Redox - Yr13 Flashcards
What is an oxidising agent?
Takes electrons
What is a reducing agent?
Loses electrons
Is Cu2+ + 2e- —-> Cu reduction or oxidation?
Reduction
Is Al —-> Al3+ + 3e- reduction or oxidation?
Oxidation
What is the overall equation for Cu2+ + 2e- —-> Cu and Al —-> Al3+ + 3e- ?
3Cu2+ + 2Al —-> 3Cu + 2Al3+
How do you write half equations
Balance the atoms - can use H2O or H+
Balance the charges - can use e-
What does the no. of electrons tell you in a half equation?
The change in the oxidation number
What is the half equation for the reduction of VO2+ to V3+?
VO2+ +2H+ + e- —-> V3+ + H2O
What is the half equation for the reduction of Cr2O7 2- to Cr3+?
Cr2O7 2- + 14H+ 6e- —-> 2Cr3+ + 7H2O
What is the half equation for the reduction of MnO4 - to Mn2+?
MnO4 - + 8H+ +5e- —-> Mn2+ + 4H2O
What is the half equation for the reduction of H2S to S?
H2S —-> S + 2H+ + 2e-
What is the half equation for the reduction of H2SO4 to H2S?
H2SO4 + 8H+ +8e- —-> H2S + 4H2O
What should the value of a hydrogen electrode always be?
0.00V
On which electrode does oxidation happen?
The half cell with the most negative standard electrode potential
What does it mean if the electrode has the more negative electrode potential?
More reactive the metal
What is the strongest reducing agent?
Li (s)
What is the strongest oxidising agent?
F2 (g)
How do you calculate electrode cell potential?
(Positive electrode) - (negative electrode)
What does it mean if there is a bigger positive voltage?
Reaction is feasible
Why may it not be feasible even if it is positive?
Conditions (not STD) may affect feasibility
Ea may be too high/rate too slow
What is the anti-clockwise rule?
Arrange redox equation so that…
Electrons on the left
Largest negative electrode on top
Reaction takes place anticlockwise
What is the overall equation for Cu2+ + 2e- —-> Cu (+0.34) and Zn2+ + 2e- —-> Zn (-0.76)?
Zn + Cu2+ —-> Zn2+ + Cu
Why is the Zn and Cu reaction feasible?
Since Zn reacted with Cu
And lost electrons
What would happen if you increased Zn2+ conc?
Equilibrium =shift left
Reduces ease of electron loss
What would happen if you increased Cu2+ conc?
Equilibrium = shift right
Increases ease of electron gain
When does it go wrong?
Changing conc
What does it mean if there is less electrons in the system?
More positive the system
What does it mean if it shifts left?
Negative
What does it mean if it shifts right?
Positive
When can an element oxidise another element?
When it is more positive
When can an element reduce another element?
When it is less positive
How do you make a standard solution for a redox titration (part 1)?
Make up 250cm3 of standard solution using 7.0g of iron sulphate
By weighing difference in volumetric flask
Record mass in table
How do you carry out a redox titration (part 2)?
Pipette 25cm3 of standard solution into conical flask
Add 10cm3 of 1.0M of H2SO4 to conical flask
Fill burette with 0.02M potassium manganate (purple)
Carry out titration until end point reached
End point = first permanent pink colour
Record results in table
Repeat until have 2 concordant results
What is the overall equation for the redox titration of potassium manganate?
MnO4 - + 5Fe2+ + 8H+ —-> Mn2+ + 4H2O + 5Fe3+
Piece of pure iron wire (0.1395g) reacts with aqueous acid to convert Fe to Fe2+. Needs 26.2cm3 of KMnO4 solution in titration. Calculate conc of KMnO4 solution
0.0191 moldm-3
0.402g of impure Fe was reacted with an aqueous aid to convert Fe to Fe2+. Needs 23.5cm3 of 0.04moildm-3 kMnO4 solution in titration. Calculate percentage by mass of Fe
65.2%
When reacted with CrO7 2- only Sn2+ to Sn4+. 10g of solder dissolved in acid to make 1.0dm3. 25cm3 needed 20cm3 of 0.0175 moldm-3 K2Cr2O7. Calculate percentage by mass
49.9%
25cm3 of hydrogen peroxide solution poured into 250cm3 volumetric flask + made up to 250cm3 with water. 25cm3 of solution acidified + titrated against 0.02 moldm-3 KMnO4, 38cm3 required. Calculate original conc of hydrogen peroxide solution
0.76 moldm-3
Why do you still divide by 25 when it was made up to 250cm3?
Because even though in 250cm3 we just want the “vodka” not the coke (water) as well
What colour is iodine change from and to in the redox titration?
Yellow/brown solution to straw colour
What is the colour change an indication of?
The completion of a redox reaction
What is a cell?
A cell that has 2 half-cells connected by a salt bridge
What does a simple half cell consist of?
Metal
Solution of compound containing metal
eg. Cu + CuSO4
What will 2 half cells produce when connected in a circuit?
Small voltage
What half cell chosen to produce “standard” potential?
Hydrogen
What are the possible half equations for the standard hydrogen half cell?
H+ + e- —-> 1/2H2
1/2H2 —-> H+ + e-
What are the conditions of the standard hydrogen half cell?
298K
1 moldm-3
1 atm
What happens if you connect a hydrogen half cell to another cell (containing a different metal)?
It will tell you the value of the electrode potential of the metal
Why use a salt bridge?
Used to connect the circuit
So free moving ions can conduct charge
What is a salt bridge made from?
Made frim filter paper soaked in potassium nitrate
Why is a salt bridge soaked in KNO3?
Unreactive with electrodes = suitable
Why is a salt bridge used instead of a wire?
Because a wire would set up its own electrode system with the solution
What is a two ion system?
When it is 2 aqueous species in a half equation
What concentration must solutions be in a two ion system?
1 moldm-3
If you aren’t given a solid in one of the half equations what must be the electrode + solution of that half cell be?
Pt = electrode
Both ions = solution
What is standard electrode potential?
The emf of a half cell compared with a standard hydrogen electrode at standard conditions - 298k,
1.0 moldm-3 + 1atm
What is a fuel cell?
A device that converts energy into electrical energy, water + heat through electrochemical reactions
What is the hydrogen fuel cell?
Hydrogen goes in + reacts with O2 = voltage
Describe hydrogen fuel cell
Fuel cell Vs Battery
Open system Anode + cathode = gases React with Pt catalyst Reactants externally supplied No recharging required
Describe galvanic cell (battery)
Fuel cell Vs Battery
Closed system
Anode + cathode = metals
Reactants internally consumed
Needs recharging
What are the similarities between fuel cell Vs combustible engine?
Both use hydrogen-rich fuel
Both use compressed air as oxidant
Both require cooling
What are the differences between fuel cell Vs combustible engine?
FUEL CELL
Output = electrochemical work
Fuel + oxidant reacts electrochemically
No pollution
What are the differences between fuel cell Vs combustible engine?
COMBUSTIBLE ENGINE
Output = mechanical work
Fuel + oxidant react combustively
Fossil fuels = pollution
What are the half equations for alkali fuel cell?
2H2O + 2e- —-> H2 + 2OH-
1/2O2 - 2e- —-> 2OH-
What are bad about hydrogen fuel cells?
Atm methanol used to make hydrogen = fossil fuels
Hydrogen gas = explosive = dangerous
