Flashcards in Redox Deck (35):
Equation for reduction of MnO4- in acidic conditions and colour changes
MnO4- + 8H+ + 5e- --> Mn2+ +4H2O
Purple to very pale pink/almost colourless
MnO4- + Fe2+ + 8H+ --> Mn2+ +4H2O + 5Fe3+
How to carry out titration with Manganate and iron ions
Solution containing Fe2+ is acidified with sulphuric acid. (HCl would react with manganate ions). Titrate like normal, end point is when excess MnO4- ions are present and this is when there is the first hint of permanent pink colour
Reaction of iodine and thiosulfate ions
2S2O32- + I2 -> 2I- + S4O62-
Titration using iodine/thiosulfate
Without an indicator, pale yellow colour of iodine disappears, leaving colourless solution. Starch is added near end point, a dark blue solution will form which will then disappear
Estimating the copper content of solutions and alloys
If a solution containing Cu2+ ions is mixed with I- a light brown/yellow solution and white precip (which appears light brown due to iodine in solution) are formed.
2I- -> I2 + 2e-
Cu2+ + e- -> Cu+
2Cu2+ + 4I- -> 2CuI + I2
Then titrated against a known conc. of sodium thiosulfate
What is the basis of all cells and batteries?
Controlled electron transfer in redox reaction, producing electrical energy
What is a copper half cell compromised of? (plus equation)
A solution containing Cu2+ ions into which a strip of copper metal is placed
Cu2+ + 2e- Cu
How is electrochemical cell made?
Through combining two half cells with different electrode potentials where one releases electrons and the other gains them
Breathalyzer relies on ethanol fuel cell in which ethanol is oxidised by oxygen to form ethanoic acid and water. a) Write balanced equation for fuel cell equation
CH3CH2OH + O2 --> CH3COOH + H2O
b) ethanol/ethanoic acid redox system has electrode potential +0.06 V and oxygen + H+/H20 system has electrode potential +1.23 V. Why does current produced increase with ethanol concentration?
Ethanol equilibrium shifts to left/more negative electrode
More electrons are released so current produced is greater
Potassium manganite (VII) preparation through two-step synthesis starting from manganese (IV) oxide. In step 2, equilibrium is set up:
3MnO42- + 2H2O 2MnO42- + MnO2 + 4OH-
How is equilibrium position shifted when carbon dioxide gas is bubbled through the mixture?
CO2 reacts with H2O to form carbonic acid,
H2CO3 + OH- --> H2O + HCO3-
Equilibrium shifts to right to restore OH-
Purpose of wire in electrochemical cell
Electrons are transferred between the half cells through the wire
Purpose of salt bridge in electrochemical cell
Allows ions to be transferred between the half cells, connects the two solutions
Diagram of copper and zinc cell
Solution of Cu2+ 1 mol dm^3
Wire and voltmeter
Solution of Zn2+ 1 mol dm^3
Non metal/non metal ion half cell
No electrode to connect the wire to (because gas??) so a platinum electrode is placed in the solution so that it is in contact with both the aqueous ions and the gas coming through.
Metal ion/ metal ions half cell, e.g Fe2+/Fe3+
Solution with equal concentrations of the two ions. Platinum electrode.
E~ Standard electrode potential of a half cell definition
The e.m.f of a half cell compared with a standard hydrogen half cell, measured under standard conditions (must be stated)
Electromotive force, the voltage produced by a cell when no current flows
How are standard electrode potentials measured?
A standard half cell for thing being measured is connected to a standard hydrogen half cell.
What is standard cell potential and how is it calculated?
The e.m.f between the two half cells making up the cell under standard conditions (state)
Standard electrode potential at positive terminal - standard electrode potential at negative terminal
What is the cell reaction?
The overall chemical reaction taking place in the cell, the sum of the oxidation and reduction half equations taking place in each cell
What does sign of electrode potential tell us?
If it is negative, the half cell is more likely to give up electrons than the standard hydrogen half cell and vice versa
Standard electrode potentials of oxidising and reduction agents
Oxidising agents: gain electrons and cause other species to lose them, they have more positive standard electrode potentials
Reducing agents: lose electrons so they have more negative standard electrode potentials
Criteria for a feasible reaction
Half equation forming oxidising agent must have a more positive standard redox potential than the half equation for the reducing agent
Limitations of predicting feasibility using standard electrode potentials
- Non-standard conditions alter the value of an electrode potential, (equilibrium is shifted)
- Activations energy may be very high so rate of reaction may be very slow
If the standard cell potential is less than 0.4 V a reaction is unlikely to take place
How do electrochemical cells produce electricity?
My means of a redox reaction, including two redox equations with different electrode potentials
Characteristics of non-rechargeable cells
Provide electricity until the chemicals have reacted to such an extent that the voltage falls. The cell is then flat and discarded (high charge density)
Characteristics of rechargeable cells
Chemicals in the cells react, providing electrical energy, the cell reaction can be reversed during recharging when the chemicals in the cell are regenerated and the cell can be used again. (Examples: Ni-Cad batteries, lithium ion and lithium polymer batteries)
Characteristics of fuel cells
The cell reaction uses external supplies of a fuel and an oxidant, which are consumed and need to be provided continuously. The cell will continue to provide electrical energy so long as there is a supply of fuel and oxidant. It is clean
Advantages of fuel cells using hydrogen rich fuels
Methanol: liquid fuel is easier to store than hydrogen gas. Methanol can be generated from biomass
Less pollution and less CO2
Storage of hydrogen, pros and cons
Can be stored as liquid under low pressure but a very low temp. is required and liquid hydrogen will need to be stored in giant flask to prevent boiling
Can be adsorbed onto the surface of a solid material in a similar way that a catalyst is able to hold gases to its surface
Can be absorbed within some solid materials
Limitations of hydrogen fuel cells
Large scale storage and transportation of hydrogen poses problems
Feasibility of storing a pressurized liquid
Current adsorbers and absorbers have a limited life time
Use toxic chemicals in the production
Have limited lifetimes
How is hydrogen produced?
Electrolysis of water or reacting methane with system
Characteristics of ethanol fuel cell
Oxygen reacts at one end as the oxidant, produces water
Ethanol at other electrode, produces CO2 and H+ s and e- s