Further Physical Chemistry: Electrochemistry session 7

00:11:11
https://www.youtube.com/watch?v=j02E-PU9MRs

Résumé

TLDRThis session delved into the kinetics at electrodes, transitioning from the earlier discussion on equilibrium to understanding how current flows when potential is varied. It primarily focuses on the relationship between potential and current, examining linearity or lack thereof. The lecture also connects traditional reaction kinetics with electrochemical processes, establishing a link between electron transfer, reactant consumption, and resulting current. A first-order rate process is demonstrated where the electrode surface area critically affects the reaction rate and current density. The concepts are tied back to the Butler-Volmer relation, illuminating how potential impacts reaction kinetics. Through potential modification, an over potential is created—higher for oxidation favoring more electron movement and lower for reduction, altering free energy barriers. Crucially, the session clarifies how these kinetic aspects define electrodes' behavior under varying conditions, including considerations for both forward and reverse reactions (oxidation and reduction).

A retenir

  • ⚡ Exploring kinetics at electrodes when current flows.
  • 🔍 Relation between potential and current, checking linearity.
  • ⚗️ Connects electron transfer to traditional reaction kinetics.
  • 📐 Electrode surface area crucial for reaction rate and density.
  • 🔄 Butler-Volmer relation ties current density with processes.
  • ⬆️ Higher potential favors oxidation, increasing rate.
  • ⬇️ Lower potential favors reduction, modifying energy barriers.
  • 🔗 Forward and reverse reactions affect electrode behavior.
  • 🪙 Equilibrium modifications due to applied potentials.
  • 🧪 First-order process demonstrated at electrodes.

Chronologie

  • 00:00:00 - 00:05:00

    In this session, the focus shifts from thermodynamics to the kinetics of electrode processes, specifically examining what happens when a current flows at the electrodes. Initially, electrochemical equilibrium was established, but now the external potential is applied to disturb that equilibrium, allowing examination of how current and potential interact. The key question revolves around whether a linear relationship exists between increased potential difference and current. To understand this, the session explores rates of reaction at the electrode, involving the transfer of electrons and reaction of moles of reactants. By understanding the rate of charge transfer (current) and reaction rates at the electrodes, the session provides insight into the process by differentiating charges over time to establish the relationship between current and reaction. Furthermore, the significance of the electrode area is considered as it affects how much reactant can interact and consequently the rate of reaction and current observed. Different areas influence how ions deposit on electrode surfaces and affect electrochemical processes.

  • 00:05:00 - 00:11:11

    The session progresses by analyzing the rate equations involving electrode processes, emphasizing how electrode area impacts the reaction rate. Key takeaways involve understanding that the current density, i.e., current per unit area, is vital for evaluating reaction kinetics. The session then explains the concept of overpotential, the difference between equilibrium potential and actual electrode potential, highlighting how positive or negative overpotential can influence oxidation and reduction processes at electrodes. The Butler-Volmer equation is introduced, linking current density to oxidative and reductive processes, considering factors like activation energy and potential influence. It concludes with a summary of electrode kinetics, indicating that at equilibrium, oxidation and reduction rates balance with no net current flow, while applied potential disturbs this balance, altering reaction rates and promoting current flow, fulfilling conditions necessary for electrochemical work.

Carte mentale

Vidéo Q&R

  • What happens when an external potential is applied?

    When an external potential is applied, it disturbs the electrochemical equilibrium, affecting the current flow and electrode reactions.

  • Why is electrode surface area important in electrochemical reactions?

    Larger electrode areas allow more cations to deposit, increasing the rate of reaction and current density.

  • What is current density in electrochemical terms?

    Current density is the current flowing divided by the area of the electrode.

  • How do oxidation and reduction relate to electrode potential?

    Positive over potential favors oxidation, while negative over potential favors reduction.

  • What is the significance of the Butler-Volmer relation?

    The Butler-Volmer relation connects current density with oxidative and reductive processes, adjusting for activation energy barriers.

  • What role does area play in rate equations for electrodes?

    Rate equations incorporate area to express reaction rates per unit area, influencing the observed current.

