Further Physical Chemistry: Electrochemistry session 3

00:18:19
https://www.youtube.com/watch?v=5dVpJ6LLDAg

Sintesi

TLDRThe video explores the role of ions in solution conductivity, emphasizing mobility and motion types: diffusion, migration, and convection. It covers key electrochemical concepts like current, voltage, resistance, power, and conductance. Notably, the video distinguishes between conductivity and conductance, specific to solutions. It delves into measuring conductivity, particularly highlighting the impact of electrolyte concentration and type on conductivity. Strong electrolytes, fully dissociated into ions, contrast with weak electrolytes, showing less dissociation and lower conductivity. Conductivity is linked to ion mobility, affected by ion interactions and solvent dynamics. Important phenomena like ionic atmosphere distortion, solvent drag, and ion pairing are introduced, influencing ion mobility. The video introduces infinite dilution for assessing molar conductivity and elucidates independent ion migration. Understanding these concepts allows the calculation of limiting molar conductivity, crucial for weak electrolyte analysis, using data from strong electrolytes.

Punti di forza

  • ๐Ÿ”ฌ Ions contribute to solution conductivity via diffusion, migration, and convection.
  • โšก Conductivity differs from conductance, with the former specific to ion-based solution current flow.
  • ๐Ÿ”‹ Measuring conductivity involves fixed-distance electrodes and applying potential difference.
  • ๐Ÿ’ก Higher concentration can lead to decreased molar conductivity due to ion-ion interactions.
  • ๐Ÿงช Strong electrolytes fully dissociate in solutions, enhancing conductivity compared to weak electrolytes.
  • ๐Ÿ” Molar conductivity at infinite dilution provides insights into maximum ion mobility.
  • ๐Ÿ“Š Ionic mobility is disrupted by distortions in ionic atmosphere and solvation shells.
  • ๐Ÿ”— Ion pairing in nonpolar solvents affects conductivity through charge interactions.
  • ๐Ÿ”„ Independent ion migration allows assessment of weak electrolytes using strong electrolyte measures.
  • ๐ŸŒ€ Proper unit conversion between SI and commonly used units is crucial in electrochemical calculations.

Linea temporale

  • 00:00:00 - 00:05:00

    In this segment, the video explores the behavior of ions in solutions and how they contribute to conductivity. It covers basic concepts such as ionic mobility, diffusion (movement due to concentration differences), migration (movement due to electric fields), and convection (movement due to thermal phenomena). The section emphasizes the importance of understanding current, voltage, resistance, and conductance in electrochemical measurements, highlighting that solution conductivity results from the mobility of ions and differs from conductance, which applies to all electrical conductors.

  • 00:05:00 - 00:10:00

    The video discusses the measurement of solution conductivity using electrodes and relates it to molar conductivity, which factors in concentration effects. It explains how conductivity and molar conductivity differ, with the latter accommodating for concentration and solution variations. The relationship between concentration and conductivity is explored, noting the unexpected decrease in molar conductivity with increased concentration. This section highlights how conductivity is influenced by the type of electrolyte, introducing the concepts of infinite dilution and limiting molar conductivity, which unify molar conductivity by eliminating ion-ion interactions at zero concentration.

  • 00:10:00 - 00:18:19

    This final segment further delves into ionic and electrolyte behavior at varying concentrations. It examines phenomena like ion pairing, the ionic atmosphere, and solvation shells, which affect ion mobility and conductivity. The session further distinguishes between strong and weak electrolytes, demonstrating how the Law of Independent Migration helps in calculating the limiting molar conductivity of weak electrolytes. It concludes by summarizing that ionic motion is governed by diffusion and migration, and limiting molar conductivity for strong electrolytes can be determined directly, whereas for weak ones, it relies on independent migration assumptions.

Mappa mentale

Video Domande e Risposte

  • What are the three types of ion motion in solutions?

    The three types of ion motion are diffusion, migration, and convection.

  • How is the conductivity of a solution defined?

    Conductivity is defined as the ability of a solution to carry a current through the motion of ions.

  • What is the relationship between conductivity and conductance?

