redox + electrode potentials Flashcards
what are the half equations for MnO4- and Fe2+
MnO4- + 8H+ + 5e-»_space; Mn2+ + 4H2O
- Mn is reduced
Fe2+»_space; Fe3+ + e-
- Fe is oxidised
what is the full equation for the redox titration reaction between MnO4- and Fe2+
MnO4- + 5Fe2+ + 8H+»_space; 5Fe3+ + Mn2+ + 4H2O
what colour change is seen in redox titrations involving manganate (VII)
a natural colour change of pink»_space; colourless is seen
- this titration is self indicating, as MnO4- = pink and everything on the RHS is colourless
when the solution turns permanently pink/colourless this indicates there is no more Fe2+/MnO4- to react with
what are the half equations for iodine I2 and thiosulfate S2O2 2-
I2 + 2e-»_space; 2I-
- iodine is reduced
2S2O3 2-»_space; S4O6 2- + 2e-
- thiosulfate is oxidised
what is the full equation for the redox titration reaction between iodine I2 and thiosulfate S2O2 2-
2S2O3 2- + I2»_space; 2I- + S4O6 2-
what indicator is used in an iodine/thiosulfate redox titration + why
iodine is naturally brown-yellow, as the titration occurs this colour fades to yellow then to colourless - as the end point is approached (solution is the colour of hay) starch is added, turning the solution blue-black, which makes the end point more easily observable
at the end point the blue-black colour will have disappeared and the solution will be colourless
oxidising agent definition
a substance that accepts electrons from another species in a redox reaction, becoming reduced itself and oxidising the donor species
reducing agent definition
a substance that donate electrons to another species in a redox reaction, being oxidised itself and reducing the receiving species
oxidised definition
the loss of electrons
reduced definition
the gain of electrons
how do you find the correct number and placement of electrons when constructing half equations
use oxidation numbers, moles and ion charges to find the total for both sides of the equation, then add electrons on either side to balance them
e.g. +8-1 on LHS and +2 on RHS
= +7 on LHS and +2 on RHS
difference is 5 so place 5e- on LHS to balance 2 sides (as +7-5=+2)
what is a trick for finding the number of water molecules made in a half equation
same as the number of oxygens available, and then add missing hydrogens as H+ ions - should be double the oxygens
what is a source of H+ ions
acid
standard electrode potential definition
the tendency to be reduced and gain electrons, relative to a hydrogen H2(g) half cell
how can you tell which half cell will be reduced and which one will be oxidised
the half cell with the more + electrode potential will be reduced
what 5 features need to be included in a diagram of 2 half cells + extra features if it is the diagram for the determination of standard electrode potential vs H2 half cell
- the 2 half equations happening at each electrode
- labels for the 1 moldm-3 solutions and solid material of the electrodes
- the voltmeter
- the salt bridge
- an indication of which is the positive and negative half cell
- the standard conditions of 298K + 100kPa
- the specific apparatus of a hydrogen half cell
what is the standard electrode potential of a hydrogen half cell
0V
how do you calculate whole cell electrode potentials
(E cell)
E anode - E cathode
what are the names of positive and negative electrodes
+ = cathode
- = anode
what is the purpose of a salt bridge
allows ions from electrolyte to travel between the 2 solutions, acting as charge carriers, completing the circuit and allowing half cells to remain electrically neutral
the salt bridge contains salt solution e.g. KNO3
at which electrodes do reduction + oxidation take place
reduction occurs at the cathode (red cat)
oxidation occurs at the anode
what are 3 limitations of predicting reactions using E values
- reaction rate - predictions for feasibility ΔG give no indication of Ea or reaction rate
- concentrations - E is measured with 1 moldm-3 solutions, if using different concs E will be different
- conditions - if standard conditions aren’t used then E will be different
3 types of cells
- primary cells
- secondary cells
- fuel cells
outline the function + uses of a primary cell
- they are non rechargeable/single use
- electrical energy is produced by oxidation and reduction at electrodes, these reactions cannot be reversed
- often used for low current, long storage devices e.g. wall clocks
outline the function + uses of a secondary cell
- rechargeable
- cell reaction that produces electrical energy can be reversed by recharging, so chemicals are regenerated
- used for car batteries, cylindrical batteries, modern appliances
outline the function + uses of a fuel cell
- do not need to be recharged
- electrical energy is generated from the reaction of a fuel with O2
- as long as fuel and O2 are available fuel cells can operate continually
electrochemical cells in phones are based on Li+ ion transport - what are 2 risks of these if phone is damaged
- risk of fire
- Li+ ions are toxic
2 reasons why methanol is a good fuel source
- it is a liquid so easy to transport, store and use
- doesn’t require extreme pressures or temps to store
3 reasons why hydrogen is not often used as a fuel source
- as a gas it is difficult to store + transport – requires high pressures + low temps
- it is highly flammable
- not as energy efficient as other fuel sources
