Chemistry - Q's that I need to work on

The increase in boiling point from CH4, SiH4, GeH4 and SnH4 is due to the increase in strength for which type of intermolecular force, hydrogen bonds, dipole-dipole forces or London dispersion forces?

London dispersion forces - These compounds are all non-polar, therefore they only have London dispersion forces. An increase in the molar mass or surface area of a molecule will result in an increase in the strength of the London dispersion forces. Going down Group 14, the central atom increases in size, therefore increasing the strength of the London dispersion forces, increasing the boiling point.

benzene ring

The special thing about benzene, the molecule in D, is the presence of alternating single and double bonds. Since they are located in this cyclic structure, the electrons in the double bonds are not confined to one location and are delocalised throughout the structure. Another way of looking at this is to see the ring as two different resonance structures. This delocalisation provides extra stability to the molecule and causes all of the carbon-carbon bond lengths to be the same, somewhere intermediate between the length of a single and double bond.

Which of the following metals has the highest melting point, Cu, K, Ba, Na?

Cu- Copper forms a 2+ ion but sodium and potassium only form 1+ ions. In addition, copper, being a transition element, is able to use its 3d electrons for bonding which further increases the strength of the metallic bond and the melting point.

Which of the following correctly identifies the general trends in melting point down Group 1 and Group 17?

Group 1 elements have metallic bonding. When metals melt, the metallic bonds between the lattice of positive ions and the delocalised electrons are starting to break. The larger the metal ions are within the metallic lattice, the weaker the attraction to the delocalised electrons. the larger the metal ion size, the weaker the metallic bond and therefore the lower the melting point. Therefore, travelling down Group 1 the melting point will decrease. Group 17 contains small covalent molecules. When small covalent molecules are melting, the intermolecular forces holding the molecule together will start to break. Travelling down Group 17, the molecule size increases. As the mass of a molecule increases, its polarisability increases, increasing the strength of the London dispersion forces between molecules. This means that the greater the size of the molecule, the stronger the London dispersion forces are between them and therefore the higher the melting point will be. Therefore, travelling down Group 17 the melting point will increase.

Identify which of the following Group 16 hydride compounds, H2O, H2S, H2Se or H2Te, would have the highest boiling point. Answer with the name of the compound.

H2O - The boiling point is determined by the strength of the intermolecular forces between molecules. All of these molecules are polar, so they have dipole-dipole forces in addition to the London dispersion forces present in all molecular compounds. Water is the only one of these molecules that also has hydrogen bonding, the strongest intermolecular force, therefore it has the highest boiling point.

Identify the hydrogen halide with the highest boiling point. HF (ANSWER)

HF has hydrogen bonding and London dispersion forces between molecules. The other hydrogen halides have only dipole-dipole and London dispersion forces between molecules. Hydrogen bonding is a much stronger intermolecular force to overcome, meaning more energy will be needed to separate molecules. This causes the boiling point of HF to be much higher than those of the other hydrogen halides.

You have samples of four different molecular compounds: ammonia (NH3), methane (CH4), silicon tetrahydride (SiH4) and phosphorus trihydride (PH3).

The boiling point is determined by the strength of the intermolecular forces between molecules. Ammonia is the only compound here that has hydrogen bonding, the strongest intermolecular force, therefore having the highest boiling point. Phosphorus trihydride is a weakly polar molecule, therefore having both London dispersion forces and dipole-dipole forces, making it have the second highest boiling point. Methane and silicon tetrahydride are both non-polar molecules, which only have London dispersion forces, the weakest intermolecular force. Since methane has a lower molar mass compared with silicon tetrahydride, it would have weaker London dispersion forces, making it have the lowest boiling point.