306 AcidBase 6titrations

00:32:49
https://www.youtube.com/watch?v=GZ4giKongJ0

Summary

TLDRCette leçon couvre les titrations acido-basiques, expliquant leur principe et leur importance pour déterminer la concentration d'une solution inconnue grâce à une procédure volumétrique. Elle souligne l'utilisation de la burette pour ajouter le titrant à l'inconnu, la nécessité d'un indicateur pour détecter le point d'équivalence, et les différences principales observées dans les courbes de titration des acides et bases forts et faibles. Des exemples de calculs de pH à divers stades de la titration (début, avant, à l'équivalence et au-delà) sont fournis, ainsi que des informations sur les indicateurs appropriés pour différentes solutions. En conclusion, la leçon met en lumière la variabilité des pH à l'équivalence selon les type d'acide et de base impliqués et comment le choix des indicateurs joue un rôle crucial dans le succès de la titration.

Takeaways

  • 🧪 Titration est une analyse volumétrique pour déterminer la concentration d'une solution inconnue.
  • 🔍 Utilisation d'indicateurs pour visualiser le point d'équivalence par un changement de couleur.
  • ⚖️ À l'équivalence d'un acide fort et d'une base forte, le pH est généralement 7.
  • 📉 Les titrations d'acides faibles par des bases fortes ont un pH d'équivalence supérieur à 7.
  • 📈 L'importance du choix de l'indicateur dépend du pH à l'équivalence.
  • 📊 Des courbes de titration montrent des changements rapides de pH près du point d'équivalence.
  • 🧮 Les calculs de pH doivent être faits à chaque étape de la titration.
  • 🧭 Les standards primaires sont essentiels pour une concentration fiable.

Timeline

  • 00:00:00 - 00:05:00

    Dans cette leçon, nous allons discuter des titrages acido-basiques, utilisant des concepts d'équilibre pour déterminer le pH de diverses solutions. Nous verrons que le titrage est une procédure d'analyse volumétrique, permettant de déterminer la concentration d'une solution inconnue en ajoutant un volume connu d'une solution standard. Il est également important de noter que les titrages ne se limitent pas aux acides et aux bases, mais peuvent également inclURE des réactions redox et des précipitations.

  • 00:05:00 - 00:10:00

    Nous passons en revue le matériel utilisé, tel que la burette pour ajouter des volumes contrôlés. La solution standard est souvent un standard primaire, comme le phosphate d'hydrogène de potassium. Nous discuterons aussi des indicateurs de pH, essentiels pour déterminer le point d'équivalence du titrage, où les acides et les bases réagissent en proportions stoichiométriques.

  • 00:10:00 - 00:15:00

    Nous comparerons aussi trois types de titrages : acide fort avec base forte, acide faible avec base forte, et base faible avec acide fort. Le premier cas est simple, présentant un changement quasi instantané du pH au point d'équivalence, qui est toujours 7. En revanche, pour les autres types, le comportement du pH diffère au fur et à mesure que l'on approche du point d'équivalence.

  • 00:15:00 - 00:20:00

    Lors d'un titrage d'un acide fort avec une base forte, la concentration finale en ions H+ à l'équivalence est nulle, produisant de l'eau et du sel neutre, avec un pH de 7. On apprend aussi à choisir le bon indicateur qui changera de couleur près du point d'équivalence.

  • 00:20:00 - 00:25:00

    Pour un acide faible titré par une base forte, le pH initial est plus élevé et il existe une zone tampon avec peu de variations. L'équivalence se produira à un pH supérieur à 7, typiquement autour de 8,72, ce qui est une caractéristique importante à reconnaître.

  • 00:25:00 - 00:32:49

    Enfin, nous abordons aussi le titrage d'une base faible par un acide fort, refusant de titrer des acides et des bases faibles à cause de la complexité. Ce titrage se déroule aussi dans une zone tampon avec une relation distincte entre les concentrations d'acide et de base, et nous soulignons l'importance de bien choisir les indicateurs en fonction du pH d'équivalence souhaité.

Show more

Mind Map

Video Q&A

  • Qu'est-ce qu'une titration acidobasique ?

    C'est un processus d'analyse où une solution de concentration connue est ajoutée à une solution inconnue pour déterminer sa concentration.

  • Quel est le but de l'indicateur dans une titration ?

    L'indicateur aide à identifier le point d'équivalence par un changement de couleur.

  • Pourquoi ne pas titrer un acide faible avec une base faible ?

    Les systèmes sont trop complexes et il n'y a pas de point d'arrêt clair.

  • Quel est le pH à l'équivalence d'une titration acido-basique forte ?

    Le pH est généralement de 7.

  • Comment calcule-t-on le pH d'une solution acide faible ?

