QUESTION 135 (5 marks)
The diagram shows a standard hydrogen electrode connected to another half-cell. The electrode is platinum, the aqueous hydrogen-ion concentration is 1.0 mol L⁻¹, hydrogen pressure is 100 kPa and temperature is 25 °C.
(a) State two roles of the platinum electrode. [2 marks]
(b) Write the hydrogen reduction half-equation and state the assigned standard electrode potential. [2 marks]
(c) When electrons flow from the other half-cell towards the hydrogen electrode, identify the hydrogen electrode as anode or cathode. [1 marks]
Practice marking scheme
Answer
Platinum provides an inert electrical conductor/surface and catalyses the hydrogen reaction. ; . It is the cathode when it receives electrons.
Working
The solid conductor connects the external wire to the reacting hydrogen/hydrogen-ion system; it does not act as a consumed metal reagent. Platinum catalyses the interconversion of hydrogen and hydrogen ions. The reference reduction potential is defined as zero under standard conditions. Incoming electrons are used in reduction, so the hydrogen electrode is the cathode.
Marking criteria
- Describes platinum as an inert conductor/reaction surface. [1 mark]
- Describes its catalytic role. [1 mark]
- Writes the balanced hydrogen reduction half-equation with states. [1 mark]
- States the assigned standard potential 0.00 V. [1 mark]
- Identifies the electrode receiving electrons as the cathode. [1 mark]
Practice question aligned to the current QCAA syllabus; review the worked solution and marking criteria.
View the QCAA syllabusCompare your working with the guide above.