Question
This question is about salts.
(a) Salts that are insoluble in water are made by precipitation.
● Lead(II) iodide, \(PbI_2\), is insoluble in water.
● All nitrates are soluble in water.
● All sodium salts are soluble in water.
You are provided with solid lead(II) nitrate, \(Pb(NO_3)_2\), and solid sodium iodide, NaI.
Describe how you would make a pure sample of lead(II) iodide by precipitation.
Your answer should include:
● practical details
● a chemical equation for the precipitation reaction.
(b) Nitrates decompose when heated.
(i) When hydrated zinc nitrate is heated, oxygen gas is given off.
Describe a test for oxygen.
test ……………………………………………………………………………………………………………………..
observations ………………………………………………………………………………………………………..
(ii) Complete the equation for the decomposition of hydrated zinc nitrate.
\(2Zn(NO_3)_2•6H_2O → …..ZnO + …..NO_2 + O_2 + …..H_2O\)
(c) Some sulfates are hydrated.
When hydrated sodium sulfate crystals, Na2SO4•xH2O, are heated, they give off water.
\(Na_2SO_4•xH_2O(s) → Na_2SO_4(s) + xH_2O(g)\)
A student carries out an experiment to determine the value of x in \(Na_2SO_4•xH_2O\).
step 1 Hydrated sodium sulfate crystals are weighed.
step 2 The hydrated sodium sulfate crystals are then heated.
step 3 The remaining solid is weighed.
(i) Describe how the student can check that all the water has been given off.
(ii) In an experiment, 1.61g of \(Na_2SO_4•xH_2O\) is heated until all the water is given off. The mass of \(Na_2SO_4\) remaining is 0.71g.
\([M_r : Na_2SO_4,142; H_2O,18]\)
Determine the value of x using the following steps.
● Calculate the number of moles of \(Na_2SO_4\) remaining.
………………………… mol
● Calculate the mass of \(H_2O\) given off.
………………………… g
● Calculate the number of moles of \(H_2O\) given off.
………………………… mol
● Determine the value of x.
x = …………………………
Answer/Explanation
Answer:
(a) (add) water (to both salts) (1)
dissolve both salts / make solutions (1)
filter (lead(II) iodide)(1)
wash (residue of lead(II) iodide) with water AND dry e.g. with filter paper / description of washing and drying (1)
\(Pb(NO_3)_2 + 2 NaI → 2NaNO_3 + PbI_2\)
OR \(Pb^{2+} + 2I^– → PbI_2\) (1)
(b) (i) glowing splint (1)
relights / rekindles (1)
(ii) 2ZnO(s) and \(4NO_2\)(g) (1)
\(12H_2O\)(g) (1)
(c) (i) heat again and weigh again / repeat steps 2 and 3 (1)
until mass is constant (1)
(ii) 0.005 (1)
0.9 (1)
(0.9 ÷ 18 =) 0.05 (1)
(0.05 ÷ 0.005 =) 10 (1)
Question
A student wanted to make some zinc chloride crystals.
The student followed the procedure shown.
step 1 Add excess zinc powder to dilute hydrochloric acid to form aqueous zinc chloride.
step 2 Remove unreacted zinc powder from the aqueous zinc chloride.
step 3 Heat the solution until it is saturated.
step 4 Allow the saturated solution to cool and remove the crystals that form.
(a) Write the equation for the reaction in step 1. Include state symbols.
(b) Explain why excess zinc powder is added in step 1.
(c) Suggest how unreacted zinc powder is removed in step 2.
(d) A saturated solution is formed in step 3.
Suggest what is meant by the term saturated solution.
(e) Explain why crystals form as the solution cools in step 4.
(f) Name two zinc compounds which react with dilute hydrochloric acid to form zinc chloride.
(g) If excess calcium metal is used instead of excess zinc powder in step 1, pure calcium chloride
crystals do not form.
Explain why.
(h) Some salts can be made by titration.
In a titration experiment, 20.0\(cm^3\) of aqueous sodium hydroxide reacts exactly with 25.0\(cm^3\) of 0.100mol/\(dm^3\) dilute sulfuric acid to make sodium sulfate.
