3.1.11 - electrode potential and electrochemical cells Flashcards
what happens when a metal is placed in a solution of its own ions?
an equilibrium is set up between the metal solid and the aqueous ions
which electrode is oxidised and which is reduced
the more negative electrode is oxidised and the more positive electrode is reduced
half equations for a Zn/Cu half cell
Zn(s) ⇌ Zn2+(aq) + 2e-
Cu2+(aq) + 2e- ⇌ Cu(s)
how do you draw a conventional cell diagram
most negative electrode is on the left and most positive on the right.
most oxidised is closest to the middle, separated by a double line which represents the salt bridge.
phase changes are represented by a single line.
if there is a change of species but no change of phase a comma is used e.g. Fe3+(aq),Fe2+
state symbols are required
what is the salt bridge made from
filter paper dipped in an unreactive solution e.g. KNO3
why is a salt bridge required
allows for ions to pass between half cells to maintain charge balance
how do you measure EMF in a cell
use voltmeter to measure potential difference between half cells as the current flows from the oxidised electrode to the reduced electrode
2 equations for Ecell
Ecell = E RHS - E LHS
Ecell = E red - E ox
what is the standard hydrogen electrode
the SHE acts as a reference electrode allowing us to make comparisons between half cells as the Ecell of the SHE is 0V
what are the conditions of the SHE
platinum electrode
hydrogen gas at 100KPa
hydrogen ion with a 1.0 moldm-3 conc usually from HCl
298K
the theoretical potential of an electrode is different from the experimental value. why?
temp is not 298K and [H+] is not 1.0 moldm-3
why is platinum used as the electrode for non metal systems
chemically inert so wont react with ions in the solution and it conducts electricity
what is the electrochemical series
list of reduction half equations in order of E (electrode potential)
E increases as you go down the table
why do certain species act a reducing/oxidising agents
increasing emf value, increasing tendency for species to act as an oxidising agent (most positive emf)
decreasing emf value, increasing tendency for species to act as a reducing agent (most negative emf)
what is the effect of concentration on Ecell
increasing concentration of reactants causes Ecell to increase
decreasing concentration of reactants causes Ecell to decrease
what is the effect of temperature on Ecell
most cells are exothermic, so an increase in temp causes a decrease in Ecell, as the equilibrium shifts backwards
what are the two types of batteries
non rechargeable - less expensive but must be replaced often. when the chemicals have been used up the emf falls to 0
rechargeable - more costly, but can be reused multiple time
example of rechargeable batteries
lithium batteries - used in phones, tablets and laptops
lead-acid
nickel - cadium
how can you recharge batteries
an external current is applied to drive electrons in the opposite direction of the circuit, reversing the reaction of both electrodes.
example of non rechargeable batteries
zinc - carbon
alkaline
what are fuel cells
cells which use energy from a reaction of a fuel with oxygen to create voltage
e.g. alkaline hydrogen fuel cell
how does a hydrogen fuel cell work
hydrogen and oxygen gases are supplied to separate platinum electrodes
electrolyte is aqueous KOH
electrons move from the negative to the positive electrode through a wire
advantages of fuel cells
+ more efficient
+cleaner emissions
+ dont need to be recharged
disadvantages of fuel cells
- expensive
- requires alot of energy to produce hydrogen and oxygen from water electrolysis
- flammability concerns: hydrogen gas is highly flammable needing careful storage.
