Spontaneous (irreversible) process

Do you know?

i. When aqueous NaOH and HCl solutions are mixed together, NaOH immediately combines with HCl to form NaCl and water: $\mathrm{NaOH(aq) + HCl(aq)} \rightarrow \mathrm{NaCl(aq) + H_2O}(l)$, $\Delta_r H^0 = -57$ kJ. No external force or energy is required for the reaction to occur — it is spontaneous, and the process stops when HCl or NaOH is consumed. When NaCl is dissolved in water, it does not react with water to produce NaOH and HCl back.

ii. Water flows from a higher level to a lower level. It is not necessary to apply external force — it is a spontaneous process. The flow ceases when the two levels become equal, or when the equilibrium is reached.

iii. Ice melts spontaneously above 0 °C.

iv. Hot coffee in a cup placed in a room cools down, releasing heat to the surroundings. This is spontaneous.