Redox II Flashcards
Standard electrode potential Conditions
- 298°K
- 100 kPa for gasses
- 1.00 mol dm^-3 concentration for ions
SHE (Standard Hydrogen Electrode) Structure
- Porous platinum foil immersed in 1 moldm^–3 HCl,
- This is enclosed in a tube containing hydrogen gas at standard pressure (100kPa).
- Platinum wire
- State temperature is 298 ° K
Why is using standard conditions important?
- The position of equilibrium can change if we alter these conditions
- In order to make a fair comparison with other metals we make the conditions the same
Why is Platinum foil covered in Porous Platinum used? for the SHE.
- Hydrogen gas can adsorb onto the platinum surface
- This allows for a Hydrogen ion solution to quickly be established
What are the components in a complete cell?
- High resistance voltmeter
- Salt Bridge (e.g. KNO3)
Why is a salt bridge needed?
- To complete the electrical circut
- Allows the movement of ions
- BALANCE THE CHARGES
- Make sure the correct salt is used for the salt bridge
How do you set up a metal half cell?
- Solid Metal dipped in a solution of 1moldm^-3 of its salt
Anode
+ve electrode site of oxidisation
Cathode
-Negative electrode site of reduction
Electromotive force (emf)
- the potential difference of a cell is measured when there is no current flowing.
- The difference between the voltages of 2 different half cells
- E(RHS) - E(LHS)
What is the significance of the +ve & -ve values of the standard electrode potential of cells?
- Tells us if it forms the positive electrode or the negative electrode
- Tells us how reactive certain elements are based on how readily they release electrons
What is meant by the term standard electrode potential?
- The emf of a cell formed with the half cell your measuring (e.g 1/2Br(aq)/Br-(aq)
- And the cell is in standard conditions (1moldm^-3, 298°K, 100kPa)
- Compared to SHE
Electrochemical cell
- Combination of two half cells to produce electricity (voltage)
Standard Hydrogen Electrode Cell Diagram (Half Cell) (ROOR)
- Pt(s) | 1/2 H2(g) | H+ (aq) | |
How do you determine the order of a cell diagram?
- -ve electrode on left
- ROOR (Reduced | Oxidised | | Oxidised | Reduced)
- If the other half cell is a SHE then the SHE is always on the left (Pt(s) | 1/2 H2(g) | H+ (aq) | |)
Why is Porous platinum foil necessary in some half cells?
- If a system does not include a metal that can act as an electrode, then a platinum electrode must be used and included in the cell diagram
- Both aqueous ions
- One state is gaseous (e.g. Hydrogen)
What is the relationship between E cell (standard cell potential) and ln(k) and TOTAL Entropy Change(∆S Total)?
- Direclty propertional
What is the equation relating ln(k) (Equilibrium constant) and the E cell?
ln(k) = (nFE Cell)/(RT)
How can changing conditions affect the E cell?
- The position of equilibirum can be shifted affecting the voltage produced
How can disproportionation reactions relate to standard electrode potentials?
- 2Cu+ -> Cu + Cu2+
- Cu+(aq) + e− -> Cu(s) Eo = +0.52 V
- Cu2+(aq) + e− -> Cu+(aq) Eo = +0.15 V
- So Eo cell = 0.52 − 0.15 = +0.37 V
- The E cell is determined by each of the reduction and oxidization reactions
What are storage cells? (secondary / rechargeable cells)
- Cell that can be recharged by passing a current through it in the opposite direction to the current generated by the cell
What is a fuel cell?
A fuel cell uses the energy from the reaction of a fuel with oxygen to create a voltage
What fuels can be used in fuel cells?
- Hydrogen
- Methanol
- Ethanol
- Other fuels
Reaction at the Electrodes of a Hydrogen Fuel cell in Acidic conditions
-ve: H2(g) -> 2H+(aq) + 2e-
+ve: 1/2O2(g) + 2H+(aq) +2e- -> H2O(l)
Reaction at the Electrodes of a Hydrogen Fuel cell in Alkaline conditions
-ve: H2(g) + 2OH-(aq) -> 2H2O(l) + 2e-
+ve: 1/2O2(g) + 2H+(aq) + 2e- -> H2O(l)
Advantages of Hydrogen fuel cells
+ Alternative to fossil fuels
+ CO2 not produced
+ More Efficient
+ Lighter engines
Disadvantages of Hydrogen Fuel Cells
- Expensive
- Preassurised Hydrogen gas is a hazard
- Finding a suitable material for the Hydrogen to adsorb too
- Infrastructre not currently present to store large quantities of hydrogen
Why is starch added only when nearly all the iodine has reacted?
- High concentrations of iodine react with the starch to form an insoluble blue complex