    En utilisant l'équation d'hydrolyse et le Ka de l'acide.

  • Qu'est-ce qu'un standard primaire ?

    C'est une substance dont la concentration peut être mesurée avec une grande confiance.

  • Pourquoi le phenolphtaléine est-il un bon indicateur ?

    Sa pKa est proche du pH d'équivalence pour de nombreuses titrations.

  • Quel type de titration donne un pH à l'équivalence supérieur à 7 ?

    La titration d'un acide faible par une base forte.

View more video summaries

Get instant access to free YouTube video summaries powered by AI!
Subtitles
en
Auto Scroll:
  • 00:00:00
    in this lesson we're going to discuss
  • 00:00:02
    acidbase titrations and this is a very
  • 00:00:04
    nice application of all the work that
  • 00:00:06
    we've been doing to calculate pH of
  • 00:00:08
    various Solutions we've calculated pH of
  • 00:00:12
    strong acid strong base Solutions we've
  • 00:00:14
    calculated pH of weak acid we weak based
  • 00:00:18
    Solutions buffers and of solutions after
  • 00:00:22
    some concentration of strong or acid or
  • 00:00:26
    base has been
  • 00:00:27
    added now you'll recall from your
  • 00:00:29
    studies in grade 11 that a titration is
  • 00:00:32
    a lab procedure where we are analyzing
  • 00:00:35
    an unknown solution we refer to this as
  • 00:00:38
    a volumetric analysis because we are
  • 00:00:41
    adding a certain volume of known
  • 00:00:43
    solution to a certain volume of unknown
  • 00:00:47
    solution and the goal is to determine
  • 00:00:49
    the concentration of an unknown solution
  • 00:00:52
    now what's important to note is that
  • 00:00:54
    titrations don't always have to be acid
  • 00:00:56
    based titrations we're going to see
  • 00:00:58
    later on in the year that we can do
  • 00:01:00
    redox titrations we can do precipitation
  • 00:01:03
    titrations so it's important to think of
  • 00:01:05
    this as a volumetric analyses procedure
  • 00:01:08
    rather than something that we use to
  • 00:01:12
    analyze acids and bases the bual rec
  • 00:01:16
    call is the glassware that we use to add
  • 00:01:20
    controlled volumes of the standard
  • 00:01:23
    solution or the titrant into our unknown
  • 00:01:27
    solution so the standard solution is the
  • 00:01:29
    one that is of known
  • 00:01:32
    concentration and you learned in grade
  • 00:01:34
    11 that there is something that we refer
  • 00:01:37
    to as a primary standard and the classic
  • 00:01:40
    primary standard in acidbase chemistry
  • 00:01:43
    is potassium hydrogen phate so it's
  • 00:01:46
    important that you understand that a
  • 00:01:49
    primary standard is one where we can
  • 00:01:51
    basically be really confident in its
  • 00:01:54
    concentration you'll recall that sodium
  • 00:01:56
    hydroxide is deliquescent or it absorbs
  • 00:01:59
    water so it's really difficult to
  • 00:02:01
    prepare a solution of known
  • 00:02:04
    concentration of sodium hydroxide
  • 00:02:07
    although it's a very useful tirant or
  • 00:02:10
    solution that we put in the buet in
  • 00:02:12
    order to study acids and bases the
  • 00:02:15
    acidbase indicator is the substance that
  • 00:02:17
    we put in order to help us to identify
  • 00:02:20
    the end point we're going to discuss
  • 00:02:22
    indicators in a little bit of detail
  • 00:02:24
    later on in the
  • 00:02:25
    lesson as you know the classic acidbase
  • 00:02:29
    indicator you've used so far is phenol
  • 00:02:31
    theine it's colorless in acid and it's
  • 00:02:34
    pink or Magenta in base now the end
  • 00:02:37
    point of the titration is the point
  • 00:02:39
    where we stop titrating so we no longer
  • 00:02:42
    add any of the titrant from the buet and
  • 00:02:45
    this occurs when we observe a color
  • 00:02:48
    change now the goal of completing a
  • 00:02:50
    titration is actually to determine the
  • 00:02:53
    equivalence point we're trying to find
  • 00:02:56
    the point where the two reactants are
  • 00:02:59
    sto metrically equal so the moles of
  • 00:03:02
    acid for example is equal to the moles
  • 00:03:05
    of base or H+ and O minus that is the
  • 00:03:10
    equivalence
  • 00:03:12
    point now we want to choose an indicator
  • 00:03:15
    that's going to change color as close as
  • 00:03:18
    possible to the end point because that's
  • 00:03:20
    what we're using to identify the
