✅14 - Redox II Flashcards
What is oxidation in terms of electrons?
Loss of electrons
What is reduction in terms of electrons?
Gain of electrons
What is oxidation in terms of oxidation number?
Increase in oxidation number
What is reduction in terms of oxidation number?
Decrease in oxidation number
Redox reactions are in…
…equilibria
What happens to the electrons when a metal is placed in water?
The electrons build up on the surface and result in a negative charge
How are positive ions attracted to a metal in water?
The build up of electrons on the surface attracts them, forming a layer of positive ions
How do some of the positive ions in solution form part of the metal?
Some of the positive ions in the layer regain their electrons from the build up of electrons below, becoming part of the metal surface
How is a dynamic equilibrium established when a metal is placed in water?
The rate at which ions are leaving the surface of the metal to go into solution is about the same as the rate at which they join the metal from solution
What is the absolute potential difference?
The potential difference between the metal and the solution
How can the absolute potential difference be measured?
By creating a reference electrode and measuring the difference in potential between this reference electrode and the metal electrode
Which reference electrode is usually used to calculate absolute potential difference?
Standard Hydrogen electrode
What is a standard hydrogen electrode?
Hydrogen gas at a pressure of 100kPa bubbling over a piece of platinum foil dipped into a solution of HCl with a hydrogen ion concentration of 1 moldm-3 at 298K
What does the platinum foil in a standard hydrogen electrode allow?
An equilibrium between hydrogen ions in solution and hydrogen gas to be established quickly due to the porous platinum
What are standard conditions?
100kPa (1 bar) gas pressure
298K
Ion concentration of 1 moldm-3
What is a complete cell made up of?
Two half cells
How could the standard electrode potential of a Magnesium ion/metal system be measured?
By connecting the standard hydrogen electrode to the Magnesium electrode via a circuit containing a high resistance voltmeter
Why is a salt bridge needed?
To complete the circuit by allowing the movement of ions
What is a salt bridge usually made of?
Potassium Nitrate as a gel or liquid
What property should the salt bridge have?
Its ions should not interfere with the components of the half cells
Why is a high resistance voltmeter used?
To avoid any flow of current through the circuit
What are the two reactions which occur in a SHE?
H+(aq) + e- ⇌ 1/2H2(g)
Mg2+(aq) + 2e- ⇌ Mg(s)
Why is magnesium the negative electrode of the cell in SHE?
Because if the voltmeter was removed, electrons would flow from the magnesium electrode to the hydrogen electrode
What does the negative sign in front of a standard electrode potential indicate?
That the equilibrium position of this reaction is further to the left than the equilibrium of the hydrogen electrode
What does a positive sign in front of a standard electrode potential indicate?
That the equilibrium position of the reaction is further to the right than the equilibrium position of the standard hydrogen electrode
What does a negative electrode value indicate about electron movement?
The more readily the metal looses electrons to form ions
What does a positive electrode value indicate about electron movement?
The less readily the metal loses electrons to form ions
What is the electromotive force (emf)?
The potential difference when no current is flowing through the external circuit
What is the standard electrode potential of a half-cell?
The emf of a cell containing the half-cell connected to the standard hydrogen electrode
The species on the right hand side of a cell is capable of being…
A reducing agent
What is the least powerful reducing agent?
F- as it is the ion with the least negative redox potential
What is the reference point when measuring standard electrode potentials?
The standard hydrogen electrode, set at zero
The more negative the E0 value….
…the more the equilibrium lies to the left and the more readily the species to the right looses electrons. The more powerful the reducing agent.
The less positive the E0 value…
…the more the equilibrium lies to the right and the more readily the species on the left loses electrons. The more powerful the oxidising agent
What do solid vertical lines in cell notation indicate?
A phase boundary
What is used to separate species of the same state in cell notation?
A comma
Where are the two reduced forms shown in cell notation?
On the outside of the notation
Where is the positive electrode shown in cell notation?
On the right hand side
How is a salt bridge represented in cell notation?
By a double dotted line
How is the E0 cell calculated?
By calculating the difference between the electrode potentials of the two half cells
Where in cell notation is the hydrogen electrode always written?
On the left hand side
Why may a reaction not take place despite it being thermodynamically feasible?
The reactants may be kinetically stable because the activation energy is very large
The reaction may not be taking place under standard conditions
What can changing conditions alter?
The electrode potential of the half cell, because the position of equilibrium may change
How can you tell if a reaction will be feasible?
If E0cell is positive
What is the relationship between Gibbs energy and E0cell?
The more positive the E0cell value, the more negative the Gibbs energy
What is the relationship between entropy and E0cell?
Proportional, so as one increases, so does the other
What is the relationship between lnK and E0cell?
Proportional, so as one increases, so does the other
What is a storage cell?
A cell that can be recharged by passing a current through it int he opposite direction to the flow of current generated by the cell.
What was one of the first storage cells to be invented?
The Nickel-Cadmium cell, NiCd
What is the positive electrode in the NiCd cell?
NiO(OH)
What is the negative electrode in the NiCd cell?
Cadmium
What is the electrolyte in the NiCd cell?
Aqueous Potassium Hydroxide
What is a fuel cell?
One which produces a voltage from the chemical reaction of a fuel with oxygen
What is the most common fuel cell?
Hydrogen
What are both electrodes coated with in the hydrogen fuel cell?
Platinum
What is the overall cell reaction for the hydrogen fuel cell in acidic conditions?
H2 (g) + 1/2O2(g) —> H2O(l)
What is the overall cell reaction for the hydrogen fuel cell in alkaline conditions?
H2 (g) + 1/2O2(g) —> H2O(l)
What are the advantages of hydrogen fuel cells?
Offer an alternative to the direct use of fossil fuels such as petrol and diesel
Products do not include pollutants, only water
Lighter and more efficient than other engines
What are the disadvantages of hydrogen fuel cells?
Compressing the gas can be dangerous
Lots of energy is needed to get it from water by electrolysis, requiring the burning of fossil fuels
Can also be produced from non-renewable sources
What is usually used as a quantitative estimation for reducing agents?
Potassium Manganate (VII)
What is the overall equation for the titration of Iron (II) with Potassium Manganate?
MnO4- + 8H+ + 5Fe2+ —> Mn2+ + 4H2O +5Fe3+
What is the colour change in the Potassium Manganate titration?
Purple to colourless
