Electrochemistry Flashcards
What is electrochemistry?
the study of chemical processes that cause electrons to move
How can a simple electrochemical cell be created?
two different metals dipped in salt solutions of their own ions joined by a salt bridge, connected to an external circuit
What are the two reactions that always occur in electrochemical cells?
oxidation and reduction
What does each beaker contain?
a half cell made up of a redox couple
What is a redox couple?
metal ions in equilibrium with its aqueous solution of ions
What is the salt bridge soaked in?
a strip of filter paper soaked in a solution of soluble ionic compound
What is the cell potential?
the voltmeter reading of potential difference between the two half-cells
What is the e.m.f (electromotive force)?
the cell potential under zero current conditions
Which direction do electrons flow?
from the more reactive metal to the less reactive metal
What does a positive voltmeter reading mean?
electrons flow left to right
What happens to the most reactive metal?
it gets oxidised and is the more negative electrode
How do you get the overall reaction?
by combining the two half equations
What is the electrochemical series?
a list of standard electrode potentials placed in order according to the value of the reduction process
Describe the layout of the electrochemical series
-most positive at the top (most strongly oxidising is positive, good at gaining electrons)
-most negative at the bottom (most strongly reducing is negative, worse at gaining electrons)
What is the formula for the oxidising agent accepting electrons?
ox + e- ⇌ red NEED STATE SYMBOLS
The E value of a redox couple gives a…
quantitative measure of equilibrium
What happens to the equilibrium if the E value is positive?
the more likely the forward reaction is to occur, correspond to reduction
What happens to the equilibrium if the E value is negative?
the more like the backward reaction is to occur, correspond to oxidation
Any species on the left hand side can oxidise…
any species on the right-hand side below it
Any species on the right hand side can reduce…
any species on the left-hand side above it
What are good reducing agents?
reactive metals
What are three types of groups of electrochemical cells?
-primary cells
-secondary cells
-fuel cells
What is a primary cell?
a primary cell is irreversible and non-rechargeable
What is a secondary cell?
a secondary cell is reversible and is rechargeable
What is a fuel cell?
a fuel cell generates electricity from the continuous oxidation of an external source of fuel
What is the function of a primary fuel cell?
to provide current to an external circuit while discharging (galvanic action)
What is a galvanic cell?
a galvanic cell is one with a positive e.m.f in which the spontaneous forward cell reaction can be used to provide electric current to an external circuit
How do secondary cells work?
they combine two opposing functions of all reversible electrical cells -
-they provide current to an external circuit while discharging (galvanic action) chemical into electrical energy
-they use current from an external circuit while charging (electrolytic action) electrical into chemical energy
In order for the system to be truly reversible, what is important about the products?
that the products resulting from both galvanic and electrolytic action are not dispersed in the cell but remain attached to the cell electrodes
What is the equation to find Ecell?
Eright - Eleft
right is the most positive, left is the most negative
Describe the lithium rechargeable cell
-one electrode is LiCoO2
-one electrode is graphite
-electrolyte is a lithium salt in an organic solvent
What are the equations for the lithium rechargeable cell?
positive electrode/reduction : Li+(aq) + CoO2(s) + e- –> Li+[CoO2]-(s)
negative electrode/oxidation : Li(s) –> Li+(aq) + e-
OVERALL : Li(s) + CoO2(s) –> Li+[CoO2]-(s)
What is a benefit of lithium?
very light
What are fuel cells?
use a supply of hydrogen or organic fuel with a supply of oxygen to provide a source of electrical power
Why are fuel cells reliable?
there are no moving parts
What is the difference between batteries and fuel cells?
fuel cells consume reactants that must be replenished, whereas batteries store and release energy is a closed system
Describe the structure of a hydrogen-oxygen fuel cell
-hydrogen as a fuel on the anode side, oxygen as an oxidant on the cathode side
-platinum-group metal/alloy as the catalyst
What are two properties of the platinum electrodes?
inert and porous - allows the passage of reactant and product gases
What can the electrolytes be in hydrogen-oxygen fuel cells?
-acidic - phosphoric acid
-alkaline - hot aqueous potassium oxide
How do acidic hydrogen-oxygen fuel cells work?
-hydrogen diffuses to the anode platinum catalyst where it splits up to make H+ ions and electrons
-hydrogen ions are conducted through the electrolyte to the cathode
-the electrons travel through the external circuit
-at the cathode platinum catalyst surface, oxygen molecules react with electrons and hydrogen ions to form water
-water is the only waste product
How do alkaline hydrogen-oxygen fuel cells work?
-at the anode, hydrogen reacts with hydroxide ions, water and electrons are produced
-electrons enter the external circuit
-at the cathode, oxygen reacts with electrons arriving from the external circuit to make hydroxide ions
In acidic conditions, what are the electrode reactions?
overall : 2H2(g) + O2(g) –> 2H2O(l)
oxidation : H2(g) –> 2H+(aq) + 2e-
reduction : O2(g) + 4H+ (aq) + 4e- –> 2H20(l)
In alkaline conditions, what are the electrode reactions?
overall : 2H2(g) + O2(g) –> 2H2O(l)
oxidation : H2(g) + 2OH-(aq) –> 2H2O(l) + 2e-
reduction : O2(g) + 2H2O(l) + 4e- –> 4OH-(aq)
Why is the e.m.f exactly the same for both acidic and alkaline reactions?
they have exactly the same overall reaction
What is the cell representation for the acidic hydrogen-oxygen fuel cell?
Pt(s)│H2(g)│H+(aq)║O2(g)│H+(aq),H2O(g)│Pt(s)
What is the cell representation for the alkaline hydrogen-oxygen fuel cell?
Pt(s)│H2(g)│OH- (aq),H2O(l)║O2(g)│H2O(l),OH-(aq)│Pt(s)
Why do hydrogen-oxygen fuel cells only work under standard conditions limitedly?
-rate of flow of electrons is slow
-usual to operate cell around 200C
-however since the reaction is highly exothermic, increasing temp decreases e.m.f
-therefore an increased pressure is used to compensate
How do we obtain hydrogen?
-oxygen can be cheaply obtained from the air
-most hydrogen is produced by the reaction between methane and steam
-high input of energy with a large carbon footprint
What is the overall reaction for the production of cycle from methane and steam?
CH4(g) + 2H2O(g) –
> CO2(g) + 4H2(g)
Are hydrogen-oxygen fuel cells really green?
water is the only product, but the hydrogen that is produced needs to be made cleanly, by the electrolysis of acidified water
-the energy for this electrolysis must come from carbon neutral sources such as solar power
What is the overall equation for an organic fuel cell?
C2H6O + 3O2 –> 2CO2 + 3H2O
What is the SHE?
standard hydrogen electrode - measuring standard for half-cell potentials
What are the conditions of a cell potential 0.00V for a SHE?
-solutions are 1.0 moldm-3 conc
-temp of 298K
-100kPa pressure
What does a SHE consist of?
hydrochloric acid, hydrogen gas, platinum electrodes