    Conductance is the ability to pass current, applicable to any material carrying electricity, while conductivity specifically pertains to the solution's ability to carry current through ions.

  • Why does molar conductivity decrease with concentration?

    Molar conductivity decreases with concentration due to increased ion-ion interactions and reduced mobility as concentration increases.

  • What role do cations and anions play in conductivity?

    Cations and anions are responsible for making solutions conductive by moving in response to an electric field.

  • How is molar conductivity at infinite dilution significant?

    It indicates the maximum molar conductivity achievable when no ion-ion interactions impede ion mobility.

  • What is independent migration of ions?

    It's the principle that at infinite dilution, ions behave independently, allowing their conductivities to be studied separately.

  • Why are strong and weak electrolytes different in terms of conductivity?

    Strong electrolytes are fully dissociated and thus more conductive, while weak electrolytes are only partially dissociated, resulting in lower conductivity.

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Sottotitoli
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Scorrimento automatico:
  • 00:00:00
    we've looked at solvents and now we need
  • 00:00:02
    to look at the behavior of ions in
  • 00:00:03
    solution so remember we just find a few
  • 00:00:06
    basic terms without terms of our ions
  • 00:00:08
    are cations cathode anions anode but
  • 00:00:10
    fundamentally ions are what gives
  • 00:00:12
    solutions our conductivity so whenever
  • 00:00:14
    we think of a current flowing in a
  • 00:00:16
    solution we have to visualize it as a
  • 00:00:18
    flow of ions not a flow of electrons
  • 00:00:20
    whenever we make conductivity
  • 00:00:21
    measurements these can all be traced
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    back to ionic interactions to how these
  • 00:00:26
    ions behave in solution the ionic
  • 00:00:28
    mobility that we discussed is a
  • 00:00:30
    phenomenon which links the measurable
  • 00:00:32
    quantities to do with the currents that
  • 00:00:34
    we observe in solution as well as the
  • 00:00:36
    theoretical quantities in terms of the
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    limiting conductivity of an ion well
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    first introduce some basic concepts to
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    cover mobility in solutions the first of
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    these is diffusion diffusion is simply
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    motion due to concentration differences
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    so if we have a concentration of ions in
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    one part of the solution we would expect
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    these to diffuse through the solution so
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    that we get an even concentration
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    throughout this applies to all molecules
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    in that solution migration by comparison
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    is motion due to electric fields so if
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    we put an electric field across our
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    solvent we would expect our ions to move
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    in the direction of that electric field
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    relevant to their charge it only applies
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    to charged particles in solution so
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    these ions that we're talking about the
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    third motion of mobility is something
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    called convection which is motion due to
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    thermal phenomena or simply stirring a
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    solution it's not considered in this
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    course but it's something to be aware of
