Redox And Electrode Potentials Flashcards
What is oxidising and reducing agent
Oxidising agent takes electrons from species being oxidised, oxidising agent contains species which is reduced
Reducing agent is reverse
How to construct a redox equation using half equation
See card 1
How to construct redox equation using oxidation numbers
Use this method when hydrogen and oxygen appear in more than one product
See card 2
How to predict products of redox reactions
In aq redox reactions H2O often formed, or H+ or OH- ions
As a final check ensure both sides of equation are balanced by charge
How it write half equation
See card 3
The two common redox reaction
Potassium manganate (VII) (KMnO4(aq)) under acidic conditions Sodium thiosulfate (Na2S2O3(aq)) for determination of iodine (I2(aq))
Manganate titrations
Standard solution of KMnO4 added to burette.
Using pipette add measured vol of solution being analysed to conical flask, an excess of dilute sulfuric acid is also added to provide H+ ions required for reduction of MnO4- ions. No indicator needed, reaction is self-indicating.
During titration manganate solution reacts and is decolourised as it is being added, endpoint of titration is judged by first permanent pink colour, indicating when there is excess of MnO4- ions present. In titrations this endpt is one of easiest to judge.
Repeat titration until u obtain concordant titres (+-0.1 cm3)
How to read meniscus
KMnO4 is deep purple, difficult to see bottom of meniscus, burette readings read from top rather than bottom of meniscus.
Titre is difference between two readings
Manganate titrations can be used for analysis of many different reducing agents e.g.
Iron ions, Fe2+ Ethanedioic acid (COOH)2
How to analyse percentage purity
See card 4
Determination of a formula
See card 5
What can be analysed
Manganate titrations can be used to analyse reducing agents that reduce MnO4- to Mn2+.
KMnO4 can be replaced with other oxidising agents, e.g. acidified dichromate
Iodine/ thiosulfate titrations
See card 6
Iodine thiosulfate titrations can be used to determine:
The ClO- content in bleach
The Cu2+ content in copper (II) compounds
Th Cu content in copper alloys
Procedure of iodine thiosulfate titrations
- Add standard solution of Na2S2O3 to burette
- Prepare solution of oxidising agent to be analysed, using a pipettes add this solution to conical flask, then add excess of potassium iodide, oxidising agent reacts with iodide ions to produce iodine, which turns solution yellow brown
- Titrate solution with Na2S2O3, during titrations iodine reduced back to I- ions and brown colour fades making it difficult to decide endpoint, so use starch indicator, when end point is being approached iodine colour has faded enough to become pale straw colour
- Starch indicator added….
Why is starch used for endpoint
When endpoint being approached and iodine colour faded enough to become pale straw colour small amount of starch indicator added. A deep blue black colour forms to assist with identification of endpoint. As more sodium thiosulfate added blue black colour fades, at endpoint all iodine reacted and blue black colour disappears
Active ingredient of bleach
Chlorate (I) ions ClO- also known as hypochlorite
Bleach is NaClO
Titration of ClO- with thiosulfate equation
See card 7
How to find conc of ClO- in bleach
See card 8
Iodine thiosulfate titrations can be used to determine
Copper content of copper (II) salts or alloys For copper (II) salts Cu2+ ion produced simply by dissolving compound in water Insoluble copper (II) compounds can be reacted with acids to form Cu2+ ions
Analysis of copper
- For copper alloys eg brass or bronze alloy is reacted and dissolved in concentrated nitric acid followed by neutralisation to form Cu2+ ions.
- Cu2+ ions react with I- to form solution I2 and white precipitate of copper (I) iodide, CuI (s). Mix appears as brown colour
- Iodine in brown mix then titrations with standard solution of sodium thiosulfate
See card 9
What is a voltaic cell
Converts chemical energy into electrical energy
What is a half cell
Contains Chemical species present in a redox half equation.
Two of these connected together form a voltaic cell, allowing electrons to flow
In the cell why are the chemicals in the two half cells kept apart
If allowed to mix electrons would flow in an uncontrollable way and heat energy would be released rather than electrical energy
Metal/ metal ion half cells
Metal rod dipped in solution of its aqueous metal ion.
Represented using vertical line for phase boundary between aqueous solution and metal
See card 10
What is equilibrium in a half cell
At the phase boundary, where the metal is in contact with its ions.
It is written so that forward reaction shows reduction and reverse reaction shows oxidation
See card 11
In an isolated half cell
No net transfer of electrons in or out of metal
When two half cells are connected direction of electron flow
Depends on relative tendency of each electrode to release electrons
Ion/ion half cells
Contains ions of same element in different oxidation states
E.g. see card 12
In this half cell there is no metal to transport electrons into or out of half cell so insert metal electrode made of platinum is used
Which electrode has greater tendency to lose electrons
In a cell with two metal/ metal ion half cells connected, the more reactive metal releases electrons more readily and is oxidised
In an operating cells which metal loses electrons
The more reactive metal loses electrons and is oxidised, this is negative electrode
Reverse for less reactive metal
Standard electrode potential
Tendency to be reduced and gain electrons.
Standard chosen is a half cell containing hydrogen gas H2(g) and a solution containing H+ (aq) ions.
Insert platinum electrode used to allow electrons in and out of half cell
Sign of standard electrode potential shows sigh of
Half cell connected to standard hydrogen electrode and shows the relative tendency to gain electrons compared with hydrogen half cell
How to measure a standard electrode potential
Half cell connected to a standard hydrogen electrode
See card 13
The more negative the E value
The greater the tendency to lose electrons and undergo oxidation.
Th greater the reactivity of metal in losing electrons
More positive the E value
The greater the tendency to gain electrons and undergo reduction.
Greater reactivity of non metal in gaining electrons
Metals tend to have … E values
Negative.
They lose electrons.
Reverse for non metals
emf measured is then a
Cell potential
How to measure a standard cell potential (the experiment)
Prepare 2 standard half cells(temp 298K): for metal half cell conc of metal ion: 1 Moldm-3, for ion cell metal ions must have same conc and inert electrode eg platinum present, for half cell contains gases gas must be at 100 kPa in contact with solution conc 1 moldm-3 and inert electrode.
Connect metal electrodes to voltmeter with wires.
Prep salt bridge, by soaking filter paper in aq KNO3.
Connect solutions with salt bridge.
Record standard cell potential from voltmeter
How to calculate standard cell potential from standard electrode potentials
See card 13
Limitations of predictions using E values
Reactions that have very large activation energy result in very slow rate, electrode potentials indicate feasibility of reaction but not rate of reaction.
Non standard conditions will affect the electrode potentials and therefore the overall cell potential.
Standard electrode potentials apply to aqueous equilibria, many reactions that take place are not aqueous.
Primary cells
Non rechargeable.
Used once only.
Chemicals eventually used up
Secondary cells
Rechargeable.
Unlike primary cells Cell reaction producing electrical energy can be reversed during recharging.
Chemicals in cell then regenerated.
Fuel cells
Uses energy from reaction of fuel with oxygen to create voltage.
Fuel and oxygen flow into fuel cell, products flow out, electrolyte remains in cell.
Can operate continuously when fuel and oxygen supplied into cell.
Do not have to be recharged.
Hydrogen fuel cells produces no CO2 only water as combustion product
Alkali hydrogen fuel cell
See card 14
Acid hydrogen fuel cell
See card 15
