5.2.3 Redox and electrode potentials Flashcards
Define oxidising agent
accepts electrons from species being oxidised and is reduced
Define reducing agent
donates electrons to species being reduced and is oxidised
What are the steps for constructing a redox equation?
- Write half equations (work out oxidation numbers and electrons, ensure charges balance)
- Then add H2O to balance oxygens
- Add H+ to balance hydrogens
- Multiply each half equation to ensure there are equal numbers of electrons in each one
- Add equations together and cancel electrons
Example: Construct redox equation for reaction where MnO4- is reduced to Mn2+ and C2O4 2- is oxidised to CO2.
MnO4- + 5e- + 8H+ -> Mn2+ +4H2O
C2O42- -> 2CO2 + 2e-
= 2MnO4- + 16H+ +5C2O4 2- -> 2Mn2+ 8H2O + 10CO2
What can manganate titrations be used for? How do they work? What is the endpoint?
Goes from —– to —— if Mn2+ is in the burette
Goes from —- to —- if MnO4- is in the burette
- iron (II) ions
- ethanedioic acid (COOH)2
- end point.= permanent pink colour
- MnO4- (purple) turns into Mn2+ (colourless).
- SELF INDICATING
- colourless to PALE PINK
What acid must be used for manganate titrations? WHy?
DILUTE SULFURIC ACID
- Insufficient volume or weak acid = MnO2 produced instead
- HCl = Cl2 produced and greater manganate volume needed
- HNO3 = oxidising agent (we need reducing agent) and smaller volume of manganate will be used
What can iodine/thiosulfate titrations be used for? What colour changes occur?
- S2O3- oxidised
- I2 reduced
- can be used for ClO- or Cu2+ ions
aqueous iodine appears yellow brown
during titration, iodine reduced back to I- ions and brown colour fades gradually.
What indicator can be used for iodine/thiosulfate titrations?
STARCH
as end point approached and a pale straw colour is seen, add it = deep blue/black
as more sodium thiosulfate added, the colour fades -> at endpoint = colourless.
How can you prepare Cu2+ ions to then analyse in iodine/thiosulfate titrations?
- soluble salts: dissolve in water
- insoluble salts: react with acids
- alloys: reacted and dissolved in conc HNO3, then neutralisation
add to iodine - brown mixture
What do electrochemical cells do?
convert chemical energy to electrical energy
based on redox reactions
How can you create an overall current and avoid heat energy?
two half cells interact to create overall current (chemicals must never mix though, since flow wouldn’t be controlled and it’d be heat energy instead)
What is a metal/metal ion half cell?
- piece of metal dipped into solution of metal ions
- reactive metals tend to form ions, more unreactive tend to have positive charge on the metal.
What is an ion/ion half cell?
- solution containing ions of same element in diff oxidation states
- platinum inert electrode used
What is used when you combine the two half cells?
- filter paper soaked in aq concentrated ionic compound in order to balance charges = SALT BRIDGE
How else do you represent electrochemical cells?
R | O | | O | R
Define standard electrode potential
e.m.f of a half cell compared with a hydrogen half cell, under standard conditions. Hydrogen half cell = 0V
Any half cell with a negative electrode potential has a greater tendency to…
Any half cell with a positive electrode potential has a greater tendency to..
undergo OXIDATION e.g. magnesium
undergo REDUCTION e.g. silver
How do you measure a standard electrode? How do you calculate the electrode potential of an electrochemical cell?
- set up the half cell against a standard hydrogen one and see voltage recorded
- E(positive) - E(negative/less positive)
Student modifies experiment to determine a combined conc of iron(II) and iron (III) ions in river water. They add excess zinc to 250dcm^3 sample of river water and warm gently. Why? E of Zinc is -0.76, E of iron (III) ions is +0.77.
To ensure all the iron (III) ions in the riverwater are reduced to iron (II) ions. Zinc is the reducing agent since it has a more negative E than Fe3+.
Student modifies experiment to determine a combined conc of iron(II) and iron (III) ions in river water. They remove the excess zinc they added in step 1 by filtration. Why? E of Zinc is -0.76, E of iron (III) ions is +0.77. MnO4- E is +1.51.
- If unfiltered, MnO4- would oxidise zinc, since system 1 is more negative than 3. We do not want that affecting our titration results ( we do not want zinc ions in the solution)
What are the limitations of electrode potential predictions?
- Rate: high activation energy might mean reaction effectively doesn’t proceed due to very slow rate
- Concentration: if not 1M, this will affect eqm and the electrode potential
- Other factors: if other conditions aren’t standard, and electrode potentials only apply to aq eqm and so many reactions that cant take place in solutions cant have values.
What are primary cells?
non rechargeable, single use
chemicals will be used up, voltage will fall, cell will be recycled or discarded.
low current, long storage devices e.g. smoke detectors
most alkaline based on Zn/MnO2 and KOH electrolyte
What are secondary cells?
rechargeable.
e.g. lead-acid car batteries, NiMH in radios, lithium-ion polymer cells in laptops and cameras
What are fuel cells?
energy from fuel and oxygen used to create voltage
fuel and o2 flows into cell and products flow out
dont have to be recharged
Describe features of hydrogen fuel cells
no CO2 produced
alkali/acid electrolyte (but produce same cell voltages for either)
State one important feature of a fuel cell that is different from a conventional storage cell
Fuel reacts with oxygen to give electrical energy
