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this course on electrochemistry will
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give an overview of the electrochemical
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behavior of species in solution the
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first question course is why are we
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interested in electrochemistry
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well chemistry is all about electrons
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and electrons of the foundation of
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electricity and we're interested in what
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happens when an electron goes from one
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place to another the example here is
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when we have sodium and chlorine
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reacting together the electron hops from
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the sodium to the chlorine and forms
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these ion pairs okay and this is what
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forms salts so we are interested in what
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happens when this electron goes from one
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place to another so fundamentally this
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movement of charge results in ions so we
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need to understand a little bit about
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how ions behave and what is it that
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controls whether or not a reaction can
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happen under these conditions
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fundamentally through better
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understanding of these electrochemical
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processes we will understand better
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about how chemical reactions happen and
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how electro statics are important
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electrochemistry is used widely from
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looking at energy storage and managing
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reactions through to our electronic
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devices anywhere that we have any level
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of electricity storage we have chemistry
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happening when we think about things
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like electron transfer is not a trivial
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subject we will be discussing more of it
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in years three and four so please do
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bear in mind that this is a little bit
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more complex than it might initially
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seem but through a better understanding
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of the electrochemical processes in
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cells we will gain a better
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understanding of the chemistry that
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we're studying to give a quick overview
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of this session we're going to be
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looking at the fundamentals of
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electrochemistry so what is it we need
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to consider when we think about
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electrochemical processes will look into
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ions and iron behavior and how they
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behave in solution will then be looking
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at how they diffuse and migrate through
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that solution will then consider the
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concepts around electrochemical
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equilibrium and in particular the
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thermodynamics that happen in
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electrochemical cells and finally we
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will be looking at electrode processes
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so thinking about how to how do the
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kinetics of an electrochemical process
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affect the rates of our reactions
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we'll need to quickly reintroduce you to
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some areas that you have covered in
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first-year this won't take very long but
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it's important that you are familiar and
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understand what this terminology that
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we'll be using it means so whether we
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think of electrode pretend
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whether we thinking standard electrodes
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cell potentials cations anions cathodes
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anodes so we need to do a very quick
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recap of this material electrical
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potential is a first-year topic which
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you may well have come across as half
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cell potentials reduction potential or
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indeed half reactions these will be
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fairly familiar to you so you'll have
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seen things that look a bit like this
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where we have copper two-plus being
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reduced via the addition of electrons to
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copper methyl with an Associated attempt
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cell potential another way of
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representing such an equilibrium is
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using this bar here so here we have zinc
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two-plus being reduced to zinc metal and
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here we have a different cell potential
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value there's a range of nomenclature
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that's used in this but by convention
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all of these cells are written as
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reductions this is simply a convention
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so X plus plus an electron going to X
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solid or X gas all of these are measured
