化学专业英语《Chemistry English》课程教学资源（PPT课件）Lecture 06

Acids and basesAcids and baseskey theory: Bronsted-Lowry theory of acids and bases:Acid-base reactions are ones in which protons are given and takenAcidsareprotondonorsBases are proton acceptors

1 Acids and bases Acids and bases key theory: Brønsted-Lowry theory of acids and bases: Acid-base reactions are ones in which protons are given and taken. Acids are proton donors Bases are proton acceptors

Acids and basesConjugate pairsEvery acid has what is called a conjugate base. This is particleleft when the acid has given away its proton. In the same way,every base has a conjugate acid, which is the particle made whenthe proton has arrived. So the conjugate base of HClis CI andthe conjugate acid of ammonia is NH t.Familiar conjugate pairsAcidBaseH,0OH-H,O+H0H,SO4HSO4SO,2-HSO4CH,COOHCH,COOHNO3NO3H,CO3HCO3CO,2-HCO32NH4+NH3

2 Acids and bases Conjugate pairs Every acid has what is called a conjugate base. This is particle left when the acid has given away its proton. In the same way, every base has a conjugate acid, which is the particle made when the proton has arrived. So the conjugate base of HCl is Cl- and the conjugate acid of ammonia is NH4 + . Familiar conjugate pairs Acid Base H2O OH￾H3O+ H2O H2SO4 HSO4 - HSO4 - SO4 2- CH3COOH CH3COO￾HNO3 NO3 - H2CO3 HCO3 - HCO3 - CO3 2- NH4 + NH3

Acids and basesK.=5.55×108H,O+ + ClHCI + H,OCONJUGATECONJUGATEBASEACIDACIDBASEK_=3.12×10-8NH + + OHH,O+NHTo be a base, a substanceTo be able to give protons,must have a lone pair whichan acid must have acan become a sigma pair ashydrogen atom which canit accepts a protonbe lost:e.g. :NH3e.g. HCl; H,SO43

3 Acids and bases To be able to give protons, an acid must have a hydrogen atom which can be lost: e.g. HCl; H2SO4 To be a base, a substance must have a lone pair which can become a sigma pair as it accepts a proton e.g. :NH3

Acids and basesK.=5.55×108H,O+ + CHCI + H,O→CONJUGATECONJUGATEBASEACIDACIDBASEK,=3.12×10-8NH + + OHH,O +NHAmpholytesThe water in the first reaction above is acting as a base. One of itslonepairs accepts aproton.In the second reaction the water is being an acid, giving one of itsprotons to ammonia.Substances like water, with hydrogens that can be lost and lonepairs that can be protonated, are called ampholytes. They can beeither acids or bases depending on what they react with.You can see some other common ampholytes in the list on the topright, because they appear in both the acid and base columns.Even ammonia can be an ampholyte in the right conditions.having NH+ as its conjugate acid and NH, as its conjugate base

4 Acids and bases The water in the first reaction above is acting as a base. One of its lone pairs accepts a proton. In the second reaction the water is being an acid, giving one of its protons to ammonia. Substances like water, with hydrogens that can be lost and lone pairs that can be protonated, are called ampholytes. They can be either acids or bases depending on what they react with. You can see some other common ampholytes in the list on the top right, because they appear in both the acid and base columns. Even ammonia can be an ampholyte in the right conditions, having NH4 + as its conjugate acid and NH2 - as its conjugate base. Ampholytes

Acids and basesSTRONGANDWEAKACIDSANDBASESThe equilibrium constants for the two reactions above are verydifferent.The first one, 5.5 X 108, tells us that the reaction lies well over to theright. That means that there are practically no hydrogen chloridemolecules left because they have almost all become ions. Thissolution would conduct strongly and so the hydrochloric acid isdescribed as a strong acid.In the second reaction the constant is 3.12 X 10-8. This tells us thatabout one in a thousand ammonia molecules have reacted withwater. There are hardly any ions in this solution and so it conductsweakly. Ammonia is called a weak baseSo in water strong acids and bases exist mainly as ions, weak acidsand bases exist mainly as molecules.5

