Chemical equilibrium

Equilibrium state [dynamic equilibrium]The equilibrium system is apparently a

stationary system but in reality dynamic .

Example: If water in a closed vessel is heated, we notice two reversible or

opposite processes. These are vaporization and condensation processes . At the

beginning of heating, vaporization takes place faster which associated with an

increase in the vapor pressure (vapour pressure is the pressure due to water

vapour in air at a certain temperature).

The vaporization process continues until the vapour

pressure equals saturated water vapour pressure

which is maximum water vapour pressure (which is

the maximum water vapour pressure in air at a

certain temperature).

A state of equilibrium is reached between the rate of vaporization and

condensation. At this time the number of water molecules which evaporate from

the liquid is equal to the number of water molecules in the condensed vapour.

vaporiztion

condensationwater(liquid) water(vapour)

Similar to the equilibrium established in physical systems, equilibrium in many

chemical reactions takes place chemical reactions can be divided into two types :

1. Complete (irreversible) reactions.

2. Reversible reactions.

1. Irreversible reaction: is the chemical reaction takes place in one direction only

which is forward, because the product can’t react together reforming reactant

once more, because one of the product escapes from the medium as gas evolved

or precipitate deposit. As in the following reactions.

AgCl 3

NaNO

3

AgNO NaCl

2 H

2ZnCl

dil 2HCl Zn

Question explain why thermal decomposition of copper nitrate is irreversible

reaction?

Cu(NO3)2 heat CuO + 2NO2 + O2 because oxygen and nitrogen dioxide escape

from the medium so the product cant react together reforming reactant once

more.

2. Reversible reaction: is the chemical reaction takes place in the two directions,

forward and backward reactions because all the product and reactant are found in

the medium and can react together reforming reactant once more, because none

of them escapes from the medium. For example the reaction between acetic acid

and ethyl alcohol to form ethyl acetate and water as following:

Chemical reactions

Irreversible reaction

Reversible reaction

5 53 2 3 2 2

CH COOH C H OH CH COOC H H O

So we can define the chemical equilibrium as the following:

It is a dynamic state in which the rate of forward reaction equals the rate

of backward reaction.

The rate of reaction is :

The concentration of reactant or the product per unit time and measured in

mole/liter.

Note that:

1- Some reactions are very fast (instantaneously) as all ionic

compounds ( reaction of sodium chloride with silver nitrate

2- Some reactions are slow as reaction of caustic soda with oil to form

soap

3- Some reactions are very slow as iron rust take many months

So there are some factors affect rate of reaction.

1. Nature of reactant:

a. Surface area

Powder reacts faster than large particles Why? Because powder has larger surface

area.

Experiment:

In a beaker contains 250 cm3 dilute hydrochloric acid add one gram of zinc block,

notice the speed of the reaction, collect amount of hydrogen gas produced in two

minutes.

Repeat the above experiment but use one gram of zinc in the form of powder, and

notice the speed of the reaction, collect the amount of hydrogen produced in two

minutes, then compare between the two experiments.

Conclusion: the rate of reaction in the 2nd stage takes place faster as the number of

zinc atom in case of powder increases so the rate of reaction with hydrochloric

acid increases.

Question: write scientific explanation:

The chemical reaction takes place faster if the reactants are finely divided or

pours?

Factors affecting

rate of reaction

Nature of reactant concentration temperature

Surface area Kind of bonds

catalyst

Ionic

covalent

pressure light

Because as the substance is in the form of powder or pourus the number of

molecules exposed to other reactant increase and the reaction takes place faster

due to the increase of surface area.

b- Type of bonding in reactants:

In case of ionic compounds the rate of reaction takes place faster than covalent

compounds, because in case of ionic compound there is exchange of ions so we

reach to equilibrium state faster.

