Tài liệu Chapter 6: Chemical Equilibrium ppt

Acids and Bases - electrolytes can be weak or strong 1.. Conjugate acids and bases: when an acid donates a proton in an acid-base reaction, it forms a con Ugate base.. Likewise, when a b

Chapter 6:

Chemical Equilibrium

l The Chemical Composition of Aqueous Solutions

Acids and Bases - electrolytes (can be weak or strong)

1 Bronsted-Lowery Theory:

> acids are proton donors

> bases are proton acceptors

2 Strong Acids and bases almost completely

ionized

3 Weak acids and bases (poorly ionized)

see Table 6-2 (memorize strong acids and bases)

a Conjugate acids and bases:

when an acid donates a proton in an acid-base

reaction, it forms a con Ugate base

Likewise, when a base accepts a proton in an acid-base reaction it forms a con Uugate acid

H,O (base) + HCOOH (acid) <===> H,O* (conj acid) + HCOO-(con) base)

b Amphiprotic solvents can be either acids or

bases:

H,O (acid) + CH,NH, (base) <===> OH: (conj

base) + CH,NH,*(con acid)

c Autoprotolysis: when amphiprotic

solvents undergo self-ionization

Acid and Base Strengths

Acid and base strengths may be determined by using

> a differentiating solvent

That is, selecting a solvent which will accept

(or donate) a proton to one acid (or base) but

not another (example: use of glacial acetic acid

as solvent to compare HCIO, and HCl acid

>a differentiating solvent

CH.COOH +HCIO, <====>CH,COOH,* + CIO,

CH.COOH +HCl <====> CH,COOH,* + CI

> aleveling solvent (H,O)

HCIO,+H,O —— H,Ot + CIO,

Table: Acids and Bases

HCIO¿ + H;O HCI + HO

AI(H;O)¿† + H;O HC2H4O› + H;O

Strongest base

Any Questions?

ll Chemical Equilibrium:

The ratio of the molar concentrations of reactants and

products is a constant at certain temperature

H,O+HCOOH <===> H,O* + HCOO-

> Partial pressure in atm if species is a gas

> Unity if species is a pure liquid, pure solid, or pure solvent (solvent in an extremely dilute

solution)

lil Types of Equilibrium Constants (See Table 6-1)

A The ion-product for water

Water is poorly dissociated, but does undergo autoprotolysis:

2 H,O <===> H,O" + OH: and,

recalling that molar concentrations = 1 for

B Solubility Product Constants

For the solubility of Fe(OH),, we can write

Fe(OH), <===> Fe** (aq) + 3 OH: (aq)

again, recalling that the molar concentration

of a pure solid = 1

we can write the solubility product:

[Fe*?] [OH:]? = K,, = 4 x 10-38

Type of Equilibrium Name and Symbol

of Equilibrium Constant Typical Example

Equilibrium-Constant Expression

Dissociation of water

Heterogeneous equilibrium

between a slightly soluble

substance and its ions in

Distribution equilibrium for a

solute between immiscible

Any Questions?

The common ion effect

¢ Based upon Le Chatelier's Principle, if we add an ion common to the solid to the

medium, the equilibrium will shift to make

the solid less soluble

THIS IS ALSO CALLED A MASS-ACTION EFFECT

Example 6-1: The common ion effect

1 The solubility, x, of Fe(OH), in pure water

({[OH-] = 10-’ M) is:

Fe(OH), <===> Fe** (aq) + 3 OH: (aq)

[Fe?”]IOH'° = [Fe”](10-? M)° = 4X 10-3, and [Fe3J= 4X 10M

2 when add © 00 ¥) \\aOl, [OH] = 1.00 M, then

[Fe*?][1.00 MỊ]° = [Fe?°](1.00)° = 4X 10 and

[Fe73J] =4 X 103M

The common ion effect

¢ In the case of the ferric hydroxide, addition

of 1.00 M KOH will make the Fe(OH) (solid)

10-2' times less soluble.

How many grams of Ba(IO;), (487 g/mol) can be dissolved in 500 mL of

water at 25°C?

The solubility-product constant for Ba(IO,), is 1.57 & 107? (Appendix 2)

The equilibrium between the solid and its tons in solution is described by the equation

Ba(IO,).(s) —— Ba** + 210;

Stoichiometric ratio | mole I mole 2 mole

K,, = [Ba**][IO;}? = 1.57 x 107°

The equation describing the equilibrium reveals that | mol of Ba** is formed

for each mole of Ba(TO,), that dissolves Therefore,

biota molar solubility of Ba(IO,), = [Ba?*] |= Ale

Substituting this last equation into the equilibrium-constant expression gives

no mmol Ba(1O;); = 7.32 x 10-4 500 mi

| The mass of Ba(IO,); in 500 mL is given by

mass Ba(IO:); = (7.32 x 10” X 500) mmelBaftO;),

| mole 1 mole 2 mole

[Ba?*] ,.1a) = [Ba?*] from Ba(IO,), + |Ba’*| from Ba(NO,),

-0

0.0200

Any Questions?

C Acid-Base Dissociation Constants

when a weak acid or base is dissolved in

water, it partially ionizes:

a

1 Dissociation constants for conjugate acid/base pairs

K.K, = [H,O*][OH'] = K,

so that as K, becomes smaller, the corresponding

K, for the acid 's conjugate base becomes

greater (or the weaker the acid, the stronger of

conjugate base)

The same ts true for weak bases.

2 Hydronium ion concentrations in solutions of

since K, usually >> K,,, we normally can neglect

the water hydronium ion contribution (but not

always, particularly if the c,,, is very low)

[A] = [H,O*], and recalling that

Cu, = [A] + [HA] (analytical concentration)

Cu, = [H,0*] + [HA] or

[HAI = caa - [H;O"]

Equilibrium expression for the acid dissociation:

Ka=

~ (Cy, -[H,0*]) [H,O*F + K,[H,O*] - K,C,, = 0

The positive solution to the quadratic equation Is:

-K,+ JK, +4K.C,,

Simplified Equilibrium Expression

Assume c,, >> [H;ạO"]):

[H,O*]* = c,K, or [H,O*]=./K,C,„

when c,,,/K, = 10%, the error is 0.5%

when c,,,/K, = 10°, the error is 1.6%

when c,,,/K, = 102, the error increases to 5%

See Table 7-4

Using CHA More Exact Percent

K, Cua Assumption K, Equation Error

1.00 X 10-4 1.00 X 107> ¡0ˆ 0.95 xX 10-5 aa

1.00 x 10-3 3.16 X 10-5 103 341 < 10> 1.6

1.00 x 10-2 1.00 x 10-3 104 9.95 x 1075 0.5

1.00 X 107! 5310 xX 10”* ¡0° 3.16 X 10-4 0.0

3 Hydronium ion concentrations in solutions of weak bases:

In the same manner we can derive:

the quadratic solved for [OH’] solved for

[OH ]=/K,C,

Any Questions?

Quiz: Multiple choices: please circle the best answer

Identify the conjugate acid of NH3

(a) NH; (b) NHy (c) NH> (d) NH, (e) none of these

2 Find the pH of a solution containing 6.0x10° M OH”

(a)5.2 (b)8.8 (c)6.0x10° (d)1.7x10” (e) none of these

Homework

¢ 6-4, 15, 16, 19, 22, 25, 30, 37, 47, 48, 53

Before working on Homework,

Practice with all examples that we discussed 1n the class and examples in the textbook!!