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under standard conditions so remember
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that our standard conditions here are
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the pressure at one atmosphere and our
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concentration M which is molality this
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is looking at moles per kilogram of
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solvent are measured up one molar per
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kilogram and the reasons for this will
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become apparent to you as we go through
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fundamentally an electrode potential
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cannot be measured in isolation we need
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to measure it relative to something else
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so this brings us onto the topic of
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standard electrodes as mentioned they
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can't be they can't be measured directly
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this is akin to the sound of one hand
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clapping we cannot look at one thing on
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its own it has to be measured in
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reference to something else so the
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concept of standard electrodes is
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important to recognize as well the
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standard electrode that we often think
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of is the standard hydrogen electrode so
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we what we do is we set up a reaction
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where we pass hydrogen at one atmosphere
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over a platinum electrode and if this is
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in a one mol per kilogram of
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hydrochloric acid this is defined as
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being zero potential
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okay so however we choose to write this
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whether it's a h+ going to h2 or h+ is
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going to h2 over our platinum electrode
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the potential is the same we define this
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as zero and everything else is measured
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relative to this it's a definition it's
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not an absolute it is impossible to
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measure potential directly of a single
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electrode so it has to be measured
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relative to that something else and we
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define the Hydra net
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electrode at zero so that everything
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else is measured relative to it so when
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we have our zinc potential from the
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previous slide that is measured relative
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to the hydrogen electrode the cell
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potential itself is worth revisiting and
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if you remember this is the result of
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combining two half-cells
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so if we have our copper and zinc cells
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here we're going to combine them
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together and remember they're both
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written as reductions so what we need to
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do is we can't add them together we need
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to subtract them because remember we
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have to eliminate the electron term so
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we subtract one half cell from the other
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so if in doubt we look to get rid of the
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electron term so we balance the charges
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here we're fortunate because we have two
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electrons in each things can be
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subtracted directly from each other and
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we subtract the cell potentials and then
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finally we rearrange the equation so
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that it all makes sense so here we have
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copper minus zinc going to copper solid
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minus zinc solid add zinc to both sides
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subtract sink ions from both sides and
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we end up seeing that if we have copper
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in solution with zinc solid we would get
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copper solid and zinc in solution and we
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can see that the cell potential we
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simply subtract minus 0.76 from 0.34 and
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we end up with our overall cell
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potential this gives us an indicator of
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the direction of thermodynamic outcome
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for a reaction such as this with regard
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to the thermodynamics of cell processes
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we can use these cell potentials to
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predict the thermodynamics of reactions
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where we use Delta G standard is minus
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NFE standard where n is the number of
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electrons in the reaction and F is the
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Faraday constant so this can be used to
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predict a wide range of reaction
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outcomes not just those happening in
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electrochemical cells because
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electrochemistry happens all over
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anywhere you have an exchange of
00:06:05
electrons we have electrochemistry but
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this only predicts the thermodynamics
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and we have to consider the kinetics of
00:06:11
a reaction as well and this study of
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electrochemistry