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    as it's something you are probably
  • 00:01:31
    already familiar with in order to make
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    measurements on electrochemical cells we
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    need to establish some basic electrical
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    concepts so this will require a little
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    bit of physics revision but it's vital
  • 00:01:42
    to understand the first of these is
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    current current is simply the flow of
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    charge and how it moves around a circuit
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    so the current can be delivered a number
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    of different ways but fundamentally it
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    is a transfer of charge voltage is
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    another one but we would tend to talk
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    about it in terms of a potential
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    difference and we'll talk about that
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    later in the course but a voltage is
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    something that can be very easily
  • 00:02:01
    measured using a voltmeter the next
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    phenomena to consider it's resistance
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    which carries the unit of ohms
  • 00:02:07
    resistance is simply the resistance to
  • 00:02:09
    carrying electrical charge the greater
  • 00:02:10
    the resistance the greater the potential
  • 00:02:12
    difference required to push
  • 00:02:13
    current through that conductor power is
  • 00:02:15
    another phenomena we need to discuss
  • 00:02:17
    power is simply the rate of transfer of
  • 00:02:20
    energy so in electrical terms it's a
  • 00:02:23
    relationship between the current and the
  • 00:02:25
    voltage the last one which is slightly
  • 00:02:26
    different is conductance G which is
  • 00:02:29
    simply the inverse of the resistance so
  • 00:02:32
    the greater the conductance the lower
  • 00:02:34
    the resistance and these are all kind of
  • 00:02:36
    fairly we're fairly comfortable with
  • 00:02:38
    these ideas but it's important to lay
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    down which ones we're going to be using
  • 00:02:43
    when we think of electrochemistry we
  • 00:02:45
    think of solution conductivity so we
  • 00:02:46
    tend not to think about what's going on
  • 00:02:48
    in the external circuit we only think
  • 00:02:50
    about what's going on in solution
  • 00:02:51
    between our two electrodes this is as we
  • 00:02:53
    said a couple times before is the result
  • 00:02:55
    of the mobility of charges it's not a
  • 00:02:58
    straightforward process so we need to be
  • 00:03:00
    aware of what it is we need to consider
  • 00:03:02
    and at this point it's worth flagging up
  • 00:03:04
    the conductivity is different to
  • 00:03:06
    conductance so I mentioned conductance
  • 00:03:08
    in the previous slide I mentioned them
  • 00:03:10
    both to avoid confusion conductivity is
  • 00:03:13
    simply the ability for a solution to
  • 00:03:15
    carry a current through motion of ions
  • 00:03:16
    where it's conductance is simply the
  • 00:03:19
    ability to pass current so conductance
  • 00:03:21
    applies to anything which carries
  • 00:03:22
    electricity but conductivity is specific
  • 00:03:25
    to the solution and it's conductivity
  • 00:03:26
    that we're going to be looking at in
  • 00:03:28
    this course measuring conductivity is a
  • 00:03:32
    slightly tricky but it's a fairly
  • 00:03:34
    straightforward procedure once we see
  • 00:03:36
    what's going on to do it we need to take
  • 00:03:38
    two equally sized electrodes so we have
  • 00:03:40
    our cathode of a given area and we have
  • 00:03:42
    an anode of equal area we make sure that
  • 00:03:44
    these are parallel and we separate them
  • 00:03:46
    by a fixed distance we can then apply a
  • 00:03:48
    potential difference across them and
  • 00:03:50
    measure the current that goes through
