redox ii Flashcards
what is oxidation
loss of electrons
when oxidation number increases
loss of hydrogen
gain of oxygen
what is reduction
gain in electrons
decrease in oxidation number
gain in hydrogen
loss of oxygen
s block metals tend to react by being …
oxidised (lose electrons to form positive ions)
p block metals can react by being …
oxidised (losing electrons to form positive ions)
p block non metals can react by being …
reduced (gaining electrons to form negative ions)
d block metals tend to get …
oxidised (losing electrons to form positive ions)
electrochemical cells make …
electricity
how can electrochemical cells be made ?
made by two different metals dipped in salt solutions of their own ions connected by the wire
what are the two reactions that always take place in a electrochemical cell ?
oxidation and reduction so its a redox process
oxidation always happens at the …
anode(positive electrode)
reduction always happens at the …
cathode(negative electrode)
which metal becomes the anode
the more reactive metal becomes the anode as it is easily oxidised because it gives up its electrons more easily
which metal becomes the cathode
less reactive metal
where do the electrons flow from ?
the anode
in zinc/copper electrochemical cell which half cell do the electrons flow from
zinc as it loses electrons more easily (so in copper half cell it receives the same amount of electrons that left the zinc half cell)
in zinc/copper electrochemical cell which which metal is oxidised which metal is reduced
zinc is oxidised as it loses electrons more easily copper is reduced as it gains electrons more easily
which way do the electrons flow in electrochemical cell
from more reactive metal to least reactive metal
what does the voltmeter do in the circuit
shows the voltage between two half cells which is called cell potential or EMF or E(cell)
what is salt bridge made from
filter paper soaked in salt solution
what is the salt bridge for
salt ions flow through the cell to complete the circuit and balance out the charges in the beaker
if neither products or reactants are solid what can you use for electrodes
has to conduct electricity
has to be inert
eg pt but its expensive so graphite is used instead
when drawing electrochemical cells where should you draw the anode and the cathode
anode(where oxidation happens) on the left and cathode (where reduction happens) on the left
the reactions at both electrodes are …
reversible
method to construct electrochemical cell
.get strip of each metal and clean the surfaces using piece of emery paper/sandpaper
.clean grease or oil from electrodes using propane don’t touch cells with your hands after this or you could transfer back the grease
.place each electrode into a beaker containing solution of ions of that metal
.create salt bridge to link solutions together by dipping filter paper in salt solution ends of filter paper should be immersed in the solution
.connect electrodes to voltmeter using crocodile clips and wires and wires
the electrode potential is a measure of ?
how easily the substance is oxidised in the half cell
the half reaction with the more positive electrode potential value goes in what direction ?
forward
the half reaction with the less positive electrode goes?
backwards
if half cell Zn^2+ + 2e^- —>Zn has electrode potential of -0.76v
and half cell Cu^2+ + 2e^- —> Cu has electrode potential of +0.34
write the electrode potential equation
Cu^2+ + Zn —> Cu + Zn^2+
you measure the electrode potential of a half cell against what ?
standard hydrogen electrode
what is the standard electrode potential ?
voltage measured under standard conditions when the half cell is connected to a standard hydrogen electrode
what are the standard conditions when measuring standard electrode potential ?
*solution of the ions you are interested in must have a concentration of 1.00 mol dm⁻³
*the temperature must be 289 K (25 C)
*the pressure must be 100 kPa
what is the equation for the reaction at the hydrogen electrode ?
2H⁺(aq) + 2e⁻ ⇌ H₂(g)
when drawing standard hydrogen electrode where is it positioned?
on the left
the standard hydrogen electrode is a reference electrode which means
it has a value of 0.00V meaning the voltage reading will be equal to the E of the other half cell
E(cell) =
E(reduction)-E(oxidation)
cell potential will always be what charge?
positive as the more negative E is being taken away from the more positive E
what conditions change the equilibrium position in a half cell?
temperature pressure and concentration
changing the equilibrium position in half cell will affect the ?
cell potential
how can you get around, equilibrium position affecting the cell potential?
by using standard conditions which means u always get the same value for electrode potential and you can compare the values for different cells
more reactive metals have a more negative or positive standard electrode potential ?
more positive
if a reaction is thermodynamically feasible then then overall potential will be ?
positive if its negative then it isn’t feasible
electrode potentials can predict wether disproportionation reactions will happen, if the overall cell potential is negative will the element disproportionate in solution
no overall cell potential is negative and so element will not disproportionate in solution
the half reaction with the more negative electrode potential value E goes
backward
Zn²⁺(aq) 2e⁻ ⟶ Zn(s) E=-0.76V
Cu²⁺(aq) + 2e⁻ ⟶Cu(s) E=+0.34
what is the full equation
Cu²⁺(aq) + Zn(s) ⇌ Cu(s)+ Zn²⁺
in Cu²⁺(aq) + Zn(s) ⇌ Cu(s)+ Zn²⁺
what is the oxidising agent and what is the reducing agent
zinc is reducing agent and copper as the oxidising agent
how do you measure the electrode potential of a half cell ?
