electrochemistry Flashcards
what can a voltmeter measure in an electrochemical cell
voltmeter needs to be of infinite resistance, you can measure the “potential” of the cell
what will the difference in reactivity produce on the voltmeter
larger the difference in reactivity the larger the reading on the voltmeter
what is an electrochemical cell made of
2 half cells
how do we determine the potential of 1 half-cell?
we must compare it to a standard. (standard hydrogen electrode)
the standard hydrogen electrode
conditions
- temp 298k
- pressure at 100kPa
- conc [H+] 1moldm-3
what is the convention rule with S.H.E
to draw the standard hydrogen electrode on the left hand side of the cell
what is the potential for the S.H.E
As it is the standard, it is 0.00v
what side is the reduction half-cell on
conventionally we put the reduction half-cell on the RHS of a cell, and the oxidation half-cell on the left
how to work out the potential of the E cell
E cell = Erhs - Elhs
E cell = Ered - Eox
what does a correct cell diagram give?
a correct cell diagram gives a positive value
what is an oxidising agent?
electron acceptor
what happens in a half-cell
metal is dipped into a solution containing its ions, an equilibrium is established between the metal ions and the metal atoms.
What is filter paper soaked in and why?
KCl(aq) or KNO3(aq) or KCl in dissolved agar gel set in a U tube.
These solutions are used as they’re soluble and neither react with either electrode.
how does a salt bridge work?
has mobile ions that complete the circuit. The salt bridge allows ions to flow through and balance the charges, completing the circuit.
why is platinum used as an electrode
- Inert & conducts electricity
- Porous platinum gives a larger surface area.
why is a high-resistance voltmeter used to measure the potential difference?
maintains the conc. of ions in the solution by not allowing any current to flow yet still measuring the p.d. of electrons trying to flow.
reducing agent
electron donor
oxidising agent
electron acceptor
what is the electrode potential
tendency to lose or gain electrons in the equilibrium state
Problems with predicting reactions:
- the conditions are not standard - changing the conc. or temp of the solution causes the electrode potential to change.
- reaction kinetics are not favourable. -ROR may be slow so the reaction might not appear to happen.
-If a reaction has a high activation energy, this stops the reaction from happening
what is An electrochemical cell
controls the electron transfer to produce electrical energy. The control transfer of electrons is the basis of all cells and batteries.
what are Non-rechargeable cells
provide electrical energy until the chemicals have reacted to such an extent that the voltage falls.
what are Rechargeable cells
chemicals in cells react to provide electrical energy. The cell reaction can be reversed during recharging.
non-rehargable cells
how do they work?
example?
use irreversible reactions
e.g. dry cell alkaline battery: useful for short-term products
pros and cons for non-rechargeable cells
cost
+ve
-ve
+ve - cheaper than rechargeable
-ve - have to be replaced regularly
pros and cons for non-rechargeable cells
lifetime
+ve
-ve
+ve - non-rechargeable lasts longer per charge
-ve - have to be disposed of
pros and cons for non-rechargeable cells
power
-ve
non-rechargeable don’t supply much power, so not good for an item that needs lots of power
e.g. phones
pros and cons for non-rechargeable cells
resource /waste
+ve
-ve
+ve - both batteries can be recycled and the metals recovered to use again or not if they are thrown away
-ve - non-rechargeable uses more resources and produces more waste
pros and cons for non-rechargeable cells
toxicity
+ve
-ve
+ve - non-rechargeable less likely to contain toxic metal so are less hazardous
-ve - could leak out and pollute water
lithium-ion cell
Positive electrode/reduction
Li+ + CoO2 + e- -> Li+[CoO2]- EƟ = +1.00V
Lithium cobalt oxide : Li+[CoO2]-
Lithium-ion cell
Negative Electrode / Oxidation
Li+ + e- -> Li
lithium-ion cell
overall equation
Li + CoO2 Li+[CoO2]-
lithium-ion cell
Conventional Cell Notation
Li|Li+||Li+, CoO2|LiCoO2|Pt
how does a lithium-ion cell work
reactants in this cell are absorbed into graphite powder which acts as a support medium. Ions react in this support medium without the need for a solvent e.g. water. It cannot be used as it would react with the lithium
what are modern fuel cells based on?
hydrogen or hydrogen-rich fuels e.g.
