electrode potentials Flashcards
what are electrochemical cells made from?
two half cells
what is a half cell?
a solid metal dipped in a salt (ionic) solution of their own ions. The solid metals are connected by an external circuit
the half cell acts as…
an electrode within an electrochemical cell
what are the requirements for an electrode?
- must be solid
- must conduct electricity
what metal is used as the electrode when there is no solid metal involved?
platinum
why is platinum used?
- it is chemically inert
- it conducts electricity
the electrode where oxidation takes place is regarded as…
the negative electrode (anode)
the electrode where reduction takes place is regarded as…
the positive electrode
what are the two half cells connected by “internally”?
a salt bridge
what is the purpose of the salt bridge?
The salt bridge has mobile ions that complete the circuit. KNO3 is used as its ions are relatively inert so they don’t partake in any reaction. It’s also useful that nitrates are soluble. Salt bridge contains spectator ions which can migrate to preserve neutrality
within electrode potentials, where are the electrons found in the equations?
on the left hand side of the reversible reaction sign
according to IUPAC rules, where is the oxidation electrode located?
The oxidation electrode (i.e. the electrode with the more negative
electrode potential) is written on the LHS. The ion / metal are
written in order of being oxidised (ions are closest to the salt bridge)
according to IUPAC rules, where is the reduction electrode located?
The reduction electrode (i.e. the electrode with the less negative
electrode potential) is written on the RHS with the components in
order of being reduced.
according to IUPAC rules, what symbol is used to represent the salt bridge?
||
according to IUPAC rules, what symbol is used to represent a phase boundary?
|
what is the phase boundary representing?
a change in physical state
according to IUPAC rules, where should the Pt electrode be positioned (if used)
to the extreme left or extreme right
according to IUPAC rules, how do you represent components in an electrode which are in the same state?
separating by commas
what is used to measure the voltage in the electrochemical circuit?
a high resistance voltmeter (we assume infinite resistance)
what is the voltage measured called?
the cell potential/EMF
what does the EMF show?
the potential difference between the two half cells
if a metal is easily oxidised, its electrode potential will be…
negative
if it is harder to oxidise a particular metal, its electrode potential will be…
positive
what factors affect the position of equilibrium?
temperature, pressure, concentration
conditions: when we measure electrode potentials, we use…
standard conditions (298K, 100KPa, any solutions having a conc of 1.00 mol dm-3)
what is used as a “reference” when measuring standard electrode potentials of half cells?
the standard hydrogen electrode
what is the definition of “standard electrode potential”?
the electrode potential of a
standard electrode connected to a standard hydrogen electrode using a high-resistance voltmeter and a salt bridge
what is the SEP of the SHE?
0.00V - it is the standard
when the SHE is present within an electrochemical cell, it is always located…
on the left hand side
the electrode on the left is always…
the negative electrode
- if both of the half cells have negative electrode potentials, the MORE NEGATIVE one goes on the left
what is the electrochemical series?
a list of the standard electrode potentials of various elements (decreases until it reaches 0.0V and then increases in negativity)
a strong oxidising agent will be…
more positive
a strong reducing agent will be…
more negative
what equation is used to measure the EMF of a cell?
EMF = EƟRHS - EƟLHS
the cell potential will always be…
positive
rechargability: lithium ion cells are an example of…
rechargeable/reversible cells
hydrogen-oxygen cell is an example of…
a fuel cell
what are fuel cells?
they are used to generate an electrical current and do not need to be electrically recharged
what is the main use for lithium cells?
they are used within phones and laptops
what is the conventional cell representation for a LiCoO2 and graphite electrode?
Li(s)|Li+(aq)||Li+(aq)|CoO2(s), Li+[CoO2]-(s), Pt(s)
lithium cobalt: what is the reaction at the positive electrode?
Li+(aq) + CoO2(s) + e- → Li+[CoO2]-(s)
lithium cobalt: what is the reaction at the negative electrode?
Li(s) → Li+(aq) + e-
what is the overall discharge equation for the lithium cobalt cell?
Li(s) + CoO2(s) —> Li+(CoO2)-(s)
why are lithium ion cells so light?
due to the low density of lithium
what is the electrolyte within lithium ion cells?
a polymer (advantage = no leaks)
what is the single cell EMF of a lithium ion cell?
3.5-4v
how do fuel cells work?
they convert the energy from combustion of a fuel such as hydrogen
pH: hydrogen fuel cells can be…`
acidic or alkaline
in a hydrogen-oxygen half cell, hydrogen is…
and oxygen is…
hydrogen = oxidised
oxygen = reduced
what is the chemical product in a hydrogen-oxygen fuel cell?
water
how do the ions move in a hydrogen-oxygen fuel cell?
through the polymer
acidic = H+ moving from hydrogen coming in on the left to oxygen coming in on the right - water out on the right
alkaline = OH- moving from oxygen coming in on the right to hydrogen coming in on the left - water out on the left
what type of batteries are the batteries we buy in shops today?
zinc-carbon batteries
describe the structure of a zinc-carbon battery
carbon rod = positive electrode
zinc canister = negative electrode
manganese iv oxide - hydrogen gas to water (reduced to Mn2+)
electrolyte paste = ammonium chloride, zinc chloride and water
what are the half equations for the zinc carbon battery?
2NH4+(aq) + 2e- ∏ 2NH3(g) + H2(g) E = roughly +0.7V
Zn2+(aq) + 2e- ∏ Zn(s) E = roughly -0.8
(we do not use standard electrode potential as the conditions are not standard)
what is the IUPAC cell representation for zinc-carbon batteries?
Zn(s) | Zn2+ || 2NH4+(aq) | 2NH3(g) , H2(g) | C(graphite)
car batteries are made of…
lead acid cells
when do car batteries recharge?
when the engine is running
what is the approximate voltage of a car battery?
12V
briefly describe the structure of a car battery?
Cell consists of a lead plate and a lead plate coated with lead oxide dipped in 6M sulfuric acid (electrolyte)
what are the reactions for an alkaline hydrogen-oxygen fuel cell?
negative: 4H2O(l) + 4e- ∏ 4OH-(aq) + 2H2(g) Eθ = -0.83V
positive: O2(g) + 2H2O(l) + 4e- ∏ 4OH-(aq) Eθ = +0.4V
