Electrode potentials Flashcards
What happens when a metal is placed in a solution of its ions?
An equilibrium is established between the ions in solution and the metal atoms
Represent the equilibrium by a half equation
M^n+(aq) + ne- ⇌ M(s)
What are the half equations always written as
Reductions, Electrons on LHS
What are two half cells combined called?
A cell
What is the standard electrode potential?
The potential difference measured when a half cell is connected to the standard hydrogen electrode under standard conditions 1moldm-3 and 298K
What are electrode potentials represented by and what are they measured in?
Represented by E⦵ and measured in Volts
What is the electrode potential?
A measure of the feasibility of the reduction occurring
What does a negative E⦵ suggest?
That the reduction will not occur unless a more feasible oxidation takes place
What is emf?
The potential difference measured when two half cells are connected
What does a positive emf indicate?
A feasible reaction
What does a negative emf indicate?
Unfeasible reaction
What does the emf always relate to?
The overall redox equation, oxidation and reduction combined
What are the five features of the standard hydrogen electrode?
- Platinum electrode
- 1.00 moldm-3 hydrogen ions in solution
- H2 gas at 100kPa
- Temperature 298K
- Standard electrode potential = 0.00V
How do you combine two half cells?
Combined using a salt bridge and an external circuit using a high resistance voltmeter to produce a cell
How do you make the salt bridge?
Filter paper soaked in potassium nitrate solution which dips into both solutions
What is the purpose of the salt bridge?
Allows electrical connection between the two half cells without them having to mix
What usually are the left and right hand cells?
Left hand cell is the one in which oxidation occurs and is the negative electrode of the cell
Right hand cell is the one in which the reduction occurs. It is the positive electrode of the cell
How do you remember where oxidation occurs
NEGATOX
Negative electrode is the one at which oxidation occurs
Draw the diagram of a standard hydrogen half cell and a zinc half cells
in book
How do you calculate the emf of a cell?
The emf of a cell is calculated by simply adding the two values together where you have switched the sign of the most negative so it is an oxidation
Describe non standard conditions in a cell
Two identical cells will show no overall charge, however, two cells that are the same except for concentration of ions will show a reduction where the ion in solution is at a higher concentration
What is an ammeter used in a non standard conditions in a cell?
The ammeter allows current to flow
What will you observe in non standard conditions?
The mass of the ion in the solution of greater concentration will increase as the metal is made
How are conventional cells representations written?
Oxidation at left hand half-cell⎮⎮ Reduction at right hand half-cell
Give an example of the conventional cell representation of
Zn2+ + 2e- → Zn E⦵ = -0.76V
Cu2+ + 2e- → Cu E⦵ = +0.34V
Rewrite as: Zn → Zn2+ +2e- E⦵ = +0.76 Cu2+ +2e- → Cu E⦵ = +0.34 Then in conventional cell representation Zn⎟Zn2+ ⎟⎟Cu2+ ⎟Cu
Overall emf = +0.74 + +0.34 = +1.10V therefore feasible
What are the three things about phase boundaries
- ⎟is used when two species are in different states
- , is used when two species are in the same state
- Pt should be included if it is used as the electrode for gaseous electrodes
What does a positive emf mean?
The reaction is feasible
What does the oxidation emf of a metal give?
The reactivity of the metal, the more positive the oxidation the more reactive the metal.
What happens to the reactivity of the halogens down the group?
Decreases, as you go down the halogens they are less easily reduced
Describe oxidising and reducing agents
on page
What are the three main types of cells
Primary cells - Non rechargeable
Secondary cells - Rechargeable
Fuel cells - continually produce electric current as long as they’re supplied with a fuel
Which way do the electrons flow in a battery
From the negative end (where oxidation occurs and electrons are lost) to the positive end (where reduction occurs and electrons are gained)
Draw a typical primary cell
in book
What is the overall ionic equation for a primary cell
Zn + 2MnO2 + 2H2O → Zn2+ + 2MnO(OH)2 + 2OH-
What is the purpose of the carbon rod?
The carbon rod carries eletrons back to the MnO2 paste
Write the reduction of ammonium ions
2NH4+ + 2e- → 2NH3 + H2
What is the overall equation for the old type of cells ?
Zn + 2NH4+ → Zn2+ +2NH3 +H2
In the nickel cadmium, lead-acid and lithium ion cell give:
- What happens at the negative electrode
- What happens at the positive electrode
- The overall ionic equation
on page
What is the conventional cell representation of the lithium ion cell
Li⎟Li+⎮⎮Li+ , CoO2 ⎟LiCoO2⎟Pt
What do fuel cells do?
Convert chemical energy into electrical energy
What are the different types of fuel cells?
Acidic hydrogen fuel cell, alkaline hydrogen fuel cell, Ethanol fuel cell
Hydrogen fuel cell alkaline
- Negative electrode
- Positive electrode
- Overall equation
- Oxidation: H2 + 2OH- → 2H2O + 2e-
- Reduction: O2 + 2H2O + 4e- → 4OH-
- 2H2 + O2 →2H2O
What is the conventional cell representation for the hydrogen fuel cell alkaline?
Pt ⎮H2⎮OH- , H2O⎟⎟O2 ⎮H2O , OH- ⎟Pt
What are environmental issues of all the different cells?
- Primary cells - source of waste
- Secondary cells - Finite lifespan
- Fuel cells - Constantly producing current while fuel is supplied
Why are phones bad
Contain precious metals and so should be recycled
What is the con of hydrogen fuel cells
Theyre carbon neutral but EXTREMELY hard to maintain