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allows us to look at the ionic behavior
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in solutions the effect of
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concentrations and in particular
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solution activities and what happens
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when reactants start moving through
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solution this diffusion happening we no
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need to consider the ions in the
00:06:27
electrodes so just to clarify
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definitions a cation is attracted to the
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cathode an anion is attracted to the
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anode worth remembering it is simple
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terminology a cation is positively
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charged so
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means it's attracted to the cathode
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which means a cathode must carry a
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negative charge these are the source of
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electrons for cations so a cation will
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migrate to the cathode pick up an
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electron and then diffuse back into
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solution as the reduced form anions are
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negatively charged and attracted to the
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anode which means the anode must be
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positively charged and it's the anode
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which collects the negative charge from
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the anion then the oxidized species
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diffuses from the anode so simple
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terminology but it's important to
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remember which way around we're
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considering it we don't need to think of
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ion-ion interactions in solution so when
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we think of ionic interactions recall
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from first year that we they're governed
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by Coulomb back for PSA's and they're
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derived from this columbic potential v
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of r which when we sketch it looks a bit
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like this this is just a simple
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potential showing how the potential
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varies q1 and q2 represents the charge
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on each of the ions while R is the
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separation between them and remember
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again from your intermolecular forces
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that force is the first derivative of
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potential with respect to R so we find
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our first derivative here and we get a
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graph that looks like this okay so the
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closer the ions are or closer oppositely
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charged ions are the more they attract
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it's particularly important to pay
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attention to the signs the negative here
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vs no negative here make sure that you
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can achieve this result so make sure
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that you can do a first derivative of
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this expression with respect to R and
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come up with this final answer if it
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helps this one over 4 PI epsilon naught
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epsilon R is simply a constant and you
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only need to worry about this section ok
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so just make sure you can reproduce that
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ions in solution can readily form ion
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pairs so no great surprise that opposite
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charges can be held together so a
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positive is held to a negative charge
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this effect is strongest for these small
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highly charged ions so magnesium 2 plus
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is a key example but we have to also
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consider the dielectric properties of
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the constant that epsilon R component
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the relative permittivity we'll talk
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more about this later on but this
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relative permittivity is key in whether
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or not iron pairs will form if we have a
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higher relative permittivity that means
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electric fields can readily permeate the
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solution we have lower ion pair
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attraction because it can feel more of
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the other ions in solution and therefore
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disperses more throughout solution
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whereas if we have a lower relative
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permittivity it cannot sense other ions
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so an ion pair will tend to form under
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these conditions
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with this columbic force it's important
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to remember I've represents it here is
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q1 q2 but you will also see ZD used to
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represent charges in equations this
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appears all over in textbooks you'll
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sometimes seeing as Q other times you'll
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see it is Zed but you do need to be
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familiar with IR
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but you do need to be familiar with
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either representation the next concept
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we wish to introduce is that of the