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    the solution this allows us to determine
  • 00:03:53
    the resistance via this V equals IR
  • 00:03:56
    relationship so just rearrange this
  • 00:03:58
    divide both sides by AI and we get a
  • 00:04:00
    value for the resistance this gives us a
  • 00:04:02
    value for that solution conductivity
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    which carries a symbol Kappa which Greek
  • 00:04:07
    letter K and it has the units of per ohm
  • 00:04:11
    per meter and we simply use this
  • 00:04:12
    equation to determine our solution
  • 00:04:14
    conductivity a solution conductivity is
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    measured but it only relates that
  • 00:04:18
    particular solution that we've measured
  • 00:04:19
    at the time a more useful measurement is
  • 00:04:21
    the molar conductivity so looking at how
  • 00:04:23
    the conductivity of a solution varies
  • 00:04:25
    with its concentration this is known as
  • 00:04:27
    the
  • 00:04:27
    molar conductivity encouraged this
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    symbol lambda so this is a capital
  • 00:04:31
    lambda with subscript M for molar
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    conductivity and this eliminates the
  • 00:04:35
    effect of concentration so this will
  • 00:04:37
    give us the molar conductivity for any
  • 00:04:39
    solution of a particular analyte that
  • 00:04:41
    we're interested in now the units of the
  • 00:04:43
    molar conductivity can be complex so
  • 00:04:45
    Kappa is in Peron per meter the
  • 00:04:48
    concentration C is in moles per cubic
  • 00:04:50
    meter we're trying to keep an SI units
  • 00:04:52
    remember we're normally used to diem per
  • 00:04:55
    diem cubed here we're interested in
  • 00:04:57
    meter cubed while limiting molar
  • 00:04:59
    conductivity in ohms per square meter
  • 00:05:01
    ohm square meter per mole these units
  • 00:05:03
    can be complex but it's important you're
  • 00:05:05
    able to move between them when we look
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    at the actual formula itself when we see
  • 00:05:08
    these units written down we normally see
  • 00:05:10
    them in the square centimeter unit so
  • 00:05:13
    it's important that you're happy with
  • 00:05:15
    this particular conversion because the
  • 00:05:17
    conversion is absolutely vital
  • 00:05:19
    remember we're stressing how to convert
  • 00:05:21
    units all the way through your course so
  • 00:05:23
    make sure you're happy with this unit
  • 00:05:24
    conversion when we think of conductivity
  • 00:05:27
    we want to look at the concentration so
  • 00:05:29
    concentration as I'm sure is no surprise
  • 00:05:31
    greatly affects the conductivity of a
  • 00:05:33
    solution and the common way of thinking
  • 00:05:35
    about it is that higher concentration
  • 00:05:37
    gives higher conductivity because the
  • 00:05:39
    reasoning is that we have more ions in
  • 00:05:41
    solution so there are more current
  • 00:05:42
    carriers this seems a perfectly logical
  • 00:05:44
    way to think however as is always the
  • 00:05:48
    case it's never quite as simple and when
  • 00:05:50
    we measure our conductivity we find that
  • 00:05:52
    actually as the concentration increases
  • 00:05:54
    we get a decrease in the molar
  • 00:05:56
    conductivity so it does vary with
  • 00:05:59
    concentration but in the opposite way
  • 00:06:02
    that we would predict so this seems
  • 00:06:03
    extremely strange so we need to under
  • 00:06:05
    unravel why that's the case we also see
  • 00:06:08
    here we've got two electrolytes we've
  • 00:06:09
    got a strong electrolyte potassium
  • 00:06:11
    chloride I've got a weak electrolyte of
  • 00:06:13
    ethanoic acid so we need to unravel this
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    a little bit because again not only does
  • 00:06:19
    it depend on concentration but it also
  • 00:06:21
    depends on the type of electrolyte
  • 00:06:23
    although in this case the fact that the
  • 00:06:26
    weak electrolyte has a lower
  • 00:06:27
    conductivity should probably doesn't
  • 00:06:29
    surprise us because we would expect
  • 00:06:30
    fewer current carriers but this is in
  • 00:06:33
    contravention with what we see it
  • 00:06:35
    increasing concentrations so let's start
  • 00:06:37
    looking at these phenomena the
