Chemistry 5070/21 — October/November 2013

Walkthrough

Aqueous ammonia is a weak alkali. When added to a solution containing a metal ion, it often precipitates the metal hydroxide. The clue says the precipitate is green. Iron(II) hydroxide, $\text{Fe(OH)}_2$, is green, so the solid must contain $\text{Fe}^{2+}$ ions. From the list, iron(II) chloride is the only iron(II) salt, so it is the answer.

Key Takeaways

• Aqueous ammonia and sodium hydroxide are used to test for cations by forming hydroxide precipitates.
• Iron(II) hydroxide is green; iron(III) hydroxide is reddish-brown.
• The colour of the precipitate identifies the metal ion.

Common Mistakes

• Choosing iron(III) chloride: iron(III) would give a reddish-brown precipitate, not green.
• Writing 'iron chloride' without the oxidation state: the question needs the iron(II) compound.

Things to Be Careful About

• The solid must dissolve in water first; iron(II) chloride is soluble.
• Aqueous ammonia is added in excess: iron(II) hydroxide does not dissolve in excess ammonia, but the green colour is enough for this clue.

Walkthrough

Limestone is mainly calcium carbonate. When heated strongly, calcium carbonate undergoes thermal decomposition:

The gas given off is carbon dioxide, which is colourless. The solid residue is calcium oxide.

Key Takeaways

• Metal carbonates decompose on strong heating to give the metal oxide and carbon dioxide.
• Carbon dioxide is a colourless gas.

Common Mistakes

• Choosing carbon monoxide: carbonate decomposition produces carbon dioxide, not carbon monoxide.
• Choosing calcium oxide: calcium oxide is the solid residue, not the gas.

Things to Be Careful About

• 'Colourless gas' rules out nitrogen dioxide, which is brown.
• The equation must be balanced and include state symbols if written.

Walkthrough

Lightning provides a very high temperature. This supplies enough energy for nitrogen and oxygen in the air to react, forming nitrogen monoxide, which is then oxidised to nitrogen dioxide. Among the listed gases, nitrogen dioxide is the nitrogen oxide formed by lightning activity.

Key Takeaways

• Lightning is a natural source of nitrogen oxides.
• Nitrogen dioxide is a brown gas and an air pollutant.

Common Mistakes

• Choosing carbon dioxide or sulfur dioxide: lightning activity forms nitrogen oxides, not these.
• Thinking the gas is nitrogen monoxide: the question asks for a compound from the list, and nitrogen dioxide is the listed nitrogen oxide.

Things to Be Careful About

• Nitrogen dioxide is the oxide usually named at O Level for this process.
• The reaction of nitrogen with oxygen requires a high temperature, such as a lightning flash or a car engine.

Walkthrough

Oxides can be classified by their acid-base behaviour. Metal oxides are usually basic; non-metal oxides are usually acidic. Calcium is a metal, so calcium oxide is a basic oxide. Carbon dioxide, sulfur dioxide and nitrogen dioxide are non-metal oxides and are acidic.

Key Takeaways

• Metal oxides tend to be basic; non-metal oxides tend to be acidic.
• Calcium oxide reacts with water to form calcium hydroxide, an alkali.

Common Mistakes

• Choosing carbon dioxide or sulfur dioxide: these are acidic oxides.
• Choosing copper(II) sulfate: it is a salt, not an oxide.

Things to Be Careful About

• 'Basic oxide' refers to an oxide that neutralises acids; calcium oxide fits.
• Calcium oxide is also called quicklime.

Walkthrough

Ethane is an alkane. Complete combustion of an alkane always produces carbon dioxide and water. For ethane:

Among the listed compounds, carbon dioxide is the gas formed. Water is not in the list.

Key Takeaways

• Complete combustion of hydrocarbons gives carbon dioxide and water.
• Incomplete combustion would give carbon monoxide and/or carbon.

Common Mistakes

• Choosing carbon monoxide: that is a product of incomplete combustion, not complete combustion.
• Choosing ethanol or ethene: these are reactants or related compounds, not combustion products.

Things to Be Careful About

• The question says 'ethane', not 'ethanol' or 'ethene'; read the fuel name carefully.
• Carbon dioxide is colourless, so it matches the 'gas' clue.

Walkthrough

We need a white salt that is insoluble in water. From the list, silver chloride and silver iodide are both insoluble silver halides. Silver chloride is white, while silver iodide is yellow. Copper(II) sulfate is blue and soluble; iron(II) and iron(III) chlorides are soluble and coloured. Therefore the answer is silver chloride.

Key Takeaways

• Most silver halides are insoluble; silver chloride is white, silver bromide is cream, silver iodide is yellow.
• Solubility rules help identify salts.

Common Mistakes

• Choosing silver iodide: it is insoluble but yellow, not white.
• Choosing calcium oxide: it is not a salt and reacts with water.

Things to Be Careful About

• The clue asks for both 'white' and 'insoluble'; both conditions must be satisfied.
• Silver chloride is used in tests for chloride ions because it is a white precipitate.