topic 14 Flashcards
standard electrode potential
- voltage measured under standard conditions when the half cell is connected to a standard hydrogen electrode
what are standard conditions
- 298K
- 100 kPa pressure
- 1 mol dm-3 concentration
features of standard hydrogen electrode
- hydrogen gas at 100 kPa
- solution with hydrogen ion at 1 mol dm-3
- temp at 298 K
why is a reference electrode needed
- impossible to measure the absolute potential of a half electrode on its own
- it must be connected to another half-cell of known potential
- reference electrode is given a potential of 0V
- the p.d. can then be measured between the two
setting up an electrochemical cell
- obtain metals being investigated and clean with sandpaper to remove impurities
- wash surface of metals with propanone and wear gloves
- place each metal into solution containing the ion of the same metal
- make salt bridge from filter paper soaked in saturated KNO3 or KCl
- connect electrodes with wires, crocodile clips & voltmeter
why is the surface washed with propanone
to remove oils
why do we clean with sandpaper
to remove any impurities
why is a salt bridge used
- to connect the circuit
- free moving ions conduct the charge
why is a wire not used to connect the circuit
- metal wire would set up its own electrode system
- wires do not allow the flow of ions
what happens if a current is allowed to flow
- reactions will occur separately at each electrode
- as V drops to 0, reactants are used up
how to set up a zinc copper electrochemical cell
- zinc is the cathode so loses electrons more easily than copper so it’s oxidised
- copper is the anode so gains the electrons
what happens at the anode
oxidation
what happens at the cathode
reduction
how is an electrochemical cell set up for different metals
- place the metals in solutions of their own ions that are connected with a wire to an external circuit to allow electron flow , form a salt bridge
how to set up an electrochemical cell that forms an ion
- use a platinum electrode - as it needs to conduct electricity but be inert so it won’t react with anything else in the half cell
- use a solution containing 2 aqueous ions of the same element
how to calculate standard emf, Ecell
Ereduction - Eoxidation
cell diagram representation
R O O R
hydrogen electrode equilibrium
H2 (g) ⇌ 2H+ + 2e-
why are standard conditions needed
- the position of the redox equilibrium will change with conditions
e.g. of eqb
Mx+ (aq) + x e- ⇌ M (s)
if Mx+ concentration increases, eqb will move to the right hence the potential is more positive
what does the more positive E cell undergo?
- it acts as an oxidising agent - accepts electrons
- so undergoes reduction
what does the more negative E cell undergo?
- it acts as a reducing agent - donates electrons
- so undergoes oxidation
how to work out the Ecell from two electrode potentials
E red - E ox
what determines whether a reaction will be feasible
- if the E cell value is positive, the reaction is likely to occur
predicting reaction feasibility
- identify which cell is being oxidised
- take oxidised equation and reverse it
- combine the two equations
- calculate the E cell value to confirm
entropy, cell potential & eqb constant
- the larger the cell potential, the larger the total entropy change
E ∝ ∆S total - entropy & eqb constant (K) are linked
E ∝lnK
effect of current on E cell
- if current is allowed to flow, the cell reaction will occur
- the E cell will fall to zero as the reaction proceeds
- the reactant concentration drops
effect of concentration on E cell
- increasing concentration of reactants will increase E cell
- decreasing will cause E cell to decrease
effect of temperature on E cell
- most cells are exothermic
- applying Le Chatelier to temp rise would result in a decrease in E cell because the eqb reactions would shift backwards
disproportionation reactions - using electrode potentials
- electrode potential values can be used to calculate EMF
- if EMF is positive, the disproportionation will occur
- only possible under standard conditions
fuel cell
- fuel cells use energy from the reaction of a fuel with oxygen to create a voltage
how do fuel cells maintain a constant concentration of reactants
- they maintain a constant voltage overtime as they are fed continuously with fresh O2 and H2
advantages of fuel cells
- less pollution and less CO2
- pure hydrogen emits only water whilst hydrogen rich fuels produce only small amounts of air pollutants and CO2
- greater efficiency
limitations of hydrogen fuel cells
- expensive
- storing & transporting hydrogen (in terms of safety, feasibility & limited life cycle of a solid adsorber or absorber)
- limited lifetime
- high production cost
- use of toxic chemicals in their production
