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The elements sodium to argon form the third period of the periodic table - Scottish Highers Chemistry - Question 2 - 2018
Question 2
The elements sodium to argon form the third period of the periodic table. (a) Explain the decrease in atom size going across the third period from sodium to argon. ... show full transcript
Worked Solution & Example Answer:The elements sodium to argon form the third period of the periodic table - Scottish Highers Chemistry - Question 2 - 2018
Step 1
Explain the decrease in atom size going across the third period from sodium to argon.
Answer
As we move across the third period from sodium to argon, the atomic size decreases due to the increasing number of protons in the nucleus, which results in a greater nuclear charge. This greater positive charge attracts the electrons more strongly, pulling them closer to the nucleus and leading to a decrease in atomic radius.
Step 2
Step 3
Explain fully why, of these three chlorides, silicon tetrachloride is the most soluble in hexane.
Answer
Silicon tetrachloride (SiCl₄) is non-polar due to its symmetrical tetrahedral structure, where the dipoles from silicon-chlorine bonds cancel out. Hexane is also non-polar, and therefore, according to the principle of 'like dissolves like', silicon tetrachloride is more soluble in hexane compared to the other polar chlorides.
Step 4
Explain fully, in terms of structure and bonding, why silicon nitride has a high melting point.
Answer
Silicon nitride (Si₃N₄) exhibits a covalent network structure, which means that it consists of a vast network of strong covalent bonds extending throughout the solid. Breaking these strong covalent bonds requires a substantial amount of energy, leading to a high melting point of 1900 °C. Moreover, the strong bonding interactions contribute to the overall stability of the compound in its solid state.
Step 5
Calculate the atom economy for the formation of silicon nitride.
Answer
To calculate the atom economy, we first determine the molar masses:
- Molar mass of Si₃N₄ = 3(28.09 g/mol) + 4(14.01 g/mol) = 140.19 g/mol
- Total molar mass of reactants = 35Cl + 16NH₃ = (35.453 g/mol + 16.042 g/mol) = 51.495 g/mol
Atom economy calculation:
ext{Atom Economy} = rac{ ext{Molar mass of desired product}}{ ext{Total molar mass of reactants}} imes 100
ext{Atom Economy} = \frac{140.19}{51.495} imes 100 = 271.31\%
Thus the atom economy is approximately 0.179 or 17.9% when calculated from the stoichiometry of the reaction.
Step 6
Complete a labelled diagram to show an apparatus suitable for carrying out this preparation.
Answer
A diagram illustrating a round-bottom flask connected to a delivery tube that goes into a gas washing bottle should be provided. Labelling should include hydrochloric acid, sodium hypochlorite solution, and an indication of heating with a Bunsen burner.
Step 7
Explain why the aluminium foil needs to be heated at the start of the preparation, despite the reaction being highly exothermic.
Answer
The aluminium foil must be heated initially to provide the necessary activation energy for the reaction to proceed. While the reaction is exothermic after initiation, the initial heat ensures that sufficient energy is supplied to overcome the activation barrier allowing the reaction between aluminium and chlorine gas to occur efficiently.