All Exam Soloutions And Notes: Chemistry Notes

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Thursday, 29 December 2016

کیا آپ آکسفورڈ یونیورسٹی کے انٹری ٹیسٹ کے سوالوں کے جوابات دے سکتے ہیں؟

Larki Set Krny Ka Asan Tareka

Sunday, 24 July 2016

Fsc part 2 chemistry chapter 1

CHAPTER # 1
Periodic Classification of Elements & Periodicity
1. Which of the following pairs are chemically
dissimilar?
(A) Na and K (B) Ba and Sr
(C) Zr and Hf (D) Ca and Zn.
2. The total number of inner transition elements
is
(A) 10 (B) 14
(C) 28 (C) 30
3. The alkali metal which is liquid at 15oC is
(A) K (B) Cs
(C) Na (D) None
4. Which of the following ion will form most water
soluble hydroxide?
(A) K+ (B) Ni2+
(B) Zn2+ (C) Al3+
5. Which of the following has greatest tendency to lose electron?
(A) F (B) Fr
(B) S (C) Be.
6. The oxide of which of the following elements will be acidic in character
(A) Mg (B) Rb
(C) Li (C) CI
7. Which of the following is isoelectronic with carbon atom?
(A) Na+ (B) Al3+
(C) O2- (D) N+
D
8. Which of the following ions are paramagnetic in character?
(A) Zn2+ (B) Cu+
(C) Ni2+ (D) Ag+
9. Ca2+ ion is isoelectronic with
(A) Mg2+ (B) Na+
(C) Ar (D) Kr
10. Gradual addition of electronic shells in the noble gases causes a decrease in their
(A) Ionization energy (B) atomic radius
(C) Boiling point (D) density.

11. Which of the following has highest first ionization potential?
(A) Carbon (B) Oxygen
(C) Nitrogen (C) Boron.
12. Which of the following has the smallest size?
(A) Na+ (B) Mg2+
(C) Al3+ (D) CI
13. Which of the following element has the maximum electron affinity?
(A) F (B) S
(C) I (D) CI.
14. Which of the following is isoelectronic as well as has the same structure as that of N2O ?
(A) N3H (B) H2O
(B) NO2 (C) CO2
15. The atomic radius increases as we move down a group because
(A) Effective nuclear charge increases
(B) Atomic mass increases
(C) Additive electrons are accommodated in
new electron level
(D) Atomic number increase.
16. Which one of the following is an incorrect
statement?
(A) The ionization potential of nitrogen is
greater than that of chlorine
(B) The electron affinity of fluorine is greater
than that of chlorine
(C) The ionization potential of beryllium is
greater than that of boron
(D) The electronegativity of fluorine is greater
than that of chlorine.
17. Electron affinity depends on
(A) Atomic size
(B) Nuclear charge
(C) Atomic number
(D) Atomic size and nuclear charge both.
18. Two elements whose eletronegativities are 1.2 and 3.0, the bond formed between them would
be
(A) Ionic (B) covalent
(C) Coordinate (C) metallic.

19. Ionic radii are
(A) Directly proportional to square of effective
nuclear charges
(B) Inversely proportional to effective nuclear
charge
(C) Inversely proportional to square of
effective nuclear charge
(D) Directly proportional to effective nuclear
charge.
20. Which of the following oxides is atmospheric in
character?
(A) CaO (B) CO2
(C) SiO2 (D) SnO2
21. Mark the correct statement:
(A) Na+ is smaller than Na atom
(B) Na+ is larger than Na atom
(C) CI- is smaller than CI atom
(D) CI- and CI are equal in size
22. Who introduced the zero groups?
(A) Lothar Meyer (B) Lockery
(C) Mendleev (D) Ramsay
23. Element, of group I-B are called
(A) Representative elements
(B) Transition elements
(C) Rare earth
(D) Coinage metals
24. The element with Z = 24 is placed in the period
(A) 5 (B) 2
(C) 3 (D) 4
25. Which is the part of metalloids?
(A) NA and K (B) F and CI
(C) None of these (D) Cu and Au
26. Which one of the following has the maximum
electron affinity?
(A) I (B) Br
(C) CI (D) F