1- Concentration: of reactants:

The rate of reaction takes place faster in cases of high concentration

because there is a great chance for collision takes place according to the law

of mass action ------------ to understand that study the following experiment

Steps:

Add iron (III) chloride solution (faint yellow) gradually to ammonium thiocynate

solution (colourless).

Observation:

The colour of the solution turns blood red due to formation of iron (III)

thiocyanate.

3 4 4 3int

3 3 ( )colourlessfa red

FeCl NH SCN NH Cl Fe SCN yellow blood

If an excess amount of iron (III) chloride is added, the red blood colour of the

solution increases indicating formation more iron (III) thiocynate

Conclusion: If the concentration of reactants (iron III)chloride increases, the rate of

forward reaction increases i.e Fe(SCN)3 is increased.

The law of mass action:- At a constant temperature, the rate of

chemical reaction is directly proportional to the product of multiplication of the

reactant concentrations , each is raised to the power of the number of

molecules .

This law discovered by Waag and Guldberhg

You can understand the law of mass action from the above experiment

by gradual addition of iron (ІІІ) chloride solution (faint yellow colour ) to

thiocyanate solution (colourless) ,the colour of the reaction mixture becomes

red blood due to the formation of iron (ІІІ) thiocyanate .

The reaction can be represented by the following equilibrium :

3 4 4 3 blood

3 3 ( )red

FeCl NH SCN NH Cl Fe SCN

If an excess amount of iron (ІІІ) chloride is added, the red colour of the solution

increases indicating formation of more iron(ІІІ) thiocyanate.

When the rate of the backward reaction (r2) equals the forward reaction (r), in the

above reaction an equilibrium state is established and both reaction rates are

expressed as follows :

r1 α * FeCl3] [NH3SCN]3

r1 = k1[ FeCl3] [NH3SCN]3

r2 α *Fe(SCN)3 ] [ NH4Cl ]3

r2 = k2 [Fe(SCN)3 ] [ NH4Cl ]3

The brackets [ ] represent the concentration ( mole / liter unit ) whereas k1 and k2

are rate constants for the forward and back ward reactions , respectively .

At equilibrium r1 = r2

k1 [ Fecl3][ NH4SCN]3 = k2[Fe(SCN)3][NH4cl]3

3

3 3

3

3 4

[ ( ) ][ ]

[ ][ ]

Fe SCN NH Cl

FeCl NH SCN

1c

2

k=K =

k

The product of 1

2

K

K is a constant value referred to as Kc and is known as the

equilibrium constant of the reaction :

Example :-

Calculate the equilibrium constant of the reaction : 2 2I + H 2HI

Provided that the concentration of І2 ,H2 and HІ at equilibrium are 0.221 , 0.221

and 1.563 mole/liter , respectively .

Solution :-

2 2

2[ ]

[ ][ ]

HIKc

H I

2(1.563)=50

0.221 0.221x

Please note that

(a) The small values of equilibrium constant (less than one) means that the

concentration of the products are less than the concentration of the reactants

which represent that the reaction is not proceed well towards the formation of the

products and that the reversible reaction has an effective role . As an example is

the solubility of silver chloride in water

+ -

(s) (aq) (aq)AgCl Ag + Cl

Kc= [Ag+][Cl-] =1.7x 10-10

(B) The value of Kc in the above reaction indicates the insolubility of silver chloride

in water . KC is so small so backward is more favored

(c) The high values of equilibrium constant indicate that the reaction proceeds

almost to its end .This means that the forward reaction is the predominant

reaction, as in the case of the reaction of chlorine with hydrogen .

2 (g) 2(g) (g)H +Cl 2HCl K c=4.4x1032

(D) The concentration of water(liquid) , solid substances and precipitates should

not appear in the equilibrium constant equation ,because their concentrations

remain constant whatever their quantities. In general the concentration of water

or solvent is considered as a constant because it does not significantly varied in the

reaction .

