The standard hydrogen electrode Flashcards
The SEP table shows us how likely a substance is to be reduced but only in comparison to the other substances on the table. To create this table, chemists needed to compare each substance to a standard or a reference point. What is this reference point?
The reference chosen was hydrogen and each substance on the table has been deemed to be more or less likely to reduce than hydrogen gas – and thus has
determined its position on the table.
How were e values generated?
In order to generate the E0 values, each 1⁄2 cell was connected to a standard hydrogen half-cell, and the potential difference measured (under standard
conditions).
What does the standard hydrogen half-cell consist of? (3)
o A platinum wire electrode over which H2 gas is bubbled at 1 atmosphere pressure.§ Pt is used as it is unreactive and adsorbs H2 gas effectively and because it catalyses the reaction between H2 and H+
o A 1 mol/dm3 solution of H+ ions
o At 250C
When being used to calculate Standard Electrode Potentials (i.e. those seen on the SEP table) it must be connected as the ___.
anode
What end of the terminal will the std hydrogen half cell be connected to?
This means that it will always be connected to the negative terminal of the voltmeter.
What happens to substances that are worse oxidising agents?
Substances that are worse oxidising agents (less likely to be reduced) than H+, will have their electrons pulled away by the H2 and so electrons will travel towards the hydrogen 1⁄2 cell. This results in a negative voltmeter reading and a negative E0 value.
What happens to substances that are better oxidising agents?
Substances that are better oxidising agents (more likely to be reduced) than H+ ions will pull electrons away from the H2 and electrons will travel away from the hydrogen 1⁄2 cell. This results in a positive voltmeter reading and a positive E0 value.
The E0 value for the reduction of H+ is __ V since it we are comparing the likelihood of reduction of H+ to itself (i.e., electrons will not transfer between the two 1⁄2 cells)
0
It is important to understand that although the Standard Hydrogen Electrode is connected as the anode, it may not necessarily be the site of ____ . That is determined solely by the chemical properties of the substances involved. Connecting the Standard Hydrogen Electrode as the anode only changes the sign of the voltmeter reading, allowing us to determine if the substance in the other 1⁄2 cell is being oxidised or reduced.
oxidation
Why is Pt used? (3)
- Reabsorbs gasses
- Good conductor
- Inert
What must we always assume?
That hydrogen always gets oxidised.
When using the Standard Hydrogen Electrode to measure the E0 value for Copper, the cell notation would be as follows:
Pt | H2 | H+ || Cu2+ | Cu
The voltmeter reading for this set-up would be +0,34 V.
When using the Standard Hydrogen Electrode to measure the E0 value for Zinc,
the cell notation would be as follows:
Pt | H2 | H+ || Zn2+ | Zn
The voltmeter reading for this set-up would be -0,76 V. The negative sign tells us that, in reality, the electrons are not moving from the H2 to the Zn2+ but are in fact
moving from the Zn to the H+. If asked for the oxidation 1⁄2 reaction, we would therefore say Zn–> Zn2+ + 2e- (even though this is not implied by the cell
notation).
Rate of reaction >
Equilibrium
E value is a measure of ___ point.
Equilibrium
