16A.1 Flashcards
oxidation is
loss of electrons
reduction is
gain of electrons
when an element is oxidized its oxidation number
increases
when an element is reduced its oxidation number
decreases
standard electrode potential is also
redox potentail
standard electrode potential is also
redox potential
what happens to metals when they are placed in water
they have a very small tendency for them to lose electrons and from a solution of positive ions
how is this (M^n+(aq) + ne^-1 eq M(s)) eq formed
when a metal is put in water it loses electrons and forms a positive ion solution, so within a short time, there will be a build-up of electrons on the surface of the metal, due to the electrostatic attraction between the electrons on the surface of the metal and the positive ions formed, some of the positive ions will regain their electrons and become part of the metal surface, eventually a dynamic equilibrium will form
the eq formed between the positive metal ion and its electrons in water is given by
M^n+(aq) + ne^-1 eq M(s)
the more reactive the metal in a half-cell is
the eq will lie further to the left, which means more significant tendencies to release electrons so greater negative charge on the metal and more positive ions in the solution
what is the absolute potential difference
the potential difference between the electrons on the surface of the metal and the solution of positive ions of that metal
what is the rs between absolute potential difference and reactivity of the metals
as the metal becomes more reactive the V increases
why can’t we measure the absolute potential difference directly
we can connect the metal electrode to one terminal (of the circuit), but the other terminal would have to be connected to the solution which isn’t possible. and we can’t connect the terminal to other metal dipped into the water because then that would create its own potential difference.
how can we measure absolute potential difference of a half cell
create a refence potential electrode (standard hydrogen electrode) then measure the potential difference between the reference electrode and the metal electrode
what is the reference electrode of choice
standard hydrogen electrode
how does the standard hydrogen electrode work
hydrogen gas at pressure of 100KPa (1 bar) being bubbled over a piece of platinum foil covered with porous platinum, dipped into a solution of acid (H2SO4) with [H^+] at 1 mol dm^-3, at temp of 298K
why do we cover platinum with porous platinum
it has a large surface area and allows the Eq between hydrogen ions in the solution and the hydrogen gas to be established quickly
what is the standard hydrogen electrode half cell (its half equation)
page 161
Importance of using student conditions
the position of Eq changes by altering the conditions so fair comparisons we need standard conditions
what are standard conditions
- 100KPa (1 bar), 298K, concentration in solution of 1 mol dm^-3
what is a half cell
it is one half that makes the electrochemical cell, and two half cells complete one cell
how does a metal ion/metal system look like
page 161
why is a salt bridge needed
to complete an electrical circuit
what is the salt bridge made from
filter or chromatography paper dipped in an a ionic salt (usually potassium nitrate), the salt ions should mot interfere with the components of the half cell
equilibrium reaction are written in
the reduced form
why is a high resistance voltmeter used in an electrochemical cell
because we don’t want the electrons (circuit) to flow through it so the potential difference measured from the voltmeter is that of the two half cells
the sign of standard electrode potential indicates
the polarity of the electrode when compared with hydrogen electrode. the sign is fixed and doesn’t change even of the equation of the half cell is reversed
the sign of the electrode depends on (metal)
if more reactive than hydrogen its negative, if less reactive than hydrogen its positive (since metal want to lose electrons0
what is the standard electrode potential
The emf measured when a half-cell is connected to a standard hydrogen electrode under standard conditions
the sign of the electrode depends on (non metal)
if more reactive than hydrogen then positive, and less reactive than hydrogen then negative (because metals like to gain electrons).
the standard electrode potential of hydrogens half cell is
0.00 under standard conditions
the potential difference provides a comparison between
the position of the metal ion/metal eq and the position of eq in the hydrogen electrode
what does a negative standard electrode potential mean
the equilibrium of the half cell reaction lies further on the left than the eq of the standard hydrogen electrode (the more readily the element loses elections to from ions)
what does a positive standard electrode potential mean
the equilibrium of the half cell reaction lies further on the right than the eq of the standard hydrogen electrode (the less readily the element loses electrons to form ions)
the electrode where oxidation takes place is
the anode
the electrode where reduction takes place is
the cathode
what is a reducing agent
a species that reduces other species by adding one or more electrons to it . when it reacts it loses electrons, and therefore oxidized
what is a oxidizing agent
a species that oxidizes another species by removing one ore more electrons from it. when it reacts it gains electrons and therefore reduced
what is emf
the measured potential difference of a cell when no current is flowing
the emf will be positive if
the hydrigen electorn is negative (cathode)
the emf will be negative if
the hydrogen electrode is positive (anode)
the standard potential difference of a cell is called
E cell, and standard cell potentail
the emf polarity depends on (in general)
the reference point against which the potential difference is measured
how can we setup a system to measure the V of a gas
the gas is bubbled into a solution of its ions, in order to establish an eq between the gas and its ions, and to provide a connection the the external circuit, we need to place a piece of platinum into the solution
gas/hydrogen system looks like
164
how can we set up a non metal elements and their ions in solution system
The non metal would be put into a solution with its ions, but in order to establish an equilibrium between the non-metal and its ions, and also provide a connection to the external circuit, we would need to place a piece of platinum into the solution.
how can we set up a ‘ions of the same element with different oxidation number’ system
a solution would be made with both ions, and a platinum metal is put in the solution in order to establish an eq between the ions, and also provide a connection to the external circuit.
what is the electrochemical series
it is an series built-up by arranging various redox equilibria in order of their standard electrode (redox) potential, with the most positive values on top
species on the right-hand side of the half cell half reaction behaves as
a reducing agents because they lose electrons.
species on the left-hand side of a half cell - half reaction behaves as
oxidizing agents because they can gain electrons
the more negative the standard electrode potential is
the more the eq position lies towards the left, so more readily the species on the right loses electrons therefore more reducing power
the more positive the standard electrode potential is
the more the eq position lies towards the right, so more readily the species on the left gain electrons therefore more oxidizing power
why cant we measure the standard electrode potential of reactive metals and other experimentally
because they can react with water to completion, so E us measured from thermodynamic data.
