Oxidation and reduction Flashcards
What determines oxidation?
- Gain of oxygen
- Loss of electrons
- Loss of hydrogen
ALO (anode, loss of electrons, gain of O2)
What determines reduction?
- Loss of oxygen
- Gain of electrons
- Gain of hydrogen
What is a redox reaction?
When either oxidation or reduction occurs, the other one will also take place too
How do you write a half-equations of Zn + Cu2+ –> Zn2+ + Cu?
- Zn –> Zn2+ + 2e- (oxidation)
- Cu2+ + 2e- –> Cu (reduction)
- They make up the original ionic equation
- The electrons on opposite sides cancel out
What are oxidation numbers? What are the used for?
- The numerical part of oxidation state, given as a roman numeral
- Tell if oxidation or reduction has taken place
- Work out what has been oxidised/reduced
- Construct half equations
What are oxidation states?
- A number which, together with its sign, indicates the gain or loss of electron control of an atom during a reaction
How do you determine the oxidation number of an ion?
- The charge of ion e.g. Na+ = +1
- A neutral atom e.g. Na = 0
- Charge (+ or -) always infront of number
How do you determine the oxidation number of compounds or molecules?
- The sum of oxidation numbers of the atoms is 0
- The more electronegative species will have a negative number
- Electronegativity increases across a period and decreases down a group
- E.g. O further right than C, hence O2 is negative
- Look at their position in the periodic table
What are rules 1, 2 & 3 to determine the oxidation states?
- Free elements e.g. O2, oxidation state = 0
- The sum of oxidation states of all atoms in a compound equal the net charge on compound (could be 0 or +3 or -2)
- The group 1 metals have O.S = +1. Group 2 metals have O.S = +2
What are rules 4, 5 & 6 to determine the oxidation states?
- Fluorine has O.S = -1
- The alkaline earth metals (Be, Mg, Ca …) and Zn in compounds have O.S = +2
- Hydrogen in compounds has O.S = +1 except in metal hydrides (NaH) O.S = -1
What are rules 7, 8, 9 & 10 determine the oxidation states?
- O2 has O.S = -2 except in peroxides where it is -1 and in F2O it has +2
- Cl has O.S = -1 unless combined with O2 or F
- Charge on metal ion is same state, Zn2+ = +2
- Sum of O.S in polyatomic ion add up to charge (CO3 2-) S.O = -2
How do you determine the oxidation number of metals?
- The number usually corresponds to the position on periodic table (group)
- Metals have a positive values in compounds
- Transition metals can have multiple oxidation numbers
How do you determine the oxidation number of non-metals?
- Mostly negative based on their usual ion
- The number usually corresponds to the position on periodic table (group)
- Hydrogen has +1
What is the oxidation number of sulfur?
- E.g. S4O6 2-
-2 x 6 = -12
4x = 10
x = 2.5, therefore the oxidation number is a fraction - This is technically not possible, single atoms can only have a whole number oxidation number
What are oxidising agents?
- Oxidising agent: a substance that oxidises other substances. Itself is reduced and gains electrons
- The oxidation number of this agent decreases
What are reducing agents?
- Reducing agent: a substance that reduces other substances. Itself is oxidised and loses electrons
- The oxidation number of the reducing agent increases
How do you identify oxidising and reducing agents?
- Look at whether their oxidation number is increased (reducing agent)
- Look at whether their oxidation number is decreased (oxidising agent)
How do you name transition metal compounds?
- Transitions have varying oxidation numbers
- If an element has a variable oxidation number, it is written in Roman numerals (stock notation)
Check book for specific examples
What are disproportionation reactions?
- The same species is oxidised and reduced simultaneously during a reaction to form 2 products
E.g. catalytic decomposition of hydrogen peroxide
Check book
Half-equations in book
What is the reactivity series? What is it used for?
- When metals are ranked in order of reactivity
- Metals higher in reactivity can displace less reactive metals from their compounds in solutions
What does the ACTIVITY series determine?
- Metals listed in order of their strength as reducing agents
- In other words the ease with which they undergo oxidation
- The activity series allows metals to be ranked from strongest reducing agents to weakest
- Found in section 25 in data booklet
Explain the chemistry behind displacement reactions.
- The more reactive a metal is, the better it is at pushing electrons into less reactive metal ions
- Metals above hydrogen can displace hydrogen ions from solution to produce hydrogen gas
What is a spectator ion?
- An ion that appears unchanged on both sides of the complete ionic equation, it is neither oxidised nor reduced
- Not included in net ionic equation
What does feasibility in the context of displacement reactions mean?
- The activity series helps predict whether a displacement reaction will take place or not (feasible)
- If one of the elements is more reactive than the one in the compound, a reaction will take place
- The more reactive metal –> oxidation
- The less reactive metal –> reduction
How are redox reactions involved in titrations?
- A titration in which the concentration of a solution (in conicle flask) is determined by titrated with solution of known concentration
- The oxidising agent is titrated against a reducing agent
Explain the redox titration of manganate (VII).
- Reaction between acidified manganate (VII) ions and iron (II) ions (reaction in book)
- Manganate (VII) has strong purple colour, disappears at end point, no further indicator needed
Explain the redox reaction of iodine-thiosulphate titration.
- Reaction between iodine and thriosulfate ions
- Iodine is naturally brown, and becomes colourless when converted to iodine ions
- Starch is added before reaction is finished to clarify end point
- Solution turns blue/black until iodine reacts, then colourless
- Aim is to find concentration of oxidising agent (iodine)
- Known quantity of sodium thiosulfate solution
What is the Winkler method and what does it determine?
- Technique used to measure the concentration of dissolved oxygen in a water sample
- Dissolved O2 used as an indicator of the health of a water body (more O2 little pollution)
- Used to determine the Biochemical Oxygen Demand (BOD), amount of O2 used to decompose the organic matter over a period of time
- A high BOD, means low level of dissolved O2
Explain the chemical aspect of the Winkler method.
- Manganese (II) sulfate is added to a water sample, NaOH is added to make the solution alkaline
- In the solution the dissolved O2 will oxidize Mn (II) ions to Mn (IV), (equation in book)
- Manganese (IV) oxide (MnO2) appears as a brown precipitate
- Then potassium iodide (KI) is added to the solution. Mn (IV) is reduced back to Mn (II) releasing iodine (equation in book)
- Thiosulphate is used as an indicator to titrate the liberated iodine (equation in book)
What do the results of the Winkler method determine?
- The number of moles of iodine produced (2 moles), we can work out the number of moles of O2 molecules present in the original water sample
- Oxygen content presented as mg/dm3 or ppm
- Check how the O2 content decreased after 5 days
Calculations found in book
See steps for Winkler method calculations in book.
