Chemistry 5070/21 — October/November 2016

Cambridge O-Level · Theory · worked solutions for every part, with the mark scheme

questions

marks

minutes

Topics Stoichiometry · Organic Chemistry · Chemical Reactions · Acids, Bases and Salts · Experimental Techniques and Chemical Analysis · Atoms, Elements and Compounds · +5 more

Q15MChemistry of the EnvironmentOrganic ChemistryAcids, Bases and SaltsExperimental Techniques and Chemical AnalysisChemical ReactionsMetalsFree sample

Choose from the following compounds to answer the questions below.

$\text{Ba(NO}_3)_2$
$\text{CaCO}_3$
$\text{CaO}$
$\text{CH}_4$
$\text{C}_3\text{H}_8$
$\text{Fe}_2\text{O}_3$
$\text{KMnO}_4$
$\text{NaNO}_3$
$\text{NO}$
$\text{SO}_2$
$\text{V}_2\text{O}_5$
$\text{Zn(NO}_3)_2$

Each of these compounds can be used once, more than once or not at all.

Which compound

(a)

is an atmospheric pollutant formed by lightning activity,

______

DifficultyEasy

Worked solution

Answer

NO (nitrogen monoxide / nitric oxide).

Final answer

NO (nitrogen monoxide)

Detailed explanation

Walkthrough

Lightning is hot enough to supply the energy needed for nitrogen and oxygen in the air to react:

\text{N}_2(\text{g}) + \text{O}_2(\text{g}) \rightarrow 2\text{NO}(\text{g})

The nitrogen oxide formed by lightning is nitrogen monoxide, NO. It is an atmospheric pollutant because it can help form acid rain and photochemical smog. So from the list, the compound asked for is NO.

Key Takeaways

Pollutants can be linked to their sources. High-temperature combustion and lightning allow nitrogen and oxygen to combine, producing oxides of nitrogen. The main oxide made directly by lightning is nitrogen monoxide, NO.

Common Mistakes

Choosing nitrogen dioxide, NO2, instead of NO. Lightning produces mainly NO, which can later be oxidised to NO2 in the air.
Giving nitrogen gas, N2, as the pollutant. N2 is the main gas in clean, dry air and is not a pollutant.

Things to Be Careful About

The exam accepts NO, nitrogen monoxide, nitric oxide or nitrogen oxide. Write the formula or the name clearly; do not write an unbalanced equation in place of the answer.

Techniques used

link a natural source to the pollutant formedidentify nitrogen monoxide from high-temperature nitrogen and oxygen reaction

(b)

is the main constituent of natural gas,

______

DifficultyEasy

Worked solution

Answer

$\text{CH}_4$ (methane).

Final answer

CH4 (methane)

Detailed explanation

Walkthrough

Natural gas is mostly methane, the simplest alkane. Methane has one carbon atom and four hydrogen atoms, so its formula is CH4. Propane, C3H8, is a hydrocarbon fuel too, but it is associated with bottled/LPG gas rather than being the main constituent of natural gas. Therefore the required compound is CH4.

Key Takeaways

Alkanes form a homologous series of hydrocarbons. Methane is the first member and is the main component of natural gas, a fossil fuel used for heating and cooking.

Common Mistakes

Choosing C3H8 (propane) because it is also a hydrocarbon fuel. Propane is not the main constituent of natural gas.
Writing CH3 or forgetting the subscript 4.

Things to Be Careful About

The mark scheme accepts CH4 or methane. Write the formula with the subscript 4 clearly shown.

Techniques used

recall the composition of natural gasidentify methane as the simplest alkane

(c)

dissolves in water to form an aqueous solution which gives a white precipitate on addition of aqueous sodium sulfate,

______

DifficultyMedium-Easy

Worked solution

Answer

$\text{Ba(NO}_3)_2$ (barium nitrate).

