Module 5: Chapter 23 (Redox and Electrode Potentials)
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What is oxidation?
loss of electrons or increase in oxidation number
What is reduction?
loss of electrons or decrease in oxidation number
What is an oxidising agent?
• takes electrons from the species that is being oxidised • contains the species that is reduced • for example: oxygen, dichromate, manganate (VII)
What is a reducing agent?
• adds electrons to the species being oxidised • contains the species that is being oxidised • for example: magnesium, zinc, sulfur dioxide, hydrogen and Fe2+
How do you write a redox equation from half-equations?
• balance the electrons • add the equations and cancel out the electrons • cancel any species that is on both sides of the equation
Write a full equation: H2O2 + 2e- -> 2OH- Cr^3+ + 8OH- -> CrO4^2- + 4H2O + 3e-
3H2O2 + 2Cr^3+ + 10OH- -> 2CrO4^2- + 8H2O
What are the rules for oxidation numbers?
• Group one elements = +1 • group two elements = +2 • F = -1 • H = +1 (unless in a metal hydride = -1) • O = -2 • Cl (and descending halides) = -1
How to write a redox equation from oxidation numbers?
• summarise any info provided into an equation • assign oxidation numbers • balance only the species that contain elements that have changed oxidation numbers • balance any remaining atoms (H+ or H2O may have to be added to balance)
Balance: S + HNO3 -> HSO4 + NO2 + H2O
S + 6HNO3 -> HSO4 + 6NO2 + 2H2O
Balance: BrO3^- + Br- + H+ -> Br2 + H2O
BrO3^- + 5Br- + 6H+ -> 3Br2 + 3H2O
Balance (in acidic solution): MnO4^- + SO3^2- -> SO4^2- + Mn^2+
6H+ + 2MnO4^- + 5SO3^2- -> 5SO4^2- + 2Mn^2+ + 3H2O
Balance (in acidic solution): Cr2O7^2- + Sn^2+ -> Sn^4+ + Cr^3+
Cr2O7^2- + 3Sn^2+ + 14H+ -> 3Sn^4+ + 2Cr^3+ + 7H2O
What happens overall in a manganate (VII) titration?
• carried out under acidic conditions - MnO4- ions are reduced to Mn2+ and the other chemical is oxidised
What is the general method for manganate (VII) titrations?
• standard solution of potassium manganate (VII) fills the burette • the other solution is pipetted into a conical flask, along with an excess of dilute sulfuric acid (acid provides the H+ ions) • during the titration the manganate (VII) solution reacts and is decolourised. (MnO4- is pink/purple, Mn2+ is very pale/pink and often appears colourless) • the end point is the first permanent pink colour (no indicator required) • Repeat until concordant titres are obtained
What is the equation for the overall manganate equation?
5Fe2+ + MnO4- + 8H+ -> 5Fe3+ + Mn2+ + 4H2O OR 5(COOH)2 + 2MnO4^- -> 16H+ -> 10CO2 + 10H+ Mn^2+ + 8H2O
Calculating the percentage purity of a product from manganate titration (e.g. FeSO4•7H2O)
(use a full balanced equation as reference) • Calculate the amount of MnO4- that reacted using mean titre in n = c x V • Determine the amount of Fe2+ that reacted using the balanced equation • Find out unknown information (mol used in the titration scaling up to what has been used in the standard solution, then find the mass of the substance in the
Question: A 0.1203g sample of (COOH)2•xH2O was dissolved in 25.0cm^3 of 1.0 moldm^-3 H2SO4 in a conical flask. The contents were heated to 60ºC and then titrated against 0.0200 moldm^-3 MnO4-. The mean titre was 19.10cm^3. Determine the value of x. Half equations: MnO4- + 8H+ + 5e- -> Mn2+ + 4H2O and (COOH)2 -> 2CO2 + 2H+ + 2e-
(COOH)2•2H2O (x = 2)
What is the equation for the iodine/thiosulfate equation?
2S2O3^2- + I2 -> S4O6^2- + 2I-
What the solution colours in an iodine/thiosulfate equation?
I2 = yellow-brown I- = colourless
What are the 2 reducing agents manganate titrations can be used to analyse for?
• Iron (II) iona, Fe^2+(aq) - Ethanedioic acid, (COOH)2(aq)
What can the iodine/thiosulfate titration be used to determine?
• the ClO- content in household bleach • the Cu2+ content in copper (II) compounds • the Cu content in copper alloys
What is the analysis of chlorate (I) ions?
ClO- + 2I- + 2H+ -> Cl- + I2 + H2O • ClO- is from NaClO (the active ingredient in bleach) • I2 produce reacts with S2O3^2- ions so therefore 1 mol of ClO- produces one mol of I2, which reacts with 2 moles of S2O3^2- • 1 mol ClO- = 2 moles of S2O3^2-
What is the analysis of copper (II) ions?
2Cu2+ + 4I- -> 2CuI + I2 • CuI is a white ppt • I2 produced is titrated with thiosulfate • 1 mol of Cu2+ is equivalent to one mol of S2O3^2-
What is a half-cell?
Half-cells contain all the chemical species present in a redox half-equation