14: Redox 2 Flashcards
What are the two options for balancing redox reactions?
Half reaction method
Change in oxidation method
What is the change in oxidation method for balancing redox equations?
Write out unbalanced reaction
Assigned oxidation numbers
Draw brackets and connect redox atoms
Find common factor, assign stoichiometry
What is the method for balancing equations in acidic conditions?
Write unbalanced equation
Assign oxidation numbers
Find common factor, assign stoichiometry
Add H2O to balance O atoms
Acidic solution balances H+ atoms from H2O
What is the method for balancing equations in basic conditions?
Write unbalanced equation
Assign oxidation numbers
Find common factor, assign stoichiometry
Add OH- to balance O atoms
H2O added to balance H and O
How is an equilibrium set up with metals?
Piece of metal is dipped into a solution of its metal ions
Metal forms positive ions and go into solution
Metal ions gain electrons and form metal
What is the half equation for a metal in a solution of metal ion?
Mn+ (aq) + ne- → M (s)
Where does the position of equilibrium effect the charge in a metal equilibrium?
Equilibrium to left, metal has -ve charge due to build up of electrons on the metal
Equilibrium to right, +ve charge builds up on metal as electrons are used up to form metal from metal ions
What is a negative and positive electrode potential in reference to equilibrium with a metal?
Eq. lies on left with -ve charge, -ve electrode potential
Eq. lies on right with +ve charge, +ve electrode potential
How does the position of equilibrium of a metal in solution show high reactivity?
More reactive metal tend to forms Mn+ ions
Negative charge builds up on the metal -ve electrode potentials
How does the position of equilibrium of a metal in solution show low reactivity?
Low reactive metal tend not to forms Mn+ ions
Positive charge builds up on the metal +ve electrode potentials
What is potential difference?
Difference between the positive and negative electrode
Measured in voltage
What are the three types of electrode?
Metal electrodes
Gas electrodes
Redox electrodes
What is a metal electrode?
Consists of metal surrounded by a solution of its ions
What is a gas electrode?
Inert metal (platinum) is electrode to allow electrons to flow Used for a gas and a solution of its ions
Which inert metal is used normally for the gas electrodes?
Platinum
What are redox electrodes?
For two ions of the same element where both are present in solution
Inert metal electrode allows flow of electrons
What is a half cell?
One of two electrodes in an electrochemical cell
What is reduction?
When a species gains electrons
Oxidation number decreases
What is oxidation?
When a species loses electrons
Oxidation number increases
How do s-block metals tend to react (terms of redox)?
Oxidised - lose electrons to form positive ions with charge same as group number
How do p-block elements tend to react (terms of redox)?
Metals react by losing electrons to form +ve ion
Non-metals react by gaining electrons to form -ve ion
How do d-block elements tend to react (terms of redox)?
Form ions with variable oxidation states
Tend to form +ve ions with +ve oxidation numbers
What reactions occur in a cell?
Always a reduction and oxidation reaction occurring
What charge is the anode in an electrochemical cell?
Negative electrode
Oxidation always occurs here
What charge is the cathode in an electrochemical cell?
Positive electrode
Reduction always occurs here
Where do electrons flow in a circuit?
From anode to cathode (more to least reactive metal)
Which electrode does the more reactive metal form?
Anode - as gives up its electrons more easily
Which electrode does the less reactive metal form?
Cathode - accepts electrons more easily
What is cell potential?
Voltage between the two half-cells Measure direction of flow of electrons
What is the convention of drawing half-cells?
Oxidation half-cell (anode) on the left Reduction half-cell (cathode) on the right
How are equations at electrodes presented?
Presented as reductions Reversible arrows show they can go in both directions
What are the steps to setting up an accurate electrochemical cell?
Strips of metals to investigate are cleaned using sandpaper
Clean grease/oil on electrodes with propanone, do not touch surface of metal with hands
Place into beaker with corresponding metal ions
Salt bridge made and electrodes connected to a voltmeter
What is a salt bridge?
Way to distribute ions to the different half-cells
What is the set up of a salt bridge?
Piece of filter paper soaked with unreactive ions Tube containing unreactive ions in an agar gel
Why is a salt bridge used?
Balance +ve and -ve charge in half cell solutions to maintain electrode potentials
Which compounds/ions are used in salt bridges?
KNO3 Forms unreactive ions
How are half-cell electrode potentials measured?
Measured relative to a particular half-cell (SHE)
What is SHE?
