Chemistry 5070/22 — October/November 2013

Walkthrough

Aqueous potassium sulfate contains $\text{K}^+$, $\text{SO}_4^{2-}$, $\text{H}^+$ and $\text{OH}^-$ ions. At the anode, anions are attracted. Sulfate ions are very stable and are not discharged in dilute aqueous solution, so hydroxide ions from water are discharged instead:

Oxygen gas is therefore liberated at the anode.

Key Takeaways

• In aqueous electrolysis, water provides $\text{H}^+$ and $\text{OH}^-$ ions.
• Sulfate ions are not discharged at the anode in aqueous solution.
• Hydroxide ions are discharged to give oxygen gas.

Common Mistakes

• Choosing chlorine because it is a halogen; no chloride ions are present.
• Thinking sulfate ions decompose at the anode; they do not in this case.
• Forgetting that water contributes ions to the electrolyte.

Things to Be Careful About

• The anode is the positive electrode.
• The gas is oxygen, formula $\text{O}_2$, not oxygen atoms.
• The half-equation must be balanced in both atoms and charge.

Walkthrough

Margarine is made by adding hydrogen across the double bonds of unsaturated vegetable oils. This hydrogenation reaction needs a catalyst, and the catalyst used industrially is finely divided nickel. Therefore the element is nickel.

Key Takeaways

• Margarine manufacture involves hydrogenation of unsaturated oils.
• Nickel is the catalyst for this hydrogenation.
• A catalyst speeds up a reaction without being used up.

Common Mistakes

• Choosing hydrogen: hydrogen is a reactant, not the catalyst.
• Choosing iron: iron is a catalyst for the Haber process, not margarine manufacture.
• Misspelling nickel or writing the wrong symbol.

Things to Be Careful About

• The answer should be the element name or symbol: nickel / Ni.
• Do not confuse this with the catalyst used in the Haber process, which is iron.

Walkthrough

Valency electrons are the electrons in the outermost shell. An atom with six valency electrons is in Group VI of the Periodic Table. From the list, both oxygen and sulfur are in Group VI, but oxygen is a gas at room temperature whereas sulfur is a yellow non-metallic solid. Therefore the element is sulfur.

Key Takeaways

• For main-group elements, the group number equals the number of outer-shell electrons.
• Sulfur has the electron configuration 2,8,6.
• Physical state helps distinguish elements in the same group.

Common Mistakes

• Choosing oxygen because it also has six valency electrons; oxygen is a gas, not a solid.
• Confusing valency electrons with total number of electrons.
• Choosing chlorine because it is a non-metal; chlorine has seven valency electrons.

Things to Be Careful About

• "Valency electrons" means outer-shell electrons.
• Sulfur exists as $\text{S}_8$ molecules but the element is simply sulfur / S.
• Non-metallic solid: sulfur is the only suitable element in the list.

Walkthrough

The reactivity series places potassium above sodium:

potassium > sodium > lithium > calcium > magnesium > aluminium > zinc > iron ...

So the element from the list that is higher than sodium is potassium.

Key Takeaways

• The reactivity series lists metals in order of how readily they lose electrons.
• Potassium is more reactive than sodium.
• In Group I, reactivity increases down the group: lithium < sodium < potassium.

Common Mistakes

• Choosing lithium: lithium is below sodium in the reactivity series, not above it.
• Confusing the order of Group I metals with their positions in the Periodic Table.

Things to Be Careful About

• The reactivity series order must be recalled accurately.
• The answer is the element name or symbol: potassium / K.

Walkthrough

The Periodic Table is arranged in periods (horizontal rows) and groups (vertical columns). Silver, Ag, is in Period 5. The other metals in the list, such as iron, nickel, vanadium and zinc, are in Period 4, and potassium is in Period 4 as well. Therefore the element in Period 5 is silver.

Key Takeaways

• A period is a horizontal row of the Periodic Table.
• A group is a vertical column.
• Silver is a transition metal in Period 5.

Common Mistakes

• Choosing a Period 4 transition metal such as iron or nickel.
• Confusing period with group.
• Writing the symbol Ag incorrectly.

Things to Be Careful About

• Period numbers are the row numbers.
• Silver is in Group 11, but the question asks for the period, not the group.

Walkthrough

An amphoteric oxide reacts with both acids and bases. Zinc oxide, ZnO, is a white solid and is amphoteric. From the list, the element that forms this oxide is zinc. Aluminium oxide is also amphoteric, but aluminium is not in the list.

Key Takeaways

• Amphoteric oxides show both acidic and basic behaviour.
• Zinc oxide is white and amphoteric.
• Its formula is ZnO.

Common Mistakes

• Choosing a metal whose oxide is basic, such as iron or copper.
• Forgetting that aluminium oxide is amphoteric but not in the list.
• Confusing "amphoteric" with "neutral".

Things to Be Careful About

• The question asks for the element, not the oxide.
• The answer is zinc / Zn.
• Zinc oxide is insoluble in water but reacts with acids and with strong alkalis.