Chöông 13
CAÂN BAÈNG ION CUÛA NÖÔÙC
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SÖÏ ÑIEÄN LY CUÛA NÖÔÙC VAØ TÍCH SOÁ ION CUÛA NÖÔÙC
• Nöôùc laø chaát ñieän ly yeáu H2O (l) H+ + OH- Haèng soá ñieän ly cuûa nöôùc
ÔÛ 220C
Do ñoä ñieän ly cuûa nöôùc quaù nhoû neân noàng ñoä cuûa H2O xem nhö khoâng ñoåi:
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• Tích soá ion cuûa nöôùc: Kn
• ÔÛ 220C, ta coù:
• Moâi tröôøng axit coù • Moâi tröôøng bazô coù • Moâi tröôøng trung tính coù
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LYÙ THUYEÁT ACID - BASE
• THUYEÁT ACID – BASE BRONSTED-LOWRY
• Döïa treân tính chaát proton, H+: khoâng coù lôùp voû electron, chæ laø haït nhaân neân kích thöôùc raát nhoû, H+ coù theå xaâm nhaäp saâu vaøo lôùp voû cuûa caùc ion, phaân töû khaùc ñeå thöc hieän phaûn öùng trao ñoåi ion.
• ÑÒNH NGHÓA: • Acid laø tieåu phaân cho proton, coøn base laø tieåu phaân
nhaän proton trong phaûn öùng.
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• Ví duï:
-
HCl H+ + Cl- H2SO4 H+ + HSO4
• Vì acid laø chaát nhöôøng H+ vaø base nhaän H+, neân
-
trong 2 ví duï treân ta coù caùc caëp acid, base: HCl/Cl- vaø H2SO4/HSO4
laø caëp
Nhöõng caëp acid/base nhö vaäy goïi acid/base lieân hôïp. Caùc acid, base Bronsted coù theå laø phaân töû trung hoaø, cation hoaëc anion.
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ACID
BASE
HCl H+ + Cl-
+ NH3 + H+ NH4
TRUNG HOAØ
H2O H+ + OH-
H2O + H+ H3O+
2- - H+ + SO4
CH3COO- +H+CH3COOH
ANION HSO4
Cl- + H+ HCl
HCO3
2- - H+ + CO3 + H+ + NH3
CATION NH4
H3O+ H+ + H2O
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• Do H+ khoâng toàn taïi ñöôïc ôû daïng töï do, neân caùc acid chæ cho proton khi coù base nhaän vaø ngöôïc laïi.
• Phaûn öùng trao ñoåi proton xaûy ra toång quaùt giöõa
hai caëp acid-base lieân hôïp nhö sau:
A1 + B2 A2 + B1
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Trong dung dòch, caùc phaân töû vaø ion do dung moâi ñieän ly ra cuõng ñoùng vai troø cuûa caëp acid-base lieân hôïp.
H3O+/H2O; H2O/OH-
Caùc hôïp chaát coù chöùa H+ luoân laø chaát löôõng tính, phuï thuoäc vaøo chaát phaûn öùng vôùi noù coù khaû naêng cho, nhaän H+ maïnh hay yeáu hôn noù.
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THUYEÁT ACID – BASE LEWIS
• ÑÒNH NGHÓA: • Base laø chaát cho caëp electron vaø acid laø chaát nhaän caëp electron ñeå taïo thaønh lieân keát hoùa hoïc.
• Khaùi nieäm naøy lieân quan ñeán lieân keát coäng hoùa
trò cho – nhaän.
• Acid Lewis:
• Laø nhöõng tieåu phaân coù dö maät ñoä ñieän tích döông, trong phaân töû (ion) coù caùc orbital troáng coù theå tieáp nhaän caëp e chuyeån ñeán töø base.
