Chapter-8A[1]

Chapter-8A[1] - Chapter 8: Acid-Base Equilibria Arhenius...

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Chapter 8: Acid-Base Equilibria Arhenius acid : a substance that increases the number of H + ions to more than that in pure water Example: Cu 2+ ( aq ) + H 2 O( l ) CuOH + ( aq ) + H + ( aq ) Arhenius base : a substance that increases the number of OH - ions to more than that in pure water Example: CN - ( aq ) + H 2 O( l ) HCN( aq ) +OH - ( aq ) Limitations : (1). These modified Arhenius acid/base definitions apply to aqueous solutions only. (2). What happens when the solvent is other than water? (3). Does not explain why some substances are strongly acidic or basic. New definitions of Acids/Bases Bronsted-Lowry Definition Lewis Definition Bronsted-Lowry Defintions : Acid is a substance that can donate a H + ion Base is a substance that can accept a H + ion Example: HCl( aq ) + H 2 O( l ) H 3 O + ( aq ) + Cl - ( aq ) Acid 1 Base 2 Acid 2 Base 1 1. Acid 1 and Base 1 are conjugate acid-base pair - Cl - is conjugate base of HCl
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Bronsted-Lowry Acids/Bases ..contd (3). Formula of conjugate base is obtained by subtracting H + from the formula of acid Formula of conjugate acid is obtained by adding H + to the formula of base (2). Acid 2, H 3 O + (aq), and base 2, H 2 O( l ), are conjugate acid-base pair H 3 O + ( aq ) is conjugate acid of H 2 O ( l ); H 2 O ( l ) is conjugate base of H 3 O + ( aq ) The above reaction is completely to the right. Then HCl ( aq ) is stronger acid than H 3 O + ( aq ) and H 2 O ( l ) is stronger base than Cl - ( aq ) Base 2 Acid 1 Example: HCl( aq ) + H 2 O( l ) H 3 O + ( aq ) + Cl - ( aq ) Acid 2 Base 1 Example 2 : CH 3 COOH( aq ) + H 2 O( l ) H 3 O + ( aq ) + CH 3 COO - ( aq ) Acid 1 Base 2 Acid 2 Base 1 This reaction is almost to the left . Only 0.4% of CH 3 COOH ( aq ) is ionized at 1M conc. CH3COO - ( aq ) is stronger base than H 2 O ( l ) CH 3 COOH ( aq ) is weaker acid than H 3 O + ( aq ) or H 3 O + ( aq ) is stronger acid than CH 3 COOH ( aq ) 3 3 - 3 - 3
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Base 2 Acid 1 Example 3 : H 2 O( l ) + NH 3 ( aq ) NH 4 + ( aq ) + OH - ( aq ) Acid 2 Base 1 NH 4 + (aq) is conjugate acid of NH 3 (aq) or NH 3 (aq) is conjugate base of NH 4 + (aq) H 2 O( l ) is conjugate acid of OH - (aq) or OH - (aq) is conjugate base of H 2 O( l ) Bronsted-Lowry Acids/Bases ..contd Example 4 : HCl( in NH 3 ) + NH 3 ( l ) NH 4 + ( in NH 3 ) + Cl - ( in NH 3 ) Acid 1 Base 2 Acid 2 Base 1 Bronsted-Lowry approach is not limited to aqueous solutions Summary: Any molecule or ion that contains hydrogen can serve as an acid if a strong enough base is available to accept that hydrogen. Any molecule or ion can act as a base if a strong enough acid is in the vicinity to force a hydrogen ion upon it.
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Bronsted-Lowry Acids/Bases ..contd Example 1 : H 2 O( l ) + H 2 O( l ) H 3 O + ( aq ) + OH - ( aq ) Acid 2 Base 1 Acid 1 Base 2 H 2 O( l ) is conjugate acid of OH - (aq) or OH - (aq) is conjugate base of H 2 O( l ) H 3 O + ( aq ) is conjugate acid of H 2 O ( l ); H 2 O ( l ) is conjugate base of H 3 O + ( aq ) Amphoterism: A substance acts as an acid as well as a base Example 2: HCO 3 - ( aq ) + H 2 O( l ) H 3 O + ( aq ) + CO 3 2- ( aq ) Acid 2 Base 1 Acid 1 Base 2 H 2 CO 3 ( aq ) + H 2 O( l ) H 3 O + ( aq ) + HCO 3 - ( aq ) Acid 2 Base 1 Acid 1 Base 2 HCO 3 - can act as a acid as well as a base
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Water and the pH Scale H 2 O( l ) + H 2 O( l ) H 3 O + ( aq ) + OH - ( aq ) auto ionization of water K w = [H 3 O + ( aq )][OH - ( aq )]= 1.0 x 10 -14 at 25 o C Let [H 3 O + ( aq )] = [OH - ( aq )] = y Then, y 2 = 1.0 x 10 -14 y = 10 -7 M In pure water, [H 3 O + ( aq )] = [OH - ( aq )] = 10 -7
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Chapter-8A[1] - Chapter 8: Acid-Base Equilibria Arhenius...

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