3- Temperature: the rate of reaction is directly proportional to any rise in

temperature because temperature activates the molecules so a great chance for

active collision takes place this rapid the reaction. This called collision theory,

chemical reaction takes place faster at active collision, it is found that a reaction

can be doubled if the temperature increased by 10℃

note that (1) pressure cooking pots used to obtain high temperature in short time

(2) also increasing temperature increases pressure so it is undesirable to heat

butagas cylinders because it may explode due to the rise in pressure

Activation energy: it is the minimum amount of energy required to start

the reaction.

Activated molecules : Are those molecules having kinetic energy equals to

or more than activation energy

Experiment to illustrate the effect of temperature on a reaction at equilibrium:

This can be explained according to the following equilibrium equation .

reddish brown

cooling2 2 4colourlessheahing

2 NO N O + heat

We deduce from this experiment that if an exothermic reaction has reached the

equilibrium state , decrease in temperature force the reaction to proceed in the

forward direction in order to liberate heat.

3- Effect of pressure :

If the reactants or products are in the gaseous state , the concentration is

expressed by using their partial pressure. For example , ammonia gas is prepared

in industry from its elements according to the following reaction:

high pressure,cooling

2(g) 2(g) 3(g)N +3H 2NH , H= -92kJ

Four molecules react to form two molecules , i.e. , the formation of ammonia gas

is accompanied by a decrease in the number of molecules and consequently a

reduction in the volume. It was found that by applying pressure and cooling , the

rate of ammonia formation increases. We can conclude from the above example

that by increasing the pressure or cooling on a gaseous reaction under equilibrium

, a shift in the direction of reducing the pressure (or the direction in which volume

is less) takes place.

In this case , the equilibrium constant is expressed by the symbol Kp to indicate

that the concentrations of the substances are expressed by the partial pressure:

2

3

3

2 2

p (NH )Kp=

p(N )xp (H )

The equilibrium constant of the above reaction can also be expressed in terms of

molar concentration .

Example: Calculate the equilibrium constant (Kp) of the reaction :

2(g) 2(g) 2(g)N + 2O 2NO

Provided that the pressures are 2 , 1 and 0,2 atmosphere for the gases NO2, O2 and

N2, respectively .

Solution:

2 2

2

2 2

2 2

p (NO ) (2)Kp = = = 20

p(N )xp (O ) 0.2x(1)

The equilibrium constant =20

From the above observations and others , Le Chatelier ,a French scientist ,

formulated a rule named after him . This rule predicts the effect of different

factors such as concentration , temperature and pressure on the systems under

equilibrium .

In the following reaction N2 + O2 2NO - energy.

What is the effect of heat, concentration of reactant and pressure on the

formation of nitric oxide.

Answer

The effect of heat according to Le Chateleir increasing of heat will shift the

reaction forward in the direction of formation of nitric oxide so concentration of

nitric oxide increases.

Le Chatelier’s principle :

The changes in any of the conditions of a chemical equilibrium such as concentration ,

pressure or temperature cause shift of the equilibrium in the direction which will oppose

this change .

The effect of concentration of reactant: If we add more of nitrogen and oxygen the

concentration of nitric oxide will increase.

The effect of pressure: the pressure at the both side is equal so it has no effect.

(5) Effect of catalysts :

Catalysts are known as" substances that cause a change in the rate of chemical

reaction without it self being changed".

Small quantity of catalyst is required to cause such change in the reaction rate

without affecting the position of equilibrium. Catalysts are used in more than 90%

of the industrial processes such as food and petrochemical industries . Catalysts

are also used in catalytic converters utilized in modern cars to convert the gaseous

combustion products, which cause air pollution , into safe products . Catalysts are

metals, metal oxides or compounds.

Enzymes (high molecular weight proteins produced in living cells) act as catalysts

for many biological and industrial processes .

Catalyst has no effect on equilibrium position because as it increases both

forward and backward reactions by the same rate.

So catalyst attains equilibrium only in short time.

