Redox and electrode potential chp 23 Flashcards
what is oxidation in terms of oxidation number
increase in oxidation number
what is reduction in terms of oxidation number
decrease in oxidation number
what will always be present in a redox reaction
- oxidising agent
- reducing agent
what is the role of oxidising agents in redox reactions
- oxidising agent takes electrons from species being oxidised (oxidising agent contains species that is reduced)
what is the procedure for carrying out manganate (VIII) titrations
- add potassium manganate, KMnO4, of known conc to burette
- add ? solution to conical flask.
- Add excess dilute H2SO4 to conical flask, H+ allows reduction of MnO4-
- End point is when slight pink colour is seen in conical flask (indicates excess MnO4-)
- repeat titration till concordant results
do not need indicator
what is the role of a reducing agent in redox reactions
- reducing agent adds electrons to species being reduced (reducing agent contains the species that is oxidised)
what is a tip to make it easier to write the overall redox equation for a reaction
- balance the electrons
^times the half equations so that the number of electrons is equal
what can manganate (VIII) titrations be used to analyse
- reducing agents that reduce MnO4^-to Mn^2+
what can iodine/thiosulfate titrations be used to determine
- the ClO- content is household bleach
- the Cu ^2+ content in copper (II) compounds
- the Cu content in copper alloys
concentrations
what colour changes can be seen when carrying out iodine/thiosulfate titrations
- when yellow/brown colour fades add starch solutions, turns blue/black
- blue/black fades as end point nears
- at end point blue/black disappears and goes clear
what is a voltaic cell
a type of electrochemical cell called a voltaic cell
what does a half cell contain
- contains the chemical species present in a redox half-equation
what is a half-cell
consists of a metal rod dipped into solution of its aqueous metal ions
how can a voltaic cell me made from half cells
by connecting 2 half cells, allowing electrons to flow
what does the flow of electrons between half cells depend upon
- the tendency of an electrode to release electrons (whether it is an oxidising or reducing agent)
what electrode is used if there is no metal to transport the electrons
- platinum
how do you know which metal electrode has a greater tendency too gain or lose electrons
- in a cell with 2 metal/metal ion electrodes the more reactive metal will release electrons more readily and will be oxidised
how can you tell which electrode is oxidised and reduced
- the electrode with more reactive metal loses electrons and is oxidised
- electrode with less reactive metal, gains electrons, is reduced
what is the standard electrode potential a measure of
whats the symbol for standard electrode potential
- the tendency to be reduced and gain electrons
- E^standard conditions
what is the standard electrode
- a half-cell containing hydrogen gas (pumped into HCl)
- an inert platinum electrode is used to allow electrons into and out of half-cell
How can the standard half-cell be used to find out the standard electrode potential of other half cells
the standard electrode potential is the electromotive force of the half-cell connected to a standard hydrogen half-cell, under standard conditions
what is the standard electrode potential of a hydrogen electrode
remember the units
- 0 v
- what must be placed between the 2 half cells to allow for a flow of ions
- what does it contain
- a salt bridge
- contains concentrated solution of an electrolyte that does not react with either solution
what does a negative standard electrode potential value mean
- the greater the tendency to lose electrons and undergo oxidation
- the less the tendency to gain electrons and undergo reduction
what does a positive standard electrode potential mean
- the greater the tendency to gain electrons and undergo reduction
- the less the tendency to lose electrons and undergo oxidation
what standard electrode potential values do metals and non-metals tend to have
- the more neg the value the greater the reactivity of a metal in losing electrons
- the more positive the value, the greater the reactivity of a non-metal in gaining electrons
what the equation to find out the overall electrode potential value of a reaction
- Ecell = E(right) - E(left)
what are some rules that can be used to write overall reactions from half equations
- the redox system with the more positive E value will react from left to right and gain electrons
- the redox system with the more negative E value will react from right to left and lose electrons
How can you predict the feasibility of redox reactions
- most negative system has the greatest tendency to lose electrons
- most positive system has the greatest tendency to gain electrons
- if this happens in reverse the reaction is not feasible
What are the limitations of using E values for prediciting redox reactions
- does not tell you any thing about the rate of reaction
- reactions can have large activation energy
- if concentrations and conditions not standard, value will differ from standard value
what are primary cells
- non-rechargeable
- only used once
- chemical energy converted to electrical energy
- reaction not reversible
what are examples of primary cells
- carbon-zinc dry cells
- alkaline dry cells
what are secondary cells
- rechargeable
- chemical energy converted to electrical energy (is a reversible reaction)
what are some examples of secondary cells
- lead-acid batteries
- lithium-ions cells
what are fuel cells
- uses energy from reaction of a fuel with oxygen to create voltage
- fuel and oxygen flow into fuel cell and products flow out, electrolyte remains in cell
- can operate continuously as far as fuel and oxygen are supplied
what are hydrogen fuel cells
- can have either alkali or acid electrolyte
- reactants are hydrogen and oxygen
- products are water