  • 00:06:39
    electrolyte itself is extremely
  • 00:06:41
    important so when we think of strong
  • 00:06:43
    electrolytes we assume them to be 100%
  • 00:06:46
    dissociated into ions so HCl KCl and so
  • 00:06:50
    on and so forth we have 100%
  • 00:06:52
    dissociation so we would expect there to
  • 00:06:54
    be a commensurate effect on the
  • 00:06:55
    conductivity but the degree of
  • 00:06:57
    dissociation depends on the solvent we
  • 00:06:59
    almost always consider water but what
  • 00:07:01
    happens if we deal with a completely
  • 00:07:02
    nonpolar solvent if we consider
  • 00:07:04
    something like dry HCl in benzene so
  • 00:07:08
    this is a very very weak electrolyte and
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    HCl will tend to clump together as ion
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    pairs by in pairs have a net charge of
  • 00:07:15
    zero and when we apply a potential
  • 00:07:16
    difference across this no current flows
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    so we need to consider what's going on
  • 00:07:21
    there we saw that the conductivity
  • 00:07:22
    decreases with concentrations so to
  • 00:07:24
    understand that we need to look into
  • 00:07:26
    these ion-ion interactions and ions
  • 00:07:28
    solvent interactions the more the ions
  • 00:07:30
    interact with themselves the less
  • 00:07:32
    current will flow but the more it
  • 00:07:33
    interacts with the solvent we'll get
  • 00:07:35
    more of an effect on the conductivity so
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    at these very low concentrations we see
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    we have an increase in the conductivity
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    which seems completely counteract
  • 00:07:42
    counter intuitive at this point we're
  • 00:07:45
    going to introduce the phenomenon of
  • 00:07:46
    infinite dilution this seems like a very
  • 00:07:48
    strange one to think of but it's a way
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    of unifying molar conductivity and
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    allows us to study it in more detail the
  • 00:07:55
    method has come up by Friedrich Kohl
  • 00:07:56
    rush who proposed our limiting molar
  • 00:07:59
    conductivity at zero this is simply an
  • 00:08:01
    extrapolation to zero concentration
  • 00:08:03
    what is our conductivity at zero
  • 00:08:06
    concentration so at zero concentration
  • 00:08:08
    we have no ion-ion interactions almost
  • 00:08:11
    by definition and our molar conductivity
  • 00:08:13
    is highest at infinite dilution because
  • 00:08:15
    there are no ion-ion interactions are
  • 00:08:17
    slowed down in migration the limiting
  • 00:08:18
    molar conductivity is simply defined as
  • 00:08:20
    the sum of the limiting molar
  • 00:08:22
    conductivity of each of the ions so that
  • 00:08:24
    the positive ion and the negative ion so
  • 00:08:27
    let's unravel this a little bit because
  • 00:08:28
    it seems a bit odd how can we measure a
  • 00:08:30
    conductivity at effectively zero
  • 00:08:32
    concentration so let's explore this a
  • 00:08:34
    bit firstly let's consider the ionic
  • 00:08:37
    atmosphere remember that we said this
  • 00:08:38
    was the surrounding atmosphere of ions
  • 00:08:40
    around a central charge it's spherical
  • 00:08:43
    and symmetric in the absence of an
  • 00:08:45
    electric field
  • 00:08:45
    now I'm not going to put all the other
  • 00:08:47
    counter ions in here but assume there
  • 00:08:49
    they are present as soon as we apply an
  • 00:08:51
    electric field so as soon as this goes
  • 00:08:53
    into
  • 00:08:54
    an electric field between two electrodes
  • 00:08:56
    this ionic atmosphere starts to be
  • 00:08:59
    distorted so think about the shape the
  • 00:09:01
    overall shape of it
  • 00:09:02
    remember the ionic caps here will have
  • 00:09:04
    an opposite charge to the central line
  • 00:09:06
    which means it's attracted to the other
  • 00:09:07
    electrode this causes drag on the eye
  • 00:09:10
    and it slows the ions migration down
  • 00:09:12
    this is something known as the
  • 00:09:14
    relaxation or the asymmetric effect but
  • 00:09:16
    when we get to low concentration to
  • 00:09:18
    remember that ionic atmosphere increases
  • 00:09:20
