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