Question

For each of the following use your knowledge of bonding theory and complete sentences to fully explain each fact provided. 1.ch3oh has a higher boiling point than h2co 2. mgcl2 has higher melting point than cacl2 3. mgs has higher melting point than nacl 4. c3h8 has a higher vapor pressure than liquid butane(c4h10) 5. molecular solids usually have lower melting points than ionic solids

Answer 1

1..Methanol has a higher boiling point than formaldehyde

Since methanol contains hydrogen atoms directly bonded to oxygen, methanol molecules form hydrogen bonds to one another. The hydrogen atom on one methanol molecule is attracted to the oxygen atom of its neighbor. The H in methanol's O-H bond is very strongly attracted to the O in a neighboring methanol's O-H bond; the boiling point will be very high. Molecules with H-F, H-O, or H-N bonds will exhibit hydrogen bonding.

This type of bonding is not present in formaldehyde, which make methanol high boiling than formaldehyde.

2. This is not correct statement, it should be Cacl2 has higher melting point than (Mgcl2 mgcl2 melting point 714 °C, Cacl2 melting point 772 °C)

In ionic compounds, the electrostatic interactions between cations and anions holds the compound together

The trend of increasing melting point and one goes down the periodic table is due to an increase in lattice energy. As you move down a group in the alkaline earth metals, the intermolecular forces between individual atoms increases. This is due to partially due to increased nuclear shielding preventing positive nuclei from getting close together (magnesium has a smaller electron cloud, so nuclei get closer together than calcium.)

The metallic lattice of calcium is stronger than in magnesium, meaning it takes more energy to break that lattice and allow the metal to enter a liquid state.

3. Mgs has higher melting point than NaCl-

Comparison of melting points of ionic compounds is generally done by considering the following two factors:

Charge of the cation/anion : More the charge of cation or anion, stronger will be the forces of attraction between the ions and higher will be the melting point.

Ionic radii: More the distance between ions, lesser will be the strength of the bond giving rise to lesser melting point.  

See the structure of Mgs- Mg2+ and S2-

NaCl- Na+, Cl-

The differenece is in number of cation and anion. Which leads to difference in melting point.

4.C3H8 has a higher vapor pressure than liquid butane(C4H10)-

First understand relation between vapour pressure and boiling point-

The substance with low boiling point will have high vapor pressure at certain temperature. This will give answer for this question.

5. Molecular solids usually have lower melting points than ionic solids-

In case of ionic compounds, there is a complete transfer of electrons from one atom of the element to another atom of the another element. So, there exists a strong electrostatic force. In case of covalent compounds, electrons are not completely dragged, the atoms of different elements are attracted by a weaker electrostatic force. Thus, ionic compounds have atoms bound by stronger electrostatic force than in case of covalent compounds. So, in general it is easy to break covalent bond than ionic. Thus, melting and boiling points of ionic compounds is greater than covalent compounds.