Acids and Bases 1

Acids and Bases 1 - Acids and Bases What are acids and...

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Acids and Bases
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02/21/08 Acids and Bases 1 2 What are acids and bases? Arrhenius: An acid can generate a proton (H + ) and a base can generate a hydroxide ion (OH ) in aqueous solution. Brőnsted-Lowry: An acid can donate a proton (H + ion) and a base can accept a proton In aqueous solution. The Arrhenius and Bronsted-Lowry models seem to lead to the same result, but the B-L approach is the most generally useful, and will be used in this course. Lewis: An acid can accept electrons to form a chemical bond (e.g., BF 3 ), while a base can donate electrons to form a chemical bond (e.g., NH 3 ).
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02/21/08 Acids and Bases 1 3 Conjugate Acid-Base Pairs Each acid has a conjugate base and each base a conjugate acid. Every acid base reaction has two conjugate acid base pairs. Thus, in the following, we have: HCO 3 (aq) + H 2 O( l ) CO 3 2- (aq) + H 3 O + (aq) This is more consistent with the definition of acid-base pairs, since the proton from HCO 3 - is donated to the solvent. Using this approach, one can readily track the acid and conjugate base pairs.
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02/21/08 Acids and Bases 1 4 In the above, HCO 3 acts as an acid , while CO 3 2- is the conjugate base , and H 2 O acts as the base , while H 3 O + is the conjugate acid . Another possible reaction is: HCO 3 (aq) + H 2 O ( l ) H 2 CO 3 (aq) + OH - (aq) Here, HCO 3 acts as a base , while H 2 CO 3 is the conjugate acid , and H 2 O acts as an acid , while OH - is the conjugate base . The first of these two reactions is important to owners of swimming pools, as it provides a way to stabilize the pH . Notice that some of the reagents can act as acid or base, depending on the circumstances. Materials capable of functioning as either acid or base are known as amphoteric .
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02/21/08 Acids and Bases 1 5 Some examples of amphoteric compounds SO 4 2- H 2 SO 4 HSO 4 - HPO 4 2- H 3 PO 4 H 2 PO 4 - CO 2 H 2 CO 3 HCO 3 - OH - H 3 O + H 2 O Conjugate Base Conjugate Acid Substance
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02/21/08 Acids and Bases 1 6 A more obvious example of a base is ammonia in solution: NH 3 (aq) + H 2 O( l ) NH 4 + (aq) + OH - (aq) Here NH 3 is a base , NH 4 + its conjugate acid . H 2 O here acts as an acid and OH - its conjugate base . The stronger the acid, the weaker its conjugate base. The stronger the base, the weaker its conjugate acid.
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02/21/08 Acids and Bases 1 7 Example (strong acid): HCl (aq) + H 2 O ( l ) H 3 O + (aq) + Cl (aq) Here, since the equilibrium is strongly biased toward the right, HCl is considered to be a stronger acid than H 3 O + , and H 2 O is a stronger base than Cl - . View this as follows: HCl easily loses a proton in solution, whereas the reverse process, the protonation of Cl - ions to make HCl molecules in solution is very unlikely.
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02/21/08 Acids and Bases 1 8 For practical purposes, we define HCl as a strong acid, because the equilibrium constant for the above reaction: 3 [ ( )][ ( )] [ ( )][ ( )] 1 [ ( )] [ ( )] H O aq Cl aq H aq Cl aq K HCl aq HCl aq + - + - = Often, we make the assumption that the reaction shifts
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This note was uploaded on 04/29/2008 for the course HIST 101 taught by Professor Wormer during the Spring '08 term at UPenn.

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Acids and Bases 1 - Acids and Bases What are acids and...

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