Chem Chapt 3: Periodicity

Outline the features of period 3 elements(Magnesium, sodium and aluminium)

Magnesium burns with a white fume, sodium forms an external layer of NaO when exposed to air. Aluminium also forms a tough layer of Al2O3 in air, and is often used in aircrafts and cans as they are tough and lighweight

Outline the features of period 3 elements(Phosphorous, sulfur and chlorine)

Phosphorous can exist as P4 which is white or just P which is red. Sulfur exists as S8 and is a bright yellow crystalline solid and chlorine is a strong oxidising agent

Outline the trend of ionic radii, atomic radii and 1st IE for period 3 elements

They generally decrease across the period as number of protons increases while inner shell electrons remain the same, hence shielding effect remains the same, this leads to a higher effective nuclear charge and hence stronger electrostatic forces of attraction between nucleus and valence electron

For MgCl2 and NaCl, explain their melting point

NaCl will have a higher melting point as Mg2+ has a higher ionic charge and smaller ionic radius and will hence have higher charge density and polarising power than Na+, conferring more covalent character for MgCl2 which weakens its ionic bonds

Outline what happens to AlCl3 at different temperatures and why AlCl3 has simple molecular structure

At 180C, AlCl3 will exist as a dimer, Al2Cl6, held by weak idid. At higher temperatures, it will dissociate to give AlCl3 which has a simple molecular structure. It is because Al3+ has high charge density and polarising power, which confers significant covalent character

Outline the products and pH when Na2O and MgO reacts in water respectively

NaOH, pH 14. Mg(OH)2, pH 9

Outline why Al2O3 does not react with water

It has high lattice energy that requires large amount of energy to overcome the strong ionic bonds.

Outline why Na2O and MgO does not react with NaOH and why Al2O3 does. state the products formed

Na2O and MgO are basic oxides whereas Al2O3 is amphoteric. It will react with NaOH to form H2O and NaAl(OH)4

Outline the products for when P4O10 and SO3 dissolves in water respectively

H3PO4, pH2 and H2SO4, pH 2

Define hydration

It is the physical attraction between the cations and anions with the negative dipole and positive dipole of the water molecules respectively, ie ion dipole interactions

Define hydrolysis

It is the chemical reaction between the cation or covalent chloride and water molecules such that the O-H bond in water is cleaved to form H3O+ ions

Outline the resulting pH of NaCl, MgCl2 and AlCl3 after hydrolysis and hydration(same pH)

7 for NaCl, 6.5 for MgCl2 and 3 for AlCl3

Explain why [Na(H2O)6]+ does not undergo hydrolysis, [Mg(H2O)6]2+ only undergoes slight hydrolysis and [Al(H2O)6]3+ undergoes full hydrolysis

This is because the relatively higher charge density of the metal cations will polarise the electron cloud of the surrounding water molecules and weaken and break the O-H bond more easily to release a proton.

Both AlCl3, SiCl4 and PCl5 does not undergo hydration but undergoes hydrolysis. Outline why they are able to do so

Si and P have vacant and energetically accessible 3d orbitals while Al has energetically accessible 3p orbitals to accept the lone pair from water molecules to form a dative bond

Explain why CCl4 does not hydrolyse in water

They do not have energetically accessible and vacant orbitals for dative bonding with water molecules. It is also small in size and is hence shielded by larger Cl atoms and there will hence be steric hindrance

Outline the products, observations and pH of the hydrolysis of AlCl3, SiCl4 and PCl5 respectively

Al(OH)3 or Al2O3, white solid and white fumes of HCl are observed, pH is 3. SiO2, white solid and white fumes of HCl are observed, pH is 2. H3PO4, white fumes of HCl are observed. pH is 2

Define why diagonal relationships exist in the second and third period

Diagonally adjacent elements have similar electronegativities since it increases across the period and decreases down the group. Their cations have similar charge densities too.

Outline the diagonal relationship that Be and Al

Both BeCl2 and AlCl3 form aqua complex ions that hydrolyses to give an acidic solution. Both BeO and Al2O3 are amphoteric oxides too

Outline the diagonal relationship between Li and Mg

Both LiNO3 and Mg(NO3)2 decomposes on heating to give metal oxide, NO2 and O2. Li and Mg hydoxides and carbonates are both thermally unstable and will decompose when heated

Outline why the thermal stability of group 2 carbonates increases down the group

Down the group, both nuclear charge and shielding effect increases but the valence electron added is located at a principal quantum shell of a larger number, hence electrostatic forces of attraction between nucleus and valence electron decreases. Size of cation increases and charge density of cation decreases. Hence electron cloud is polarised to a lesser extent and C-O bond is weakened to a lesser extent

Outline the colour and state of Cl2, Br2 and I2 at rtp

Greenish yellow gas, reddish-brown liquid and black solid

Outline the colour of Br2 in aqueous state and when in organic solvent

Orange in aqueous states and orange red in organic solvents

Outline the colour of Cl2 in aqueous and in organic solvents

Both pale yellow