Question

1. The dissolution of water in octane (C8H18) is prevented by ________. A) London dispersion forces between octane molecules B) hydrogen bonding between water molecules C) dipole-dipole attraction between octane molecules D) ion-dipole attraction between water and octane molecules E) repulsion between like-charged water and octane molecules
2. For a substitutional alloy to form, the two metals combined must have similar ________. A) ionization potential and electron affinity. B) number of valance electrons and electronegativity. C) reduction potential and size. D) atomic radii and chemical bonding properties. E) band gap and reactivity.
3. As a polymer becomes more crystalline, ________. A) its melting point decreases B) its density decreases C) its stiffness decreases D) its yield stress decreases E) None of the above is correct.
4. List the substances CCl4, CBr4, and CH4 in order of increasing boiling point and give the reason.

Thanks a lot

Answer 1

1) B

Hydrogen bonding between water molecules

Explanation:

Octane (C8H18) is a non-polar molecule while water (H2O) is a polar molecule. According to the principle of like-dissolves-like, water and Octane are not soluble in each other due to different nature.

But what is the underlying reason behind this insolubility?

In order to dissolve octane in water, the water molecule must move further and accommodate octane molecules. However, the energy required by water molecules to move further cannot be provided by octane molecules. This is because London dispersion forces exist between water and octane molecules and they are very weak than hydrogen bonding. Therefore, octane is not dissolved in water molecules.

2) D

Atomic radii and chemical bonding properties.

Explanation:

When two molten metals are mixed with each other, they can combine by two ways:

• Atom exchange
• Interstitial mechanism

By definition, substantial alloys are those which are formed by the atom exchange method. The sizes of the atoms are extremely important in determining which particular way the metals will combine. If atoms are of relatively similar sizes, some atoms of one metal crystal can be easily exchanged with other’s crystals. For example, the substantial alloys formed from the mixing of brass and bronze, where some of the copper atoms are replaced with tin or zinc atoms.

Similarly, chemical bonding between two metals is also an important factor for the mixing of atoms to form alloy.  The chemical bonding depends on the covalent bonding between two atoms and sharing of metallic electrons. As metals contain free electrons, they share the more easily they share the electrons with each other; the stronger will be the bonding between them. Excellent chemical bonding between the metals results in the formation of excellent substantial alloys.

3) E

None of the above

Explanation:

All polymers cannot undergo crystallization due to their specific structure and properties. But some types of polymers like polyethylene (PE) can go through crystallization. The process of crystallization depends on two factors:

• Time  
• Cooling rate
• Type of polymers

Polymers do not become 100 % crystalline and some part is melted while other becomes solid. The part which becomes crystalline contains ordered arrangement of polymer chains. So when the ordered structure is formed, polymers chains arrange to form bigger crystals. Now it’s easy to infer that these crystals are not easily melted and have high melting temperature  

Crystalline structures are formed due to stiff chains of polymer are strong intermolecular forces. The ordered arrangement of polymer chains enhances the secondary bonds between chains which increases the overall density of the crystal. Therefore, high crystallinity enhances the density as well as overall stiffness of the structure of polymers.

The crystals of polymers can also be formed by stretching them to highest point. In this process, crystals are formed because molecules are stretched in the direction of stretch.  The crystal is formed when polymers are stretched beyond their yield point. Therefore, all the strain during the process increases the stress on polymer.

Therefore, the best suitable choice is E, None of the options is correct.

4) The order of the molecules based on the increasing boiling points, will be as:  

                                   CH4 <CCl4< CBR4

Explanation:

The boiling point depends upon the London dispersion force operating between molecules. These forces are the very weak and temporary, which arise when electrons of two adjacent atoms position themselves in such a way that dipoles are formed. They are also referred to as induced dipole forces because they are produced one moment and vanished other moment.  

If we talk about methane (CH4), here carbon is attached to the four atoms of hydrogen to form a small molecule. The ability of Hydrogen to attract the neighboring carbon’s atoms is very less therefore the London dispersion forces are very weak. As a result, CH4 has lowest boiling point than CCl4 and CBR4 due to smaller molecular size and weaker intermolecular forces respectively.

The molecule of carbon-tetra chloride (CCl4 )is comparatively larger in size. Chlorine also has far more electronegativity (ability to attract electrons)  and bigger size than hydrogen atoms. Therefore, the large size of chlorine will help attract the electron and produce relatively stronger London dispersion forces as compared to the CH4. So, the boiling point of CCl4 is greater than CH4.

Applying same criteria on carbon-tetra bromide (CBr4), it has the highest boiling point among CCl4,CH4 and CBR4. This is because bromine is greater in size than hydrogen as well as chlorine. It has the greatest ability to attract the electrons of neighboring carbon atom. This will create the relatively strongest Londen dispersion forces between the molecules of CBR4.