Topic 14: Redox II 1️⃣ Flashcards
What is the absolute potential difference referring to?
The potential difference between the metal and the solution
Why is it not possible to measure the absolute potential difference between the metal and solution ?
How is this overcome?
Although one terminal of a voltmeter can be connected to the metal electrode the other end can’t be connected to the solution
Solution : create a reference electrode and then measure the difference in potential between the reference electrode and the metal electrode
What is the reference electrode of choice ?
Standard hydrogen electrode
Consists of hydrogen gas bubbling over a piece of platinum foil dipped into a solution of hydrochloric/sulfuric acid
All under standard conditions : 100kPa, 298K, H+ ion conc of 1moldm-3
Describe the steps in setting up apparatus to mesure standard electrode potential
Connect the standard hydrogen electrode to the magnesium electrode via a circuit
Contains a high resistance voltmeter (no flow of electrons around external circuit hence both half cell reactions are in equilibrium)
Two components are known as half cells
Salt bride needed to complete the circuit (allows the movement of ions)
What does the sign of a standard electrode potential indicate ?
Polarity go the electrode relative to the hydrogen electrode
A negative sign (-) indicates that the metal electrode is negative with respect to the hydrogen electrode whilst a positive sign (+) indicates the reverse
What do E° values tell us ?
Provide a method of comparing the positions of equilibrium when metal atoms lose electrons to form ions in solution
- the more negative the E° value, the further the equilibrium lies to the left
- the more positive the E° value, the further the equilibrium lies to the right
If the E° values mean the equilibrium lies further left what does this mean ?
The more readily the metal loses its electrons to form ions
eg. Mg2+ (aq) + 2e- ⇌ Mg (s)
If the E° values mean the equilibrium lies further right what does this mean ?
The less readily the metal loses its electrons to form ions
eg. Mg2+ (aq) + 2e- ⇌ Mg (s)
What is the definition of emf
Electromotive force (emf) = the standard electrode potential of a half cell (measured under standard conditions: 298K, 100kPa, 1moldm3) connected to a standard hydrogen electrode
When will emf values be positive/negative
In a cell in which the hydrogen electrode is the positive electrode, the emf will have a negative value
In a cell in which the hydrogen electrode is the negative electrode, the emf will have a positive value
Describe how you would set up apparatus to measure standard electrode potentials of systems involving gases
Example : 1/2Cl2 (g) + e- ⇌ Cl (aq)
- Set up half cell where chlorine gas is bubbled into solution containing chloride ions
- To establish equilibrium between chlorine molecules and chloride ions and provide electrical connection to external circuit platinum is added to solution
- Half cell is then connected to standard hydrogen electrode and emf is measured
Describe how you would set up apparatus to measure standard electrode potentials of systems involving ions of same element with different oxidation numbers
Example : Fe3+ (aq) | Fe2+ (aq)
- half cell containing a solution of both Fe2+ and Fe3+ (each conc of 1moldm3) & platinum
- connected to standard hydrogen electrode
What is the electrochemical series
The arrangement of various redox equilibria in order of their standard electrode potentials
Found in data booklet
Explain the trend in power of reducing agents on the electrochemical series
The most powerful reducing agents have the most negative E° values (those at the top of the chart)
Their equilibrium lie furthest left (can more readily lose electrons)
Describe how to draw a complete cell diagram
The relatively more positive half cell goes on the right and more negative goes on left
Example:
Zn2+ (aq) + 2e- ⇌ Zn (s) E° = -0.76V
Cu2+ (aq) + 2e- ⇌ Cu (s) E° = +0.34V
Zn(s)|Zn2+(aq)||Cu2+(aq)|Cu(s)
When the electrode, ie. the electrical connection between the solution and the external circuit, is a piece of platinum foil how are cell diagrams denoted ?
Pt(s) | Fe3+(aq), Fe2+(aq)
Give the cell diagram of the standard hydrogen electrode
H+(aq) | 1/2H2(g) | Pt (s)
How do you calculate the E°cell values form the complete cell diagram below:
Zn(s)|Zn2+(aq)||Cu2+(aq)|Cu(s)
note: standard electrode potential value for Zn = -0.76V and for Cu = +0.34V
Right - left
so…
+0.34V - (-0.76V) = +1.10V
When do we break cell convention when drawing cell diagrams
When drawing diagrams to reprint the measurement of s standard electrode potential, the standard hydrogen electrode is always written on the left hand side
Example:
Pt(s) | 1/2H2(g) | H+(aq) || Zn2+(aq) |Zn(s)
Give the direction of energy flow between half cells
Electrons flow form the half cell with the more negative E° value to the half cell with the less negative E° value
How can you determine from the Ecell value whether a reaction is thermodynamically feasible
If value is positive then reaction is feasible
E cell =
E (reduced) - E (oxidised)
Give the steps in setting up an electrochemical cell
- obtain metals under investigation and clean with sandpaper to ensure impurities are removed
- wash surface with propanone to removed grease from surface
- wear gloves throughout cleaning process
- place each metal into a solution containing the ion of the same metal
- if using an oxidising agent that contains oxygen you need to add acid
-make salt bridge from filter paper soaked in KNO3 - connect the electrodes to the voltmeter using wires
Why is a Pt electrode used for half cells of aqueous ions eg Fe2+ (aq) | Fe3+ (aq)
Inert but electrically conductive