  • 00:03:22
    equivalence
  • 00:03:23
    point we're going to examine three
  • 00:03:26
    different types of acidbase titrations
  • 00:03:28
    we're going to look at the the titration
  • 00:03:31
    of a strong acid by a strong base we'll
  • 00:03:34
    look at the titration of a weak acid by
  • 00:03:37
    a strong base and we'll look at the
  • 00:03:39
    titration of a weak base by a strong
  • 00:03:43
    acid last year the focus was when you
  • 00:03:47
    created titration curves the focus was a
  • 00:03:49
    strong acid and strong base and you're
  • 00:03:52
    going to see from your Advanced
  • 00:03:53
    knowledge of acid based chemistry that
  • 00:03:55
    these ones the last two have a bit of a
  • 00:03:58
    difference and you're going to notice
  • 00:04:00
    that we don't do a titration with a weak
  • 00:04:03
    base and a weak acid the systems are far
  • 00:04:05
    too
  • 00:04:07
    complex and so they're not
  • 00:04:11
    ideal so this should be familiar to you
  • 00:04:14
    this is the picture of the titration
  • 00:04:18
    curve for a strong acid with a strong
  • 00:04:21
    base okay a few features of the curve
  • 00:04:25
    you can tell by looking at the pH that
  • 00:04:28
    we're starting with an a a so in this
  • 00:04:30
    case the titrant or the solution in the
  • 00:04:33
    buet is the base so we are adding base
  • 00:04:37
    in increments and we're measuring the pH
  • 00:04:40
    at each of those increments and graphing
  • 00:04:43
    the
  • 00:04:46
    results now when you have a titration
  • 00:04:48
    curve there is always a very steep part
  • 00:04:52
    of the titration curve and that always
  • 00:04:53
    occurs around the equivalence point the
  • 00:04:57
    point where the moles of acid is equal
  • 00:05:00
    to the moles of Base in this case we're
  • 00:05:02
    titrating A.1 M hydrochloric acid
  • 00:05:06
    solution with A.1 molar sodium hydroxide
  • 00:05:09
    solution we're starting with 25
  • 00:05:12
    milliliters of the acid and so the
  • 00:05:15
    equivalence point here occurs at 25
  • 00:05:18
    milliliters of base and the pH of this
  • 00:05:21
    solution is going to be
  • 00:05:24
    seven the reason it's seven is because
  • 00:05:26
    the products here are water and Salt N
  • 00:05:30
    ACL and as you know from your study of
  • 00:05:32
    salts that if a salt is produced from a
  • 00:05:36
    strong acid and strong base like NAC is
  • 00:05:39
    it will not affect the pH and therefore
  • 00:05:41
    the pH is
  • 00:05:43
    seven this point of the curve the
  • 00:05:46
    equivalence point is actually an an
  • 00:05:48
    inflection point so it's the point where
  • 00:05:51
    the curve changes from being a curve
  • 00:05:53
    that's increasing to a curve that is
  • 00:05:57
    decreasing and when we're studying in
  • 00:05:59
    these graphs in order to find the
  • 00:06:01
    equivalence point we actually need to
  • 00:06:03
    take the halfway point of the Steep
  • 00:06:06
    point of the curve okay so we take look
  • 00:06:09
    at the Steep part of the curve and the
  • 00:06:11
    halfway point is where the equivalence
  • 00:06:13
    point is which in this case is pH of s
  • 00:06:18
    now what we're going to do is look at
  • 00:06:20
    how we can put everything together that
  • 00:06:22
    we've used already to determine the pH
  • 00:06:26
    calculated at different points
  • 00:06:28
    throughout the titration
  • 00:06:32
    so let's look at the pH before the
  • 00:06:34
    titration begins we started with a
  • 00:06:36
    solution that is 25 milliliters in .1
  • 00:06:40
    molar hydrochloric acid hydrochloric
  • 00:06:43
    acid is the only solution present and
  • 00:06:46
    it's a strong acid so we know from this
  • 00:06:50
    that the concentration of the H+ is
  • 00:06:52
    going to be equal to the concentration
  • 00:06:54
    of the
  • 00:06:55
    HCL so for this particular solution the
  • 00:06:59
    pH H is simply the negative log of the
  • 00:07:03
    concentration of the H+ which is .1 okay
  • 00:07:08
    so initially the pH of the solution is
  • 00:07:14
    one which if we look at the table
  • 00:07:17
    associated with the Curve it
  • 00:07:21
    matches now let's look at the the
  • 00:07:25
    concentration of the H+ and subsequently
  • 00:07:28
    the pH
  • 00:07:31
