Where is the enthalpy change for lattice formation from
(Direct method) or through a number of steps (Indirect method). (b) Direct method: Let be the enthalpy of formation of 1 mol of NaCl(s) from its constituent species.Īccording to Hess's law, the enthalpy of formation of one mole of sodium chloride should be the same irrespective of the fact whether it takes place directly in one step. process is exothermic, therefore is always taken as a negative quantity. Since the process is exothermic, therefore electron gain enthalpy is taken as a negative quantity.Įnthalpy is released in this process i.e. Since the process is endothermic, therefore, ionisation enthalpy is taken as a positive quantity. Minimum the energy, Maximum the stability of the system. The process is endothermic, therefore is taken as a positive quantity. M +(g) + X (g) MX (s) (U -ve) When any system gets stability, it released energy. Since enthalpy is required for the process (endothermic), therefore is taken as a positive quantity. (a) Indirect method: Various terms involved are: Thus, we may conclude that negative value of ∆H is not the sole criterion for determining the spontaneity of the reaction.įormation of NaCl may be done by two different methods: There are reactions where ∆H = 0 but still they are spontaneous. (ii) Water evaporates with the absorption of heat.Īgain there are spontaneous reactions which do not go to completion even though ∆H throughout remains negative. (i) Ammonium chloride dissolves in water with the absorption of heat i.e. There are many endothermic reactions (∆H is +ve) which occur spontaneously. Thus, a tendency to acquire minimum energy (–∆H) may be taken as the cause of spontaneity.Īlthough for most of the spontaneous processes, ∆H is negative, yet it cannot be taken as the only criterion of spontaneity. Now a process in which energy of the system decreases has a negative value of ∆H (Exothermic reaction). In all these cases, the system tends to change from higher energy state to lower energy state. Through the decrease of energy, for example, No, it is a common observation that all systems tend to acquire a state of greater stability
0 kommentar(er)
