Chemistry 5070/21 — October/November 2020

Walkthrough

The clue describes the test for nitrate ions. When a nitrate is warmed with aqueous sodium hydroxide and aluminium, the nitrate ions are reduced and ammonia gas is produced. Ammonia turns damp red litmus paper blue and has a sharp smell. In the list, silver nitrate is the only nitrate salt, so it must be the answer. The aluminium is the reducing agent; the sodium hydroxide provides the alkaline conditions.

Key Takeaways

• Nitrate ions can be detected by warming with aqueous sodium hydroxide and aluminium.
• Ammonia gas is the product that confirms the presence of nitrate.
• Silver nitrate has the formula $\text{AgNO}_3$ and contains nitrate ions.

Common Mistakes

• Choosing aluminium iodide because aluminium appears in the test. The aluminium is the reagent, not the compound being identified.
• Confusing this nitrate test with the ammonium test (ammonium salts give ammonia with warm sodium hydroxide alone).

Things to Be Careful About

• Give the compound name from the list, not just the ion.
• The nitrate test needs aluminium as well as sodium hydroxide; without aluminium, nitrate ions do not give ammonia.

Walkthrough

Many fertilisers contain nitrate ions because nitrogen is an essential plant nutrient. The nitrate ion is $\text{NO}_3^-$ and has a single negative charge. The list contains only one nitrate salt: silver nitrate. Potassium phosphate contains phosphate ions, $\text{PO}_4^{3-}$, which have a 3- charge, and sodium sulfate contains sulfate ions, $\text{SO}_4^{2-}$, with a 2- charge. So the compound that contains a 1- ion present in fertilisers is silver nitrate.

Key Takeaways

• Nitrate ions are a common source of nitrogen in fertilisers.
• The nitrate ion has a 1- charge.
• A compound's name tells you which ions it contains.

Common Mistakes

• Choosing potassium phosphate because fertilisers contain phosphorus. Phosphate is a 3- ion, not 1-.
• Thinking all fertiliser ions are 1-; ammonium is 1+ and phosphate is 3-.

Things to Be Careful About

• The question asks for the compound, not the ion.
• Silver nitrate can be used for both part (a) and part (b) because the instruction allows each compound to be used more than once.

Walkthrough

Magnesium is in Group II of the Periodic Table, so it forms a 2+ ion, $\text{Mg}^{2+}$. Magnesium carbonate has the formula $\text{MgCO}_3$ and contains $\text{Mg}^{2+}$ and $\text{CO}_3^{2-}$ ions. None of the other compounds has a 2+ cation: lead(IV) chloride contains $\text{Pb}^{4+}$, sodium sulfate and potassium phosphate contain 1+ cations, and the remaining compounds contain 1+ cations or are covalent. So the answer is magnesium carbonate.

Key Takeaways

• Group number often tells you the charge on a metal ion: Group II metals form 2+ ions.
• Magnesium carbonate contains the magnesium ion, $\text{Mg}^{2+}$.

Common Mistakes

• Choosing lead(IV) chloride because the Roman numeral IV suggests a 4+ ion, not 2+.
• Choosing sodium sulfate because sulfate has a 2- charge; the question asks for a 2+ ion, not a 2- ion.

Things to Be Careful About

• Distinguish cations (positive) from anions (negative).
• Read the Roman numeral in lead(IV) chloride carefully: it is 4+, not 2+.

Walkthrough

Chlorine is above bromine in Group VII, so chlorine is more reactive than bromine. When chlorine water is added to an aqueous bromide, chlorine displaces bromide ions and forms bromine:

Bromine dissolved in water is orange. The only bromide in the list is lithium bromide, so lithium bromide is the compound that gives an orange colour with chlorine.

Key Takeaways

• Halogens become less reactive down Group VII.
• A more reactive halogen displaces a less reactive halogen from its salt solution.
• Aqueous bromine is orange.

Common Mistakes

• Choosing a chloride or an iodide instead of a bromide; the orange colour is specific to bromine.
• Thinking the orange colour is chlorine; chlorine water is pale green.

Things to Be Careful About

• The reaction happens in aqueous solution, so the bromide must be soluble. Lithium bromide is soluble.
• The equation shows bromide ions being oxidised to bromine, but at O Level it is enough to describe it as displacement.

Walkthrough

The bacterial decay of vegetable matter in the absence of oxygen produces methane, $\text{CH}_4$. This is why methane is called marsh gas and is the main component of biogas. Methane is a hydrocarbon because it contains only carbon and hydrogen. Ethanol and glucose are organic compounds but they also contain oxygen, so they are not hydrocarbons. The answer is methane.

Key Takeaways

• Methane is the simplest alkane and a hydrocarbon.
• Anaerobic bacterial decay of plant material produces methane.
• A hydrocarbon contains only carbon and hydrogen.

Common Mistakes

• Choosing ethanol or glucose because they are organic; they contain oxygen, so they are not hydrocarbons.
• Writing the formula instead of the name; the question asks for the compound.

Things to Be Careful About

• "Hydrocarbon" means only carbon and hydrogen atoms.
• Methane is formed by decay, not by combustion.