Everything Science Grade 12

4.4 CHAPTER 4. ELECTROCHEMICAL REACTIONS

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[H 3 O+]

(1 mol.dm−^3 )

Pt^25 ◦C

H 2 gas

(1 x atmospheric pressure)

Figure 4.3: The standardhydrogen electrode

A standard hydrogen electrode consists of a platinum electrode in a solution containing
H+ions. The solution (e.g.H 2 SO 4 ) that contains the H+ions has a concentrationof 1
mol.dm−^3. As the hydrogen gas bubbles over the platinumelectrode, an equilibrium
is set up between hydrogen molecules and hydrogen ions in solution. The reaction is
as follows:

2 H+(aq) + 2e−� H 2 (g)

The position of this equilibrium can change if you change some of the conditions (e.g.
concentration, temperature). It is therefore important that the conditions for the stan-
dard hydrogen electrodeare standardised as follows: pressure = 100 kPa(1atm); tem-
perature = 298 K (25◦C) and concentration =1 mol.dm−^3.

In order to use the hydrogen electrode, it needs to be attachedto the electrode system
that you are investigating. For example, if youare trying to determinethe electrode
potential of copper, you will need to connectthe copper half cell tothe hydrogen
electrode; if you are trying to determine the electrode potential of zinc,you will need
to connect the zinc half cell to the hydrogen electrode and so on. Let’s look at the
examples of zinc and copper in more detail.

1. Zinc
   Zinc has a greater tendency than hydrogen toform ions, so if the standard hy-
   drogen electrode is connected to the zinc halfcell, the zinc will be relatively
   more negative becausethe electrons that are released when zinc is oxidised will
   accumulate on the metal. The equilibria on thetwo electrodes are as follows:

Zn2+(aq) + 2e−� Zn (s)

2 H+(aq) + 2e−� H 2 (g)