Redox And Electrode Potential Flashcards
Reducing agents
Donates electrons and decreases oxidation number of another species.
Oxidising agent
Accepts electrons and increase oxidation number of another species.
Standard electrode potential (E•)
E.m.f of a half-cell compared with a standard hydrogen half cell
- 1 mol dm-3
- 298K
- 100kpA
- Platinum electrode
Salt bridge
Allows ions to move
Wire
Allows electrons to flow
What is the process taking place at the positive and negative electrodes?
Electrons flow away from the least positive electrochemical cell and flows to the most positive.
- Least positive loses electrons, most positive gains
What does a more negative electrode potential tell us ?
- Greater tendency to lose electrons and undergo oxidation
- Less tendency to gain electrons and undergo reduction
- Equilibrium shifts to the left
Cell potential equation
(most positive)-(least positive)
What value do all feasible electrode potential reactions have?
+ V
When will a reaction occur
If the oxidising agent has a more positive value than the redox system of the reducing agent
Steps of comparing feasibility in equations
- identify which is being oxidised
- Reverse oxidised equation
- Combine 2 equations to obtain feasible reaction
- Compare equation to one stated in question
Limitations of predicting feasibility (concentration)
Non standard conditions alter value for E, half equations are at equilibria so changes in concentration will shift the position which affects electron transfer
Limitations of predicting feasibility (rate of reaction)
Reaction rate may be very slow due to a high activation energy and electrode potential give no indication of rate of reaction.
Limitations of predicting feasibility: Aqueous solutions
Standard electrode potentials apply to aqueous equilibria but some reactions that may take place are not aqueous.
Three types of electrochemical cells
- Non rechargeable cells (1• non-reservable)
- Rechargeable cells (2• reversible)
- Fuel cells
What are fuel cells
Electricity is generated by a continuous external supply of fuel and oxygen (acid/alkali)
What products are made in fuel cell reaction?
Water
Advantages of fuel cells
- don’t need to recharge
- less environmental impact as water is only product
- no combustion: less energy dissipates as heat = more efficient
Disadvantages of fuel cells
- High flammable: has safety and economic issues with transportation
- Involves electrolysis- an energy intensive process (involves burning of fossil fuels)
Why don’t you need an indicator in some redox titration
They are self indicating, permanent colour change= endpoint
What is a good oxidising agent to find the conc of reducing agent such as Fe2+, (COOH)2
KMnO4
What is the colour change in KMnO4 and Fe2+
Purple to colourless to permanent pink
Why do we add excess acid to the unknown concentration?
To allow the oxidising agent to fully reduce
Why does the solution containing Fe3+ and Mn2+ remain pink?
The reaction has reached an endpoint as all the Fe2+ had reacted with MnO4- so anymore MnO4- is in excess
Why do we take reading from the top of the miniscus in KMnO4 titration?
KMnO4 is very dark so it hard to see
Why is starch added to iodine-sodium thiosulfate reaction?
For a sharper colour change, continues titration until colourless to blue black which indicated all I2 had reacted
Iodine-sodium thiosulfate titration colour change
Colourless to pale yellow
Which solution do you put in the burette?
The one with the known concentration
Steps to finding out the concentration of oxidising agent?
1- Use mean titre to find moles of Na2S2O3 in equation 2
2- Find the moles of Iodine in equation 2 to find the moles of Iodine in equation 1
3- Use the moles of Iodine in equation 1 to find the moles of IO3-
