1

Acids and Bases

Acids & Bases are one of the most important classes of chemicals

Acids and bases have been know to human for a long time Acids taste sour (in fruit), change colour of certain dye

Bases taste bitter and feel slippery (like in soap, lime water)

Acids and bases are widely present in nature, especially in plants, electrolyte balance in life system cycle etc

Acids and bases are widely used in industry for various purpose

Dissolving chemicals, e.g. HF, aqua regia (HCl:HNO3=3:1)

Reagents for producing various chemicals

Catalysing various types of reactions

Titration in volumetric analysis

etc

Chemical Reactions

2

Acids and Bases - Definition

Classical definition

Acids - Substances that, when dissolved in water, increase the concentration of H+ ions

e.g. HCl(g) H+(aq) + Cl-(aq)

Note: H+, which is a proton only (no e- ), is actually bond with water molecule forming H3O+, the rxn is

HCl(g) + H2O (l) H3O+(aq) + Cl-(aq)

For simplicity, we often use H+ instead of H3O+.

Bases - Substance that, when dissolved in water, increase the concentration of OH- ions

e.g. NaOH OH-(aq) + Na+(aq)

NH3 + H2O NH4+ + OH-

Brønsted-Lowry definition

Acid is proton donor and Base is proton acceptor

(because H+ is a proton and OH- of a base reacts with H+ giving water)

Chemical Reactions

H2O

H2O

3

Conjugate Acid and Base Pairs An acid & a base always work together to transfer proton (donate-accept). A substance

can function as an acid only if another substance behaves simultaneously as a base. When an acid or a base is dissolved in water, ions are released - this process involves

proton transfer. To mark the process and link the ions with its original acid or base, conjugate acid-base pairs are defined.

Acid and conjugate base always appear in pair; likewise base and conjugate acid appear in pair When an acid losses proton (H+) it becomes the conjugate base of that acid (e.g. HX to X -)

when a base receives a proton (H+) it becomes the conjugate acid of that base (H2O to H3O+)

If an acid dissolves in water, H2O is a base; if a base dissolves in water, H2O becomes an acid.

Chemical Reactions

remove H+

HX(aq) + H2O (l) X-(aq) + H3O+(aq) acid base conjugate base conjugate acid

add H+

remove H+

HCl(aq) + H2O(l) Cl-(aq) + H3O+(aq) acid base conjugate conjugate

base acidadd H+

add H+

NH3(aq) + H2O(l) NH4+(aq) + OH-(aq)

base acid conjugate conjugate acid base

remove H+

4

Strengths of Acids and Bases The strength of acids and bases

The strength of an acid is the ability to donate proton, or increase [H+] when acid is dissolved in water.

likewise, the ability to accept proton, or [OH-], determine the strength of a base

Common acids and their relative strengthsStrong acids, paired with bases with negligible basicity

- Able to completely transfer their proton to water

- Their conjugate bases are the weakest, with negligible tendency to accept proton

Weak acids, paired with week bases

- These acids are partially dissociated to ions

- Their conjugate bases are also weak, with limited ability of accepting proton

Acids with negligible acidity, paired with strong bases

- These class of acids, though carrying H, give out no [H+]

- Their conjugate bases, however, are strong bases Water can act as acid as well as base

Chemical Reactions

acid base

HCl Cl-

H2SO4 HSO4-

HNO3 NO3-

H3O H2O

HSO4 SO42-

H3PO4 H2PO4

HF F-

HC2H3O2 C2H3O2-

H2CO3 HCO3-

H2S HS-

H2PO4 HPO42-

NH4 NH3

HCO3 CO32-

HPO4 PO43-

H2O OH-

OH O2-

H2 H-

CH4 CH3-

acid

str

eng

th in

crea

se

base stren

gth

increase

negligible

weak

strong

neg

ligib

lew

eak

st

rong

5

Acid and Base Equilibrium

The extent of ionisation of an acid or a base in water Some acids (or bases) ionise in water completely, leaving no molecules behind

Other acids (or bases) ionise partially in water, forming an equilibrium between

molecules and ions

e.g. HF(aq) + H2O (l) F-(aq) + H3O+(aq) (1)

NH3(g) + H2O (l) NH4+(aq) + OH-(aq) (2)

The tendency of ionisation of an acid (or a base) varies with the type of acids, we

can use the concept of reaction equilibrium to indicate the degree of ionisation.

