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Acid-Base Chemistry

Acid-Base Chemistry. Definitions Arrhenius Theory Bronsted -Lowry Theory Amphoteric / Amphiprotic Substances Polyprotic Acids. Earlier Historians. 1811 - Humphry Davy. Antoine Lavoisier. 1661 - Robert Boyle.

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Acid-Base Chemistry

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  1. Acid-Base Chemistry Definitions Arrhenius Theory Bronsted-Lowry Theory Amphoteric/Amphiprotic Substances Polyprotic Acids

  2. Earlier Historians 1811 - Humphry Davy Antoine Lavoisier 1661 - Robert Boyle Challenged Lavoisier’s belief that all acids contained oxygen by noting that Hydrochloric acid (HCl) did not contain oxygen but still was an acid. Other non-oxygen acids were also found HBr, HF, HI. Characterized acids as: corrosive, sour tasting, changes litmus from blue to red, and becomes less acidic when mixed with base. Characterized bases as: slippery feeling, turns litmus from red to blue, and becomes less basic when mixed with acid. Believed that all acids contained oxygen after studying several acids. Examples: Sulfuric acid (H2SO4) and Nitric acid (HNO3)

  3. 1884-1887 : Svante Arrhenius • Arrhenius proposed that water could dissolve many substances into their individual ions. • He suggested that acids are compounds that contain hydrogen and can dissolve into water to release hydrogen ions into solution. • He defined bases as substances that dissolve in water to release hydroxide ions into solutions. • The Arrhenius definition also explains Boyle’s observation that acids and bases counteract each other. This idea that a base can make an acid weaker, and vice versa was later found to be called neutralization.

  4. Arrhenius Definition • Acid = proton donorHA = H+ + A- • Base = hydroxide donorBOH = B+ + OH- • Dilemma: NH3

  5. Dilemma • Though Arrhenius helped explain the fundamentals of acid/base chemistry, unfortunately his theories have limits. For example, the Arrhenius definition does not explain why some substances, such as common baking soda (NaHCO3) or ammonia(NH3), can act like a base even though they do not contain hydroxide ions.

  6. 1923 - Johannes Brønsted and Thomas Lowry • Brønsted and Lowry separately proposed a new set of definitions for acids and bases which are known as either Brønsted acids and bases or Brønsted-Lowry acids and bases. • For Acids: Any substance that can donate a proton, H+ ion to a base. (Proton donor, or hydrogen-ion donor) • For Bases: Any substance that can accept a proton, H+ ion from an acid. (Proton acceptor or hydrogen-ion acceptor)

  7. The Brønsted-Lowry model of acid and bases helps develop the concept of Conjugate acid-base pairs. The part of the acid remaining when an acid donates a proton is called the conjugate base. The Acid formed when a base accepts the proton is called the conjugate acid. Strong acids have weak conjugate bases. Strong bases have weak conjugate acids.

  8. Bronsted-Lowry Definition • Acid = proton donor • Base = proton acceptorNH3 + H+ = NH4+

  9. Amphoteric Substances • An Amphoteric substance can act as either an acid or a base depending on their surroundings • Examples include: Water, Al(OH)3 and Be(OH)2 Amphiprotic Substances • An amphiprotic substance can accept and donate protons • Examples include: Water, hydrogen carbonate(HCO3-) and hydrogen sulfate(HSO4-)

  10. Polyprotic Acids • Are acids that are able to donate more than one proton per acid molecule • Example H2SO4- Sulfuric acid H2SO4 + H2O HSO4- + H3O+ Ka1= very large HSO4- + H2O SO42- + H3O+ Ka2=1.3X10-2 • Ka1 is greater than Ka2 • Polyprotic acids can be diprotic(able to donate 2 protons), triprotic(able to donate 3 protons) or can be more.

  11. Conjugate acid-base example Consider the reaction of acid hydrogen fluoride with water. HF+H2O HF is donating its proton to the water molecule. In the reaction a proton is transferred from HF to H2O : HF+H2O H3O+ + F- H2O is acting as a base in the reaction because it is accepting a proton. After it accepts the proton the water becomes a hydronium ion. It now has a proton, which it can donate, therefore it is an acid.

  12. Example continued • When a base accepts a proton, it becomes an acid because it now has a proton that it can donate. • When an acid donates a proton it becomes a base, because it now has room to accept a proton. • These are what we call conjugate pairs of acids and bases • So in this example the HF and F- are a conjugate acid-base pair and H2O and H3O+ are a conjugate acid-base pair.

  13. Another Example • H2SO4 + H2O • So in this example H2SO4 is the acid so it will get rid of a proton and form a conjugate base which is willing to accept a proton: HSO4- • H2Ois acting as the base so when it accepts a proton it becomes a conjugate acid: H3O+ • So the full reaction can be represented by: H2SO4 + H2O HSO4- + H3O+

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