Chemistry 5070/22 — May/June 2014

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

questions

marks

minutes

Topics Metals · Organic Chemistry · Stoichiometry · Chemical Reactions · Chemistry of the Environment · Acids, Bases and Salts · +5 more

Q14MMetalsChemistry of the EnvironmentOrganic ChemistryFree sample

Choose from the following gases to answer the questions below.

$\text{CClF}_3$
$\text{CH}_4$
$\text{CO}$
$\text{CO}_2$
$\text{H}_2$
$\text{N}_2$
$\text{NH}_3$
$\text{O}_2$
$\text{SO}_2$

Each gas can be used once, more than once or not at all.

Which gas is

(a)

used in making steel,

______

DifficultyEasy

Worked solution

Answer

$\text{O}_2$

Final answer

Detailed explanation

Walkthrough

The question asks for the gas used in making steel. In the blast furnace, iron is extracted from its ore, but the iron produced still contains impurities such as carbon. To make steel, oxygen is blown through the molten iron. The oxygen reacts with the impurities, especially carbon, removing them as carbon dioxide. Therefore the gas needed from the list is oxygen, $\text{O}_2$ .

Key Takeaways

Oxygen is used in steelmaking to remove impurities such as carbon from molten iron.
When a question gives a list of gases, read each clue and match it to the known use or source.

Common Mistakes

Writing the name 'oxygen' instead of the formula. The mark scheme allows the name, but the formula $\text{O}_2$ takes precedence.
Choosing carbon dioxide because carbon is removed as $\text{CO}_2$ ; the gas used is oxygen, not the product.

Things to Be Careful About

The answer blank expects a gas from the list, so give the formula $\text{O}_2$ .
Do not confuse the use of oxygen in steelmaking with the use of oxygen in respiration or combustion; the clue specifically mentions steel.

Techniques used

recall that oxygen is used in steelmakingidentify oxygen from the list of gases

(b)

made by the bacterial decay of vegetable matter,

______

DifficultyEasy

Worked solution

Answer

$\text{CH}_4$

Final answer

CH4

Detailed explanation

Walkthrough

When vegetable matter decays in the absence of oxygen (anaerobic decay), bacteria produce methane gas. Methane is the main component of natural gas and is also formed in swamps, marshes and landfill sites. From the list, the gas is $\text{CH}_4$ .

Key Takeaways

Methane is produced by the bacterial decay of vegetable matter in anaerobic conditions.
Methane is a greenhouse gas and is the main component of natural gas.

Common Mistakes

Choosing carbon dioxide because decay releases carbon compounds; the specific gas named in the clue is methane.
Writing the name 'methane' instead of the formula; the formula is preferred.

Things to Be Careful About

The decay must be described as anaerobic (without oxygen) for methane to form.
Use the formula $\text{CH}_4$ , not $\text{CH}_3$ or any other formula.

Techniques used

recall that anaerobic decay of vegetable matter produces methaneidentify methane from the list of gases

(c)

responsible for ozone depletion in the upper atmosphere,

______

DifficultyEasy

Worked solution

Answer

$\text{CClF}_3$

Final answer

CClF3

Detailed explanation

Walkthrough

Ozone in the upper atmosphere is depleted by chlorofluorocarbons (CFCs). CFCs are compounds containing carbon, chlorine and fluorine. In the list, $\text{CClF}_3$ is a CFC, so it is the gas responsible for ozone depletion.

Key Takeaways

CFCs (chlorofluorocarbons) damage the ozone layer.
$\text{CClF}_3$ is a CFC because it contains carbon, chlorine and fluorine.

Common Mistakes

Choosing carbon dioxide or methane because they are greenhouse gases; the clue is specifically about ozone depletion, not global warming.
Choosing chlorine gas, which is not in the list; the CFC is a compound, not chlorine.

Things to Be Careful About

The formula $\text{CClF}_3$ must be written with the correct subscripts: one carbon, one chlorine and three fluorine atoms.
Do not confuse ozone depletion with global warming; they are different environmental problems.

Techniques used

recall that CFCs deplete the ozone layeridentify CClF3 as a CFC from the list

(d)

used to manufacture margarine?

______

DifficultyEasy

Worked solution

Answer

$\text{H}_2$

Final answer

Detailed explanation

Walkthrough

Margarine is made by hydrogenating unsaturated vegetable oils. Hydrogen gas is added across the carbon–carbon double bonds in the oil, making the oil more saturated and turning it into a solid fat. Therefore the gas used is hydrogen, $\text{H}_2$ .

Key Takeaways

Hydrogen is used to hydrogenate unsaturated vegetable oils to make margarine.
Hydrogenation is an addition reaction across carbon–carbon double bonds.

Common Mistakes

Choosing oxygen or nitrogen; hydrogenation specifically uses hydrogen.
Writing the name 'hydrogen' instead of the formula; the formula is preferred.

Things to Be Careful About

The formula is $\text{H}_2$ , not H.
Do not confuse hydrogenation with oxidation; hydrogen is being added to the oil.

Techniques used

recall that hydrogen is used to hydrogenate unsaturated oilsidentify hydrogen from the list of gases

The rest of this paper

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Q4Electrochemistry6M
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