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• Ví duï: Ña soá caùc cation laø acid Lewis (Ag+, Co3+,
Cr3+…), hay caùc halogen, hydrua cuûa B, Al…
• Base Lewis:
• Laø nhöõng tieåu phaân coù khaû naêng cho ñi caëp e. Ví duï: Caùc anion (Cl-, Br-, OH-…), caùc phaân töû trung hoaø hoaëc ion trong thaønh phaàn coù caùc nguyeân töû coøn caëp e chöa lieân keát nhö N, O (NH3, röôïu, cetone). Ag+ + NH3 [Ag(NH3)2]+ Acid Base BF3 + F- [BF4]- HCl + NH3 NH4Cl
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CHÆ SOÁ HYDRO (pH)
• Quy öôùc:
• Trong caùc dung dòch coù noàng ñoä cao hoaëc caùc dung
dòch axit bazô maïnh thì:
• Moâi tröôøng axit pH < 7, bazô pH > 7 trung tính pH = 7
hay
• Ta coù:
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TÍNH pH CUÛA CAÙC DUNG DÒCH ÑIEÄN LY
• pH CUÛA DUNG DÒCH ACID MAÏNH
• Acid maïnh ñieän ly hoaøn toaøn
HA H+ + A-
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pH ACID YEÁU ÑÔN BAÄC
HA H+ + A- Luùc ñaàu Ca 0 0 Caân baèng Ca-x x x Haèng soá caân baèng ñieän ly ñöôïc goïi laø haèng soá axit, Ka
Do acid yeáu
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• pH DUNG DÒCH ACID YEÁU ÑA BAÄC
• Do K1>>K2>>K3 (ví duï vôùi H3PO4, K1=10-2,12, K2=10-7.21, K3=10-12.38), do ñoù ñeå tính pH, ta chæ tính cho baäc phaân ly ñaàu. Neân caùch tính cuõng gioáng tröôøng hôïp acid yeáu ñôn baäc.
• pH DUNG DÒCH BASE MAÏNH
MOH M+ + OH- COH- Cb
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• pH DUNG DÒCH BASE YEÁU ÑÔN BAÄC
• Laäp luaän töông töï tröôøng hôïp acid yeáu ñôn baäc. Ta coù:
• pH DUNG DÒCH BASE YEÁU ÑA BAÄC
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CHAÁT CHÆ THÒ MAØU
• Chaát chæ thò maøu coù nhieàu loaïi: Chaát chæ thò maøu pH,
chæ thò maøu Oxy hoùa – khöû, chæ thò maøu phöùc…ÔÛ ñaây ta xeùt chaát chæ thò maøu pH.
• Chaát chæ thò maøu pH laø caùc hôïp chaát hoaù hoïc coù khaû naêng thay ñoåi maøu theo pH (theo noàng ñoä H3O+, hay H+-theo Arrhenius). Thöôøng ñaây laø caùc acid hay base höõu cô yeáu.
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• Kyù hieäu chaát chæ thò maøu Acid yeáu laø HInd
HInd
H+
+ Ind-
Maøu daïng acid Maøu daïng base
IndOH
+ Ind+
Kyù hieäu chaát chæ thò maøu laø Base yeáu laø IndOH OH- Maøu daïng base Maøu daïng acid Maøu daïng acid khaùc vôùi maøu daïng base.
• Ví duï: phenolphthalein (HP) hay quì tím (HQ) + P-
HP
H+
Acid khoâng maøu Base maøu hoàng
HQ
H+
+ Q-
Acid maøu ñoû Base maøu xanh
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Haèng soá ñieän ly cuûa chaát chæ thò maøu
Chuyeån qua pH
CH+ taêng thì maøu daïng axit chieám öu theá, pH giaûm thì
ngöôïc laïi.
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DUNG DÒCH ÑEÄM
• Ñònh nghóa: Dung dòch ñeäm laø dung dòch coù giaù trò pH xaùc ñònh vaø haàu nhö khoâng thay ñoåi khi pha loaõng, hay theâm vaøo moät löôïng nhoû acid hoaëc base maïnh.