    after a certain amount of Base has been
  • 00:07:33
    added we're talking about the region
  • 00:07:35
    before the equivalence point
  • 00:07:38
    here so we're dealing with a strong acid
  • 00:07:41
    and base and if we're before the
  • 00:07:43
    equivalence point we have a scenario
  • 00:07:45
    where we have a limiting reactant so
  • 00:07:49
    what we need to do is look at the
  • 00:07:52
    reaction that we have and remember
  • 00:07:54
    anytime you have a reaction with a
  • 00:07:57
    strong acid or a strong base so in this
  • 00:07:59
    case we've got both it's going to be a
  • 00:08:02
    complete reaction so we're going to have
  • 00:08:04
    a full Arrow here in order to organize
  • 00:08:07
    the data to see what we have left over
  • 00:08:10
    we're going to set up an ICF
  • 00:08:13
    chart now what we have is we've got a
  • 00:08:16
    volume of 35 milliliters for the total
  • 00:08:21
    because remember we started with 25
  • 00:08:24
    milliliters of the acid and we've added
  • 00:08:27
    10 milliliters of the base so a very
  • 00:08:30
    very important initial step in all of
  • 00:08:33
    the calculations that you're going to do
  • 00:08:34
    in this section will involve you
  • 00:08:37
    diluting the given Solutions because
  • 00:08:40
    during a titration we're not adding
  • 00:08:42
    solid quantities we're adding volumes of
  • 00:08:47
    solution so if you do the C1 V1 equals
  • 00:08:50
    c2v2 with the given 10mil and 25ml
  • 00:08:55
    solutions what you see is that you get a
  • 00:08:58
    concentration of sodium hydroxide which
  • 00:09:00
    is
  • 00:09:02
    0.285 and a concentration of HCL which
  • 00:09:06
    is
  • 00:09:10
    0.714 now in this case you can see that
  • 00:09:12
    the sodium hydroxide is limiting which
  • 00:09:15
    makes sense because we're before
  • 00:09:19
    the equivalence point and so the
  • 00:09:22
    limiting reactant is going to be
  • 00:09:24
    completely
  • 00:09:26
    consumed HCL is in a one one ratio with
  • 00:09:30
    the NaOH so the concentration of the HCL
  • 00:09:34
    that's left over here is
  • 00:09:37
    0.04
  • 00:09:40
    28 now when we were dealing with buffers
  • 00:09:43
    we would have needed to look at this
  • 00:09:45
    side of the
  • 00:09:47
    solution and look at the ions that are
  • 00:09:50
    put into the solution but because sodium
  • 00:09:53
    chloride does not affect the pH and H2O
  • 00:09:56
    is a pure liquid it doesn't affect our
  • 00:09:58
    calculations
  • 00:10:00
    there's no need for us to look at how
  • 00:10:02
    much nhcl is in this
  • 00:10:05
    solution now if we look at this what we
  • 00:10:07
    have left over in the solution at this
  • 00:10:09
    point is strong acid this is where the
  • 00:10:13
    key decision making comes in you always
  • 00:10:15
    need to be conscious that you're making
  • 00:10:18
    a decision that you've got a strong acid
  • 00:10:21
    the strong acid will dissociate
  • 00:10:23
    completely so that will put into our
  • 00:10:26
    solution a concentration of H+ at0
  • 00:10:32
    0.428 okay and as expected it's
  • 00:10:35
    decreased originally it was1 so it's
  • 00:10:38
    decreased a little bit so the pH
  • 00:10:41
    here is
  • 00:10:46
    1.36 okay so so far the pH has risen
  • 00:10:49
    from 1 to
  • 00:10:53
    1.36 and it's not too large of an
  • 00:10:55
    increase the beginning part of a
  • 00:10:58
    titration the pH changes
  • 00:11:03
    gradually let's look at just before the
  • 00:11:06
    equivalence point so when we've added
  • 00:11:12
    24.90 milliliters of the sodium
  • 00:11:15
    hydroxide solution which if you look at
  • 00:11:17
    the beginning it's 0.1 in its
  • 00:11:21
    concentration okay so right now our
  • 00:11:24
    total
  • 00:11:26
    volume is 49
  • 00:11:33
    9 so again we've got the reaction of
  • 00:11:37
    NaOH with
  • 00:11:45
    HCL okay I'm going to do my ICF
  • 00:11:49
    chart the concentration of sodium
  • 00:11:52
    hydroxide in the solution is 0.499
  • 00:12:02
    and the concentration of HCL is
  • 00:12:11
    0.0
  • 00:12:16
    501 and so the sodium hydroxide again is
  • 00:12:19
    our limiting reactant so it's going to
  • 00:12:21
    go to zero and the concentration of HCL
  • 00:12:24
    that I'm left with is 2 * 10 the4
  • 00:12:29
    so the pH still hasn't risen too too
