Define the following terms:
(a) Standard enthalpy of formation
(b) Bond enthalpy
( c) Zeroth law of thermodynamics
(d) Reversible and irrversible process

Explanation

Standard enthalpy of formation: The enthalpy change accompanying the formation of one mole of a compound from its constituent elements in their respective standard states is called the standard enthalpy of formation of that compound. It is denoted by `Delta_(f)H^(Θ)`.
The standard enthalpy change when one mole of water `(H_(2)O)` is formed by the combination of hydrogen and oxygen is called standard heat formation of water.
`Delta_(f)H^(Θ)(298K)=-286 kJ mol^(-1)`
b. Bond enthaply: The bond enthalpy is the amount of energy necessary to break bonds in mole of gaseous covalent substance to form products in the gaseous state. The bond enthalpy for `H_(2)` is `435.0 kJ mol^(-1)`
c. Zwroth law of thermody namics: According to it two system separately in equilibrium with a third system are also in equilibrium with each other.
Thus, if two system `A` and `B` are separately in equilibrium with a third system C, these are also in equilibrium with each other.
d. Reversible process: A process is said to be reversible un case it is carried out infinitesimally slowly so that all changes occurring in the direct process can be exactly reversed and the system and surroundings remain in a state of equilibrium at any stage in the process.
Irreversible process: A process in which the change is brought about rapidly and the system dies not have chance to achieve equilibrium. Actually, in an irreversible process, the force which drives the reactants towards the products (called driving force) is greater than the opposing force which is to carry the reverse process.