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The enthalpy change depends upon the following factors:

(i) Chemical composition of the reactants:- The amount of heat released or absorbed depends upon the strength of the chemical bonds of the products relative to those of the reactants.

(ii) Amount of the reactants:- The value of heat change in a reaction depends upon the amounts of the reactants. The value of ΔH is expressed in such away that it indicates the amount of material as well as quantity of heat.

(iii) Conditions of a reaction:- The amount of heat change in a reaction also depends upon the physical state of the reactants as well as products. Therefore, it is essential to indicate the physical state of every reactant and product of a reaction.

Phase changes involve energy changes. Ice, for example, requires heat for melting. Normally this melting takes place at constant pressure (atmospheric pressure) and during phase change, temperature remains constant (at 273 K).

H₂O (s) → H₂O (l);

Δ_fusH^Θ = 6.00 kJ mol^-1

Here Δ_fus H^Θ is enthalpy of fusion in standard state. If water freezes, then process is reversed and an equal amount of heat is given off to the surroundings, that is

Δ_freezeH^Θ = -Δ_fus H^Θ

This enthalpy is called enthalpy of fusion or molar enthalpy of fusion.

Δ_fus H^Θ = H^Θ_liquid - H^Θ_solid

Melting of a solid is endothermic, so all enthalpies of fusion are positive.

Amount of heat required to vaporize one mole of a liquid at constant pressure and temperature is called its enthalpy of vaporization or molar enthalpy of vaporization. Here constant temperature is its boiling point, T_P.

In case of water vaporization

H₂O (l) → H₂O (g); Δ_vapH^Θ = + 40.79 kJ mol^-1

Sublimation is a direct conversion of a solid into its vapour. Solid CO₂ or 'dry ice' sublimes at 195 K with Δ_subH^Θ = 25.2 kJ mol^-1, napthalene sublimes slowly and for this Δ_subH^Θ = 73.0 kJ mol^-1.

Because enthalpy is a state property, the enthalpy of sublimation can be expressed as

Δ_subH^Θ = Δ_fusH^Θ + Δ_fusH^Θ.

Extensive properties:- The property whose value depends upon the quantity of matter contained in any system is called extensive property. Thus, an extensive property depends upon the size of the system. The properties such as mass, volume, area, energy, heat capacity etc, are extensive properties.

Intensive properties:- The property whose value depends upon the nature of the substance, and in independent of its amount in the system is called an intensive property. Thus, an intensive property does not depend upon the size of the system. The properties such as density, temperature, pressure, concentration, viscosity, refractive index, surface tension, etc. are some intensive properties.

The equation ΔE = q + W is a mathematical statement of the first law of thermodynamics. In this equation ΔE is the increase in the internal energy of the system, q is the heat absorbed by the system and W is the work done on the system. Delta (Δ) is not used before q and W, because these depend on path and there is no initial and final heat or work on the system. The concept of heat or work comes into picture only during the change. If there is no change, there is no heat or work. But the system has energy.

Molar heat capacity, cm = Specific heat capacity x molar mass

therefore, cm (ethanol) = 2.46 Jg^-1°C^-1 x 46 g mol^-1

= 113.2 J mol^-1°C^-1

The molar heat capacity of ethanol is 113.2 J mol^-1°C^-1

(or 113.2 J mol^-1 K^-1).

ΔE is regarded as state function because it depends upon initial and final states of the system are not on the path e.g., CO₂ obtained from various methods has some value of ΔE.

Heat (g) and work (w) are not the state functions because they are path dependent.

Reaction are feasible in the direction of decrease in energy. Thus the reactions which proceed with a decrease in enthalpy (ΔH_negative) should be spontaneous. This is found to be so in many cases. However, there are some spontaneous reactions should process with an increase in entropy. Once again there are many spontaneous reactions which proceed with a decrease in entropy (ΔS_negative).

Thus, it appears which explains the spontaneity of any reaction is called free energy change ΔG. This is related to ΔH and ΔS by the reaction.

ΔG = ΔH - TΔS

The reactions which proceed with negative ΔG are spontaneous.

Data : I = 2 A, R = 200 Ω, t = 10 min = 10 × 60

s = 600 s, M = 5 kg, S = (1 kcal/kg) (4186 J/kcal) 

= 4186 J/kg

Q = ∆ U + W = MS ∆T + W ≃ MS ∆T ignoring W..

Also, Q = I² Rt 

∴ I² RT = MS∆T

∴ The rise in the temperature of water = ∆T = \(\frac{I^2Rt}{MS}\)

= \(\frac{(2)^2(200)(600)}{(5)(4186)}\)°C = 22.93°C

Zero. Because, the heat absorbed and change in internal energy are zero.

Relation between molar specific heat at constant pressure and that at constant volume for an ideal gas is,

C_p – C_v = R,

where C_p is the molar specific heat at constant pressure, C_v is the molar specific heat at constant volume and R is the Universal gas constant.

Zero. Because as the heat absorbed by the gas is entirely used to raise its internal energy.

A system is said to be in thermal equilibrium if the macroscopic variables that characterise the system to not change whit time.

Zeroth law of thermodynamics states that – two systems in thermal equilibrium with a third system separately by are in thermal equilibrium with each other.

An isotherm is a P – V curve for a fixed temperature.