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Sous-titres
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    in the last session we explored the
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    thermodynamics of electric processes in
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    this session we're now going to look at
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    the kinetics of what goes on at the
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    electrode so we looked at what happens
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    when we have equilibrium established at
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    the electrodes but now let's consider
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    what happens when we allow a current to
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    flow at the electrodes so we've
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    established what happens with electro
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    chemical equilibria but now what we're
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    going to do is explore what happens when
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    we disturb that equilibrium by applying
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    an external potential fundamentally we
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    want to look at what happens to the
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    current as the potential varies what we
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    expect to happen and what predictions
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    can we make if we apply a higher
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    potential difference do we get a higher
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    current is there a linear relationship
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    between them and should there be a
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    linear relationship between them through
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    exploring these questions we will start
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    tricks find out what's going on at the
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    electrode fundamentally in chemistry we
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    want to consider rates of reaction so
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    when we think of regular reactions we
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    apply the principles of these rate laws
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    so we're familiar with forming a rate
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    equation such as this the rate of change
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    of the concentration of a with respect
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    to time is equal to the negative of the
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    rate constant times concentration this
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    is for a first order process we can
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    apply similar principles to electrode
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    processes so at the electrode what's
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    going on well let's think first of all
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    we are transferring a certain number of
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    electrons if we're transferring a
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    certain number electrons we can then
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    consume a certain fraction a certain
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    proportion of reactants so we have n
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    moles of reactant being consumed this of
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    course it assumes a single electron
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    process so this means that we can find
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    the overall charge transferred be Q as a
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    number of electrons times Faraday's
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    constant multiplied by the number of
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    moles of reactant consumed so this is a
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    simple relationship between charge and
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    the amount of reaction that we're
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    working with when we think of charge
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    transfer we start to think of what's
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    going on with the electric current
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    because the current is the thing that we
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    measure remember that current is simply
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    the rate of charge transfer so the rate
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    of transfer charge with respect to time
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    this gives us the current that we can
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    observe but we also have our rate of
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    reaction which is the rate of change of
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    our reactant as a function of time as
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    well so fundamentally we are looking at
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    a rate equation whether it's a rate of
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    transfer of charge or a rate of
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    consumption of
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    reacting looking at this equation here
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    between the overall charge transferred
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    and the amount of reaction we have
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    present all we need to do is find our
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    derivative our first derivative with
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    respect to time so if we differentiate
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    both sides we'll get an expression the
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    rate of charge transfer with respect to
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    time our current can be equated to the
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    rate of the reaction and the faraday
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    constant so this is a fairly
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    straightforward way to look at the
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    current flowing through our
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    electrochemical cell and the reactions
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    happening at the electrode to understand
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    a bit more we need to go back to the
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    area considerations remember we looked
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    at the areas of electrodes because this
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    is quite an important aspect of it many
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    considerations that we work with involve
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    using conductivity and conductivity
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    fundamentally is an area function so
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    working with these cross-sectional areas
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    becomes very very useful we apply
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    similar principles to the electric