    in size and becomes more diffuse so the
  • 00:09:22
    more diffuse it is the less drag it
  • 00:09:24
    causes so lower concentrations get less
  • 00:09:26
    drag we get less of an effect on the
  • 00:09:28
    mobility of the ion which increases the
  • 00:09:31
    molar conductivity the next thing we're
  • 00:09:33
    going to look at is the solvent so
  • 00:09:35
    remember solvation shells so around a
  • 00:09:37
    central ion the solvent will organize
  • 00:09:38
    itself and be tied into a solvation
  • 00:09:40
    shell so whenever an ion in solution is
  • 00:09:42
    solvated and it migrates it's going to
  • 00:09:45
    be carrying the solvent molecules with
  • 00:09:46
    it
  • 00:09:46
    which gives it more mass which also
  • 00:09:49
    increases the drag in solution so as it
  • 00:09:52
    migrates towards the opposite charged
  • 00:09:54
    electrode it's dragged more it's held
  • 00:09:57
    back more by the solvent in solution
  • 00:09:59
    this is something called the electro
  • 00:10:01
    phoretic effect and it's in informally
  • 00:10:04
    termed solvent drag at lower
  • 00:10:07
    concentrations we get less drag well why
  • 00:10:09
    is this well if we think about lots of
  • 00:10:11
    these solvated ions moving together they
  • 00:10:15
    have to push past each other they have
  • 00:10:16
    to jiggle past each other but at lower
  • 00:10:19
    concentrations there is more free
  • 00:10:21
    solvent which means there is more space
  • 00:10:23
    between the ions and it's easier for
  • 00:10:25
    those salvation' shells to slip past
  • 00:10:26
    each other the next thing we're looking
  • 00:10:28
    at is the effect of ion pairing so
  • 00:10:30
    remember that direct ion ion pairs
  • 00:10:33
    occur when we don't get salvation so if
  • 00:10:36
    we imagine this ion ion pair forming
  • 00:10:38
    here but we also have some ions free in
  • 00:10:40
    solution as well the ion pair carries no
  • 00:10:43
    charge
  • 00:10:43
    so when we put it into an electric field
  • 00:10:45
    count free counter ions move but the ion
  • 00:10:48
    pair stays locked together when we have
  • 00:10:51
    low concentrations we get less pairing
  • 00:10:53
    happening so they're proportionally
  • 00:10:55
    fewer uncharged ion pairs consequently
  • 00:10:57
    if you have fewer ion pairs the net
  • 00:10:59
    mobility of ions in solution increases
  • 00:11:01
    as well so we have all of these
  • 00:11:03
    different effects happening we have the
  • 00:11:05
    ionic atmosphere becomes more diffuse at
  • 00:11:07
    low concentrate
  • 00:11:08
    increasing mobility the solvent
  • 00:11:11
    salvation shells interact less with each
  • 00:11:13
    other so there's increased mobility at
  • 00:11:15
    low concentrations and we get less iron
  • 00:11:17
    pairing at low concentration which
  • 00:11:19
    increases the mobility of the ions so
  • 00:11:21
    all these effects serve to increase the
  • 00:11:24
    mobility which increases the molar
  • 00:11:26
    conductivity when we think about weak
  • 00:11:28
    electrolytes they are different the
  • 00:11:30
    strong electrolytes but
  • 00:11:31
    electrochemically it's important to
  • 00:11:33
    consider what they're doing as well so
  • 00:11:35
    let's consider our acetic acid
  • 00:11:37
    dissociation so we have a seating acid
  • 00:11:39
    plus water dissociates into the
  • 00:11:41
    hydronium ion and the acetate r9 these
  • 00:11:44
    have lower conductivity and strong
  • 00:11:46
    electrolytes this shouldn't be a
  • 00:11:47
    surprise to us there are fewer charge
  • 00:11:49
    carriers due to the lower dissociation
  • 00:11:50
    and consequently we can ignore the ion
  • 00:11:52
    ion interactions but as the
  • 00:11:55
    concentration drops the dissociation
  • 00:11:58
    constant remains a constant at a given
  • 00:12:00
    temperature but the proportion of the
  • 00:12:03
    acid dissociated changes
  • 00:12:05
    this may seem slightly odd but if we
  • 00:12:07
    look at this particular equation here
  • 00:12:08
    we're looking at the only thing we're
  • 00:12:10
    interested in the concentration of water
  • 00:12:11
    we're going to assume as constant will
  • 00:12:13
    eliminate that we have our dissociation
  • 00:12:16
    constant but it takes the form of x
  • 00:12:19
    squared over Y so we've got two values
  • 00:12:21
    that are squared remember the hydronium