• Noùi chung dung dòch ñeäm ñöôïc taïo thaønh baèng caùch troän moät acid yeáu vôùi muoái cuûa noù (heä ñeäm acid) hoaëc troän moät base yeáu vôùi muoái cuûa noù (heä ñeäm base).
• Ví duï: • Heä ñeäm acid: CH3COOH + CH3COONa • Heä ñeäm base: NH4OH + NH4Cl
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Cô cheá taùc duïng cuûa dung dòch ñeäm
• Xeùt heä: CH3COOH CH3COO- + H+
(1) (2)
CH3COONa
CH3COO- + Na+
Khi theâm acid maïnh: HA H+ + A- thì theo nguyeân lyù Le Chatelier caân baèng (1) seõ dòch chuyeån theo chieàu nghòch (töùc laø H+ taùc duïng vôùi CH3COO- ôû (2) taïo CH3COOH) laøm giaûm H+.
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Khi theâm base maïnh: MOH M+ + OH- , thì OH- seõ keát hôïp vôùi H+ (ôû caân baèng 1) laøm giaûm noàng ñoä OH-, ñoàng thôøi caân baèng (1), do bò giaûm H+, cuõng seõ dòch chuyeån theo chieàu thuaän ñeå taïo ra H+.
Vôùi heä ñeäm base laäp luaän töông töï.
Toùm laïi, khi cho H+ hay OH- vaøo heä ñeäm thì pH dung dòch thay ñoåi raát ít.
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TÍNH pH DUNG DÒCH ÑEÄM
• Heä ñeäm acid • Ví duï heä acetate treân, goïi Cm vaø Ca laø noàng ñoä muoái vaø
acid.
• Khi heä ñaõ ñaït traïng thaùi caân baèng (ôû 1), ta coù:
• Trong ñoù,vì noàng ñoä muoái khoâng coù caùch bieät lôùn so vôùi
•
noàng ñoä acid, maø acid laø chaát keùm ñieän ly, do ñoù [CH3COO-] = Cm + Ca Cm (vì <<1)
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• Cuõng vì acid keùm ñieän ly, <<1:
• Thay vaøo coâng thöùc tính [H+] ñeå tính pH, ta coù:
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• Heä ñeäm base • Lyù luaän töông töï treân, ta coù:
• Pha cheá dung dòch ñeäm: • Tröôùc heát choïn axit hoaëc bazô coù pKa, 14-pKb gaàn vôùi pH caàn coù, sau ñoù tính tæ soá noàng ñoä muoái vaø acid (base) ñeå coù pH caàn thieát.
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Indicator Low pH color Transition pH range High pH color
Gentian violet (Methyl violet) yellow 0.0–2.0 blue-violet
Leucomalachite green (first transition) yellow 0.0–2.0 green
Thymol blue (first transition) red 1.2–2.8 yellow
red 2.9–4.0 yellow Methyl yellow
yellow 3.0–4.6 purple Bromophenol blue
blue-violet 3.0–5.0 red Congo red
red
3.1–4.4
yellow
Methyl orange
yellow 3.8–5.4 blue-green Bromocresol green
red 4.4–6.2 yellow Methyl red
red 4.5–8.3 blue Azolitmin
yellow 5.2–6.8 purple Bromocresol purple
yellow
6.0–7.6
blue
Bromothymol blue
yellow 6.8–8.4 red Phenol red
red 6.8–8.0 yellow Neutral red
7.3–8.7 colorless to reddish greenish to blue Naphtholphthalein
yellow 7.2–8.8 reddish-purple Cresol Red
yellow
8.0–9.6
blue
Thymol blue (second transition)
colorless 8.2–10.0 pink Phenolphthalein
colorless 9.3–10.5 blue Thymolphthalein
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yellow 10.2–12.0 red Alizarine Yellow R
Leucomalachite green (second transition) green 11.6–14 colorless