The ‘equilibrium constant’ used to describe the degree of ionisation of an acid is

called acid-dissociation constant, Ka, which is defined as

for equili. (1)

for equili. (2)

Chemical Reactions

[HF]

]][H[For

[HF]

]O][H[F -3

-

aeqa KKK

][NH

]][OH[NH

3

4

eqa KK

ions

molecule

The higher the Ka value, the higher

ion conc., the higher acidity/basicity

6

Quantifying the Strength of Acids and Bases

[H+] and [OH-] are the measure of the strengths of acids and bases We know that an acid when dissolved in water releases [H+] and a base gives [OH-] We also know that the strengths of an acid or a base depend on the [H+] and [OH-] It comes naturally that [H+] & [OH-] are used to indicate the strengths of acids/bases

The range of [H+] and [OH-]

Dilute aqueous solutions at 25°C always give,

Kw=[H+][OH-]=1.0x10-14 For an acid [H+]>[OH-], Kw=[H+][OH-]=1.0x10-14

For a base [OH-]>[H+], Kw=[H+][OH-]=1.0x10-14

For pure water, which is neutral

[H+]=[OH-]=1.0x10-7, Kw=[H+][OH-]=1.0x10-14

pH scale For convenience the low value of [H+] and [OH-], we use the scale of log10 [H+]

define pH= -log10[H+] Scale: 1-14. Acid pH=0-7 [H+]>[OH-]; strong acids have low pH

Base pH=7-14 [OH-]>[H+]; strong bases have high pH

Note: When using [OH-] (which is less used), we have pOH= -log10[OH-] (=14-pH)

Chemical Reactions

water can act as an acid as well as a baseat equilibrium H2O H+ + OH-

Equili. constant at 25 °C is found to be

Further examine other aqueous solution the same relation holds

14-

2

101.0]][OH[HO][H

]][OH[H

wK

In pure water [H2O] is constantKnown [H+], [OH-] can be calculated by this eqn.

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Calculation of pH

Example 1: Calculate pH of 0.05M HNO3 solution

HNO3 + H2O H3O+ + NO3-

HNO3 is a strong acid, HNO3 ionizes completely in water, i.e. [H3O+]= 0.05M

pH = - log10[0.05] = 1.3

Example 2: Calculate pH and pOH of 0.05M NaOH solution

NaOH + H2O Na+ + OH-

NaOH is a strong base, NaOH ionizes completely in water, i.e. [OH -]=0.05M,

Kw = [H3O+][OH-] = 1 x10-14 M2

[H3O+] = 1 x10-14 M2 / [OH-] = 1 x10-14 M2 / 0.05 M = 2 x 10-13 M

pH = - log10[2 x 10-13] = 12.7

pOH = 14 - pH = 14 - 12.7 = 1.3

(why is this result the same as that of example1?)

Chemical Reactions

8

Calculation of pH

Example 3: What is the [OH-], in mol/L, in a solution whose pH is 9.72?

Known: pH = - log10[H3O+] = 9.72 [H3O+] = 1.9 x 10-10 (mol/L)

for any aqueous solution Kw = [H3O+][OH- ] = 1.0 x 10-14 (mol/L)2

[OH- ] = Kw / [H3O+] = 11.0 x 10-14 (mol/L)2 / 1.9 x 10-10 (mol/L) = 5.3 x 10-5 (mol / L)

Example 4: The acid-dissociation constant, Ka, of hydrofluoric acid is 6.8x10-4. What is

the [H3O+] in a 2M HF solution? What is the pH of the solution?