(6) Effect of light :

Some chemical reactions are affected by light , the photosynthesis is an example in

which chlorophyll in plants absorbs light and form carbohydrates in the presence

of carbon dioxide and water .

Photographic films contain silver bromide in a gelatinous layer .When light falls on

such films , silver ions accept electrons from bromide ions and are converted into

silver metal . Bromine is absorbed in the gelatinous layer.

The increase in light intensity is associated with an increase in the amount of silver

formed .

+ -Ag + e Ag

Ionic equilibrium.

To understand the concept of ionic equilibrium you have to know the meaning of

electrolyte.

The electrolytic solution is the solution that conducts electricity by movement of

its ions.

Strong electrolyte: it is completely ionize in the solution, such as strong

acids and strong alkalis and some salts

Weak electrolyte: is the solution not completely ionized as some covalent

compounds

Experiment to show the difference between weak electrolyte and strong electrolyte

Dissolve 0.1 mole of hydrogen chloride gas in one liter of water,

Also dissolve 0.1 mole of pure ethanoic acid in water to make two acid

solutions of equal concentration.

Electrolyte

Strong electrolyte

Weak electrolyte

a solution of HClAcetic acid

Test the electrical conductivity of each solution.

Observation

You can notice that in the case of hydrochloric acid, the lamp gives a strong

illumination, but gives a faint illumination with ethanoic acid.

Conclusion: A solution of HCl in water is strong electrolyte and completely

ionizes 3HCl HOH H O Cl while a solution of acetic acid is weak

electrolyte and partially ionized 3 3 3CH COOH HOH CH COO H O

Ionization: is the process in which all unionized molecules converts into ions

Exp 2Explain experiment to show the relation between ionization

and dilution (Verify Ostwald law)

Dissolve 0.1 mole of hydrogen chloride gas in one liter of water,

also dissolve 0.1 mole of pure ethanoic acid in water to make two acid

solutions of equal concentration.

a solution of HClAcetic acid

Test the electrical conductivity of each solution.

Observation

you can notice that in the case of hydrochloric acid, the lamp gives a strong

illumination, but gives a faint illumination with ethanoic acid

Repeat the above experiment

Dilute each solution to 0.01 molar and test the electrical conductivity, then

further dilute each solution to 0.001 molar and test the conductivity of each

solution again. You will notice that illumination of the lamp is not affected in the

case of dilution of hydrochloric acid, but the illumination increases in the case

of the dilution of acetic acid.

Conclusion.

Electrical conductivity of HCl doesn’t affect by dilution but ethanoic acid

increase, so the degree of ionization of weak electrolyte only increases by

dilution

Ionization constants of some weak acids are given in the following table:

Acid Chemical formula Ionization constant (ka)

Sulphurous H2SO3 1.7 × 10 -2

Hydrofluoric HF 6.7 × 10 -4

Nitrous HNO2 5.1 × 10 -4

Acetic (Ethanoic) CH3COOH 1.8 × 10 -5

Carbonic H2CO3 4.4 × 10 -7

Boric H3BO3 5.8 × 10 -10

In case of weak acid KA is directly proportion to strength of the acid

Hydroxonium ion:

No free hydrogen ion (protons); is present in aqueous of ionized acids .This ion

attract to the lone pair of electrons on the oxygen atom of the water molecule and

connected to a water molecule by a co-ordinate covalent bond. This proton is

called hydrated proton or hydroxonium ion [H3O] +.

+ -

2 3HCl +H O H O +Cl

Ionic equilibrium:-

It is the equilibrium arising between molecules of a weak electrolyte

and ions resulting from it.

The law of mass action can be applied only in the case of weak electrolyte

solutions. Strong electrolyte solutions do not contain un-dissociated molecules,

since they are completely ionized.

In 1888 Ostwald described the relation between the extent

of ionization - alpha ( α ) and concentration ( C ) per mole/litre.

