Studyledge

Redox Reactions: Question 1

Syllabus 6.4

Multiple choice Core 1 mark

Tungsten metal, used to make the filaments inside some specialist lamps, is manufactured by heating tungsten(VI) oxide in a stream of hydrogen gas:

WO3+3H2→W+3H2O\text{WO}_3 + 3\text{H}_2 \rightarrow \text{W} + 3\text{H}_2\text{O}

Which substance is reduced in this reaction?

Choose an answer to check it, then compare with the worked solution below.
Show worked solution Hide worked solution

Worked solution

Step 1: Recall the oxygen-based definitions

At Core level, oxidation and reduction are defined in terms of oxygen:

  • Oxidation = gain of oxygen
  • Reduction = loss of oxygen

Step 2: Track the oxygen in the equation

WO3+3H2→W+3H2O\text{WO}_3 + 3\text{H}_2 \rightarrow \text{W} + 3\text{H}_2\text{O}

  • Tungsten(VI) oxide, WO3\text{WO}_3, starts with three oxygen atoms per formula unit and ends up as tungsten metal, W\text{W}, with none. It has lost oxygen.
  • Hydrogen, H2\text{H}_2, starts with no oxygen and ends up as water, H2O\text{H}_2\text{O}, which contains oxygen. It has gained oxygen.

Step 3: Apply the definitions

Losing oxygen is reduction, so tungsten(VI) oxide is the substance that is reduced. Hydrogen, which gains oxygen, is oxidised. This is why hydrogen is acting as the reducing agent in this reaction (the reducing agent is always the substance that is itself oxidised).

Final answer

  • Tungsten(VI) oxide, WO3\text{WO}_3, is reduced, option D.