1 / 6

Colour I

Acid-Base Eqm (5): Acid-Base Indicators. p.01. H 3 O + + In -. HIn + H 2 O. [H 3 O + (aq)][In - (aq)]. Colour II. Colour I. K In =. [HIn(aq)]. In alkali, H 3 O + is used up and the eqm shifts FW, so the colour seen is colour II.

noel
Download Presentation

Colour I

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Acid-Base Eqm (5):Acid-Base Indicators p.01 H3O++ In- HIn + H2O [H3O+(aq)][In-(aq)] Colour II Colour I KIn = [HIn(aq)] In alkali, H3O+ is used up and the eqm shifts FW, so the colour seen is colour II. In acid, the excess H3O+ shift the eqm BW, so the colour seen is colour I. Acid-Base Eqm (5):Acid-Base Indicators An indicator (HIn) is a WEAK ACID whose conjugate base is a different colour. C. Y. Yeung (CHW, 2009)

  2. p.02 RED YELLOW PINK COLOURLESS Examples: Phenolphthalein (pKIn = 9.3) Methyl Orange (pKIn = 3.70)

  3. p.03 H3O++ Ph- HPh + H2O COLOURLESS PINK [H3O+(aq)][Ph-(aq)] KIn = this ratio controls the colour shown! [HPh(aq)] 1 1  [Ph-(aq)] [Ph-(aq)] [Ph-(aq)] Colourless when - log pH pKIn = 10 10 [HPh(aq)] [HPh(aq)] [HPh(aq)]  Redwhen When does it change colour? (1) Phenolphthalein (pKIn = 9.3)

  4. p.04 When [H3O+], eqm shifts FW, [Ph-] >> [HPh]. When [H3O+], eqm shifts BW, [Ph-] << [HPh]. H3O++ Ph- HPh + H2O   PINK ! Colourless ! when when [Ph-(aq)] [Ph-(aq)] + log + log pKIn pKIn pH = pH = At that time … [HPh(aq)] [HPh(aq)] 1 + log 9.3 = 10 10 10 1 + log 9.3 = [Ph-(aq)] [Ph-(aq)] 1 10 1 [HPh(aq)] [HPh(aq)] = 10.3 = 8.3 When does it change colour? (2) At that time …

  5. 8.15 2.70 p.05 3.70 9.15 10.15 4.70 7 7 pH pH pink colourless pale pink Methyl orange yellow red orange “Working Range” [where colour change is observable] of Indicators Phenolphthalein pH 8.15 to 10.15 = “Working Range” of phenolphthalein. pH 8.15 to 10.15 = “Working Range” of methyl orange.

  6. p.06 Important note about the use of Indicator … Next …. Titration Curve (Book 2 p. 168 – 169) Indicator is actually an acid (a very very weak acid), only a minimal amount is added during a titration, so that the pH of the solution would not be affected.

More Related