5 Acids and bases STRONG AND WEAK ACIDS AND BASES The equilibrium constants for the two reactions above are very different. The first one, 5.5×108 , tells us that the reaction lies well over to the right. That means that there are practically no hydrogen chloride molecules left because they have almost all become ions. This solution would conduct strongly and so the hydrochloric acid is described as a strong acid. In the second reaction the constant is 3.12×10-8 . This tells us that about one in a thousand ammonia molecules have reacted with water. There are hardly any ions in this solution and so it conducts weakly. Ammonia is called a weak base. So in water strong acids and bases exist mainly as ions, weak acids and bases exist mainly as molecules

Acids and basesCommon strong acids are:HCl; H,SO4; HNO3Common weak acids are:ethanoic, CH,COOH; carbonic, H,CO3Common strong bases are:NaOH; KOHAmmonia is a weak base6

6 Acids and bases Common strong acids are: HCl; H2SO4 ; HNO3 Common weak acids are: ethanoic, CH3COOH; carbonic, H2CO3 Common strong bases are: NaOH; KOH Ammonia is a weak base

Acids and basesWordsWords and Expressionsdonor; acceptor; donate; acceptconjugate: conjugate acid; conjugate baseAmpholyte7

7 Acids and bases Words donor; acceptor; donate; accept conjugate: conjugate acid; conjugate base Ampholyte Words and Expressions

Strength of acidsStrength ofacidsThe strength of an acid (how well it protonates) is measured bythe equilibrium constant for the reaction:HA+H,O →H.O++A[H3O+] × [A-][H,O] is nearly constant at 55.5 mol[HA] × [H2O]dm-3 in most bench solutions, so thisexpression can be simplifiedKa= K× 55.5_[H30+] ×[A-][HA]K, is called the acidity or dissociation constant for the acid.If K, is greater than 102 the acid is strong, while ifit is lessthan 10-2 the acid is weak.8

8 Strength of acids Strength of acids The strength of an acid (how well it protonates) is measured by the equilibrium constant for the reaction: HA + H2O → H3O+ + A- [H2O] is nearly constant at 55.5 mol dm-3 in most bench solutions, so this expression can be simplified Ka is called the acidity or dissociation constant for the acid. If Ka is greater than 102 the acid is strong, while if it is less than 10-2 the acid is weak

Strength of acidsIn the same way, it is possibleFor an acid HA:to write a Kh expression for aHA+H2O=H3O++A-base and a Kw expression for[H3O+] [A-]water. Compare the three:Ka=[HA]For the conjugate base A-:For waterA-+H2O=HA++OH-H2O+H2O=H3O++OH[HA] [OH-]Kb=Kw= [H3O+] [OH-][A-]Now look what happens[HAT[OH-][H3O+][A-]KaXKp=when we multiply the[AL[HATfirst two together and= [H3O+] [OH-] = Kwcancel out terms on theKw is called the dissociation constant fortop and bottom lineswater and has the value91 X 10-14 mol2 dm-6 at 250C

9 Strength of acids For an acid HA: For the conjugate base A- : In the same way, it is possible to write a Kb expression for a base and a Kw expression for water. Compare the three: For water Now look what happens when we multiply the first two together and cancel out terms on the top and bottom lines: Kw is called the dissociation constant for water and has the value 1×10-14 mol2 dm-6 at 25oC

Strength of acidsComplementary strengths of acids and their conjugate bases.K, X Kp = Kw, which is a constant. So it follows that if K, is large.K, must be small, and if K, is small Kp must be large. This meansthat if an acid is strong, its conjugate base must be weak, and viceversa.So strong acids have weak conjugate bases10

10 Strength of acids Complementary strengths of acids and their conjugate bases. Ka×Kb = Kw, which is a constant. So it follows that if Ka is large, Kb must be small, and if Ka is small Kb must be large. This means that if an acid is strong, its conjugate base must be weak, and vice versa. So strong acids have weak conjugate bases