# What is the standard enthalpy of formation of water vapor?

The $\text{standard molar enthalpy of formation}$, $\Delta {H}_{\text{f}}^{\circ}$, is the enthalpy associated with the formation of $1 \cdot m o l$ of substances from its constituent elements in their standard states under standard conditions...........
${H}_{2} \left(g\right) + \frac{1}{2} {O}_{2} \left(g\right) \rightarrow {H}_{2} O \left(l\right) + \Delta {H}_{f}^{\circ}$.
I should make you dig for this, but in this scenario $\Delta {H}_{f}^{\circ} \left({H}_{2} O\right) = - 285.6 \cdot k J \cdot m o {l}^{-} 1$.........What does the negative value signify?