Standard hydrogen electrode Electrode chosen to be the primary standard which all others are measured against
Why are electrodes measured under standard conditions?
Various factors affect the electrode potential of a half-cell Due to half cells being in equilibrium
What are the standard conditions for a half-cell?
Cell conc - 1 M of ions involved in half-equation
Cell temp - 298K
Cell pressure - 100kPa (only affects half-cells with gases)
High resistance voltmeter
Why is a high resistance voltmeter used?
Must be done under zero-current conditions Allows measurement of full pd (emf), no current drawn from cell
What are the standard conditions in a SHE?
1M solution of H+ atoms
298K
H2 at 100kPa
Salt bridge
Platinum electrode
What is the half equation for the SHE?
2H+ (aq) + 2e- -> H2 (g)
What are the conventions to drawing a cell?
|| separates half cells
Reduction is on the right
[] show substance flowing over inert metal
R|O||O|R
separates species/phases
What is the convention for a hydrogen and magnesium electrochemical cell?
Pt[H2 (g)] | 2H+ (aq) || Mg2+ (aq) | Mg(s)
What is the standard electrode potential of a half-cell?
Voltage measured under standard conditions when half-cell connected to SHE
What is the electrode potential of a SHE?
0V
What is E(cell)?
Cell potential of an electrochemical cell
What is the equation for E(cell)?
E(cell) = E(reduction) - E(oxidation)
Why is the E(cell) value always positive?
More negative E value is being subtracted from more positive E value
What is the electrode potential of more reactive metals and why?
More negative standard electrode potential Loses electrons more easily
What is the electrode potential of less reactive metals and why?
More positive standard electrode potential Gains electrons more easily
When is a reaction thermodynamically feasible?
The overall E(cell) value is positive
How can E values be used to predict disproportionation?
If E(cell) is positive for different redox reactions then disproportionation occurs
Why might the E(cell) value predict something is not feasible but in practice it is?
Conditions are not standard Reaction kinetics aren’t favourable - slow rate of reaction so may not seem to occur, or high activation energy
What is cell potential related to?
E(cell) is directly proportional to ΔS(total)
E(cell) is therefore directly proportional ln(K)
How are pipettes used?
Measure only one volume of solution Fill pipette just above line and drop level carefully to the line
How is a burette moved?
Burettes measure different volumes and let you add solutions drop by drop
How are titrations used to calculate conc of acid/alkali?
Know volume of an alkali with unknown conc titrated with acid of known conc Volume of acid needed to neutralise acid used to calculate the conc of the alkali Or vice versa with acid/alkali
What does an oxidising agent do?
Accepts electrons and is reduced
What does a reducing agent do?
Donates electrons and is oxidised
Why are d-block transition metals used as oxidising/reducing agents?
Good at changing oxidation numbers Readily give out/accept electrons
How can the amount of reducing agent be calculated from an impure source?
Measure amount of impure product
Add dilute sulphuric acid to make up 250cm3
Titrate against oxidising agent such as MnO4- or C2O7 2-
Calculate moles
What are the oxidation agents used which have a colour change in titration?
Potassium manganate KMnO4, goes from purple to colourless
Potassium Dichromate K2Cr2O7, goes from orange to green
What are the ratios for iron and common oxidising agents?
Manganate - 5Fe2+ : MnO4-
Dichromate - 6Fe2+ : Cr2O7 2-
What are the oxidation states of Vanadium?
+5 = VO3- or VO2+
+4 = VO2+
+3 = V3+
+2 = V2+
What are the colours of the oxidation states of Vanadium?
Yellow = VO3- or VO2+ (oxidation state +5)
Blue = VO2+ (oxidation state +4)
Green = V3+ (oxidation state +3)
Violet = V2+ (oxidation state +2)
What is the electronic configuration of Vanadium?
1s2 2s2 2p6 3s2 3p6 3d3 4s2
What is the half equation for the changing on oxidation state of vanadium between +5 and +4?
VO2+ + 2H+ + e- -> VO2+ + H2O
What is the half equation for the changing on oxidation state of vanadium between +4 and +3?
VO2+ + 2H+ + e- -> V3+ + H2O
What is the half equation for the changing on oxidation state of vanadium between +3 and +2?
V^3+ + e- -> V^2+
How is thiosulphate used for a titration?
XS iodide ions react with oxidising agent, liberating iodine
Iodine titrated against a standard solution of sodium thiosulphate
What is the equation linking iodine and thiosulphate?