  • 00:12:33
    much but it is making its way upwards
  • 00:12:37
    and the pH at that point is
  • 00:12:41
    3.9 oh sorry it's
  • 00:12:45
    3.7 put it
  • 00:12:47
    over okay so it's
  • 00:12:55
    3.7 now if we looked at the point
  • 00:12:59
    24.99 we would see that the pH would
  • 00:13:02
    rise to
  • 00:13:05
    4.70 at the equivalence
  • 00:13:08
    point with the addition of only 0.01
  • 00:13:11
    milliliters of Base the pH jumps to
  • 00:13:15
    seven and we've got a solution of just
  • 00:13:21
    NAC okay so small additions right around
  • 00:13:24
    the equivalence point do cause rapid
  • 00:13:27
    changes in PH
  • 00:13:30
    and it's really important to know always
  • 00:13:32
    that when you have the titration or the
  • 00:13:34
    reaction of a strong acid and strong
  • 00:13:36
    base at the equivalence point the pH is
  • 00:13:39
    going to be seven okay there's no
  • 00:13:41
    calculation necessary for that now if we
  • 00:13:44
    look at beyond the equivalence point so
  • 00:13:47
    when we've added
  • 00:13:50
    one very small quantity of sodium
  • 00:13:54
    hydroxide in excess so I'm talking about
  • 00:13:58
    25 5.01 milliliters of Base being added
  • 00:14:03
    at this point so my total volume is
  • 00:14:11
    50.01 at this point now the sodium
  • 00:14:14
    hydroxide is going to
  • 00:14:17
    be our limiting reactant okay and it's
  • 00:14:20
    important that as you can see when we're
  • 00:14:22
    dealing with such
  • 00:14:25
    small differences here you need to keep
  • 00:14:28
    lots of sign ific diges in your
  • 00:14:30
    preliminary calculations so that you can
  • 00:14:32
    actually see the difference so now at
  • 00:14:35
    this point beyond the equivalence point
  • 00:14:37
    the sodium hydroxide is actually in
  • 00:14:40
    excess in a concentration of 2 *
  • 00:14:45
    105 so this being a strong base
  • 00:14:47
    dissociates completely so I know the
  • 00:14:50
    hydroxide concentration is 2 * 10 to
  • 00:14:53
    the5 so I can calculate the P of 4.70
  • 00:15:00
    and the pH is therefore
  • 00:15:05
    9.30 okay so as you can see between
  • 00:15:10
    24.99 and 25.01.2013
  • 00:15:29
    and that's what really drives the pH
  • 00:15:31
    upwards when you're doing a titration it
  • 00:15:34
    doesn't necessarily mean that you're
  • 00:15:37
    going to have an incorrect calculation
  • 00:15:40
    if you've added you know half a drop in
  • 00:15:42
    excess okay because the amount of volume
  • 00:15:46
    that causes the pH to jump significantly
  • 00:15:49
    is really really
  • 00:15:57
    small now when we're hydrating a weak
  • 00:16:00
    acid by a strong base we're going to be
  • 00:16:02
    dealing with a variety of situations
  • 00:16:05
    we're going to deal with weak acid
  • 00:16:07
    calculations we're going to deal with
  • 00:16:08
    buffer calculations we're going to deal
  • 00:16:10
    with the addition of a strong base to a
  • 00:16:13
    buffer the scenario that we're looking
  • 00:16:15
    at here is with the titration of
  • 00:16:17
    vinegar okay and B sodium
  • 00:16:22
    hydroxide as you can see here with a
  • 00:16:25
    weak acid the pH initially is a little a
  • 00:16:28
    little bit higher you're also going to
  • 00:16:31
    see that this here is the buffer region
  • 00:16:34
    so there's a very uh small changes in PH
  • 00:16:38
    over this range as we move towards the
  • 00:16:41
    equivalence point the steepness around
  • 00:16:45
    the equivalence point the height of
  • 00:16:48
    it steepness is the same but the length
  • 00:16:51
    of the section where it's steep is not
  • 00:16:53
    uh quite as high because of the lower
  • 00:16:57
    initial pH here and this is where it's
  • 00:17:00
    going to be really important for us to
  • 00:17:02
    use the technique of taking the middle
  • 00:17:06
    the midpoint of the Steep section in
  • 00:17:09
    order to determine the equivalence point
  • 00:17:11
    pH one of the major differences of
  • 00:17:14
    titrating with weak Solutions is that
  • 00:17:17
    the equivalence point doesn't actually
  • 00:17:18
    lie at seven in this case the
  • 00:17:21
    equivalence point is at
  • 00:17:23
    8.72 and we'll see in our calculations
  • 00:17:25
    why that makes
  • 00:17:27
    sense so we're going to go through a
  • 00:17:29
