what is oxidation
the process of electron loss involving an increase in oxidation number
what is reduction
the process of electron gain involving the decrease in oxidation number
the oxidation numbers in a compound add up to
zero
all uncombined elements have an oxidation number of
zero
the oxidation number of a monoatomic ion is equal to
the ionic charge
in a polyatomic ion, the sum of the individual oxidation numbers of the elements adds up to
the charge on the ion
what is the invariable oxidation number of group 1 metals
+1
what is the invariable oxidation number of group 2 metals
+2
what is the invariable oxidation number of aluminium
+3
what is the invariable oxidation number of hydrogen and its exception
+1 except in metals hydrides where it is -1
what is the invariable oxidation number of flourine
-1
what is the invariable oxidation number of the halogens (Cl, Br, I) and its exception
-1 except in compounds with oxygen and flourine
what is the invariable oxidation number of oxygen and its exception
-2 except in peroxides and compounds with flourine where it is -1
a reduction equation shows
parts of the chemical equation involved in reduction and the electrons are on the left
the oxidising agent is
an electron acceptor
an oxidation half equation shows
the parts of the chemical equation involved in oxidation and the electrons are on the right
the reducing agent is the
electron donor
what is a reducing agent / reductant
the species that causes another element reduce and is itself oxidised
what is an oxidising agent / oxidant
the species that causes another element to oxidise and itself is reduced
for reduction add electrons to the
to the reactants
for oxidation add electrons to the
products
what does OILRIG stand for
Oxidation Is Loss Reduction Is Gain
in acidic conditions use
H+ and H2O
how do you balance an equation where the substance being oxidised/reduced contains a varying amount of Oxygen
by adding H+, OH- and H2O
to combine half equations into a full redox equation what must be equal on both sides of the equation and why
numbers of electrons so that they cancel out
what is the faraday's constant
measure of the charge of 1 mole of electrons
what is the value of the faraday's constant
96320 coulombs
what is the value of the Avogadro's constant
6.022 x 10^23
what is the charge of an electron, e
1.60 x 10^-19 Coulombs
what happens when a simple ionic compound is electrolysed in the molten state using inert electrodes
the salt splits
metal ion moves to the negative electrode
negative ion moves to the positive electrode+
what is the name of the negative electrode
cathode
what is the name of the positive electrode
anode
what happens at the cathode
reduction as the positively charged ions gain electrons to become atoms
usually metals
what happens at the anode
oxidation as the negatively charged ions lose electrons to become atoms
ions usually end in
-ide
what does ion discharge depend on when electrolysing an aqueous solution
the electrode potentials of the ions involved
how to find the number of molecules of a substance produced by electrolysis
find number of moles and then multiply by avogadro's constant
in aqueous solutions, where the metal is more negative in the electrochemical series than hydrogen, which is discharged first
hydrogen gas
at the anode, which is discharged first between a halogen and oxygen
halogen
in an aqueous solution, what changes the product evolved
concentration of reactants
Charge (C) =
Current x time (s)
moles of electrons =
charge (C) / Faradays
number of electrons passed =
charge (C) / electronic charge of the ion
finding the moles of electrons in the equation =
moles of electrons / moles of the metal evolved
moles of gas formed=
gas volume / molar volume (24dm)
equational avogadro's constant =
number of electrons / moles of electrons
components of an electrochemical cell
two half cells
salt bridge
electrodes
electrolytes
high resistance voltmeter
what causes voltage to form
difference in number of electrons at each electrode creating a potential difference
why is a high resistance voltmeter used
to stop current from flowing in the circuit hence no reaction occurring and maximum potential difference can be measured
use of a salt bridge
to connect the circuit and allow movement of ions
components of a salt bridge
filter paper soaked in an inert salt solution (NaNO3)
why isnt a wire used instead of a salt bridge
a wire would act as an electrode
what happens if current is allowed to flow
the reactions will happen separately and reactants used up hence the voltage will fall to zero
definition of a standard cell potential
the potential of a cell composed of two electrodes under standard conditions
what is a Standard Hydrogen Electrode
a half cell assigned the potential of 0 volts used to calculate potentials of other half cells by comparison
components of a SHE
hydrogen gas at 100kPa
solution of 1M hydrogen ions
298K
Platinum black electrode
why is platinum black used in a SHE
it is porous and can absorb the hydrogen gas
why are standard conditions needed
the position of the redox equilibrium will change with the conditions
what is a secondary standard
A standard used for comparison instead of a SHE which has been calibrated against a SHE itself
eg silver
definition of the standard electrode potential
the potential difference measured when an electrode system is connected to the SHE in standard conditions
what are the standard conditions
all ion solutions at 1M
298K
gases at 100KPa
no current flowing
in half cell equations, the more oxidised form is on the
left
when two ions are present in a solution , which electrode is used
platinum
the more negative half cell will always
oxidise (go bakcwards)
the more positive half cell will always
reduce (go forwards)
Ecell =
Ered - Eox
to get the full redox equation
add the two half cells together
electrons must cancel out
as more +ve increasing tendency, species on the left to
act as oxidising agents
as more -ve increasing tendency, species on the right to
act as reducing agents
what is Ecell
a measure of how far from equilibrium the cell reactions lies.
the more positive the Ecell
the more likely the reaction is to occur
the relationship between reactant concentration and Ecell
directly proportional
the relationship between temperature and Ecell
inversely proportional
why are temperature and Ecell inversely proportional
most cells are exothermic
the reaction may not occur even if Ecell is positive if
the activation energy is high
use of the Nernst equation
enables the electrode potential for a system with non-standard concentrations to be calculated
the Nernst equation
E = E⦵ + (0.059/z) log [oxidised species] /[reduced species]
in the Nernst equation Z represents
the number of electrons transferred
in the Nernst equation if the reduced species is a solid , the value is
1
use of electrochemical cells
commercial source of electrical energy
types of electrochemical cells
non- rechargeable
rechargeable
fuel cells
cells are non-rechargeable when the reactions that occur with in them are
non-reversible
what is used in the absence of water as a solvent
powdered graphite
what is a fuel cell
uses the energy from the reaction of a fuel with oxygen to create voltage
what electrolyte does the hydrogen fuel cell use
potassium hydroxide
difference between fuel cells and ordinary cells
fuel cells maintain a constant voltage as reactants are readily available whereas the voltage of the ordinary cells drops as the reactants are used up
what are the conditions of the Hydrogen fuel Cell
higher temperatures (to increase rate)
higher pressures (to counteract effect of temp increase)
advantages of fuels cells over Conventional petrol/diesel vehicles
less pollution / less CO2
greater efficiency
limitations of hydrogen fuel cells
storing and transporting hydrogen (safety, feasibility of pressurised liquid, limited life cycle of absorber)
limited lifetime and high production costs
use of toxic chemicals