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Zadanie 3b

5 pktotwarteśrednie (szac.)Paper 3, maj 2023, TZ2

Wzorowane na: Chemistry HL Paper 3, May 2023, TZ2, pytanie 3(b). Treść, dane i kontekst są zmienione; sprawdzane umiejętności, format i punktacja jak w oryginale.

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Polecenie

Beryllium metal is produced by the electrolysis of beryllium chloride, BeCl2, dissolved in molten sodium chloride.

(i) Write the half-equation for the process at the electrode where beryllium metal is produced.

(ii) Calculate the atom economy of this process for making beryllium from beryllium chloride. Assume that neither product reacts with the electrodes. Use section 1 of the data booklet.

(iii) Suggest one factor, apart from atom economy, which shows that producing beryllium from its ore has a major environmental impact.

(iv) Pure molten beryllium chloride is a very poor conductor of electricity, which is why sodium chloride is added. Deduce why pure molten beryllium chloride conducts poorly. Use sections 9 and 17 of the data booklet.

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Odpowiedź z klucza

(i) Be2+ + 2 e− → Be ✔

(ii) «9.019.01+2×35.45×100=9.0179.91×100=\dfrac{9.01}{9.01 + 2 \times 35.45} \times 100 = \dfrac{9.01}{79.91} \times 100 =» 11.3 «%» ✔

(iii) any one of:

high energy consumption «for the electrolysis» ✔

OR

chlorine, a toxic gas, is formed as the by-product ✔

OR

large amounts of waste «from mining and purifying the ore» ✔

OR

beryllium compounds/dust are toxic ✔

OR

greenhouse gases/pollution from transport/machinery/electricity generation ✔

(iv) average electronegativity «=1.6+3.22== \dfrac{1.6 + 3.2}{2} =» 2.4 AND electronegativity difference «= 3.2 − 1.6 =» 1.6 ✔

the compound plots on the boundary between the ionic and polar covalent regions «about 50 % covalent», so the bonding is not fully ionic and the melt contains few free ions ✔

Schemat punktowania

(i) [1]: do not penalize an equilibrium arrow; ignore state symbols. (ii) [1]: accept 0.113 or 11.28 %. (iii) [1]: any one factor. (iv) M1: both the average electronegativity and the difference. M2: the position at the border of ionic and polar covalent bonding (or partial covalent character) linked to the lack of free ions. Accept "high covalent character" for M2. Award [2] for a correct calculation of about 50 % covalent character with the conclusion.

Umiejętności w zadaniu