measure it against standard hydrogen electrode
what is the standard electrode potential
E of a half cell is the voltage measured under standard conditions when the half cell is connected to a standard hydrogen electrode
what are the standard conditions when measuring electrode potential of a cell
*the solutions of the ions you’re interested in must have a concentration of 1.00 mol dm⁻³
*the temperature must be 298 K (25°c)
*the pressure must be 100 kPa
what is the equation for reaction at the hydrogen electrode
2H⁺(aq) 2e⁻ ⇌ H₂(g)
when drawing the standard hydrogen electrode where is it positioned
on the left
standard hydrogen electrode is a reference electrode what does this mean?
the hydrogen half cell has a value of 0.00V which means voltage reading will be equal to E of the other half cell
E(cell)=
E(reduction - oxidation)
cell potential will alway be positive or negative ?
positive because the negative E is always being taken from bigger E
what conditions affect equilibrium position in a half cell
temperature pressure concentration
what does changing the equilibrium position In a half cell do ?
changes the cell potential
to get around equilibrium position changing cell potential what can you do ?
use standard conditions to measure electrode potentials, using these conditions means you always get the same value for the electrode potential and you can compare values for different cells
more reactive metals have more ?
negative standard electrode potentials
more reactive non metals have more?
positive standard electrode potentials
if a reaction is thermodynamically feasible the overall potential will be?
more positive. a reaction isn’t feasible if E is negative
for a disproportionation reaction to happen the cell potential has to be ?
positive
a prediction using E only states if a reaction is possible under standard conditions the prediction might be wrong if ?
*the conditions are not standard
*the reaction kinetics are not favourable.(rate of reaction may be slow that the reaction might not appear or reaction has a high activation energy which stops it happening)
entropy change is bigger the ?
the bigger the cell potential
What are energy storage cells
Batteries
How can you work out the voltage of the energy storage cells
Using electrode potentials of the substances in the cell
when drawing conventional representation which half cell goes on the left
the one with the more negative potential
when drawing conventional representation what goes in the middle of the centre of the diagrams
not the salt bridge
the oxidised forms
when drawing conventional representation what goes on the outside of the diagrams
the reduced forms
when drawing conventional representation what do double vertical lines represent
salt bridges
when drawing conventional representation what do single lines represent
different physical states
when drawing conventional representation when do you use commas
when species in the same half cell are in the same physical state
when drawing conventional representation if you have standard hydrogen electrode where should it go
on the left
when drawing conventional representation what should you do if you use platinum lead or other inert electrode
put it on the outside
when should you use inert or platinum electrode
when a solution is fully aqueous
Zn²⁺(aq) + 2e⁻ ⟶ Zn(s) E=-0.76
Cu²⁺(aq) + 2e⁻ ⟶ Cu(s) E=0.34
draw the conventional representation
Zn┆Zn²⁺(aq)┆┆Cu²⁺(aq)┆Cu(s)
in fuel cells where are the chemicals that generate electricity
in the electrodes and electrolyte
cells are non rechargeable when
reactions that occur within them are non reversible
a fuel cell uses
the energy from the reaction of a fuel with oxygen to create a voltage
hydrogen oxygen fuel cells will maintain a constant voltage over time if
they are fed fresh O₂ and H₂
advantages of fuel cell
*less pollution and less CO₂
greater efficiency
limitations of hydrogen fuel cells
*expensive
*has a limited lifetime requiring regular replacement
*high production cost
*uses toxic chemicals in their production
hydrogen can be stored in fuel cells
*as a liquid
*adsorbed on the surface of a solid material
*absorbed within a solid material
what advantages do ethanol fuel cells have over hydrogen fuel cells
*ethanol made from renewable sources in carbon neutral way
*materials used to produce ethanol by fermentation are abundant
*ethanol less explosive and easier to store than hydrogen
*new petrol stations would not be needed as ethanol is a liquid fuel
where are chemicals stored in a fuel cell
separately outside the cell
when are the chemicals fed into a fuel cell
when electricity is required
what can a alkaline hydrogen oxygen fuel cell power
used to power electric veichles
in a alkaline hydrogen oxygen fuel cell what are the chemicals used
hydrogen and oxygen
in a alkaline hydrogen oxygen fuel cell where are the hydrogen and oxygen fed into
two separate platinum containing electrodes
what does a anion exchange membrane allow
allows an ions and water to pass through it but not the chemicals
in hydrogen oxygen fuel cell what does the platinum catalyst do
splits the H₂ into protons and electrons
in hydrogen oxygen fuel cell what odes polymer electrolyte membrane do
only allows H⁺ across forcing electrons to travel around the circuit
in hydrogen oxygen fuel cell what does the electrons travelling around do
creates electric current which can power things
in hydrogen oxygen fuel cells what happens at cathode
O₂ combines with H⁺ and electrons making water which is the only waste product
iodine sodium thiosulfate titrations are a way of finding
the concentration of oxidising agent
CH₃OH + H₂O→
CO₂ + 6e– + 6H⁺
6H⁺ + 6e– + 1·5O₂ →
3H₂O
what do titrations allow you to do
find out how much acid is needed to neutralise a quantity of alkali
a known volume of alkali is titrated with an
acid of known concentration
volume of acid of known concentration needed to neutralise alkali can be used to work out
concentration of the alkali
what are pipette used for
to measure only one volume of. solution
how do you use a pipette
fill pipette just above the line then take pipette out of solution and drop the level down carefully to the line
what are burette used for
to measure different volumes of solutions and let you drop the solution drop by drop
An oxidising agent
accepts electrons and gets reduced.
