C23 Redox And Electrode Potentials Flashcards
What’s an oxidising agent
Electron acceptor
A species that’s reduced in a reaction and causes another species to be oxidised
What’s a reducing agent
Electron donor
A species that’s oxidised in a reaction & causes another species to be reduced
Define oxidation
Loss of electrons
An increase in oxidation number
Define reduction
Gain of electrons
Decrease in oxidation number
What happens in a redox reaction
Electrons transferred from 1 species to another
1 element is oxidised whilst other is reduced
Why’s 2CrO4^2- + 2H+ —> Cr2O7^2- + H2O not a redox reaction
Chromium is oxidised whereas hydrogen remains the same oxidation state (no element is reduced)
Define standard electrode potential
The e.m.f of a half-cell compared with a standard hydrogen half-cell measured at 298K with solution concentration of 1 mol dm^-3 and a gas pressure of 100kPa
What’s happens when a rod of a metal is dipped into a solution of its own ions?
An equilibrium is set up between the solid metal and the aqueous metal ions
Write a half-equation for zinc(s) to zinc(II)
Zn(s) <=> Zn^2+(aq) + 2e-
Write a half-equation for copper(II) to copper(III)
Cu^2+(aq) <=> Cu^3+(aq) + e-
What’s a standard hydrogen half-cell made of?
HCl 1mol dm^-3
Hydrogen gas at 100kPa
Inert platinum electrode
Why’s is hydrogen half-cell used as a standard half-cell
Easy to control its purity and reproducibility
How to make a simple salt bridge
Soak a piece of filter paper in an aq of KNO3 or NH4NO3
Why are salt bridges necessary
To complete the circuit by connecting the 2 solutions.
This enables charge to be transferred between the half-cells. They don’t react with electrodes.
Why might you use other standard electrodes occasionally?
They’re cheaper/easier/quicker to use and can provide just as good a reference.
Platinum is expensive
If an E^o value is more negative, what does it mean in terms of oxidising/reducing power
Better reducing agent (easier to oxidise)
If an E^o value is more positive, what does it mean in terms of oxidising/reducing power
Better oxidising agent (easier to reduce)
How do you calculate the emf of a cell from E^o values
E^o cell = E^o positive - E^o negative
When would you use a platinum electrode
When both the oxidised and reduced forms of the metal are in aq
Why is platinum chosen
Inert and good conductor to complete circuit
How would you predict if a reaction would occur
Take the 2 half equations
Find the species that is being reduced
Calculate its E^o value minus the E^o value of the species that is being oxidised
If E^o overall > 0.4V, reaction will occur
What are the 3 main types of electrochemical cells
Non-rechargeable cells
Rechargeable cells
Fuel cells
Describe how non rechargeable cells work
They provide electrical energy until all the chemicals have reacted
Describe how rechargeable cells work
Chemicals in the cell provide electrical energy. When recharging the reactions of the cells can be reversed
Give some examples of rechargeable cells
Nickel and cadmium batteries
Lithium ion batteries
Lithium polymer batteries
Explain why lithium is used in laptop batteries
Lithium has low density so the electrode is light and its very reactive
What are the drawbacks of using lithium batteries
They’re toxic if ingested
Rapid discharge of current can cause fore or even explosions
Describe how fuel cels work
The cell uses external supplies of fuel and an oxidant.
These external supplies need to be continuously supplied.
Modern fuel cells are based on what type of fuels
Hydrogen
Hydrogen rich fuels e.g. methanol
What are the reactions that take place at the 2 electrons in an alkaline hydrogen fuel cell
2H2 + 4OH^- —> 4H2O + 4e-
O2 + 2H2O + 4e^- —> 4OH-
What are the disadvantages of fuel cells
Hydrogen is a flammable gas with a low b.p. —> hard and dangerous to store and transport —> expensive to buy fuel cells have a limited lifetime and use toxic chemicals in their manufacture
What’s the reason that some cells cannot be recharged
Reaction of the cells is not reversible - a product is produced that either dissipates or cannot be converted back into the reactants
Why might the e.m.f of a cell change after a period of time
Concentrations of the ions change - the reagents are used up
How can the e.m.f of a cell be kept constant
Reagents are supplied constantly, so the concentrations of the ions are constant; E^o remains constant
