If a solid line represents a covalent bond and a dotted line represents intermolecular attraction, which of the choices shows a hydrogen bond? 1. H−H
2. H4C⋯H−F
3. H3N⋯H−O−H
4. H2O⋯H−CH3

3

Explanation:

Hydrogen bonding occurs between molecules in which hydrogen is bonded to a highly electronegative atom such as nitrogen, oxygen, chlorine, etc.

In option 3, the two compounds are H3N and H2O. In both molecules, hydrogen is directly attached to highly electro negative atoms(nitrogen and oxygen respectively). Hence, it is only in option 3 that we observe hydrogen bonding.

the enthalpy of formation of water is given as -285.8kj/mol, which states that the enthalpy of formation is negative.

negative enthalpy of formation indicates that the heat is released during the reaction process. the heat is released from reaction to the surroundings the enthalpy is expressed with a negative sign with the energy given out in the reaction process.

the formation of water takes place by giving out heat which is -285.8 kj/mol.