absorbed in a chemical reaction when the number of moles of the reactants as represented by the chemical equation have completely reacted, is called the enthalpy of reaction.

Hess's law states that the change in enthalpy for a reaction is the same whether the reaction takes place in one or a series of steps.

Path I :

C H_{4} (g) + 2 C l_{2} (g) \to C H_{2} C l_{2} (g) + 2 H C l (g) Δ H_{10} = - 202.3 k J

Path II :

C H_{4} (g) + C l_{2} (g) \to C H_{3} C l (g) + H C l (g) Δ H_{20} = - 98.3 k J

C H_{3} C l (g) + C l_{2} (g) \to C H_{2} C l_{2} (g) + H C l (g) Δ H_{30} = - 104.0 k J

Adding two steps

C H_{4} (g) + 2 C l_{2} (g) \to C H_{2} C l_{2} (g) + 2 H C l (g) Δ H^{0} = - 202.3 k J

Thus whether we follow path I or path II, the enthalpy change of the reaction is same.

Δ H_{10} = Δ H_{20} + Δ H_{30} = - 202.3 k J

Define the following terms:[a] State variables of asystem [b] Internal energy of system

State variables of a system are physical quantities whose change in value only depends on initial and final state of system and doesn't follow on the path which system follows during change.

Internal Energy of a system is a sum of all kinds of energies possessed within it.

Calculate the free energy change and entropy change per mole when liquid water boils against 1 atmospheric pressure [ for water d H = 2.0723 kj/gm]

$Δ H_{v a p} o f w a t e r = 2.0723 18 = 37.30 K J m o l^{1}$

$Δ S_{v a p} = \frac{Δ H_{v a p}}{T_{b}} = \frac{37.30 K J m o l^{1}}{373 K} = 0.1 k J m o l^{1} K^{1}$

$Δ G_{v a p} = Δ H (k J m o l^{1}) T (K) Δ S (k J m o l^{1})$

$= 37.30 373 0.1 = 0$

Illustrate with an example what is meant by standard free energy change of formation of compound State the relation between standard free energy change and equilibrium constant of a chemical reaction.

Standard free energy of formation is the free energy change that occurs when 1 mole of a compound forms from its component elements at standard conditions.

The standard change in free energy, ΔG, for a reaction is related to its equilibrium constant, K, by the equation ΔG = -RTlnK.

Distinguish between intensive and extensive property with examples.

The differences between Intensive and Extensive properties are mentioned below.

Intensive property	Extensive property
It is independent of amount of system and depends on nature od system.	It is dependent on the amount of system.
Eg: Refractive index, Sp heat capacity	Eg: Mass, enthalpy

What is meant by state function? Give its example.

State function of a system is the thermodynamic function that depends on the state but not on the path along which the system is carried out. example: Pressure, Temperature, Volume.

Distinguish between enthalpy of combustion and enthalpy of formation.

Enthalpy of combustion	Enthalpy of Formation
It may be defined as the amount of heat released when one mole of the substance is completely burnt in the presence of oxygen.	It is defined as the change in the enthalpy during the formation of one mole of the substance from its constituents elements involved in the reaction.
It tells us about the amount of heat that is added or released during the chemical reaction.	It tells us about the change involved in the enthalpy during the...

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