91- 6303239042
The standard enthalpy of formation, $$\Delta H_{f}^{\circ}$$, is the enthalpy change when one mole of a compound is formed from its elements in their standard states (1 bar, usually 298.15 K):
$$\mathrm{elements\ (standard\ states)}\;\to\;\mathrm{1\ mol\ compound},\quad \lt br/\gt \Delta H = \Delta H_{f}^{\circ}$$
Example: formation of liquid water:
$$\tfrac12\mathrm{H_{2}(g)} + \tfrac12\mathrm{O_{2}(g)} \;\to\; \mathrm{H_{2}O(l)},\quad \lt br/\gt \Delta H_{f}^{\circ} = -285.8\;\mathrm{kJ\,mol^{-1}}$$