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    processes and the reasoning for that
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    will become clear later on but we are
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    looking at the rate of a reaction per
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    unit area so if we take our rate of
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    reaction we simply divide it by the area
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    of the electrode we get an expression
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    which relates our current to the area of
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    those electrodes the important thing we
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    need to recognize here is that the
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    current that flows is a representation
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    of that rate of reaction if the reaction
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    happens faster we would expect to get a
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    faster rate of transfer of electrons
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    which would lead to a higher current
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    observed in the circuit around the cell
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    why is the area important well let's
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    look at what's going on at the electrode
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    so if we think about what's going on at
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    this electrode let's consider the
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    cathode at first so cations these A plus
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    ions will gather at the surface of that
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    electrode and they'll just deposit in a
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    layer remember we saw this when we were
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    looking at the inner Helmholtz plane and
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    the diffuse double there in one of our
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    earlier sessions so electrons will
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    transfer from our electrode to our
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    caffeine reducing it and it will deposit
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    this atomic a at the surface so what
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    happens to the surface so as each layer
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    of cations is deposit at the surface and
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    fundamentally reduced this surface will
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    advance so as each layer gets there we
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    extend the electrode surface and we get
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    a similar process happening in Reverse
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    at the anode so when we consider the
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    overall area of the electrode the
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    greater the area the more cations can
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    access the electrode and deposit metal
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    atoms at the surface
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    the rate expression therefore has to
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    include an element of the area and we
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    see that that rate expression
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    demonstrates the area deposition the
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    overall area of that electrode we want
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    to think of the rate of this reaction
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    the reaction that we consider is
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    first-order all that needs to happen is
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    the cation just needs to get to the
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    surface what that means for our rate
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    equation is that it becomes the amount
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    of material produced per unit area per
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    unit time so the bigger the electrode
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    the faster the rate the smaller the
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    electrode the slower the rate so if we
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    have a larger electrode area we get more
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    sites and larger area as we've said
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    gives us a higher rate this means
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    overall our rate equation which we're
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    familiar with becomes this rate whatever
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    that might be is a rate constant times
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    the concentration of X the more of X we
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    have in solution the faster rate it's
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    going to go it's worth considering the
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    units of the rate constant of
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    centimeters per second make sure that
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    you can rationalize this given that the
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    concentration could be considered as
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    moles per cubic centimeter slightly
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    different to the moles per decimeter
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    that you're used to but it is a
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    congruent unit once we've considered
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    both processes we can now start to look
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    at the overall rate of reaction at any
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    electrode we need to consider both
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    processes we need to consider the
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    oxidative process and the reductive
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    process happening at that electrode at
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    equilibrium at that electric and that
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    electrochemical equilibrium these
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    processes are equal because there's no
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    net flow of charge but as we change the
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    potential one of them begins to dominate
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    whether we raise the potential or
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    whether we lower the potential so we can
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    consider this equilibrium in this manner
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    so the overall rate is a balance of each
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    process the cathodic process where the
  • 00:06:09
    oxidated species is reduced and becomes
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    the reduced species so the rate of this
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    the forward reaction is what's happening
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    in a cathodic process while the analytic
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    process the reduced species goes
