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2. NSC1951 Lecture 18 Acids & Bases

3. Objectives Properties of acids and bases The pH scale
Distinguish between strong and weak acids and list the clinical uses of these acids
Distinguish between strong and weak bases and list the clinical uses of these acids
Understand neutralisation and the clinical applications of neutralisation

4. 1. PROPERTIES OF ACIDS & BASES
Produce hydrogen ions (H+) in H2O
Taste sour Marieb, Fig 15.1
Turn blue litmus (vegetable dye)  red
Act as electrolytes in solution
Neutralise solutions containing hydroxide ions (OH -)
React with several metals releasing H2(g)  corrosion
React with carbonates releasing CO2(g)
Destroy body tissue

5. Bases Produce or cause an increase in hydroxide ions (OH-) in H2O
Taste bitter Marieb, Fig 15.1
Turn red litmus  blue
Act as electrolytes in solution
Neutralise solutions containing hydrogen ions (H +)
Have a slippery, ‘soapy’ feel
Destroy body tissue/ dissolve fatty (lipid) material

6. 2. THE pH SCALE Ion Product of Water Pure H2O at 25°C
Some molecules ionise
H2O  H+ + OH-
[H+ ] = 1 x 10-7 M = [OH- ]

7.  Ion Product of H2O:
[H+ ] x [OH- ] = [1 x 10-7 ] x [1 x 10-7 ]
* Add exponents
= 1 x 10-14

8. Acidic solution [H+ ] > [OH- ] Neutral solution [H+ ] = [OH- ]
Basic solution
[H+ ] < [OH- ]
Timberlake, Fig 9.3

9.  acid-base concentration p  potential or Power H  Hydrogen
Using the pH scale
Exponential values for [H+ ] & [OH- ] inconvenient in a clinical workplace
Simplify  pH scale
 acid-base concentration
p  potential or Power
H  Hydrogen
Timberlake, Figs 9.4, 9.5 Marieb, Fig 2.12

10. pH describes [H+ ] & [OH- ]
Indicates if a fluid is :
0 Acidic [H+ ] = [OH- ] =10-14
7 Neutral [H+ ] = [OH- ] =10-7
14 Basic [H+ ] = [OH- ] = 100

11. On the pH scale, values below 7 are acidic, a value of 7 is neutral, and values above 7 are basic.
Marieb, 2.12

12. 3. STRENGTHS OF ACIDS Strong Acids (very few) Eg HCl Hydrochloric Acid
Marieb, Fig 26.11
Strong Acids (very few) Eg
HCl Hydrochloric Acid
~ Stomach acid
HNO3 Nitric Acid
~ May be used to cauterise warts
~Drugs, explosives, fertilisers, dyes
H2SO4 Sulphuric Acid
~  conc. to treat stomach hypoacidity
~ Fertilisers, dyes, glues

13. Strong acids are:
Strong electrolytes
~ 100% ionisation  good conductors
Severe burns to body tissue
*** Stomach lining protected against HCl by mucus

14. Dissociation in Water : Strong acids Polar covalent molecules  ions
Eg.
HCl(l) H+(aq) + Cl-(aq)
HNO3(l) H+ (aq) + NO3- (aq)
H2SO4(l) H+ (aq) + SO42- (aq)
H2O
H2O
H2O

15. Weak Acids (most acids in nature)
CH3COOH Acetic Acid
~ Antimicrobial solution  ears, plastics, dyes, insecticides
H2CO3 Carbonic Acid
~Bicarbonate buffer system, carbonated drinks
H3PO4 Phosphoric Acid
~ Drugs, fertilisers, soaps, detergents, animal feed
See lecture 19; Marieb, Fig 26.11

16. Dissociation in Water : Weak acids Polar covalent molecules
Weak acids are:
Weak electrolytes
Small % ionisation  weak conductors
Dissociation in Water : Weak acids
Polar covalent molecules
 Mainly stay as molecules

17. Dissociation in water : Weak acids (cont)
CH3COOH (l) H+ (aq) + CH3COO- (aq)
H2CO3 (l) H+ (aq) + HCO3-(aq)
H3PO4 (l) H+ (aq) + H2PO4- (aq)
H2O
H2O
H2O

18. Marieb, Fig 26.11

19. 4. STRENGTHS OF BASES Strong Bases Eg NaOH Sodium Hydroxide
~ Removes grease – drains, ovens
Mg(OH)2 Magnesium hydroxide
~ Antacid ~ Laxative
Al(OH)3 Aluminium hydroxide
~ Antacid
~ Absorbs toxins, gases,
~ Causes constipation

20. Strong bases are:
Strong electrolytes
~ 100% dissociation in water  good conductors
Severe damage to skin & eyes
(Group 1A elements)

21. Dissociation in Water : Strong bases Metal hydroxides  ions Eg.
NaOH(s) Na+(aq) OH-(aq)
Mg(OH)2(s) Mg 2 + (aq) OH- (aq)
Al(OH)3(s) Al 3+(aq) OH- (aq)
H2O
H2O
H2O

22. Weak Bases Eg
NH3 Ammonia
~ Waste product of protein break down in body.
CO3 2- In antacids
HCO3 – In antacids, buffers
HPO4 2- In buffers

23. Reaction with Water : Weak bases NH3(g) + H2O NH4 + (aq) + OH – (aq)
Weak bases are:
Weak electrolytes
Do not contain OH – but react with H2O  small numbers of OH –
Reaction with Water : Weak bases
NH3(g) + H2O NH4 + (aq) + OH – (aq)
HCO3 – (aq) + H2O H2CO3 (aq) OH-(aq)

24. 5. ACID-BASE NEUTRALISATION
Neutralisation Reaction
Acid + Base  Salt + Water
HCl + NaOH  NaCl + H2O
H+ + OH –  H2O
Neutralise each other
Must be equal concentrations

25. Antacids – clinical applications (Check for side effects!!)
~ Neutralise excess stomach acid
~ Raise stomach pH > 4
Pepsin inactive
~ Assist with ulcer treatment
~  solubility in H2O but still produce high % of ions

26. Eg
Mg(OH) 2 Milk of Magnesia & in Mylanta
2HCl + Mg(OH) 2  MgCl H2O
Al(OH) 3 In Mylanta
3HCl + Al(OH) 3  AlCl H2O

27. CaCO 3
2HCl + CaCO 3  CaCl H2O + CO 2 (g)
~Also a Ca 2 + supplement
Long term overuse  Ca 2 + levels
 risk kidney stones (renal calculi)

28. NaHCO 3 Baking Soda
Not recommended!!
HCl + NaHCO  NaCl + H2O + CO2 (g)
~ Elderly tend to OD
 Stomach can ‘explode’

29. Key Concepts: Acids and Bases Acid Base Ionization in water undergo
produce
produce
Neutralization
gives
H + ions
OH- ions
to form
OH- H+
100%
Small %
Salt
&
100%
Small %
product
Water
[H+] x [OH-]
Strong acid
Weak acid
Strong base
Weak base is pH

30. REFERENCES Cree & Rischmiller, 2001 Ch. 8 pp 205-216 Marieb, 2004

31. Review questions List the properties of acids & bases.
Discuss the pH scale: *Define the ion product of water & indicate how this is determines the pH scale * Use the pH scale to determine if a given solution is acidic, neutral or basic.
Distinguish between strong acids & weak acids: List clinical uses of these acids & write equations for their dissociation in water

32. Review questions (cont)
Distinguish between strong & weak bases; List clinical uses of these bases & write equations for their dissociation in water.
Complete simple equations for the neutralisation reaction of an acid & a base; Discuss clinical applications of acid-base neutralisation.