Lect 4. Chemical Reactions-2010

Lect 4. Chemical Reactions-2010 - Classification of...

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Classification of Reactions Acid-Base Reactions Precipitation Reactions Oxidation-Reduction Reactions
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Electrolytes and Non Electrolytes Strong Electrolytes Weak Electrolytes Non Electrolytes
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Polarity of Water O H H
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Solvation of Ions
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Because of Solvation by water, many ionic compounds dissociate (ionize) into their constituent ions when dissolved in water Compounds that “freely” ionize into independent ions in aqueous solution are called electrolytes because their aqueous solutions are capable of conducting an electric current. NaCl(s) Na + (aq) + Cl - (aq) H 2 O
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Some molecular compounds also dissociate into ions when they react with water. ) aq ( Cl ) aq ( H ) aq ( HCl - + + Since the resulting solution is electrically conducting , the molecular substance is also classified as an electrolyte .
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Some molecular compounds dissolve but do not dissociate into ions. ) aq ( O H C ) glucose ( ) s ( O H C 6 12 6 O H 6 12 6 2 → These compounds are referred to as non-electrolytes since t hey dissolve in water to give a non-conducting solution. ) ( OH H C ethanol) ( ) ( OH H C 5 2 5 2 2 aq l O H
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A weak electrolyte is an electrolyte that dissolves in water to give a relatively small percentage of ions. ) aq ( OH ) aq ( NH ) aq ( OH NH 4 4 - + + Most soluble molecular compounds are either nonelectrolytes or weak electrolytes .
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Acids and Bases
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The Arrhenius Concept The Arrhenius concept defines acids as substances that produce hydrogen ions, H + , when dissolved in water. An example is nitric acid, HNO 3 , a molecular substance that dissolves in water to give H + and NO 3 - . ) aq ( NO ) aq ( H ) aq ( HNO 3 O H 3 2 - + + →
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The Arrhenius Concept The Arrhenius concept defines bases as substances that produce hydroxide ions, OH - , when dissolved in water. An example is sodium hydroxide, NaOH, a substance that dissolves in water to give OH - and Na + . H 2 O NaOH(s) Na + (aq) + OH - (aq)
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The molecular substance ammonia, NH 3 , is a base in the Arrhenius view, because it yields hydroxide ions when it reacts with water. ) aq ( OH ) aq ( NH ) l ( O H ) aq ( NH 4 2 3 - +  + + The Arrhenius Concept Because the reaction of NH 3 with water only reacts (at equilibrium) to a small extent (~5%) and produces relatively small amounts of OH - , it is described as a weak base (i.e. it is a weak electrolyte) water.
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Acetic acid, HC 2 H 3 O 2 , is an acid in the Arrhenius view, because it dissociates to produce H + ions when it reacts with water. ) ( ) ( ) ( 2 3 2 2 3 2 aq O H C aq H l O H HC - + + → The Arrhenius Concept However, HC 2 H 3 O 2 , is a weak acid because only a small amount (~4%) of the acetic acid molecules ionize to produce H + ions. i.e. it is a weak electrolyte in water. Most of the acetic acid remains in its unionized molecular form.
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The Brønsted-Lowry Concept The Brønsted - Lowry concept of acids and bases involves the transfer of a proton (H
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Lect 4. Chemical Reactions-2010 - Classification of...

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