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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 one moves from sodium to argon in the periodic table, the atomic size decreases. This is primarily due to the increasing number of protons in the nucleus which enhances the positive nuclear charge. The increased nuclear charge attracts the electrons more strongly, pulling them closer to the nucleus and reducing the atomic radius. Thus, the effect of the stronger nuclear pull outweighs the electron shielding effect, leading to a decrease in size.
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 because of its symmetrical tetrahedral structure, which means the polar bonds cancel out. Hexane, being a non-polar solvent, effectively interacts with silicon tetrachloride due to the principle 'like dissolves like'. Thus, the non-polar nature of both substances allows silicon tetrachloride to dissolve well in hexane.
Step 4
Explain fully, in terms of structure and bonding, why silicon nitride has a high melting point.
Answer
Silicon nitride (Si3N4) has a high melting point due to its covalent network structure. It consists of strong covalent bonds that extend throughout the solid, requiring a large amount of energy to break these bonds during melting. The presence of a covalent lattice structure also contributes to its high thermal stability, making it resistant to heat.
Step 5
Calculate the atom economy for the formation of silicon nitride.
Answer
The atom economy can be calculated by the formula:
ext{Atom Economy} = rac{ ext{Molar Mass of Desired Product}}{ ext{Total Molar Mass of Reactants}} imes 100
Given: Molar mass of Si3N4 = 140.3 g, The total mass of reactants is calculated as follows:
Molar mass of 35Cl = 35 * 1 = 35 g, Molar mass of 16NH3 = 16 + (3*1) = 19 g; So, Total = 35 + 19 = 54 g (approx)
Using the equation we find the atom economy:
ext{Atom Economy} = rac{140.3}{54} imes 100 = 259.26 ext{%} But the correct calculation is to use moles:
Total molar mass = 170 + 16 = 186 g, Final calculation gives atom economy of 0.179 or 17.9%.
Step 6
Complete a labelled diagram to show an apparatus suitable for carrying out this preparation.
Answer
The diagram should include a flask with a delivery tube connected to a heat source that passes hydrochloric acid over sodium hypochlorite, and a side arm for any escaping chlorine gas. Ensure proper labelling of each component such as flask, heat source, delivery tube, and any safety measures.
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 commence. Although the reaction is exothermic overall, sufficient energy is required to break any bonds in the reactants and to initiate the reaction; otherwise, the reaction might not start spontaneously at room temperature.