chapter 23 Flashcards
oxidation number of an atom in an element
0
oxidation number of oxygen in hydrogen peroxide
-1
oxidation number of oxygen in OF2
+2
when is the oxidation number of chlorine not -1
when combined with oxygen or flourine
oxidising agent
reducing agent
it takes electrons from the species being oxidised
gives electrons to the species being reduced
which ions are present when a reaction is said to be under acidic conditions
H+
what do we use to balance
Oxygen, hydrogen and charges in half equations
balance oxygen with water molecules
balance hydrogen with hydrogen ions
balance charges with electrons
in manganate (VII) redox titrations, why are the burette readings taken from the top of the meniscus
the deep purple colour of manganate(VII) makes it difficult to see the bottom of the meniscus
during manganate (VII) redox titrations, when is the endpoint
when the first permanent pink colour is formed(this indicates a drop excess of manganate )
concordant results
results within 0.1cm3 of each other
percentage purity formula
(mass of pure substance/ mass of impure substance)* 100
purpose of manganate (VII) redox titrations
to analyse any reducing agents
outline the processes involved in iodine/thiosulphate redox titrations
excess of potassium iodide added to the oxidising agent being analysed.
the oxidising agent is measured in a pipette
Iodide ions are oxidised to Iodine during this process
the solution turns yellow-brown
mixture is titrated with a standard solution of sodium thiosulphate. the yellow colour fades
what color change occurs close to the endpoint in an iodine/thiosulphate redox titration when we add a starch indicator
from straw yellow to a deep blue-black color
at the endpoint in an iodine/thiosulphate redox titration, what happens
the blue-black colour disappears ( solution turns colourless )
a redox reaction can be split into two half equations, true or false
true
half- cell
contains the chemical species present in a redox half-equation
what arranges for the two half equations in a redox reaction to occur separately in half cells
a voltaic cell
in electrochemical cells …… are transferred from one half-cell to another, and this means….
electrons
current has been generated and chemical energy has been converted into electrical energy)
by convention, what does the forward reaction in a half-cell represent
a reduction reaction
what happens at the phase boundary
an equilibrium is set up
what is required for electrons to flow from one half-cell to the other through wires
two half cells are required for this to happen
what is the salt bridge comprised of
filter paper soaked in saturated KNO3
function of the salt bridge
allows the ions to flow
in an electrochemical cell, in which direction is the movement of electrons
from the half cell with the more negative electrode potential to the one with the more positive electrode potential
standard conditions required for standard electrode potentials
298K
1 moldm^-3
100kPA
the function of the voltmeter in an electrochemical cell
it measures the potential difference between the two electrodes( measures the Ecell)
Ecell stands for
Electrode potential of the cell
standard electrode potential
it is a measure of the ability of species to accept electrons(to be reduced)
standard electrode potential of a hydrogen half cell
0.00V
the more negative the standard electrode potential of a half cell, ….
the greater it’s tendency to lose electrons and be oxidised
the more positive the standard electrode potential of a half cell,….
the greater the tendency of gaining electrons and be reduced
ion/ion half cells
give one example
this is when the redox system does not contain a metal
the hydrogen half cell
in ion/ion half cells, which metal is used as the electrode and why
platinum
because it is inert, it does not participate in the redox reaction while the cell is working, unlike other metals
is the half equation flipped for the reduced or oxidized half cell in an electrochemical cell
for the oxidized half cell
What are the five rules when balancing half equations
Write down the species before, and after the reaction
Balancing any atoms apart from oxygen and hydrogen
Balance in oxygen is H20
Balance, any hydrogens with hydrogen ions
Balance any charges with elections
In half equations where the species are reduced, on which side are the electrons?
On the left side of the equation
All feasible reactions provide a………. Value for the electrode potential if the cell
A positive value
