1. Acid, Base and Buffers

2. What are Acid and Bases?
Definition of Svante Arrhenius (Sweden) in 1884
“An Acid is a substance that can release a proton or hydrogen ion (H+) when dissolved in water”
HCl H+ + Cl-
“ A Base is a substance that can release a Hydroxyl ion when dissolved in water”
NaOH Na+ +OH-

3. According to Thomas Lowry (England) or J. N
According to Thomas Lowry (England) or J.N. Broّnsted (Denmark) working independently in 1923:
“An Acid is a material that donates a proton:
HCl H+ + Cl-
CH3COOH CH3COO- + H+
NH NH3 + H+
“A Base is a material that can accept a proton
OH- + H H2O
CH3COO- + H CH3COOH
NH3 + H NH4+
Every ion dissociation that involves a hydrogen or hydroxide ion could be considered an acid- base reaction

4. The G.N. Lewis (1923) idea of acids and bases is broader than the Lowry- Broّnsted model.
The Lewis definitions are:
“Acids are electron pair acceptors.
H+ + e H
“Bases are electron pair donors.
OH OH + e-

5. Acid/Base definitions
Definition #1: Arrhenius (traditional)
Acids – produce H+ ions (or hydronium ions H3O+)
Bases – produce OH- ions
(problem: some bases don’t have hydroxide ions!)

6. Acid/Base Definitions
Definition #2: Brønsted – Lowry
Acids – proton donor
Bases – proton acceptor
A “proton” is really just a hydrogen atom that has lost it’s electron!

7. Acids & Base Definitions
Definition #3 – Lewis
Lewis acid - a substance that accepts an electron pair
Lewis base - a substance that donates an electron pair

8. One can accept a proton and the other can donate a proton.
Each ionizable pair has a proton donor and a proton acceptor. Acids are paired with bases.
One can accept a proton and the other can donate a proton.
Each acid has a proton available (an ionizable hydrogen) and another part, called the conjugate base.
When the acid ionizes, the hydrogen ion is the acid and the rest of the original acid is the conjugate base e.g:
Nitric acid, HNO3, dissociates (splits) into a hydrogen ion and a nitrate ion. The hydrogen almost immediately joins to a water molecule to make a hydronium ion.
The nitrate ion is the conjugate base of the hydrogen ion. In the second part of the reaction, water is a base (because it can accept a proton) and the hydronium ion is its conjugate base.
HNO3 + H2O NO H3O+
acid base conjugate conjugate base acid

9. A Brønsted-Lowry acid is a proton donor
A Brønsted-Lowry base is a proton acceptor
conjugate acid
conjugate base
base
acid

10. Conjugate Pairs

11. Label the acid, base, conjugate acid, and conjugate base in each reaction:
HCl + OH-    Cl- + H2O
H2O + H2SO4    HSO4- + H3O+

12. Properties of acids
Acids release a hydrogen ion into water (aqueous) solution
Acids neutralize bases in a neutralization reaction
An acid and a base combine to make a  salt and water. A salt is any ionic compound that could be made with the anion of an acid and the cation of a base.
HCl + NaOH NaCl + H2O
Acids corrode active metals.
Acids turn blue litmus to red.
Litmus is the oldest known pH indicator. It is red in acid and blue in base.
Acids taste sour.

13. Some Properties of Acids
Produce H+ (as H3O+) ions in water (the hydronium ion is a hydrogen ion attached to a water molecule)
Taste sour
Corrode metals
Electrolytes
React with bases to form a salt and water
pH is less than 7
Turns blue litmus paper to red “Blue to Red A-CID”

14. Examples of Some Acids Stomach acid is hydrochloric acid.
Acetic acid is the acid ingredient in vinegar.
Citrus fruits such as lemons, grapefruit, oranges, and limes have citric acid in the juice.
Sour milk, sour cream, yogurt, and cottage cheese have lactic acid from the fermentation of the sugar lactose.
Carbon dioxide formed in the body, dissolves in water to form an acid carbonic acid:
CO2 + H2O H2CO3 [carbonic acid]
Proteins are acidic at pH below their isoelectric point, and can give out hydrogen ions.

15. Properties of bases Bases release a hydroxide ion into water solution
Bases neutralize acids in a neutralization reaction.
HCl + NaOH NaCl + H2O
Bases denature protein.
Bases turn red litmus to blue.
Bases taste bitter.