    series of calculations here looking at
  • 00:17:32
    the pH before the titration begins okay
  • 00:17:36
    uh during the titration halfway to the
  • 00:17:38
    equivalence point and at the equivalence
  • 00:17:41
    point so before the titration begins we
  • 00:17:44
    have a solution of 25 milliliters of
  • 00:17:47
    acetic acid it's important to make a
  • 00:17:51
    conscious recognition that this is a
  • 00:17:53
    weak acid so if I want to know the pH of
  • 00:17:57
    a weak acid solution I need
  • 00:18:01
    to do a hydrolysis
  • 00:18:08
    calculation so we've got a 0.1 molar
  • 00:18:12
    solution got none of this at the
  • 00:18:14
    beginning plus X Plus
  • 00:18:18
    X this is - X 0.1 minus X and it's
  • 00:18:23
    approximately equal to 0.1 and then I
  • 00:18:27
    can use my Ka so I've got x^2 is equal
  • 00:18:31
    over .1 is equal to 1.8 *
  • 00:18:35
    105 and we get a pH here of
  • 00:18:40
    2.87 okay so initially before anything's
  • 00:18:43
    been added it's simply a weak acid
  • 00:18:48
    calculation next if we want to calculate
  • 00:18:50
    the pH during the titration so we're
  • 00:18:53
    before the equivalence
  • 00:18:55
    point we are adding strong base
  • 00:18:59
    to a solution so we need to write the
  • 00:19:02
    reaction for the acid that's going to
  • 00:19:04
    react with the base which is the vinegar
  • 00:19:07
    plus the
  • 00:19:11
    hydroxide this will produce
  • 00:19:15
    acetate and
  • 00:19:18
    water because we're adding a strong base
  • 00:19:20
    this is a complete
  • 00:19:23
    reaction and we're going to use an ICF
  • 00:19:26
    chart to organize our data
  • 00:19:30
    the concentration of the sodium
  • 00:19:32
    hydroxide that we're using is
  • 00:19:34
    0.1 and the volume that we have at this
  • 00:19:37
    point is 30 milliliters because we've
  • 00:19:39
    added 5 milliliters to our initial 25mil
  • 00:19:47
    solution okay so we see that we've got a
  • 00:19:50
    0.833 and
  • 00:19:54
    0.167 got none of this initially the
  • 00:19:57
    water I'm not worried about okay so the
  • 00:20:00
    hydroxide is going to be our limiting
  • 00:20:05
    reactant and so it goes down to zero and
  • 00:20:08
    it helps us know
  • 00:20:10
    what our x value is which is
  • 00:20:21
    0.0167 so we can figure out what the
  • 00:20:23
    concentration of the acetic acid and the
  • 00:20:25
    acetate ion are in the solution once the
  • 00:20:28
    hydroxide has been consumed now at this
  • 00:20:31
    point whenever ever you've completed
  • 00:20:33
    your ICF chart you need to look at it
  • 00:20:35
    and decide what you have so right now
  • 00:20:38
    we've got a solution that contains 0.0
  • 00:20:42
    666 mole per liter in the vinegar and
  • 00:20:46
    0.1 0167 mole per liter in the acetate
  • 00:20:50
    so this you must must must recognize is
  • 00:20:54
    a buffer and so you can
  • 00:20:57
    use the h hassle block
  • 00:21:01
    equation to very quickly and easily
  • 00:21:05
    calculate the pH so the pH at this point
  • 00:21:09
    is
  • 00:21:17
    4.14 okay now next we're going to look
  • 00:21:21
    at halfway to equivalence point
  • 00:21:38
    halfway to equivalence point is a
  • 00:21:40
    special point in the
  • 00:21:42
    titration so the volume at this point
  • 00:21:48
    is
  • 00:21:50
    37.5 milliliters
  • 00:22:02
    so this is the point when half of the
  • 00:22:04
    vinegar has been neutralized so we
  • 00:22:07
    started with 25 milliliters of the acid
  • 00:22:10
    at 0.1 mole per liter and so that is
  • 00:22:13
    half titrated or half neutralized when
  • 00:22:17
    we've added 12.5 milliliters of sodium
  • 00:22:21
    hydroxide of the same
  • 00:22:23
    concentration so at this point when you
  • 00:22:27
    look at the halfway to equivalence point
  • 00:22:31
    you're going to notice that this is a
  • 00:22:33
    really special point in the
  • 00:22:40
    titration and it will eventually become
  • 00:22:42
    a very quick
  • 00:22:46
    calculation okay so when we've added
  • 00:22:49
    37.5 milliliters what we have is our
  • 00:22:52
    initial concentration of the vinegar is
  • 00:22:56
    0.667
  • 00:22:58
    we've
  • 00:22:59
    added half that amount of
  • 00:23:07
    Base our base is still
  • 00:23:10
    our limiting reactant so it goes to zero
