physical - electrode potential Flashcards
what happens when a rod of metal is dipped into a solution containing its own ion?
an equilibrium is set up between the solid metal and the aqueous metal ions
the arrangement of a Zinc half cell
Zn (s) ⇌ Zn2+ (aq) + 2e
this is called a half cell
two half cells combined form..
an electrochemical cell
the more readily electrons are released by a metal
the better of a reducing agent it is in the electrochemical cell
* it is getting oxidized itself
describe the diagram of an electrochemical cell
- two half cells
- one cathode (reducing agent) negative anode
- one anode (oxidizing agent) positive anode
- 1 mol dm-3 of solution containing aqueous metal ions
- a salt bridge
- a voltmeter
function of salt bridge and what is it made of
- completes circuit, allowing electrons to flow
- filter paper soaked in saturated solution of potassium nitrate (KNO3)
why is a salt bridge necessary
- salt bridges do not perform electrochemistry
- this avoids the further metal/ion potentials
- allows ion movement to balance the charge
- it also does not react with the electrodes
what symbol is used to represent a salt bridge in standard notation
||
function of voltmeter
measures and reads the potential difference (voltage) created between the two half cells when they are connected
how is a potential difference (voltage) produced
- when two half cells are connected together
- the metal with the highest tendency to release electrons (cathode) will dissolve in the solution and release OXIDISED ions
e. g. Zn (s) –> Zn2+ (aq) + 2e - more electrons build up in the cathode
- therefore this leads to a potential difference between both anodes, which creates a “push” which tends to make electrons flow
- that is what the voltmeter measures; by preventing the flow of electrons
what happens when voltmeter is removed and circuit is completed by a bulb
- electrons then flow through the salt bridge
- combining with the metal aqueous ions found in the solution to REDUCE them
e.g. Cu2+ (aq) ions become solid Cu (s) deposits
Cu2+ (aq) + 2e –> Cu (s) - the spontaneous redox reactions generate electricity; bulb lights up
write the half reaction of the reaction between a Zinc half cell and a Copper half cell
and then the overall redox equation
half equations:
- Zn (s) –> Zn2+ (aq) + 2e
- Cu2+ (aq) + 2e –> Cu (s)
overall equation:
- Zn (s) + Cu2+ (aq) + 2e –> Zn2+ (aq) + 2e + Cu (s)
- cancel out 2e as they exist on both sides
- Zn (s) + Cu2+ (aq) –> Zn2+ (aq) + Cu (s)
what is the standard hydrogen electrode used for
- to compare the tendency of different metals to release electrons
- the potential of the SHE is defined as 0
- so when it is connected to another electrode, the voltage measured is the electromotive force (emf) of the half cell of the metal
- therefore all other emf values are compared against the emf of the SHE
- when the conditions of second cells are kept constant too, the emf is represented is given the symbol Eq
constant conditions that a SHE is used in
- 298K temperature
- 100kPa
- {H+} of 1.00 mol dm -3
describe the process of building a SHE
- hydrogen gas is bubbled into a solution of H+ (aq) ions
- hydrogen is a non metal, therefore it does not conduct electrical activity
- electrical contact is made via a piece of unreactive (inert) platinum metal wire
- the platinum wire is coated with finely divided platinum
why is the platinum wire coated with finely divided platinum
- to increase the surface area and allow any reaction to proceed quickly
why might other standard electrodes be used
- platinum is expensive
- they are cheaper/easier/quicker to use and can provide just as good reference
the SHE equilibrium
H2 (g) ⇌ 2H+ (aq) + 2e-
conventional representation of the SHE
Pt | H2 (g) | H+ (aq)
how is the conventional representation of the cell laid out
e.g. Zn(s) | Zn2+ (aq) || Cu2+ (aq) | Cu(s)
- salt bridged is represented as ||
- the most oxidized species is on the far left from the salt bridge
the | indicates the phase boundary (solid/liquid/gas)
- the most reduced species goes on the far right
- half cell equation written on either side
-
reactivity series: strongest to weakest reducing agents
- K - Potassium
- Na - Sodium
- Li - Lithium
- Ca - Calcium
- Mg - Magnesium
- Al - Aluminium
- Zn - Zinc
- Fe - Iron
- Pb - Lead
- H2 - Hydrogen
- Cu - Copper
- Ag - Silver
- Au - Gold
- Pt - Platinum
Please Send Lions, Cats, Monkeys And Zebras Into Lovely Hot Countries, Signed General platinum
which electrode has the negative Eq value?
left hand electrode (cathode) - where oxidisation occurs
how to identify the best reducing agent
the more negative the Eq value, the better of a reducing agent the species is - usually arranged at the top of an electrochemical series table
* number of electrons released does not affect value of Eq, it is the tendency to release electrons
how to identify the best oxidising agent
the more positive the Eq value, the better of an oxidising agent the species is - usually arranged at the bottom of an electrochemical series table
what factors will affect the Eq value
- concentration of ions
- temperature
