《分析化学》课程教学资源（课件讲稿，英文）Chapter 4

归东理工大军 Analytical Chemistry To master the requirements of the titration error; To know about the classical applications and calculation of all kinds of titrations. 2022/10/26 2

Analytical Chemistry ØTo master the requirements of the titration error; Ø To know about the classical applications and calculation of all kinds of titrations. 2022/10/26 2

G 归东龙子大图 Analytical Chemistry $4.1 Acid-base equilibria 1.Acid-base theory The acidity or basicity of a solution is frequently an important factor in chemical reactions.Fundamental acid-base equilibria are important in understanding acid-base titration and the effect of acids on chemical species and reactions. According to Bronsted theory: An acid is a substance that can give up protons; A base is any compound or ion that can accept protons. 2022/10/26

Analytical Chemistry §4.1 Acid-base equilibria 1. Acid-base theory The acidity or basicity of a solution is frequently an important factor in chemical reactions. Fundamental acid-base equilibria are important in understanding acid-base titration and the effect of acids on chemical species and reactions. According to Bronsted theory: ØAn acid is a substance that can give up protons; ØA base is any compound or ion that can accept protons. 2022/10/26 3

归东理工大军 Analytical Chemistry A strong acid or base is completely dissociated in aqueous solution; A weak acid is one that is only partially dissociated in water,all weak acids HA react with water by donating a proton to h2O: HA+H,OU H.O+A The acid dissociation constant is K,H][A】 [HA] 2022/10/26

Analytical Chemistry ØA strong acid or base is completely dissociated in aqueous solution; ØA weak acid is one that is only partially dissociated in water, all weak acids HA react with water by donating a proton to H2O: The acid dissociation constant is 2022/10/26 4

G 归东理子大溪 Analytical Chemistry For a strong acid,the k is large and small for a weak acid. And the base dissociation constant is expressed as Aute that the acid and the base constitute conjugate acid-base pair. And for any acid-base pair, K。Kb=K for polyprotic acids,such as H3A, Kal xKb3 =Ka2 xKb2 =Ka3 XKo1=Kw 2022/10/26 5

Analytical Chemistry For a strong acid, the Ka is large and small for a weak acid. And the base dissociation constant is expressed as Kb Note . that the acid and the base constitute conjugate acid-base pair. And for any acid-base pair, for polyprotic acids, such as H3A, 2022/10/26 5

归东龙工大军 Analytical Chemistry 2.Proton balance equation Acid-base reactions essentially involve the proton transferring process. And the number of protons that the acid gives up is always equal to that the base accepts. For example,in a Na,CO3 solution,the proton balance equation is: [H]+[HCO3]+2[HCO3]=[OH] For c mol/LNaH2PO solution, PBE [H]+〔H3P04)=(HP042-)+2P043-)+〔OH) 2022/10/26

Analytical Chemistry 2. Proton balance equation Acid-base reactions essentially involve the proton transferring process. And the number of protons that the acid gives up is always equal to that the base accepts. For example, in a Na2CO3 solution, the proton balance equation is: For c mol/LNaH2PO4 solution, PBE [H+ ]+ 〔H3PO4〕 = 〔HPO4 2-〕+ 2[PO4 3-〕 +〔OH-〕 2022/10/26 6

G 归东我子大军 Analytical Chemistry 3.Calculation of pH (1)Solutions of strong acids and bases In an aqueous solution of strong acid (HA),the proton balance equation is [H]=[A]+[OH] Suppose the concentration of the acid is c mol/L, as we know the strong acids dissociate completely in aqueous solution,if c>10-6 mol/L,then, c=[H]=[A] 2022/10/26

Analytical Chemistry 3. Calculation of pH (1) Solutions of strong acids and bases In an aqueous solution of strong acid (HA), the proton balance equation is Suppose the concentration of the acid is c mol/L, as we know the strong acids dissociate completely in aqueous solution, if c>10-6 mol/L, then, 2022/10/26 7

归东龙工大军 Analytical Chemistry if c<10-6 mol/L, The autoprotolysis of water should not be neglected, [H]=c+ H] [H1=c+e+4K 2 For strong bases,similar equations can be derived for calculation of [OH-]. 2022/10/26

Analytical Chemistry if c<10-6 mol/L, The autoprotolysis of water should not be neglected, For strong bases, similar equations can be derived for calculation of [OH- ]. 2022/10/26 8

G 归东理子大图 Analytical Chemistry (2)Solutions of weak acids and bases The pH of a weak acid can be calculated from the dissociation constant,K.Similarly,the pH of a weak base can be calculated from its Kp value. For a strong bases,similar equations can be derived for calculation of [OH-]. For a weak acid,HA,we can write its proton balance equation: H]=[A]+[OH] 2022/10/26

Analytical Chemistry (2) Solutions of weak acids and bases The pH of a weak acid can be calculated from the dissociation constant, Ka . Similarly, the pH of a weak base can be calculated from its Kb value. For a strong bases, similar equations can be derived for calculation of [OH- ]. For a weak acid, HA, we can write its proton balance equation: 2022/10/26 9

归东理工大 Analytical Chemistry H']= Ko[HA]K [H] [H] We obtain [H']=K[HA]+K If K,is not extremely small,that is cK。320K The K can be neglected,so [H']K[HA]=K.(e-[H']) H=K+K+4K,c 2 2022/10/26 10

Analytical Chemistry We obtain If Ka is not extremely small, that is The Kw can be neglected, so 2022/10/26 10

G 归东龙子大图 Analytical Chemistry If c/K,≥400，the concentration of HA can be considered c,the analytical concentration of the weak acid,then [H']=Kc This is the simplified equation. Similarly,the simplified equation of a weak base concentration is [OH ]=K,c 2022/10/26 11

Analytical Chemistry If c/Ka ≥400, the concentration of HA can be considered c, the analytical concentration of the weak acid, then This is the simplified equation. Similarly, the simplified equation of a weak base concentration is 2022/10/26 11