I2 + 2S2O32- -> 2I- + S4O62-
What is the method for sodium thiosulphate titration?
Known volume of oxidising agent pipetted into a conical flask
Similar vol of dilute sulphuric acid added, with XS solid KI, swirled to ensure all oxidising agent reacts
Burette filled with standard solution of sodium thiosulphate
Thiosulphate added until brown fades to straw
Few drops of starch added to make it blue-black
Thiosulphate added until decolourised
What colour change is seen when using thiosulphate?
Brown to pale straw before starch
Black-blue to colourless after starch
What occurs when starch is added to an iodine solution?
Iodine forms complex with starch Forms a blue-black colour
Why is manganate used instead of dichromate for some titrations?
Clearer colour change
What is the method for titration with potassium manganate?
Known volume (usually 25cm3) of reducing agent is pipetted into a conical flask
Approx 25cm3 of dilute sulphuric acid is added
Burette filed with standard solution of potassium manganate
Manganate (purple) added to reducing agent bit by bit where it turns colourless
End-point when a permanent pink colour is seen
What is the reaction which occurs in a potassium manganate titration?
Fe2+ reacts with MnO4 - ions turning it colourless
End-point when no Fe2+ is left in the solution
What are the main types of cells?
Non-rechargeable
Rechargeable
Fuel cells
What is a battery?
More than one cell which is joined together
Why are non-rechargeable cells not used more than once?
Chemicals used up over time and emf drops
Once one or more chemicals run out it cannot be used
What are the common chemicals found in non-rechargeable cells?
Zinc-carbon cell - standard and cheap with shorter life
Alkaline cell - higher cost but longer life
How do rechargeable cells work?
Reactions are reversible
Reversed by applying an external current and regenerating the chemicals
What are common types of rechargeable cells?
Lead-acid
Nickel-cadmium
What is a fuel cell?
Cell which have a continuous supply of the chemicals into the cell
Chemicals are stored separately outside the cell
What do the fuel cells not require like conventional cells?
Does not run out of chemicals
Not need recharging
What is the most common fuel cell?
Hydrogen-oxygen fuel cell
What is the set up of a hydrogen-oxygen fuel cell?
Hydrogen and oxygen gases fed into two separate platinum-containing electrodes
Electrodes separated by an anion-exchange membrane allowing only ions to move through it
What are the two types of hydrogen-oxygen fuel cells?
Alkaline or acidic
What are the half equations in an alkaline hydrogen-oxygen fuel cell?
H2 + 2OH- -> 2H2O + 2e-
O2 + 2H2O + 4e- -> 4OH-
Overall: 2H2 + O2 -> 2H2O
What are the half equations in an acidic hydrogen-oxygen fuel cell?
H2 -> 2H+ + 2e-
O2 + 4H+ + 4e- -> 2H2O
Overall: 2H2 + O2 -> 2H2O
What is the cell emf of a hydrogen-oxygen fuel cell?
Acidic and alkali: +1.23 V
Which is the anion-exchange membrane and polymer electrolyte membrane?
Surface which allows passage of H+ or OH- ions to flow between electrodes in hydrogen-oxygen fuel cells
Which electrode is hydrogen and oxygen delivered to in a hydrogen-oxygen fuel cell?
Hydrogen delivered to the anode Oxygen delivered to the cathode
How is electricity generated in an acidic hydrogen-oxygen fuel cell?
H2 forms H+ ions at anode, donating electrons to it
Electrons move to cathode, creating a pd H+ ions move between electrodes through electrolyte to cathode
Electrons at cathode used to combine H+ and O2 to form water
How is electricity generated in an alkali hydrogen-oxygen fuel cell?
O2 at cathode reacts with water and uses electrons from circuit to form OH-
OH- pass through anion-exchange membrane to negative anode
OH- reacts with H2 to form H2O and produces e- which go into the circuit and produces a pd
What are the advantages/disadvantages to using cells?
Portable source of electrical energy
Waste issues
What are the advantages/disadvantages to using non-rechargeable cells?
Cheap
Waste issues
What are the advantages/disadvantages to using re-chargeable cells?
Less waste, cheaper in long run, lower environmental impact
Some waste issues at end
What are the advantages/disadvantages to using hydrogen fuel cells?
Only waste product is water, no recharging, very efficient
Need constant supply of fuels, hydrogen is flammable and explosive and made using fossil fuels, high cost of fuel cells
Define standard electrode potential
Emf of a half-cell measured relative to SHE All standard conditions (1M solution, 100kPa gas, 298K)
