(a) (i) Name three different methods for preparing salts.
(ii) Give one example of a balanced equation for each of the methods named in (a)(i).
(iii) State two uses of sodium trioxocarbonate (IV).
(b) If you were given some impure copper, describe how you would obtain a specimen of the pure metal by electrolysis.
(c) Given that sodium chloride has a solubility of 36.3 at 30 and 39.0 at 100 and that of silver nitrate is 297.0 at 30 and 952.0 at 100.
(i) Calculate the percentage of each substance in the saturated solution at 100 that is deposited on cooling to 30
(ii) Deduce which of the two salts can be purified more efficiently by crystallization.
(a)(i) Three methods: neutralization (acid + alkali/base); action of an acid on a metal; precipitation (double decomposition).
(ii) One equation for each:
- Neutralization: \(NaOH + HCl \rightarrow NaCl + H_2O\)
- Acid on metal: \(Zn + H_2SO_4 \rightarrow ZnSO_4 + H_2\)
- Precipitation: \(AgNO_3 + NaCl \rightarrow AgCl + NaNO_3\)
(iii) Two uses of sodium trioxocarbonate(IV), Na2CO3: manufacture of glass; softening of hard water (also soap/detergent and paper manufacture).
(b) Purifying copper by electrolysis: use the impure copper as the anode and a thin strip of pure copper as the cathode, dipped in acidified copper(II) tetraoxosulphate(VI) solution as electrolyte. On passing current, the anode dissolves (\(Cu \rightarrow Cu^{2+} + 2e^-\)) and pure copper is deposited on the cathode (\(Cu^{2+} + 2e^- \rightarrow Cu\)). Impurities collect below the anode as anode sludge.
(c)(i) Take the solubility (g per 100 g water) as the mass dissolved at each temperature.
Sodium chloride: deposited \(= 39.0 - 36.3 = 2.7\) g.
Percentage of the dissolved salt deposited \(= \dfrac{2.7}{39.0} \times 100 = \mathbf{6.9\%}\).
Silver trioxonitrate(V): deposited \(= 952.0 - 297.0 = 655.0\) g.
Percentage of the dissolved salt deposited \(= \dfrac{655.0}{952.0} \times 100 = \mathbf{68.8\%}\).
(ii) Silver trioxonitrate(V) can be purified more efficiently by crystallization, because a far larger fraction of it (about 68.8%) crystallizes out on cooling than for sodium chloride (about 6.9%).
(a)(i) Three methods: neutralization (acid + alkali/base); action of an acid on a metal; precipitation (double decomposition).
(ii) One equation for each:
- Neutralization: \(NaOH + HCl \rightarrow NaCl + H_2O\)
- Acid on metal: \(Zn + H_2SO_4 \rightarrow ZnSO_4 + H_2\)
- Precipitation: \(AgNO_3 + NaCl \rightarrow AgCl + NaNO_3\)
(iii) Two uses of sodium trioxocarbonate(IV), Na2CO3: manufacture of glass; softening of hard water (also soap/detergent and paper manufacture).
(b) Purifying copper by electrolysis: use the impure copper as the anode and a thin strip of pure copper as the cathode, dipped in acidified copper(II) tetraoxosulphate(VI) solution as electrolyte. On passing current, the anode dissolves (\(Cu \rightarrow Cu^{2+} + 2e^-\)) and pure copper is deposited on the cathode (\(Cu^{2+} + 2e^- \rightarrow Cu\)). Impurities collect below the anode as anode sludge.
(c)(i) Take the solubility (g per 100 g water) as the mass dissolved at each temperature.
Sodium chloride: deposited \(= 39.0 - 36.3 = 2.7\) g.
Percentage of the dissolved salt deposited \(= \dfrac{2.7}{39.0} \times 100 = \mathbf{6.9\%}\).
Silver trioxonitrate(V): deposited \(= 952.0 - 297.0 = 655.0\) g.
Percentage of the dissolved salt deposited \(= \dfrac{655.0}{952.0} \times 100 = \mathbf{68.8\%}\).
(ii) Silver trioxonitrate(V) can be purified more efficiently by crystallization, because a far larger fraction of it (about 68.8%) crystallizes out on cooling than for sodium chloride (about 6.9%).
