Chemistry 5070/22 — October/November 2017

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

questions

marks

minutes

Topics Chemistry of the Environment · Organic Chemistry · Experimental Techniques and Chemical Analysis · Stoichiometry · Chemical Reactions · Atoms, Elements and Compounds · +5 more

Q19MChemical ReactionsChemistry of the EnvironmentOrganic ChemistryExperimental Techniques and Chemical AnalysisAtoms, Elements and CompoundsFree sample

(a)

Choose from the following elements to answer the questions.

aluminium
carbon
hydrogen
iron
magnesium
nitrogen
oxygen
sodium
vanadium

Each element may be used once, more than once or not at all.

Which element:

(i)

is a catalyst in the Haber process,

______

DifficultyEasy

Worked solution

Answer

iron

Final answer

iron

Detailed explanation

Walkthrough

The Haber process makes ammonia from nitrogen and hydrogen. The catalyst that speeds up this reaction is iron. From the list of elements, the correct answer is iron.

Key Takeaways

The Haber process uses an iron catalyst.

Common Mistakes

Writing "ammonia" instead of the catalyst. The question asks for an element from the list.

Things to Be Careful About

The answer can be given as the name "iron" or the symbol "Fe".

Techniques used

recall the catalyst used in the Haber processidentify iron from the given list

(ii)

makes up 21% of dry air,

______

DifficultyEasy

Worked solution

Answer

oxygen

Final answer

oxygen

Detailed explanation

Walkthrough

Clean, dry air is roughly 78% nitrogen, 21% oxygen and about 1% other gases. The element that makes up 21% of dry air is therefore oxygen.

Key Takeaways

Oxygen makes up about 21% of clean, dry air; nitrogen makes up about 78%.

Common Mistakes

Choosing nitrogen because it is the largest component. The question specifically asks for the 21% component.

Things to Be Careful About

The answer can be written as "oxygen" or "O₂".

Techniques used

recall the percentage composition of dry airidentify oxygen from the given list

(iii)

can be formed when hydrocarbons are cracked,

______

DifficultyEasy

Worked solution

Answer

hydrogen

Final answer

hydrogen

Detailed explanation

Walkthrough

Cracking breaks long-chain hydrocarbons into smaller, more useful molecules. Hydrogen gas can also be produced during cracking, along with smaller alkanes and alkenes. From the list, the element is hydrogen.

Key Takeaways

Cracking can produce hydrogen as well as smaller alkanes and alkenes.

Common Mistakes

Choosing carbon, because hydrocarbons contain carbon. The question asks for an element that can be formed as a product of cracking, not one present in the original hydrocarbon.

Things to Be Careful About

The answer can be given as "hydrogen" or "H₂".

Techniques used

recall the products of cracking hydrocarbonsidentify hydrogen from the given list

(iv)

forms aqueous ions with a 3+ charge which give a white precipitate when added to aqueous ammonia,

______

DifficultyEasy

Worked solution

Answer

aluminium

Final answer

aluminium

Detailed explanation

Walkthrough

Aluminium forms $\text{Al}^{3+}$ ions in aqueous solution. When aqueous ammonia is added to a solution containing aluminium ions, a white precipitate of aluminium hydroxide forms. This precipitate is insoluble in excess aqueous ammonia. From the list, the element is aluminium.

Key Takeaways

$\text{Al}^{3+}$ gives a white precipitate with aqueous ammonia.

Common Mistakes

Choosing magnesium because magnesium ions also give a white precipitate with sodium hydroxide. The clue "3+ charge" points specifically to aluminium.

Things to Be Careful About

The answer is the element name "aluminium", not the formula of the precipitate.

Techniques used

recall the ammonia test for aluminium ionsidentify the 3+ ion from the given list

(v)

has an atom with only three electrons in its outer shell?

______

DifficultyEasy

Worked solution

Answer

aluminium

Final answer

aluminium

Detailed explanation

Walkthrough

Aluminium has atomic number 13, so its electron configuration is 2,8,3. The outer shell contains three electrons. From the list, the element is aluminium.

Key Takeaways

Group number tells you the number of outer-shell electrons for a main-group element. Aluminium is in Group III and has three outer-shell electrons.

Common Mistakes

Choosing sodium, which has configuration 2,8,1, or magnesium, which has 2,8,2. Only aluminium has three outer-shell electrons.

Things to Be Careful About

Count the last number in the shell configuration. For aluminium, 2,8,3 means three electrons in the outer shell.

Techniques used

write the electron configuration of the elementsidentify the element with three outer-shell electrons

(b)

Complete the table to show the number of electrons and neutrons in the potassium atom and in the oxide ion.

	number of electrons	number of neutrons
$^{41}_{19}\text{K}$
$^{17}_{8}\text{O}^{2-}$

DifficultyMedium-Easy

Worked solution

Answer

particle	number of electrons	number of neutrons
$^{41}_{19}\text{K}$	19	22
$^{17}_{8}\text{O}^{2-}$	10	9

Final answer

K-41: 19 electrons, 22 neutrons; O-17 oxide ion: 10 electrons, 9 neutrons

Detailed explanation

Walkthrough

For $^{41}_{19}\text{K}$ , the bottom-left number 19 is the atomic number, so there are 19 protons. A neutral potassium atom has the same number of electrons as protons, so it has 19 electrons. The mass number is 41, so the number of neutrons is $41 - 19 = 22$ .

For $^{17}_{8}\text{O}^{2-}$ , the atomic number is 8, so there are 8 protons. A neutral oxygen atom has 8 electrons, but the $2-$ charge means the ion has gained two extra electrons, so it has $8 + 2 = 10$ electrons. The mass number is 17, so the number of neutrons is $17 - 8 = 9$ .

Key Takeaways

The atomic number gives the number of protons. In a neutral atom, the number of electrons equals the number of protons. For an ion, add electrons for a negative charge and subtract electrons for a positive charge. The number of neutrons is always mass number minus atomic number.

Common Mistakes

Using the mass number as the number of electrons. Forgetting to add two electrons for the oxide ion. Subtracting the mass number from the atomic number instead of the other way round.

Things to Be Careful About

For a negative ion, the electron count is larger than the proton count. The neutron count does not depend on the charge. Remember the atomic number is the smaller number written at the bottom left of the nuclide notation.

Techniques used

use the atomic number to find the number of protonscalculate neutrons from mass number minus atomic numberadjust electron number for the ion charge

The rest of this paper

8 more questions

Q2Atoms, Elements and Compounds · Electrochemistry · Experimental Techniques and Chemical Analysis13M
Q3Metals · Stoichiometry · Chemistry of the Environment9M
Q4Organic Chemistry · Experimental Techniques and Chemical Analysis9M
Q5Stoichiometry · Acids, Bases and Salts · Chemical Energetics · Organic Chemistry5M
Q6Chemical Reactions · Acids, Bases and Salts · Organic Chemistry · States of Matter10M
Q7Atoms, Elements and Compounds · Stoichiometry10M
Q8Stoichiometry · Experimental Techniques and Chemical Analysis · Chemistry of the Environment10M
Q9Chemical Reactions · Chemistry of the Environment · Chemical Energetics10M

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