  • 00:12:23
    ion and the acetate ion are an equal
  • 00:12:27
    concentration so x squared divided by
  • 00:12:29
    the concentration of our acetic acid
  • 00:12:31
    what happens as each thing varies
  • 00:12:34
    well let's rearrange this equation let's
  • 00:12:36
    make the concentration of the acetic
  • 00:12:38
    acid the subject so this varies with x
  • 00:12:41
    squared if we increase the stock
  • 00:12:43
    concentration by 4 this means the
  • 00:12:46
    hydroxo nehemiah the thing we're
  • 00:12:48
    interested in one of the charge carriers
  • 00:12:50
    only increases by two whereas let's go
  • 00:12:53
    the other way if we decrease the
  • 00:12:55
    concentration of this by a hundred the
  • 00:12:57
    concentration of the kept charge
  • 00:12:59
    carriers only decreases by 10 so because
  • 00:13:02
    of this we get this greater and greater
  • 00:13:05
    dissociation at lower and lower
  • 00:13:07
    dilutions to a point where at infinite
  • 00:13:09
    dilution we have 100% dissociation we
  • 00:13:12
    get the same behavior as for strong
  • 00:13:14
    electrolytes however measuring the
  • 00:13:16
    conductivity of weak electrolytes
  • 00:13:17
    becomes a challenge in order to overcome
  • 00:13:20
    this we need to consider the
  • 00:13:21
    of independent migration this describes
  • 00:13:24
    how ions behave and its core assumption
  • 00:13:27
    is that electrolyte behavior an infinite
  • 00:13:30
    dilution is identical so remember that
  • 00:13:32
    we had these definitions for the sums of
  • 00:13:34
    it for infinite dilutions so at infinite
  • 00:13:37
    dilution we would have for HCl we would
  • 00:13:40
    say that the overall molar conductivity
  • 00:13:43
    at infinite dilution is equal to the sum
  • 00:13:45
    of each of the molar conductivity of
  • 00:13:47
    each of the constituent ions so that
  • 00:13:50
    seems fairly straightforward that's
  • 00:13:52
    absolutely fine we're okay with that but
  • 00:13:54
    a thing to remember with the independent
  • 00:13:55
    migration is that whatever we're working
  • 00:13:57
    the conductivity of the proton is the
  • 00:14:00
    same regardless of where it comes from
  • 00:14:01
    regardless of whether it comes from HCl
  • 00:14:03
    sulfuric acid whether it comes from
  • 00:14:05
    ethanoic acid or whether it even comes
  • 00:14:07
    from water itself this allows us to
  • 00:14:09
    determine the limiting molar
  • 00:14:10
    conductivity for any weak electrolyte we
  • 00:14:12
    can't measure it directly because we
  • 00:14:14
    can't measure a limiting contact if we
  • 00:14:16
    have to predict it based on the
  • 00:14:17
    observations we make but because this
  • 00:14:19
    isn't fully dissociated because the
  • 00:14:22
    ethanoic acid isn't fully dissociated we
  • 00:14:24
    can't measure it directly so we need to
  • 00:14:26
    work around a little bit but we use this
  • 00:14:28
    idea that the molar limiting molar
  • 00:14:30
    conductivity for any ion is the same
  • 00:14:32
    regardless of where it comes from all we
  • 00:14:35
    need to do to calculate the limiting
  • 00:14:37
    molar conductivity for acetic acid is
  • 00:14:39
    just to find strong electrolytes that
  • 00:14:41
    provide the data we need as it happens
  • 00:14:43
    there's a complete set we can use we can
  • 00:14:45
    use the limiting molar conductivity of
  • 00:14:46
    hydrochloric acid this will give us our
  • 00:14:48
    limiting molar conductivity of a proton
  • 00:14:50
    which is what we need and we can use the
  • 00:14:52
    limiting molar conductivity of sodium
  • 00:14:54
    acetate this is a strong electrolyte
  • 00:14:57
    remember it's 100% associated this will
  • 00:15:00
    give us the limiting molar conductivity
  • 00:15:01
    of the ethanoate ion and then we can
  • 00:15:05
    simply just substitute use these into
  • 00:15:07
    the original equation to get our final
  • 00:15:09
    expression you've used a method very
  • 00:15:11
    similar to this whenever you were doing
  • 00:15:13
    hess cycles for solving simultaneous
  • 00:15:15
    equations in maths and so on simply
  • 00:15:17
    we're looking for the terms that we can
  • 00:15:19
    substitute into our equation remember we
  • 00:15:21
    can treat chemical equations just like a
  • 00:15:23
    mathematical one so let's do this it's
  • 00:15:26
    fairly straightforward to get the proton