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ionic atmosphere so when we look at a
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solution of ions
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they'll be dispersed throughout solution
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they'll be in constant movement very few
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ions are actually held in iron pairs in
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aqueous solution most of them are
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solvated so most of these ions will have
00:09:38
water molecules around them in aqueous
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solution but for any given area in this
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space any one iron will be surrounded by
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a very slight excess of counter ions so
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if we take this one in the center this
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positive charge if you look at this
00:09:52
cation and we draw a circle around it we
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can see if there is a slight excess of
00:09:57
the counter ions this is something we
00:09:59
call the ionic atmosphere yeah ionic
00:10:01
atmosphere is simply a result of thermal
00:10:03
motion which distributes material
00:10:04
equally throughout the solution but also
00:10:06
considering the coulombic interactions
00:10:08
so like charges will repel while
00:10:12
opposite charges will attract and this
00:10:14
means that around any one ion there will
00:10:16
be an ionic atmosphere with a very small
00:10:18
but opposite charge around it it's
00:10:21
something that is dynamic in nature
00:10:23
these are in constant motion these
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things are constantly moving through the
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solution there are a number of different
00:10:29
ways of considering the ionic atmosphere
00:10:30
there are two main ways that I'm going
00:10:32
to cover but neither is more or less
00:10:34
valid than any other but they have the
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same general features so when we think
00:10:37
of an ionic app sphere if we define a
00:10:39
volume in our solution this particular
00:10:42
volume will have zero charge so I'm just
00:10:44
going to draw a circle to define an area
00:10:47
in this particular solution it will
00:10:49
consist of a central ion of a given
00:10:50
charge and a surrounding atmosphere of
00:10:52
equal and opposite charge so inside the
00:10:55
sphere the whole thing has zero charge
00:10:57
but it's centred on a particular on a
00:10:59
positive charge here which means a ionic
00:11:01
app sphere has a net negative charge to
00:11:03
compensate when we start to change the
00:11:05
concentration of our solution the ionic
00:11:06
apps
00:11:07
changes as well so at lower
00:11:08
concentrations we find that we have to
00:11:10
go further to find anionic atmosphere of
00:11:13
equal and opposite charge so if you have
00:11:15
a high concentration we end up with a
00:11:16
smaller ion caps fear because we have to
00:11:18
don't have to go as far to find an equal
00:11:20
opposite charge whereas at low
00:11:22
concentrations we have to extend the
00:11:23
boundary a long way out to capture an
00:11:26
atmosphere of equal opposite charge the
00:11:27
ionic atmosphere serves to stabilize
00:11:29
this central iron fundamentally an ion a
00:11:32
free ion is unstable but if we can
00:11:35
surround it by an equal opposite charge
00:11:36
we offer a degree of stability however
00:11:39
the fact that it is an equal and
00:11:40
opposite charge around it means it is
00:11:42
held loosely around that iron and
00:11:45
fundamentally affects its motion if the
00:11:47
charge moves then an atmosphere has to
00:11:49
move with it another concept to cover is
00:11:52
the idea of salvation shells so whenever
00:11:54
we have an ion in solution the presence
00:11:56
of solvent dipoles affects the solvation
00:11:58
of these ions we normally consider water
00:12:01
as a solvent which is a very highly
00:12:03
polar medium but the dipoles of water
00:12:06
align differently around cations and
00:12:08
anions if we take a cation for example
00:12:11
the Delta minus that's present on the
00:12:13
oxygen atom in water will preferentially
00:12:15
aligned with the cation while an anion
00:12:19
will tend to see the Delta positives
00:12:21
aligning with the anion so we get a
00:12:24
slightly different orientation the
00:12:26
overall alignment will vary with
00:12:27
distance from the central iron and how
00:12:29
big this central iron is at different
00:12:31
levels different distances from this the
00:12:33
degree of alignment will vary so the
00:12:35
next shell out won't be quite so well
00:12:37
aligned and we get to a point where we
00:12:39
just have free water in solution which
00:12:41
isn't affected by any of the solvation
00:12:43
shells
00:12:44
so these solvent molecules here are
00:12:46
locked and held in a solvation shell and
00:12:49
they will move with the charge they're
00:12:51
not free in solution they are tied to
00:12:53
the charge they will move with that ion
00:12:56
so these solvation shells also affect
00:12:58
the ion mobility the next thing to
00:13:00
consider is this dielectric constant of
00:13:02
carrying the symbol Epsilon it's often
00:13:04
seen as a measure of solvent polarity
00:13:05
but it's fundamentally a bulk property
00:13:08
in the solvent it represents the ability
00:13:10
for a solvent to carry an electric field
00:13:12
so whenever we think of charges we have
00:13:15
electric field lines which by convention
00:13:17
go from positive to negative if we have
00:13:20
hi relative permittivity that means the
00:13:22
solvent has a high ability to transmit
00:13:24
that electric field which facilitates
00:13:26
long-range
00:13:27
ion-ion interactions if however we have
00:13:31
a very low relative permittivity then
00:13:34
there is very little transmission and
00:13:35
the field is not able to permeate so we
00:13:38
only end up with short-range ion-ion
00:13:40
interactions what this means is for a
00:13:42
low epsilon we will tend to form ion
00:13:44
pairs because as soon as one charge
00:13:47
encounters another they'll stick
00:13:49
together there's nothing else to
00:13:50
stabilize them whereas in a high
00:13:52
relative permittivity environment these
00:13:55
ions can detect each other from a long
00:13:57
range and they offer long-range
00:13:59
stabilization so it's important to
00:14:01
remember this dielectric constant and
00:14:02
the role it plays in forming ion ion
00:14:04
pairs so just to summarize the primary
00:14:07
concepts that we've discussed here ionic
00:14:10
interactions are covered by Coulomb's
00:14:11
law they're predictable and they're
00:14:12
well-defined and under ionic
00:14:14
interactions in water direct ion
00:14:17
ion-pairing is very infrequent we need
00:14:19
to consider the importance of ionic
00:14:20
salvation so when we're thinking about
00:14:23
an ionic atmosphere where we have an
00:14:25
equal opposite charge around in this ion
00:14:27
cap sphere we also have solvent
00:14:29
molecules tied up in solvation shells
00:14:31
around it and both of these affect ionic
00:14:33
mobility they affect the ability at for
00:14:35
it to feel the electric field outside
00:14:37
the atmosphere and the mass that we move
00:14:39
with the solvation shells is increased
00:14:41
which also affects the mobility and
00:14:43
finally the dielectric constant which
00:14:45
affects the ratio of a direct ion ion
00:14:47
pairing to iron salvation in the context
00:14:50
of this course will only be considering
00:14:51
aqueous environments however we can do
00:14:53
electrochemistry anywhere that we have a
00:14:55
solution so other solvents can be
00:14:57
considered as well so it's worth bearing
00:15:00
in mind the effect that all of these
00:15:01
have
