CHE 141 1st Edition Lecture 15Current OutlineQuantifying Acid/Base Strength, Behavior, Structure- A strong acid fully ionizes with Ka>>1- A weak acid partially ionizes with Ka<1- The larger the Ka value, the stronger the acid- A strong base fully ionizes with Ka>>1 - A weak acid partially ionizes with Kb<1- The larger the Kb value, the stronger the base- We can rank the strength of the acids and bases according to their Ka or Kb values- Molecule structure affects acid-base behaviorTrends of Binary Acids H-X- For a generic binary acid, HX, the strength of the acid will depend on how easily the H+ is removed from the H-X bond- This is affected by two factors:o The polarity of the H-X bond: depends on the electronegativity difference between H and Xo The strength of the H-X bond: depends on the size of X (smaller atoms form stronger bonds)- Across a period the strength of binary acids increases due to increasing polarization of theH-X bondo Electronegativity of species X increaseso Higher electronegativity difference between H and Xo The H-X bond is more polarizedo Easier to lose H+o More H+ is ionizedo Acid is strongero H2O is a weaker acid than HF- Down a group the strength of binary acids increases due to decreasing strength of the H-X bondo The larger X iso The weaker the H-X bond iso The H is held less tightly o Easier to lose H+o Acid is strongerThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.o Consider HX where X=halogen, HI is the strongest acid HI>HBr>HCl>HFWhat about HF?- HCl, HBr, and HI are all strong acids- You might think HF would also be a strong acid due to the electronegativity difference between H and F- However, it does not fully dissociate:- H-F bond is very strong due to small size of F- H3O+ and F- tend to form hydrogen-bonded cluster H3O+, F-- HF is still highly corrosive and is acutely toxicTrends of Oxyacids- Acids containing OH groups are called oxyacids- For example, sulfuric acid contains two O-H bonds:- Consider a generic oxyacid H-O-Y, where Y is some atom which may in turn have other groups attached to it- Acid strength is determined by the polarization of the H-O bond- Increasing electronegativity of the Y atom (and any additional oxygen atoms bonded to it)o The more electronegative Y iso Pulls electron density from the O-H bond, further polarizing ito Increases the ease with which hydrogen atom is ionizedo Increase acid strength- Increasing number of O atoms bonded to Yo The more O atoms bonded to Yo Y becomes more electronegative o The H-O-Y bond becomes more polarized o Increase acid
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