Redox Chemistry Flashcards
What are oxidising and reducing agents?
Substance reduced is oxidising agent
Titration of potassium manganate seven with iron two ions
KMnO4 in burrette
Acidified FeSO4 in conical
Solution in the conical flask remains colourless until all Iron two ions oxidised
Addition turns solution pink
Titration of potassium manganate seven with ethanedioic acid
Reaction is very slow because it’s catalysed by manganese ions which aren’t present at the beginning
Heated up to 60°C to speed up reaction
Autocatalysis- as Mn ions increase reaction speeds up
What is the equation for the reaction between sodium thiosulphate and iodine solution?
2(S2O3)2- + I2 –> (S4O6)2- + 2I-
Titration of sodium thiosulfate and iodine solution
Endpoint when changes from yellow – brown to colourless
Difficult to tell so a small amount of starch is added which reacts with free iodine to form a deep blue black colour
It’s the start is added to whirly, the iodine is absorbed strongly into the starch which reduces accuracy
How does bleach work?
Contains sodium chlorate I Which acts as an oxidising agent in acidic solution
ClO- + 2H+ +2e- –> Cl- + H2O
Effectiveness depends on ability to oxidise coloured compounds in stains
How can the concentration of bleach be determined?
Reacted with excess iodide in acidic solution
ClO- + 2H+ + 2I- –> I2 + Cl- + H20
Determine the quantity of iodine produced by back titrating with sodium thiosulfate
Work backwards to determine the [ClO-]
Titration of copper two ions
Add Excess KI to known solution of copper to ions
Titrate the iodine solution with Na2S2O3 to find [I-]
Work backwards to find the concentration of copper two ions
Use to find a percentage of copper in an alloy
What can electric potential to be used for?
To determine if a reaction will take place between two systems, and which direction it will go in
What is an electrochemical cell?
A reaction can be separated into two half cells which can be used to harness energy through the flow of electrons between the half cells
How can the potential difference of half cells be shown?
Lamp can be replaced with a volt meter to show electromotive force (e.m.f.)
What happens in an electrochemical cell between zinc and copper?
As copper ions reduced, The copper electrode slowly decreases
Flow of electrons can be shown by lamp
What is half cell notation?
Zn(s) | Zn2+ (aq) // Cu2+ (aq) | Cu (s) E=+1.10V
Dashed line represents salt bridge
Most negative on the left
What does a positive e.m.f. show?
A change which occurs spontaneously
A negative value may occur in the other direction
Strong oxidising agent
What does a more positive electrode potential tell you?
The greater its tendency to attract electrons
What is a reference half cell used for?
Impossible to find electrode potentials for individual half-cells
Used to compare against
Assigned electrode potential of OV under standard conditions of 1moldm-3
What is the oxidation number of an element?
The charges they would have if the electrons in each bond of the molecule or ion the belonged to the more electronegative element
What is the standard hydrogen electrode?
Hydrogen gas at 1atm pressure and 298K bubbling around a platinum electrode in 1.00moldm-3 H+ ions
2H+(aq, 1moldm-3) +2e- H2(298K, 1tm) E=0V
Platinum electrode coated with finely divided platinum to act as catalyst
How must reactions which uses inert electrodes be displayed?
2H+(aq) , H2 (g) | Pt(s)
Least oxidised species is next to electrode
What is the convention for redox reactions for half-cells? What does this mean?
Species with higher oxidation number must appear on left
Ecell =E(RH electrode) - E(LH electrode)
What is the e.m.f. independent of? What must you do when you reverse an equation?
Number of electrons being transferred
Reverse the sign of E standard
How can you tell if a reaction is feasible and positive from Estandard cell?
X> +0.6V completion
0
How does Ecell relate to entropy?
Directly proportional to total entropy change
More positive value, more energetically favourable
What limitations do standard electrode potentials have?
Concerned with energetic stability, not kinetic stability so gives no indication of visible change
Reaction might not be under standard conditions
How does concentration affect electrode potentials?
Increasing conc Cu2+ move equilibrium to right, reducing compounds and removing electrons from the system
Makes electrode potentials more positive
What is a hydrogen fuel cell?
Two electrodes separated by membrane which allows H+ to pass through but not H2 and O2
Electrons released at hydrogen electrode are used up at oxygen electrode
Potential difference set up pushes the electrons through external circuit where they do work
What are the half-equations for a hydrogen fuel cell?
2H2(g) –> 4H+(aq) + 4e-
02(g) + 4H+(aq) + 4e- –> 2H20 (l)
How can methanol be made?
Non-renewable fossil fuels or agricultural waste such as straw and biomass
Chemical recycling of CO2
What advantages and disadvantages does it have compared to hydrogen?
Easier to store (no high pressures or low temps)
Gives off more energy that highly compressed H2
Efficiency is low as alcohol can pass through proton exchange membrane
What happens at the anode and cathode in methanol-based fuel cells?
Anode: H2O + CH3OH –> CO2
Cathode: O2 –> H2O
Ethanol-based fuel cells
Ethanol can be used instead of methanol as it is less toxic and more easily produced
Currently, an expensive platinum catalyst is used
Polymer effectively acts as electrolyte
What reaction do dichromate 6 breath analysers use?
3C2H5OH(g) + 2(Cr2O7)2-(aq) + 16H+ (aq) –> 3CH3COOH(aq) + 4Cr3+(aq) + 11H2O(l)
Ethanol in breath oxidised to ethnic acid which reduced orange dichromate to green
Second tube for comparison
Electric current causes needle in meter to move
How do fuel-cell breath analysers work?
Breath flows along one side of fuel cell and any alcohol is reduced to ethanoic acid, H+ and electrons
H+ move through lower part of fuel cell and combine with oxygen and electrons on other side to form water
More ethanol is oxidised, the larger the electric current is
Why are inferred breath analysers used?
Both dichromate and fuel-cell analysers are preliminary testing so further analysis must be made
Used to measure concentration
Gives very accurate results
What are the problems with breath analysers?
Preliminary analysers can’t be used in court and must be calibrated regularly
Test the air breathed out so alcohol may come from mouth, throat or stomach
Often officers have to wait to give another test
