explain the difference in melting point in terms of intermolecular forces involved:

1) Hexane(C6H14) m.p. = -96*C) vs octadecane(C18H38) m.p. = 28 - 30*C)

2) toluene((Ph-CH3) m.p. = 5.5*C) vs benzoic acid((Ph-CO2H) m.p. = 122.4*C)

3) benzoic acid((Ph-CO2H) m.p. = 122.4*C ) vs sodium benzoate((Ph-CO2Na) m.p. = 300*C)

1. Longer chain means more tangling of the chain is possible.

Look up IM forces in the text or on the web and try the other two yourself.

To understand the difference in melting points of the given compounds, we need to analyze the intermolecular forces involved. The strength and type of intermolecular forces determine how tightly the molecules are held together and therefore influence the melting points.

1) Hexane (C6H14) and octadecane (C18H38):
Both hexane and octadecane are nonpolar hydrocarbons composed of carbon and hydrogen atoms. The primary intermolecular force in these substances is London dispersion forces, which are caused by temporary fluctuations in electron density. As the size and shape of the molecules increase from hexane to octadecane, the London dispersion forces also increase. Consequently, the molecules in octadecane are more attracted to each other, resulting in higher melting points compared to hexane.

2) Toluene (Ph-CH3) and benzoic acid (Ph-CO2H):
Toluene is a nonpolar molecule, while benzoic acid is a polar molecule due to the presence of a carboxylic acid group (-CO2H) which can form hydrogen bonds. The intermolecular forces in toluene are primarily London dispersion forces, similar to hexane. On the other hand, benzoic acid experiences both London dispersion forces and strong hydrogen bonding between the polar -CO2H groups. Hydrogen bonding is a stronger intermolecular force than London dispersion forces. Consequently, benzoic acid has higher melting points compared to toluene.

3) Benzoic acid (Ph-CO2H) and sodium benzoate (Ph-CO2Na):
Benzoic acid and sodium benzoate are both derivatives of benzoic acid, but sodium benzoate has an ionic structure due to the presence of sodium ion (Na+). Benzoic acid can form intermolecular hydrogen bonds, whereas sodium benzoate forms ionic bonds between the negatively charged CO2- ion and the positively charged Na+ ion. Ionic bonds are much stronger than hydrogen bonds, resulting in stronger intermolecular forces in sodium benzoate. Therefore, sodium benzoate has a significantly higher melting point compared to benzoic acid.

In summary, the difference in melting points between these compounds can be explained by the strength and type of intermolecular forces involved. As the strength of intermolecular forces (such as hydrogen bonding or ionic bonding) increases, the melting point also increases.