Chapter 4 - Acids and Bases

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• According to the Brasted-Lowry definitions, a conjugate acid-base pair is any pair of molecules or ions that may be interconverted via proton transfer.

  • When an acid transmits a proton to a base, the acid is transformed to the base's conjugate.
  • A base is transformed to its corresponding acid when it takes a proton.
  • The fact that the constituents of a conjugate acid-base pair differ by a proton is important to these definitions.

• ^^The image attached shows the relationships among conjugate acid-base pairs by examining the reaction of hydrogen chloride with water to form chloride ion and the hydronium ion.^^

  

• The transfer of a proton from an acid to a base may be shown using a curved arrow, the same curved arrow symbol used in the image attached above as it depicts the relocation of electron pairs among resonance contributing structures.

  • We describe the Lewis structure of each reactant and product for acid-base reactions, displaying all valence electrons on reacting atoms.
  • Curved arrows are then used to depict the change in location of electron pairs during the reaction.
  • The new position of the electron pair is shown by the head of the curved arrow. A curving arrow beginning from a lone pair and pointing to a nearby atom.

• For example, it suggests the development of a new bond, whereas an arrow originating at a bonding electron pair and pointing toward a previously bound atom signals the breakdown of that link.

  • %%The curving arrow on the left in this equation indicates that an unshared pair of electrons on oxygen shifts position to establish a new covalent bond with hydrogen.%%
  • The curved arrow on the right indicates that the H!Cl link breaks and that its electron pair is completely given to chlorine, resulting in the formation of a chloride ion.
  • As a result of the HCl-H2O reaction, a proton is transported from HCl to H2O. An O!An H bond is formed and an H!Cl bond breaks throughout the process.

• It is critical to remember that arrows represent the flow of electrons, not atoms.

  

• %%The equilibrium constants can also be estimated using the pKa values (Ka).%%

  • For example, if an acid's pKa is close to zero, the equilibrium constant for the reaction of that acid protonation of water is close to one.
  • Acids with negative pKa values have equilibrium constants more than one, while acids with positive pKa values have equilibrium constants less than one.

• Each unit variation in pKa values reflects a tenfold increase or reduction in the strength of the acids under consideration.

• Values of pKa in aqueous solution ranging from 2 to 12 can be measured.

  • In aqueous solution, pKa values in the range of 2 to 12 may be determined quite precisely.
  • Because very powerful acids, such as HCl, HBr, and HI, are totally ionized in water, the only acid present in solutions of these acids is H3O1.
  • Less basic solvents such as acetic acid or mixes of water and sulfuric acid are employed for acids that are too strong to be reliably measured in water.

• ^^Although none of the halogen acids are entirely ionized in acetic acid, HI exhibits a higher degree of ionization than either HBr or HCl.^^

  • Transition state: refers to the highest energy point on a reaction coordinate diagram.
  • The chemical structure at this point is commonly called the activated complex.
  • The majority of chemical reactions take place as a result of collisions.

• %%Consider combining two compounds, A-H and B2, in a reaction vessel with a solvent.%%

  • Compounds A-H and B2 will travel through the solvent by jostling about, colliding with, and bouncing off individual solvent molecules.
  • On rare occasions, A-H and B2 will clash. The kinetic energy of the molecules determines their movement through the vessel (kinetic meaning motion).

• Higher-energy collisions occur when molecules with more kinetic energy collide.

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