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The sign of enthalpy of formation of a highly stable compound is Negative.

(i) In process of evaporation, liquid changes to vapour state and disorder increases, ΔS = +ve.

(ii) In freezing process liquid changes into solid and disorder decreases; ΔS = -ve.

A system which can exchange matter as well as energy with its surroundings is called open system.

The presence of reactants in an open beaker is an example of an open system. A system which can exchange only energy but not matter with its surrounding is called a closed system.

Pressure cooker is an example of closed system. A system which can exchange neither energy nor matter with its surrounding is called an isolated system. The presence of reactants in a thermos flask is an example of an isolated system.

The internal energy is the total amount of kinetic and potential energy possessed by the system. It is denoted by the symbol U. The internal energy is made up of different kinds of energies like translational energy of molecules, rotational energy of the molecules, vibrational energy of the molecules, electronic energy, nuclear energy and gravitational energy. Internal energy of any substance depends on temperature and amount of the sample. 

The portion of enthalpy of a system that is available for useful work is called Gibbs free energy.

Gibbs equation:∆G = ∆H-T∆S

• It helps to calculate the enthalpies of formation of those compounds which cannot be determined experiementally. 
• It helps in determine the enthalpy of allotropic transformation like C (graphite) → C (diamond) 
• It helps to calculate the enthalpy of hydration.

A process in which there is no exchange of heat between the system and surrounding is known as adiabatic process.

The bond enthalpy is the amount of energy necessary to break bonds in one mole of a gaseous covalent substance to form products in the gaseous state.

State function is one whose value depends only on initial and final state of the system and is independent of the path followed. Examples are E, H, S, G, T, P i.e., internal.

The state of thermodynamic system may be defined by specifying the values of state variables like temperature, pressure, volume.

ΔS = {ΔH_fusion}/{Melting point of solid in kelvin}

ΔS must be positive.

A process is said to be non-spontaneous if it does not occur of its own under given condition and can occur only when an external force is continuously applied.

A process is said to be isothermal if the temperature of the system remains constant during each step of the process. 

A process which is carried out infinitesimally slowly so that it can be reversed at any instant of time by reversing the driving force by infinitesimal amount is called a reversible process.

A rapid process which cannot be reversed at any instant of time by reversing the driving force by small amount is called irreversible process.

(a) +ve (because gases are highly random than liquid)

ΔS = S(g) - S(e) = +ve

(b) +ve (because ions are in more random state in aqueous solution as compared to solid state).

ΔS = S(aq) - S(s) = +ve .

(c) -ve (because gases are highly more random than solid).

ΔS = S(s) - S(g) = -ve.

(d) zero, diamond and graphite both are solid and in crystalline state.

It states that the enthalpy change of the reaction remains the same whether the reaction is carried out in one step or several step i.e.....,

ΔH = ΔH₁ + ΔH₂ + ΔH₃ + ......

A reaction during which heat is evolved (∆H=-ve) is called exothermic reaction.

A reaction during which heat is absorbed (∆H=+ve)is called endothermic reaction. 

ΔU will become -ve, i.e., internal energy will decrease.

Because it depends only on the state of system and not upon the oath how the system attains that state.

Entropy change (ΔS) is positive. It is because disorder or degree of freedom increases on mixing.