\(2NaOH(aq) + H_2SO_4(aq) → Na_2SO_4(aq) + 2H_2O(l)\)
(i) Circle the name of the type of reaction that takes place.
decomposition neutralisation precipitation reduction
(ii) Calculate the concentration of the aqueous sodium hydroxide in g/\(dm^3\) using the following steps.
● Calculate the number of moles of dilute sulfuric acid used.
………………………… mol
● Determine the number of moles of sodium hydroxide which react with the dilute
sulfuric acid.
………………………… mol
● Calculate the concentration of the aqueous sodium hydroxide in mol/\(dm^3\)
.
………………………… mol/\(dm^3\)
● Calculate the concentration of the aqueous sodium hydroxide in g/\(dm^3\)
.
………………………… g/\(dm^3\)
Answer/Explanation
Answer:
(a) \(Zn(s) + 2HCl(aq) \rightarrow ZnCl_2(aq) + H_2(g)\)
\(ZnCl_2\) or \(H_2\) as a product (1)
correct equation (1)
states (1)
(b) to make sure all the (hydrochloric) acid reacts
(c) filtration
(d) a solution that can dissolve no more solute (1)
at a given temperature (1)
(e) solubility (of ZnCl2 / solids) decreases (as temperature decreases)
(f) zinc oxide
zinc carbonate
(g) Ca will also react with water
(h) (i) neutralisation
(ii) 0.100 × 25 / 1000 = 0.0025(0) (1)
0.0025 × 2 = 0.005(00) (1)
0.005 × 1000 / 20 = 0.25(0) (1)
Mr = 40 (1)
0.25 × 40 = 10.(0) (1)
Question
Aluminium is extracted from its ore. The ore is converted into pure aluminium oxide, which then
undergoes electrolysis as shown.
(a) (i) Name an ore of aluminium.
(ii) What is meant by the term electrolysis?
(b) Aluminium oxide has a melting point of about 2000°C, but the electrolysis process operates at
about 900°C.
(i) Name the compound added to aluminium oxide to reduce the operating temperature.
(ii) Suggest one benefit to the environment of reducing the operating temperature.
(iii) Write the ionic half-equation for the reaction taking place at:
the negative electrode (cathode) …………………………………………………………………………….
the positive electrode (anode) ………………………………………………………………………………..
(iv) Explain why the anodes need frequent replacement.
(c) Aluminium oxide reacts with acids and with alkalis.
(i) What term is used to describe an oxide that reacts with acids and with alkalis?
(ii) Aluminium oxide reacts with dilute sulfuric acid to form a salt.
State the name and write the formula of the salt formed.
name …………………………………………………………………………………………………………………..
formula ………………………………………………………………………………………………………………..
(iii) Aluminium oxide reacts with dilute sodium hydroxide to form a salt and one other product.
Name the other product.
(iv) Aluminium hydroxide, \(Al(OH)_3\), decomposes when heated to form aluminium oxide and
water.
Write the chemical equation for this reaction.
(v) Suggest the names of two other aluminium compounds that decompose when heated to
form aluminium oxide.
Answer/Explanation
Answer:
(a) (i) bauxite
(ii) breakdown by (the passage of) electricity (1)
of an ionic compound in molten / aqueous (state) (1)
(b) (i) cryolite
(ii) less \(CO_2\) emission
(iii) \(Al^{3+} + 3e^- \rightarrow Al\)
any positive Al species gainig electron(s) (1)
correct species and balance (1)
\(2O^{2–} \rightarrow O_2 + 4e^–\)
any negative O species losing electron(s) (1)
correct species and balance (1)
(iv) anodes or carbon / graphite react with oxygen / \(O_2\) (1)
(form) carbon dioxide (1)
(c) (i) amphoteric
(ii) aluminium sulfate (1) \(Al_2(SO_4)_3\) (1)
(iii) water
(iv) \(2Al(OH)_3 \rightarrow Al_2O_3 + 3H_2O\)
species (1) balance (1)
(v) aluminium carbonate (1)
aluminium nitrate (1)