HF(aq) + H2O (l) F-(aq) + H3O+(aq)

initial 2 0 0

at equili. 2 - x x x

By definition

Solve the eqn for x ( = [H3O+])

pH = - log10[H3O+] = - log10(0.0365) = 1.44

Chemical Reactions

43-

1086-2

[HF]

]O][H[F

.x

xxKK eqa

M 03650 108621086 442 .x.x.x

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Aqueous Equilibria and Some Applications In chemistry many aqueous systems involve equilibria

Human body fluids are in electrolyte equilibria in order to function properly Electrolyte: aqueous solutions that contain ions

Plants contain weak acids, which maintain right balance for plants to grow

Many properties of a solution that has ions are affected by its equilibrium state.

etc. (In a broad sense, harmony=balance=equilibria)

Many phenomena in chemistry can be studied by means of equilibria.

We will look at: The behaviour of an equilibrated electrolyte solution when other ions are added

Applications Buffer effect

Acid-base titration

Solubility of ionic substances and the factors affecting it

Chemical Equilibria

10

The Common-Ion Effect from Equilibrium Considering the following two cases

Case 1. What is the pH of 0.3M acetic acid HC2H3O2 solution, (Ka=1.8x10-5)?

HC2H3O2(aq) H+ (aq) + C2H3O2-(aq)

initial 0.3 0 0

at equilibrium 0.3-x x x

By definition

Solve eqn for x

Chemical Equilibria

5

232

-232 1081

-0.3

]OH[HC

]OH][C[H

.x

xxKa

6421032-log]-log[HpH

M 1032][H

3

3

..

.x

Note: HC2H3O2 is a weak acid (Ka<<1) and the water solutn of HC2H3O2 is an electrolyte solution.

11

The Common-Ion Effect from Equilibrium (cont’d)

Case 2. What is the pH of solutn contains 0.3M acetic acid HC2H3O2 & 0.3M NaC2H3O2?

HC2H3O2(aq) H+(aq) + C2H3O2-(aq)

initial 0.3 0 0

at equilibrium 0.3-x x 0.3+x

By definition

Solve equ for x

Compare cases 1 & 2:

The extent of ionisation of HC2H3O2 is reduced by the presence of NaC2H3O2 (which has

C2H3O2- ion in common with HC2H3O2)

This is called the Common-ion Effect. It works in many equilibrated electrolyte solutions

such as buffer solutions, solubility of ionic compounds etc.

Chemical Equilibria

5

232

-232 1081

-0.3

30

]OH[HC

]OH][C[H

.x

x.xKa

Note:

NaC2H3O2 ionises in water completely

NaC2H3O2(aq) Na+(aq) + C2H3O2-aq)

7441081-log]-log[HpHM 1081][H

5

5

...x

Note: The presence of NaC2H3O2 & Na+ does not change Ka value

Note: C2H3O2- is the conjugate base

of HC2H3O2

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Buffered Solutions Behaviour of a solution containing a weak conjugate acid-base pair

equilibrium of weak acid HX(aq) H+(aq) + X-(aq)

acid-dissociation constant

If a base, OH-, is added, OH-(aq) + HX(aq) H2O(aq) + X-(aq) [HX] & [X-]

If an acid, H+, is added, H+(aq) + X-(aq) HX(aq) [X-] & [HX]

When the addition of OH- or H+ is small compared to [HX] & [X-], the change to [HX] & [X-] is very

small, so does the ratio [HX] / [X-] the [H+] thus pH will remain almost constant.

A Buffered Solution (also called Buffer) contains a weak conjugate acid-base

pair. It can resist drastic change of pH upon the adding strong acid or base.

Buffers solutions are widely used in biology and biochemistry because of the

need of maintaining certain pH for some reactions/process to occur properly.

Note: Buffer solutions can be made for all pH ranges. The amount of acid or base it can

neutralise before pH begins to change (called buffer capacity) depends on the [HX] & [X-].

Chemical Equilibria

As the HC2H3O2 and C2H3O2

- pair in case 2

][X

[HX]][H

[HX]

]][X[H-

-

aa KK