16. Some Properties of Bases
Produce OH- ions in water
Taste bitter, chalky
Are electrolytes
Feel soapy, slippery
React with acids to form salts and water
pH greater than 7
Turns red litmus paper to blue “Basic Blue”

17. pH
pH is a way to express acidity or alkalinity of an aqueous solution.
It is the negative log of the hydrogen ion concentration.
It is a measure of the concentration of protons in solution [H3O+]
pH = - log 10 [H+] or
pH = - log 10 [H3O+]
pH Scale:
10-14 M H3O+
10-1 M H3O+
10-7 M H3O+
Basic
Acidic 7
Neutral

18. The pH scale is a way of expressing the strength of acids and bases
The pH scale is a way of expressing the strength of acids and bases. Instead of using very small numbers, we just use the NEGATIVE power of 10 on the Molarity of the H+ (or OH-) ion. Under 7 = acid = neutral Over 7 = base

19. (Remember that the [ ] mean Molarity)
Calculating the pH
pH = - log [H+]
(Remember that the [ ] mean Molarity)
Example: If [H+] = 1 X pH = - log 1 X 10-10
pH = - (- 10)
pH = 10
Example: If [H+] = 1.8 X 10-5 pH = - log 1.8 X 10-5
pH = - (- 4.74)
pH = 4.74

20. i.e pulls electrons towards
Dissociation of Water
Dissociation of water
H2O H+ + OH-
Water Proton + Hydroxyl ion
H+ + H2O H3O+
Proton + Water Hydronium ion
In water there is a strong partial negative charge on the side of the oxygen atom and partial positive charge on the hydrogen side
H H O
Each hydrogen ion unites with a water molecule to produce a hydronium ion, H3O+ - +
O2 is electronegative
i.e pulls electrons towards
its self

21. H2O can function as both an ACID and a BASE.
In pure water there can be AUTOIONIZATION
Equilibrium constant for water = Kw
Kw = [H3O+] [OH-] = x at 25 oC

22. Kw = [H3O+] [OH-] = 1.00 x 10-14 at 25 oC
Autoionization
Kw = [H3O+] [OH-] = x at 25 oC
In a neutral solution [H3O+] = [OH-]
so Kw = [H3O+]2 = [OH-]2
and so [H3O+] = [OH-] = 1.00 x 10-7 M

23. 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- ]

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

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

26. 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

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

28. Blood 7.4 Pure water 7.0 e.g At Neutral pH, H3O is 10-7 M
pH = -log [H3O+]
pH = -log [10-7] = -[-7] = +7
pH is a log scale and one number represents a H3O+ concentration that is either 10 times greater or ten times smaller in magnitude then the next. e.g M is ten times greater than pH
H+ conc [M]
Gastric Juice
1 - 2
to 10-2
Coca cola 3
10-3
Urine
5-8
10-5 to 10-8
Saliva
6.4
4 x 10-7
Blood
7.4
4 x10-8
Pure water
7.0
10-7

29. Strong acids and strong bases
Strong acids that are almost one hundred percent ionized in aqueous solution . HCl H Cl-
eg:
HNO3 - nitric acid HCl - hydrochloric acid H2SO4 - sulfuric acid HClO4 - perchloric acid
Strong bases are almost one hundred percent ionized in aqueous solution. NaOH Na OH-
LiOH - lithium hydroxide NaOH - sodium hydroxide KOH - potassium hydroxide
Mg(OH)2 - magnesium hydroxide Ca(OH)2 - calcium hydroxide

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

31. 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

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

33. Weak Acids and Weak Bases
Weak acids and weak bases dissociate partially in aqueous solution
HA H+ + A-
CH3COOH CH3COO H+
BOH B+ + OH-
NH3 + H NH4+

34. Weak Acids (most acids in nature)
CH3COOH Acetic Acid
~ Vaginal jellies, antimicrobial solution  ears, plastics, dyes, insecticides
H2CO3 Carbonic Acid
~Bicarbonate buffer system, carbonated drinks
H3PO4 Phosphoric Acid
~ Drugs, fertilisers, soaps, detergents, animal feed

35. 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

37. 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)

38. Key Concepts: Acids and Bases Base Acid 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