Question

Which member in each pair of liquids has the lower vapour pressure at a given temperature? Explain.

(a) ${\mathbf{HOCH}}_{\mathbf{2}}{\mathbf{CH}}_{\mathbf{2}}\mathbf{OH}$or${\mathbf{CH}}_{\mathbf{3}}{\mathbf{CH}}_{\mathbf{2}}{\mathbf{CH}}_{\mathbf{2}}\mathbf{OH}$

(b) ${\mathbf{CH}}_{\mathbf{3}}\mathbf{COOH}$or${\left({\mathbf{CH}}_{\mathbf{3}}\right)}_{\mathbf{2}}\mathbf{C}\mathbf{=}\mathbf{O}$

(c) HF or HCl

Short answer

1. ${\mathbf{HOCH}}_{\mathbf{2}}{\mathbf{CH}}_{\mathbf{2}}\mathbf{OH}$or${\mathbf{CH}}_{\mathbf{3}}{\mathbf{CH}}_{\mathbf{2}}{\mathbf{CH}}_{\mathbf{2}}\mathbf{OH}$ : ${\mathbf{HOCH}}_{\mathbf{2}}{\mathbf{CH}}_{\mathbf{2}}\mathbf{OH}$vapour pressure is lower than the ${\mathbf{CH}}_{\mathbf{3}}{\mathbf{CH}}_{\mathbf{2}}{\mathbf{CH}}_{\mathbf{2}}\mathbf{OH}$.
2. ${\mathbf{CH}}_{\mathbf{3}}\mathbf{COOH}$or ${\left({\mathbf{CH}}_{\mathbf{3}}\right)}_{\mathbf{2}}\mathbf{C}\mathbf{=}\mathbf{O}$:${\mathbf{CH}}_{\mathbf{3}}\mathbf{COOH}$ vapour pressure is lower than the ${\left({\mathbf{CH}}_{\mathbf{3}}\right)}_{\mathbf{2}}\mathbf{C}\mathbf{=}\mathbf{O}$.
3. HF or HCl: HF vapour pressure is lower than the HCl.

Step-by-step solution

Intermolecular Forces 

Intermolecular forces are the interaction which are formed by the attraction of the two having opposite charges (partial positive and partial negative charge). The opposite charge are formed by the presence of the electron-negative atom in the molecule. Due to the presence of an electron-negative atom, there is and induced partial positive charge is generated on the electron-positive charge (or less electron-negative charge) atom. There will be an attraction between the both oppositely charges to form a bond.

Vapour pressure may be defined as the pressure exerted by the vapour over a certain area. The vapour pressure is inversely proportional to the intermolecular forces.

Vapour Pressure Explanation

(a)${\mathbf{HOCH}}_{\mathbf{2}}{\mathbf{CH}}_{\mathbf{2}}\mathbf{OH}$or ${\mathbf{CH}}_{\mathbf{3}}{\mathbf{CH}}_{\mathbf{2}}{\mathbf{CH}}_{\mathbf{2}}\mathbf{OH}$:Hydrogen bond is the bond between the electronegative atom and electropositive atom. The molecule ${\mathbf{CH}}_{\mathbf{3}}{\mathbf{CH}}_{\mathbf{2}}{\mathbf{CH}}_{\mathbf{2}}\mathbf{OH}$shows hydrogen bonding between electronegative O-atom and electropositive H-atom whereas in the ${\mathbf{HOCH}}_{\mathbf{2}}{\mathbf{CH}}_{\mathbf{2}}\mathbf{OH}$molecule there is hydrogen bonding from both sides of the molecule. As the hydrogen bonding is greater than in ${\mathbf{HOCH}}_{\mathbf{2}}{\mathbf{CH}}_{\mathbf{2}}\mathbf{OH}$, due to the more force of attraction the vapour pressure is lower.

(b)${\mathbf{CH}}_{\mathbf{3}}\mathbf{COOH}$or ${\left({\mathbf{CH}}_{\mathbf{3}}\right)}_{\mathbf{2}}\mathbf{C}\mathbf{=}\mathbf{O}$:Hydrogen bond is the bond between the electronegative atom and electropositive atom. The molecule ${\left({\mathbf{CH}}_{\mathbf{3}}\right)}_{\mathbf{2}}\mathbf{C}\mathbf{=}\mathbf{O}$shows hydrogen bonding between electronegative O-atom and electropositive H-atom whereas in the ${\mathbf{CH}}_{\mathbf{3}}\mathbf{COOH}$molecule there is hydrogen bonding from both sides of the molecule. As the hydrogen bonding is greater than in , ${\mathbf{CH}}_{\mathbf{3}}\mathbf{COOH}$due to the more force of attraction the vapour pressure is lower.

(c) HForHCl: The hydrogen bond is the bond between the electronegative atom and the electropositive atom. The molecule HCl shows hydrogen bonding between electronegative O-atom and electropositive H-atom whereas in the HF molecule there is hydrogen bonding from both sides of the molecule. As the hydrogen bonding is greater than in HF, due to the more force of attraction the vapour pressure is lower.