A reducing agent
donates electrons and gets oxidised.
Transition (d-block) elements are good at changing
oxidation number
because transition elements good at changing oxidation number they are good as oxidising and reducing agents
as they readily give out and receive electrons
how do you work out concentration of reducing agent
you titrate it against a known volume of a oxidising agent of known concentration
how to find out how many manganate(VII) ions (MnO₄–) are needed to react with a reducing agent
*measure quantity of reducing agent using pipette
*add some dilute sulphuric acid
*do rough titration adding MnO₄–using burette swirling conical flask
*stop when mixture in flask becomes tainted with purple colour
*record volume added repeat and run few accurate titrations and calculate mean volume of MnO₄–
when doing titrations you need to watch out for
sharp colour changes
what can make a colour change easier to spot
doing reaction in front of white surface
the more concentrated an oxidising agent the more
ions will be oxidised by a certain volume of it
iodine sodium thiosulfate titration with potassium iodate (V)
*measure out volume potassium iodate(V) solution
*add this to excess of acidified potassium iodide iodate(V) ions in potassium iodate(v) solution oxidise some of the iodide ions to iodine
*find out how many moles of iodine have been produced by titrating resulting solution with sodium thiosulfate
*work out concentration of potassium iodate(v)
step 1 in iodine sulphate thiosulfate
use sample of oxidising agent to oxidise as much iodide as possible
what is step 2 in iodine sodium thiosulfate titration
find out how many of moles of iodine have been produced
I₂(aq) + 2S₂O₃²⁻(aq)⟶
2I⁻ (aq) + S₄O₆²⁻ (aq)
what is stage 3 in odine sodium thiosulfate titration
calculate the concentration of oxidising agent
can use titration to find percentage of
copper in an alloy
copper(II) ions will
oxidise iodide ions to iodine
stage 1 in finding percentage of copper in an alloy
use sample of oxidising agent to oxidise as much iodide as possible
stage 2 in finding percentage of copper in an alloy
find out how many moles of iodine have been produced
stage 3 in finding percentage of copper in an alloy
calculate the concentrations of the oxidising agent
how finding percentage of copper in an alloy
*dissolve weighed amount of alloy in conc nitric acid
*pour in 250 cm³ volumetric flask and make up to 250 with deionised water
*pipette 25cm³ of solution to flask
*slowly add sodium carbonate solution to neutralise any nitric acid
*keep going until slight precipitate forms
*removed if you add few drops of ethanoic acid
*add excess of potassium iodide solution
*white precipitate of copper(I) iodide forms
*titrate product mixture against sodium thiosulfate solution to find number of moles of iodine present
what are the sources of error in finding the percentage of copper in alloy
*starch indicator needs to be added at right time when most of iodine has reacted or else blue colour will be slow to disappear
*starch solution needs to be freshly made or it won’t behave as expected
*percipitate of copper (I) iodide makes seeing colour of solution quite hard
*iodine produced in reaction can evaporate from solution giving false titration reading so final percentage of copper would be too low so solution should be kept cool
when does starch indicator need to be added and why
when most of the iodine has reacted or else the blue colour will be slow to disappear
why does starch solution need to be freshly made
or else it won’t behave as expected
iodine produced in reaction can evaporate what does this mean and what should be done
false titration reading is given meaning final figure of percentage of copper would be too low solution should be kept cool
