Electrode Potentials Flashcards
(39 cards)
describe the link between E° and oxidation
- the more negative the E° value of a substance is, the more likely it is to be oxidised (better reducing agent)
what are the main components of a cell
- high resistance voltmeter
- salt bridge
- electrodes (half cell)
what would a salt bridge typically be made out of and why
- KNO₃
- doesn’t react with any of the ions in the solution
- allows the movement of ions
what are the standard conditions in an electrochemical cell
- 298 K
- 100 kPa
- 1.00 moldm⁻³
what electrode goes on the left
- the negative electrode except from when measuring the SHE where hydrogen electrode goes on the left
what are the components of a metal electrode
- high resistance voltmeter
- metal electrode
- solution containing aqueous metal ion (1.00 moldm⁻³)
- salt bridge
what are the components of a gas electrode
- high resistance voltmeter
- salt bridge
- gas pump with gas (100kPa)
- solution containing aqueous ion (1.00 moldm⁻³)
- solid platinum block in solution
what are the components of a redox electrode (different ions of same element)
- solution containing both aqueous ions (1.00 moldm⁻³)
- solid platinum block in solution
- high resistance voltmeter
- salt bridge
what is the E° for the standard hydrogen electrode (SHE) and why
- 0
- by definition
what is the half equation for the SHE
H⁺ (aq) + e⁻ ⇌ 0.5H (g)
what does a more negative E° value suggest
- its a better reducing agent
what does a more positive E° value suggest
- its a better oxidising agent
how does the E° value affect the direction of a reversible reaction
- more negative E° goes left
- more positive E° goes right
how to explain if a reaction is feasible or not using E° values
- E° x/x⁺ > E° y/y⁺ so x⁺ is reduced to x and y is oxidised to y⁺
how to calculate the E° for a cell
E°cell = E°(reduction) - E°(oxidation)
(positive E° - negative E°)
what does the E° value of a cell say about the feasibility of the reaction
- positive E° means the reaction is feasible, negative means its not
- more positive E° = more feasible
what does the double line in a conventional cell diagram represent
- salt bridge
when do we use a line/comma between components of a cell
- line - cell components are in different phases (states)
- comma - cell components in different phases or are liquid and aqueous
how does a conventional cell diagram look when measuring the SHE
- the standard hydrogen electrode always appears on the left hand side of the conventional cell diagram
how does the concentration affect the E° value of the half cell
- the change in concentration of a substance causes equilibrium to shift towards a certain direction
- shift in the oxidation direction (leftwards) makes the E° value more negative
- shift in the reduction direction (rightwards) makes the E° value more positive
what is an example of a non-rechargeable cell
- zinc/carbon cells (also known as Daniell cells)
what equations occur in zinc/carbon cells
half equations:
Zn + 2NH₃ ⇌ Zn(NH₃)₂²⁺ + 2e⁻
2MnO₂ + 2H⁺ + 2e⁻ ⇌ Mn₂O₃ + H₂O
overall equation:
Zn + 2NH₃ + 2MnO₂ + 2H⁺ → Mn₂O₃ + H₂O + Zn(NH₃)₂²⁺
why does the the zinc/carbon cells often leak after long usage
- zinc is oxidised to Zn²⁺ causing the case (made of zinc) to wear away
what is the difference between zinc/carbon cell and alkaline batteries
- longer life but costs more