  • 00:15:28
    so if we look at this is the equation
  • 00:15:30
    we're interested in we want to find the
  • 00:15:31
    limiting molar conductivity of ethanoic
  • 00:15:33
    acid so
  • 00:15:35
    let's look at our proton first very
  • 00:15:37
    straightforward if we consider
  • 00:15:38
    hydrochloric acid is the limiting molar
  • 00:15:41
    conductivity of hydrochloric acid is
  • 00:15:42
    simply the sum of the proton and the
  • 00:15:44
    chloride let's rearrange that subtract
  • 00:15:46
    the chloride from both sides and we get
  • 00:15:48
    an expression here which is simply the
  • 00:15:50
    molar conductivity of the proton we can
  • 00:15:52
    just take this and replace the proton
  • 00:15:55
    conductivity in the original equation
  • 00:15:57
    which gives us this expression here so
  • 00:15:59
    we've got the limiting molar
  • 00:16:01
    conductivity of ethanoic acid expressed
  • 00:16:02
    in terms of a strong electrolyte which
  • 00:16:05
    we can determine fairly easily and then
  • 00:16:07
    we've got two ions which we need to
  • 00:16:08
    consider now so let's go back to our
  • 00:16:10
    next electrolyte it's harder to spot the
  • 00:16:13
    earth animate and ion conductivity but
  • 00:16:15
    remember that the sodium if analyte is a
  • 00:16:18
    strong electrolyte as well so it is 100%
  • 00:16:20
    dissociated therefore we do the same
  • 00:16:22
    thing we set up the equation where we
  • 00:16:25
    have the limiting molar conductivity of
  • 00:16:26
    sodium ion and the athan away time
  • 00:16:29
    rearranged and we've got a value that we
  • 00:16:31
    can simply take this and plug it in for
  • 00:16:33
    our a fan away tine here and this gives
  • 00:16:36
    us yet another strong electrolytes that
  • 00:16:39
    we can use to work backwards to
  • 00:16:41
    calculate the limiting molar
  • 00:16:42
    conductivity of ethanoic acid the last
  • 00:16:45
    one we need to unify is the chloride and
  • 00:16:47
    the sodium well this seems fairly
  • 00:16:49
    straightforward again sodium chloride
  • 00:16:51
    provides the data we need however it's
  • 00:16:54
    worth noting that these are negative
  • 00:16:55
    signs that we need to consider but we
  • 00:16:58
    already know how to handle this in terms
  • 00:16:59
    of mathematics we simply subtract the
  • 00:17:02
    expression for the limiting molar
  • 00:17:03
    conductivity of sodium chloride put it
  • 00:17:05
    into our equation and remember that
  • 00:17:07
    we've got a reversed sign fundamentally
  • 00:17:09
    what was shown here is that we can
  • 00:17:11
    express any limiting molar conductivity
  • 00:17:12
    as a sum of other limiting molar
  • 00:17:15
    conductivity we cannot measure directly
  • 00:17:18
    the limiting molar conductivity of a
  • 00:17:21
    weak electrolyte but we can always find
  • 00:17:23
    it in terms of strong electrolytes we
  • 00:17:25
    can always find a strong electrolyte
  • 00:17:27
    containing the ions that we need and do
  • 00:17:29
    the appropriate measurements so we can
  • 00:17:31
    look at any limiting molar conductivity
  • 00:17:33
    however it is much easier to determine
  • 00:17:36
    the limiting conductivity for a strong
  • 00:17:37
    electrolyte we can take measure the data
  • 00:17:39
    directly and abstract to find
  • 00:17:41
    values in summary for this session ions
  • 00:17:45
    move by the diffusion migration and
  • 00:17:47
    convection through solution the only two
  • 00:17:50
    were interested in is diffusion and
  • 00:17:51
    migration convection we leave relate the
  • 00:17:54
    solution conductivity is affected by
  • 00:17:55
    concentration and the strength of the
  • 00:17:57
    electrolyte but not in the way we
  • 00:17:58
    predict at higher concentrations we see
  • 00:18:00
    a reduction in the conductivity due to
  • 00:18:03
    the effect of the solvation and the
  • 00:18:05
    ionic atmosphere and we remember that
  • 00:18:06
    while we can determine limiting molar
  • 00:18:08
    conductivity for strong electrolytes we
  • 00:18:10
    can't directly measure the conductivity
  • 00:18:12
    of weak electrolytes so we need to use
  • 00:18:14
    this idea of independent migration of
  • 00:18:16
    ions
Tag
  • Ions
  • Conductivity
  • Electrolytes
  • Ionic Mobility
  • Solution Chemistry
  • Diffusion
  • Migration
  • Conductivity Measurement
  • Convection
  • Limiting Molar Conductivity