Final answer

Ba(NO3)2 (barium nitrate)

Detailed explanation

Walkthrough

When barium nitrate dissolves in water it forms an aqueous solution containing Ba2+ and NO3− ions. Adding aqueous sodium sulfate provides sulfate ions, SO4^2−. Barium sulfate is insoluble, so a white precipitate of BaSO4 forms:

\text{Ba}^{2+}(\text{aq}) + \text{SO}_4^{2-}(\text{aq}) \rightarrow \text{BaSO}_4(\text{s})

This is the reverse of the usual sulfate test: here the clue tells you that a sulfate is being added, so the required salt must contain barium ions. From the list, the soluble barium compound is barium nitrate.

Key Takeaways

Barium sulfate is one of the classic insoluble salts. A white precipitate with sulfate ions identifies a barium compound. Solubility rules help: all sodium salts are soluble, but barium sulfate is not.

Common Mistakes

Choosing sodium nitrate because it is a soluble nitrate. Sodium sulfate is also soluble, so no precipitate would form.
Confusing calcium sulfate with barium sulfate. Barium sulfate is the classic white precipitate used in the sulfate test.
Answering with the precipitate BaSO4 instead of the compound from the list, Ba(NO3)2.

Things to Be Careful About

The clue says "dissolves in water to form an aqueous solution". Barium nitrate is soluble, and the white precipitate is due to the barium ion reacting with sulfate. State the formula or name of the compound, not the precipitate.

Techniques used

recognise the sulfate test as a white barium sulfate precipitateidentify a soluble salt containing barium ions from the list

(d)

is a catalyst in the contact process for the manufacture of sulfuric acid,

______

DifficultyEasy

Worked solution

Answer

$\text{V}_2\text{O}_5$ (vanadium(V) oxide / vanadium pentoxide).

Final answer

V2O5 (vanadium(V) oxide)

Detailed explanation

Walkthrough

The Contact process makes sulfur trioxide from sulfur dioxide and oxygen:

2\text{SO}_2(\text{g}) + \text{O}_2(\text{g}) \rightleftharpoons 2\text{SO}_3(\text{g})

The catalyst used is vanadium(V) oxide, V2O5. It speeds up the reaction without being used up. SO2 in the list is a reactant, not the catalyst, so the correct compound is V2O5.

Key Takeaways

Industrial processes have specific catalysts. The Contact process uses vanadium(V) oxide, V2O5, for the manufacture of sulfuric acid. The Haber process, which makes ammonia, uses iron as its catalyst.

Common Mistakes

Choosing iron as the catalyst because iron is used in the Haber process. This question is specifically about the Contact process.
Choosing SO2 because it appears in the equation. SO2 is a reactant, not a catalyst.
Writing the formula incorrectly, e.g. VO or V2O5 with wrong subscript.

Things to Be Careful About

The mark scheme accepts V2O5, vanadium pentoxide or vanadium(V) oxide. Use the Roman numeral in the name because vanadium can form more than one oxidation state.

Techniques used

recall the catalyst used in the Contact processidentify vanadium(V) oxide from the list

(e)

is a product of the thermal decomposition of limestone?

______

DifficultyEasy

Worked solution

Answer

$\text{CaO}$ (calcium oxide).

Final answer

CaO (calcium oxide)

Detailed explanation

Walkthrough

Limestone is mainly calcium carbonate, CaCO3. When heated strongly it thermally decomposes:

\text{CaCO}_3(\text{s}) \rightarrow \text{CaO}(\text{s}) + \text{CO}_2(\text{g})

The solid product is calcium oxide, CaO, also called quicklime. Carbon dioxide is also a product, but CO2 is not one of the compounds in the list, so the required answer is CaO.

Key Takeaways

Heating metal carbonates strongly decomposes them into the metal oxide and carbon dioxide. Limestone gives calcium oxide and carbon dioxide. Calcium oxide is an important industrial chemical.

Common Mistakes

Choosing CaCO3, which is the reactant, not a product.
Choosing carbon dioxide, which is also a product but is not in the given list.
Calling CaO slaked lime. Slaked lime is calcium hydroxide, Ca(OH)2; CaO is quicklime.

Things to Be Careful About

The mark scheme accepts CaO or calcium oxide. The question says "a product", and CaO is the solid product listed among the compounds.

Techniques used

recall the thermal decomposition of limestoneidentify calcium oxide as the solid product

The rest of this paper

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