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    backwards to the oxidized species and
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    gives us an alternative rate constant so
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    looking at these rates at a single
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    electrode remember we can consider that
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    the overall rate is the sum of the
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    forward and back processes so we have
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    our forward process and our back process
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    which all add up to give us an overall
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    rate of reaction which we
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    earlier defined as this derivative
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    because we have this area function this
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    I over a it's easier to think of a
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    current density rather than the current
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    itself
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    remember back in session three we talked
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    about charge flux and how that passes
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    through a particular area of solution if
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    we consider it if we now substitute this
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    J term in our current density this makes
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    our overall equation for a single
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    electric process where N equals one is
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    simply the Faraday constant times the
  • 00:07:13
    rates that we've disturbed there are a
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    number of factors which affect our rate
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    constant the main one we're going to be
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    looking at is the effect the applied
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    potential but there are a number of
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    things that can affect it whether it's
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    looking at temperature effects or
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    potential effects remember also that
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    your rate constant depends on the
  • 00:07:32
    energies of activation so this is
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    something you're familiar with from your
  • 00:07:35
    Arrhenius relationship this Delta G
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    dagger is the energy of activation you
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    previously knew as EA but you'll find
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    that in textbooks much as Delta G dagger
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    a fraction of this free energy helps the
  • 00:07:46
    oxidation so one proportion of it
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    enhances the oxidative process but the
  • 00:07:51
    remaining fraction of that Delta G
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    inhibits the reduction so it's blocks
  • 00:07:56
    reduction from happening this leads us
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    to the butler-volmer relation which
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    simply relates our observed current
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    density to the oxidative process and the
  • 00:08:07
    reductive process where we have this
  • 00:08:09
    enhancement of oxidation due to alpha
  • 00:08:13
    while the remaining is their inhibition
  • 00:08:15
    of the reductive process it's worth
  • 00:08:17
    briefly drawing your attention to the
  • 00:08:19
    fact that we have introduced another
  • 00:08:20
    symbol so we've introduced this ETA term
  • 00:08:23
    which we've seen already to represent a
  • 00:08:25
    viscosity but in this case it represents
  • 00:08:28
    an over potential if you're ever in any
  • 00:08:30
    doubt as to what the symbol might mean
  • 00:08:32
    look at the units remember that the
  • 00:08:34
    overall units of this expression have to
  • 00:08:36
    go to zero so the over potential here is
  • 00:08:38
    measured in volts and that will allow
  • 00:08:40
    your units to cancel so what is this
  • 00:08:43
    over potential well put simply the over
  • 00:08:46
    potential is the difference between the
  • 00:08:47
    elec were Librium potential and the
  • 00:08:49
    actual potential of an electrode so if
  • 00:08:51
    you think of your equilibrium cell
  • 00:08:54
    potential you
  • 00:08:54
    calculate that remember we did that in
  • 00:08:56
    part two but the difference once we find
  • 00:09:00
    our equilibrium cell potential we simply
  • 00:09:02
    subtract that from the applied potential
  • 00:09:04
    what we're applying to that particular
  • 00:09:06
    electrochemical cell we can apply that
  • 00:09:08
    potential from a voltage source whether
  • 00:09:10
    it's a battery or a potential stat and
  • 00:09:12
    this has an effect on the Fermi level as
  • 00:09:14
    we spoke about in the last session if we
  • 00:09:17
    have a positive over potential that
  • 00:09:19
    creates a higher potential which has the
  • 00:09:22
    effect of lowering the electron energy
  • 00:09:24
    so the Fermi level decreases and that
  • 00:09:26
    reduces the free energy barrier for
  • 00:09:28
    oxidation so our species then can easily
  • 00:09:31
    be oxidized at the electrode and that's
  • 00:09:33
    our reaction proceeds however if we
  • 00:09:36
    create a negative over potential it
  • 00:09:38
    creates a lower potential which
  • 00:09:40
    increases the electron energy it has the
  • 00:09:43
    effect of raising the Fermi level once
  • 00:09:45
    we have raised the Fermi level it
  • 00:09:47
    reduces the free energy barrier for
  • 00:09:48
    reduction so our material is then
  • 00:09:51
    reduced at the electrode to summarize
  • 00:09:55
    our overview of the kinetics of
  • 00:09:56
    electrodes we have to consider the fact
  • 00:09:59
    that the rates of oxidation and
  • 00:10:00
    reduction depend on the available
  • 00:10:02
    surface area we manage this by using our
  • 00:10:05
    current density this J term which is
  • 00:10:09
    simply the current flowing divided by
  • 00:10:10
    the area available in the electrodes
  • 00:10:12
    Ritz our first order of that electrode
  • 00:10:15
    service this allows us to very easily
  • 00:10:17
    and simply qualify what's going on at
  • 00:10:19
    the electrode at equilibrium no current
  • 00:10:23
    flows around our cell so we have this
  • 00:10:25
    constant exchange of charge at an
  • 00:10:27
    electrode interface where the rate of
  • 00:10:29
    reduction is equal to the rate of
  • 00:10:30
    oxidation so the charge transfer cancels
  • 00:10:33
    out on both sides once we start applying
  • 00:10:36
    potential we start to change the rate of
  • 00:10:38
    what's going on at that electrode if we
  • 00:10:40
    have a positive over potential ie we
  • 00:10:43
    have a more positive potential than the
  • 00:10:46
    equilibrium State we will get oxidation
  • 00:10:49
    of the species favored at that electrode
  • 00:10:52
    while if we lower the potential if you
  • 00:10:54
    have a negative over potential it will
  • 00:10:55
    favour the reduction of what's going on
  • 00:10:58
    once we've applied that potential if we
  • 00:11:00
    then complete the circuit we then
  • 00:11:02
    satisfy the conditions and the current
  • 00:11:04
    will flow from the anode to the cathode
  • 00:11:06
    and will
  • 00:11:06
    do electrical work as it does so
Tags
  • Electrode Kinetics
  • Current Flow
  • Electrochemical Reactions
  • Potential Influence
  • Butler-Volmer Equation
  • Reaction Rate
  • Current Density