Question

Intermolecular Forces and Physical Properties of Pure Substances Which has the highest enthalpy of fusion? Li2O MgO HCl H2O Which has the smallest vapour pressure at 25°C? H2O SO2 SiO2 CO2 Which has the least viscosity? CH3(CH2)3CH3 HOCH2CH2OH HOCH2CHOHCH2OH CH3(CH2)4CH3 Which has the lowest normal freezing point? NH3 H2 N2 CH4 Which has the lowest heat of vaporization? PH3 H2S NH3 AsH3

Answer 1

(1) HCl is gas, H₂O is liquid and MgO and Li₂O are solids. Stronger the intermolecular forces greater is the enthalpy of fusion.

(2) Smallest vapour pressure. SiO₂ is a solid and hence has lowest vapour pressure. Vapour pressure decreases as: Gases > Liquids > Solids

(3) VISCOSITY:

CH₃(CH₂)₃CH₃ , HOCH₂CH₂OH , HOCH₂CHOHCH₂OH , CH₃(CH2)₄CH₃

GREATER THE NUMBER OF HYDROXYL GROUPS IN A LIQUID GREATER WILL BE THE INTERMOLECULAR FORCE ( HYDROGEN BONDING), HENCE GREATER WILL BE THE VISCOSITY. THUS HOCH₂CHOHCH₂OH WILL HAVE HIGHER VISCOSITY.

(4) FREEZING POINT:

Smaller the intermolecular forces smaller will be the freezing point. Thus H2 will have minimum freezing point.

(5) PH₃ will have lower heat of vaporization. NH3 bonds are polar hence greater intermolecular force than PH3.