- methanol CH3OH
- methanoic acid HCOOH.
how does a fuel cell work and what are the benefits?
uses energy from the reaction of a fuel with oxygen to create a voltage. The development of fuel cells results in the pollution of water. Greater Efficiency from 40-60%
Storage of Hydrogen for use in fuel cells
- Gas / very low boiling point – store as a liquid under pressure.
- H2 can be adsorbed onto the surface of a solid material
- H2 can be absorbed within some materials
Limitations of the Hydrogen fuel cell:
- Large-scale storage and transportation poses problems as storing pressurised liquid.
- Current adsorbers and absorbers of hydrogen have a limited lifetime.
- Current fuel cells have a limited lifetime, need regular replacement and disposal
- use toxic chemicals in their production.
- H2 is an energy carrier not an energy source.
- manufactured by electrolysis of water or by reacting methane with steam. More energy may be used in making the hydrogen than saved by its use.
Methanol for fuel cells:
- can be generated from biomass.
- liquid is easier to store than H2 gas.
- Less pollution and less CO2 / incomplete combustion produce toxic CO which must be removed by a catalytic converter.
- Hydrogen-rich fuel produces only small amounts of CO2 and air pollutants.
Methanoic Acid for fuel cells
HCOOH <=> HCOO- + H+
- liquid and is easy to transport
- HCOOH gives a greater cell voltage
- HCOOH has more political acceptance than hydrogen as a fuel.
what are Hydrogen rich fuels (methanol, natural gas and petrol) mixed with
with water and converted into hydrogen gas by an onboard “reformer”. The reformer operates at 250-300c to generate the H2 gas.
Hydrogen-oxygen fuel cell with alkaline electrolyte (aka alkali fuel cell)
Oxidation/ negative terminal
2H2(g) + 4OH–(aq) -> 4H2O(l) + 4e–
Eϴ = -0.83V
Hydrogen-oxygen fuel cell with alkaline electrolyte (aka alkali fuel cell)
Reduction/ positive terminal
O2(g) + 2 H2O(aq) + 4e– -> 4 OH- (aq) Eϴ = +0.40 V
Hydrogen-oxygen fuel cell with alkaline electrolyte (aka alkali fuel cell)
Overall cell reaction:
2H2(g) + O2(g) 2H2O(l)
EMF: 0.40 –(-0.83) = 1.23V
why does the hydrogen-oxygen fuel cell work?
- The electrolyte is the alkali potassium hydroxide, KOH(aq).
- reactants flow in and products flow out while the electrolyte remains in the cells.
- operate continuously as long as there is a supply of H2 and O2
alkaline hydrogen-oxygen fuel cell
Cell notation
Pt|H2|OH-, H2O||O2|H2O, OH-|Pt
acidic hydrogen fuel cell
at the negative electrode
H2 -> 2H+ + 2e-
acidic hydrogen fuel cell
at the positive electrode
4H+ + 4e- + O2 -> 2H2O
acidic hydrogen fuel cell
overall equation
2H2 + O2 -> 2H2O
E cell = +1.23v
acidic hydrogen fuel cell
cell notation
Pt|H2|H+||O2|H+, H2O|Pt
ethanol fuel cell
C2H5OH + 3O2 -> 2CO2 + 3H2O
ox: C2H5OH + 3H2O -> 2CO2 + 12H+ + 12e-
red: 12H+ + 12e- + 3O2 -> 6H2O
mole fraction
proportion of all the moles in a system is each gas
e.g. moles of N2/ total moles
partial pressure
the contribution of the gas to the total partial pressure in a system
partial pressure of A = moel fraction of A x total pressure
factors affecting Kp
temperature
if the pressure has increased in Kp
the equilibrium shifts to decrease the pressure. this will favour the left-hand side, and there are fewer moles.
what happens in T is increased
Equilibrium shifts to decrease temperature. This will favour the forward, endothermic reaction, so Kp will increase