Ostwald law

At constant temperature, the degree of ionization ( ) increases

by dilution

To understand Ostwald law assume that a mono-protonic weak acid (HA)

dissociates in water according to the equation :

+ -HA H +A

By applying the law of mass action on this equilibrium system , the following

relation is obtained

[ ][ ]

[ ]

H AKa

HA

Then at equilibrium :

Degree of dissociation = Number of dissociated moles

Total number of moles before dissociation

If the number of dissociated moles is ( ) mole then, the number of un

dissociated moles from HA= (1- ) and the number of moles of H- and A-produced

equals mole.

The concentration (c) = Number of moles

Volume by liter (V)

The concentration of the substance (mole/ L) at equilibrium is

+ - HA H +A

(1 )

V

V

V

Substitution in the equation of mass action gives :

2

1

[ ][ ]

[ ] (1 )

v v

v

Kav

This relation is known as Ostwald Law for dilution which illustrate the quantitative

relationship between the degree of ionization ( ) and dilution . it can be seen that

At constant temperature, the degree of ionization ( ) increases

by dilution (so that Ka value remains constant ) . In case of weak electrolytes , the

degree of ionization ( ) is small enough and can be neglected. Consequently the

value (1- ) is considered approximately one and the relation becomes :

2

Kav

Since the concentration of the weak acid (c) =1

vmale /liter , the above equation

becomes: Ka= 2 x c

This means that increasing dilution (decrease in concentration )

causes an increase in dissociation degree and vice versa .

Example : calculate degree of ionization of 0.1 molar hydrocyanic (HCN) acid

solution at 25°C , providing that the equilibrium constant of the acid Ka= 7.2x10-10

Solution :

+ -

2 3HCN + H O H O +CN

Applying Ostwald Law : Ka= 2 x c

7.2x 1010 = 2 x 0.1

-102 -107.2x10= =72x10

0.1

= -1072x10 = 8.5x10-5

Calculation of hydroxonium ion concentration of weak acids :

+

3

+

3 A

[H O ] = C x Ka

[H O ]= .C

Example :

Calculate the hydrogen ion concentration in 0.1 molar acetic acid solution at

25oC , provided that the equilibrium constant of the acid is 1.8 x 10-5

Solution :

- +

3 3

3

[CH OO ] [H O ]Ka =

[CH COOH]

+ -5 -3

3[H O ] = 1.8 x 10 x 0.1 = 1.342 x10 molar

Calculation of the hydroxyl ion of weak bases :

Partially dissociated bases in aqueous solutions are know as weak bases . The

concentration of hydroxyl ion can be calculated as following -

b b [OH ]= C xK

2. Ionization of water:

Pure water is a very poor conductor of electricity, but some conductivity is

detected, so some ionization of water must take place as shown in the following

equation :

+ -

2 3H O H O +OH

For simplification , this equation can be written :

+ -

2H O H +OH

The equilibrium constant is expressed by the relation

+ --14

2

[H ] [OH ]Kc = =10

[H O]

From the value of the equilibrium constant, it is clear that a very small number of

water molecules ionizes.

The number of unionized water molecules can be considered as a constant value.

Therefore, the above relation can be rewritten as follows ;

Kw = [H+][OH

-] = 10

-14

Since water is neutral ( no action on litmus ), the concentration of H+ ( responsible

for the acidic properties ) equals the concentration of OH- ( responsible for the

basic properties ).

Then Kw = [10-7

][ 10-7

] = 10-14

K w( ionic product of water ):

The ionic product of water is the resultant of multiplying the concentration of the

hydrogen ion [H+] times that of hydroxide ion [OH-] .It is a constant value equals

10-14 mole/liter.

pH value

It is the negative logarithm of hydrogen ion concentration:

pH = -log [H3O+]

The symbol (p) means (-log) .

This is a way for expressing the degree of acidity or alkalinity for aqueous solution.