  • 00:23:14
    and it helps us to determine what our x
  • 00:23:18
    value
  • 00:23:19
    is and now what we see here is that
  • 00:23:22
    we've got a buffer so we still have some
  • 00:23:24
    of the vinegar and the acetate ions
  • 00:23:27
    present
  • 00:23:29
    this is a particularly important point
  • 00:23:31
    of the buffer though because at this
  • 00:23:33
    point in the titration the concentration
  • 00:23:37
    of the acetate is equal to the
  • 00:23:39
    concentration of the
  • 00:23:40
    vinegar and when we use the Henderson
  • 00:23:43
    hul
  • 00:23:48
    equation we've got our point where
  • 00:23:54
    the log portion of the Henderson Hassel
  • 00:23:57
    block equation isal equal to zero
  • 00:23:59
    because if our base concentration and
  • 00:24:02
    our acid concentration are equal then
  • 00:24:05
    we'll have log of one which is equal to
  • 00:24:07
    zero so halfway to equivalence the pH is
  • 00:24:11
    always equal to the pka of the
  • 00:24:19
    acid so you'll be able to if you
  • 00:24:22
    recognize this special scenario you'll
  • 00:24:24
    be able to save yourself some
  • 00:24:26
    significant time in making calculations
  • 00:24:29
    because you can simply report that if
  • 00:24:32
    you're halfway to equivalence the pH is
  • 00:24:34
    equal to the
  • 00:24:40
    pka at the equivalence point is another
  • 00:24:43
    special calculation this one is the most
  • 00:24:46
    challenging of the calculations that you
  • 00:24:52
    do so the equivalence point is the point
  • 00:24:56
    whereby the amount of acid is
  • 00:24:59
    stochiometrically equal to the amount of
  • 00:25:05
    Base so we're dealing with a one:1 ratio
  • 00:25:07
    here and our concentrations are the same
  • 00:25:11
    so this is the point where 25
  • 00:25:13
    milliliters of Base has been added so
  • 00:25:16
    our total
  • 00:25:18
    volume is 50
  • 00:25:21
    milliliters and so our
  • 00:25:24
    concentrations are 0.05 and 0.5
  • 00:25:27
    respectively
  • 00:25:29
    so in this case there is no limiting
  • 00:25:30
    reactant they both go to zero they're
  • 00:25:33
    both consumed but the acetate ion is
  • 00:25:36
    still being
  • 00:25:39
    produced so when we looked at the
  • 00:25:41
    equivalence point of a strong acid
  • 00:25:43
    strong base hydration we automatically
  • 00:25:45
    knew that we had a pH of seven and that
  • 00:25:49
    was because of the nature of the salt
  • 00:25:51
    that was produced in that reaction this
  • 00:25:55
    salt coming from a we acid affects the
  • 00:26:00
    pH okay this is the conjugate base of
  • 00:26:07
    the acetic acid solution so what we need
  • 00:26:10
    to do is now look at the hydrolysis
  • 00:26:14
    reaction that
  • 00:26:17
    occurs with the acetic
  • 00:26:25
    acid okay the concentration of
  • 00:26:30
    the acetate ion is
  • 00:26:38
    0.05
  • 00:26:41
    this
  • 00:26:51
    hydroxide and now the other major Point
  • 00:26:54
    here is that we've got the KA for the
  • 00:26:57
    acetic acid but we need to calculate the
  • 00:27:00
    KB using Ka is equal
  • 00:27:05
    to sorry Ka time KB is equal to KW so
  • 00:27:09
    the KB in this case is
  • 00:27:12
    5.56 * 10 the
  • 00:27:15
    -10 it's small enough to make the
  • 00:27:17
    Assumption so we've got that is equal to
  • 00:27:19
    x^2 over
  • 00:27:21
    0
  • 00:27:23
    05 so then you can calculate the p
  • 00:27:28
    at
  • 00:27:30
    5.28 and the
  • 00:27:32
    pH at
  • 00:27:36
    8.72 so calculating the pH at the
  • 00:27:39
    equivalence point of a weak acid
  • 00:27:43
    titrated by a strong base it's going to
  • 00:27:46
    be slightly basic because of the
  • 00:27:48
    presence of the acetate ion which is the
  • 00:27:50
    conjugate of the
  • 00:27:54
    vinegar we can do a similar series of
  • 00:27:57
    calculations
  • 00:27:59
    with the titration of a weak base by a
  • 00:28:02
    strong
  • 00:28:03
    acid so there is space here for you to
  • 00:28:08
    walk through the
  • 00:28:10
    calculations okay it's very similar I'm
  • 00:28:12
    going to ask you to change this
  • 00:28:16
    to10 mole per
  • 00:28:19
    liter so your job is to go through each
  • 00:28:22
    of
  • 00:28:23
    the scenarios so you're going to look at
  • 00:28:26
    before the titration during the
  • 00:28:28