Referring to the ionic product of water and using the negative logarithm of this

equation, the above relation becomes:

-log Kw=-log [H+] –log [OH-] =-log 10-14

Substituting the value (-log ) by the symbol (p) , the above equation becomes :

pH + pOH= 14

In case of a neutral solution , the pH=7 and pOH =7. For solution of pH 5 , the

pOH=9 and for solution has a pH 8 , the pOH =6. Consequently , the pH value of

the acidic solutions is < 7 and the POH is >7 and for basic solutions the pH is >7 and

pOH is<7.

Hydrolysis of some salts

Hydrolysis is opposite to neutralization and defined as decomposition of salt using

water to form acid and base one of them at least is weak.

Acid + base salt + water

(neutralization) (hydrolysis)

Hydrolysis of sodium chloride

Sodium chloride solution has neutral effect to litmus solution, as it consists of

strong acid HCl and strong alkali NaOH, and hydrolysis can be explained as follow:

HOH H + OH

NaCl Na+ + Cl

---------------------------------------------------------------------

By adding:NaCl + HOH Na+ + OH + H + Cl

salts

Acidic if it consist of strong acid and weak base

Basic if the base is strong and the acid is weak

Neutral if both acid and alkali in the same strength

We found that neither proton H+ nor hydroxide ions disappear from the solution

so that sodium chloride has neutral effect.

Hydrolysis of ammonium chloride NH4Cl

Ammonium chloride has acidic effect to litmus solution as it consists of strong acid

and weak base and hydrolysis can be explained as following:

HOH H + OH

4 NH Cl 4

NH + Cl by adding the two equation we find that

NH4Cl + H2O H + Cl + NH4OH.

Hydrolysis of sodium carbonate.

Sodium carbonate has alkaline effect to litmus solution since it hydrolyzes to form

strong alkali (NaOH) and weak acid (H2CO3) as following:

2HOH 2 H + 2 OH

Na2CO3 2Na+ + CO3-2

By adding the two equations

Na2CO3 + 2H2O H2CO3 + 2Na+ + 2 OH

According to Le Chatileir principle hydrogen ions disappear so water ionizes more

to produce more hydroxide ions which is responsible for alkaline effect

Hydrolysis of ammonium acetate

Ammonium acetate has neutral effect to litmus solution since it consists of weak

acid and weak base, the hydrolysis process takes place as following:

HOH H + OH

CHCOONH4 CH3COO +

4NH

By adding:

CH3COONH4 + HOH CH3COOH(weak acid) + NH4OH (weak base)

We found that both hydrogen ion and hydroxide ion disappear from the solution

so that ammonium acetate has neutral effect to litmus solution

Note that the strong alkalis are from group 1A and the strong acid HCl, H2SO4

and nitric acid HNO3

Solubility product

The dissolution of the sparingly soluble lead bromide in water is given by relation :

2+ -

2Pb Br Pb +2Br

The equilibrium constant is given by the relation:

2+ - 2

2

[Pb ][Br ]Kc =

[ Pb Br ]

The solid Pb Br2 concentration remains almost constant :

Ksp =[Pb 2+] [ Br -]2

Ksp is known as solubility product .

Solubility product for any sparingly soluble ionic compound is the

product of multiplication of the concentration (expressed by male /

liter )of its ions raised to power number of ions which exist in

equilibrium with its saturated solution

Example :If the degree of solubility of silver chloride (AgCl)is 10-5 mole / liter ,

calculate the solubility product of Agc1 . AgCl

Solution :

AgCl Ag C l

Ksp = [A g +] [C l-]

Ksp = 10-5x 10-5=10-10

Example

Calculate KSP of calcium phosphate Ca3(PO4)2 given that concentration of calcium ions is ( 81 10 ) mole

/liter and concentration of phosphate ions is 30.5 10 mole /liter

2 3

3 4 2 4( ) 3 2Ca PO Ca PO

KSP= 2 3 3 2

4[ ] [ ]Ca PO = 8 3 3 2 31[10 ] [0.5 10 ] 2.5 10