    titration halfway to
  • 00:28:31
    equivalence and at the equivalence
  • 00:28:35
    point you can compare the values that
  • 00:28:37
    you get to the
  • 00:28:39
    chart that is right here you should be
  • 00:28:42
    getting the same pH as that we have at
  • 00:28:44
    the particular points and a few features
  • 00:28:47
    of this you can see that we've got our
  • 00:28:48
    buffer region here where the pH doesn't
  • 00:28:51
    change too graph drastically leading up
  • 00:28:53
    to our equivalence point you're starting
  • 00:28:56
    with an acid because we're sorry we're
  • 00:28:58
    starting with a base the ammonia
  • 00:29:00
    solution and you're adding acid to it so
  • 00:29:02
    the pH is grad gradually changing over
  • 00:29:05
    time the equivalence point lies at the
  • 00:29:09
    midpoint of the Steep portion of the
  • 00:29:18
    curve now if you move ahead in the notes
  • 00:29:21
    there's a few little points here so
  • 00:29:23
    first of all we already discussed that
  • 00:29:25
    we would not titrate a weak acid with a
  • 00:29:27
    weak base
  • 00:29:28
    there's not really a clear end point
  • 00:29:30
    because of the weak base that's put into
  • 00:29:34
    the solution by the salt of this weak
  • 00:29:37
    acid and the weak acid that's put into
  • 00:29:40
    the solution by the salt of this weak
  • 00:29:43
    base so it's a competing reaction so
  • 00:29:47
    it's easier to avoid this kind of
  • 00:29:49
    complex
  • 00:29:52
    scenario now when we titrate with a pH
  • 00:29:55
    meter we're going to be able to actually
  • 00:29:58
    determine the KA of a particular acid
  • 00:30:02
    you're going to do a lab where you
  • 00:30:04
    determine the dissociation constant of
  • 00:30:07
    an acid by using a half
  • 00:30:10
    titration because we know that the pH is
  • 00:30:13
    equal to the pka halfway to the
  • 00:30:15
    equivalence point so it makes for a very
  • 00:30:17
    simple method to calculate the
  • 00:30:22
    KA now selecting an indicator to do a
  • 00:30:25
    titration is a really important lab
  • 00:30:29
    technique and what we are looking for is
  • 00:30:32
    we are looking for a solution that's
  • 00:30:35
    going to change color as close to the
  • 00:30:39
    equivalence point as
  • 00:30:42
    possible now indicators themselves are
  • 00:30:45
    actually weak assets and so the presence
  • 00:30:47
    of H+ in the solution is actually
  • 00:30:50
    shifting a small equilibrium to change
  • 00:30:53
    the color of the particular indicator
  • 00:30:58
    the color
  • 00:31:00
    change
  • 00:31:01
    occurs when at the breaking point
  • 00:31:05
    between when the concentration of one
  • 00:31:08
    color is actually equal to the
  • 00:31:10
    concentration of the other color so it's
  • 00:31:14
    a point where the concentration of the
  • 00:31:17
    acidic form of the indicator is equal to
  • 00:31:20
    the basic form of the
  • 00:31:22
    indicator this point actually occurs at
  • 00:31:27
    the pka of the particular
  • 00:31:29
    indicator so when we're looking for a
  • 00:31:33
    indicator we want to look for an
  • 00:31:35
    indicator that will have a PKA close to
  • 00:31:39
    the pH of our equivalence
  • 00:31:44
    point so if we look at these indicators
  • 00:31:47
    here okay phenol theine the indicator
  • 00:31:50
    that you use most commonly has a pka of
  • 00:31:54
    9.3 so this is ideal
  • 00:31:58
    for the titration of a weak acid by a
  • 00:32:02
    strong base because the pH at
  • 00:32:06
    equivalence is close to
  • 00:32:09
    9.3 the other reason that phenol phalion
  • 00:32:12
    is such a good indicator is because it
  • 00:32:14
    has such a drastic color change so those
  • 00:32:17
    two features we're looking for okay but
  • 00:32:20
    a classic AP question is to identify
  • 00:32:23
    which indicator is appropriate for a
  • 00:32:26
    particular acidbase hydration by using a
  • 00:32:30
    set of PKA values okay so if you have
  • 00:32:33
    something that the equivalence point is
  • 00:32:36
    around four okay bromoenol blue might be
  • 00:32:40
    an appropriate indicator for that
  • 00:32:41
    titration
Tags
  • titration
  • acido-basique
  • burette
  • pH
  • point d'équivalence
  • indicateurs
  • acide faible
  • acide fort
  • base forte
  • analyses volumétriques