Chemistry part 2 chap 1 mcqs....file 2
26. Which one of the following has the maximum
electron affinity?
(A) I (B) Br
(C) CI (D) F
27. On electrolysis of NaH, hydrogen is liberated
(A) At anode (B) in the electrolyte
(C) At cathode (D) none of them
28. Elements with greater number of electrons
have _________values of ionization energy.
a) Only one b) More than one
c) Zero d) Infinite
29. Which of the following possess maximum
hydration power?
a) Na+ b) K+
c) Mg+2 d) Ca+2
30. Higher value of electron affinity means_________
Atom will lose electron easily
Atom will gain electron easily
Atom may form di-positive ion
The reason is unknown
31. Melting points of VII-A group ___________ down the group
a. Increase b Decrease
c. Remain constant d. No regular trend
32. Oxidation state of an atom represents______
No. of electrons gained
No. of electrons lost
No. of electrons gained or lost
d. None of above correctly represent it
33. Mendeleevs periodic table was based on
a) Atomic number b) Atomic mass
c) Atomic volume d) Electronic configuration
34. Elements present in a same group have the same
a) Atomic number
b) Molecular weight
c) Chemical properties
d) Electronic configuration
35. s and p block elements are also called
Transition elements
Inert elements
Typical elements
Rare earth elements
36. What is the symbol of the element with only three electrons and three protons?
a) Li b) C
c) Ag d) Cu
37. Elements with seven electrons in their valence shell are known as
a) Inert b) Lanthanides
c) Halogens d) Alkali metals
38. Which of the following pairs of elements are chemically most similar?
a) Na and Al b) Cu and Cu
c) S and F d) Sc and Zn
39. A student of chemistry will identify positively the following symbols as sodium
a) b)
c) d)
40. In the periodic table each period begins with a metal, which is
Most electronegative
Most electropositive
Less electropositive
Less electronegative
41. Which one of the following is not a coinage metal?
a) Au b) Cu
c) Ag d) Pd
42. Which is the most metallic element of 2nd period?
a) Lithium b) Beryllium
c) Boron d) Carbon
43. The outer most orbital involved in chemical bonding is called
a) Molecular orbital b) Complete orbital
c) Valence orbital d) Free orbital
44. Elements, which form basic oxides are
a) Electropositive b) Electronegative
c) Inert d) None of these
45. Which of the following has the most basic character?
a) Na2O b) MgO
c) Al2O3 d) P2O3
46. Which of the following is smallest in size?
a) K+1 b) O-2
c) F-1 d) Na+
47. Ionization energy is lowest for
a) Inert gases b) Halogens
c) Alkali metals d) Alkaline earth metals
48. An isotope of hydrogen is
a) Neptunium b) Plutonium
c) Thorium d) Tritium
49. With respect to chlorine, hydrogen will be
a) Electropositive b) Electronegative
c) Neutral d) None of these
50. Which of the following has the highest electron affinity?
a) 1S2 2S2 2P3 b) 1S2 2S2 2P6 3S1
c) 1S2 2S2 2P5 d) 1S2 2S2 2P5
51. Excluding hydrogen and helium, the smallest elements in the periodic table is
a) Lithium b) Fluorine
c) Cesium d) Iodine
52. Which halogen has the smallest electron affinity?
a) F b) Cl
c) Br d) I
53. The element with atomic number 7 is likely to have same properties to the element whose atomic number is
a) 11 b) 2
c) 15 d) F
54. Which of the following will have largest size?
a) Br b) I -1
c) I d) F
55. In its chemical properties, calcium is most similar to
a) Cs b) Cu
c) Sc d) Sr
56. Which two of the following are iso electronic with one another?
a) Na+ and O b) Na+ and K+
c) Na+ and Ne d) Ne and O
57. Which of the following is a transuranic element?
a) Americium b) Plutonium
c) Neptunium d) All of these
58. The hydrogen, which is present in the atmosphere of sun and stars in a large amount, is
a) H2 b) H
c) H+ d) H-
59. Cesium and Francium are liquids above
a) 10oC b) 15oC
c) 20oC d) 30oC
60. In an aqueous solution the hydrides of alkali metals are
a) Stable b) Unstable
c) No change d) None of these
61. As the atomic number of the halogen increases, the halogens
Lose their outermost electrons less easily
Become less dense
Becomes lighter in color
Gain electrons less easily
62. The electron affinity of fluorine is
a) 348.8 kj/mol b) 337 kj/mol
c) 337 kj/mol d) 348.8 kj/mol
63. Which ionic halides have high m.p and b.p?
a) Fluoride b) Chloride
c) Bromide d) Iodide
64. Which gas is used in the making of tungsten bulb filaments?
a) H2 b) N2
c) O2 d) CO2
65. The ionic halides in order of decreasing m.p and b.p can be arranges as
Iodide>bromide>chloride> fluoride
Bromide>chloride>fluoride>iodide
Chloride>bromide>iodide>fluoride
Fluoride>chloride>bromide>iodide
66. A hydride ion and helium atom has same number of
a) Protons b) Neutrons
c) Electrons d) All of these
67. Ionic hydrides are also called
a) Saline hydrides b) Salt like hydrides
c) Both a & b d) None of these
68. The hydrides are acts as powerful reducing agents are
a) Ionic b) Covalent
c) Interstitial d) Complex
69. The hydrides which are non stoichiometric in nature are
a) Ionic b) Covalent
c) Interstitial d) Complex
70. Which one is the example of complex hydride?
a) PH3 b) NaH
c) LaH3 d) NaBH4

Chemistry part 2 chap 1 mcqs.....file 3
71. The adsorption of hydrogen by platinum is known as
a) Hydrogenation b) Dehydrogenation
c) Occlusion d) Substitution
72. From left to right in a periodic table charge to mass ratio increases therefore the hydration energy
a) Decreases b) Increases
c) Remains constant d) None of these
73. ______ elements have been discovered so
far.
a) 100 b) 110
c) 120 d) 150
74. _______ classified the then known elements
into metals, non metals and their derivatives.
a) Dobreiner b) Al Razi
c) Newlands d) Mendeleeve
75. Dobreiners work led to the law of triads which states that ______
a) Atomic weight of any one element was
found to be approximately the mean of the
other two elements of triad.
b) Atomic weight of the middle element was
found to be approximately the mean of the
other two elements of a triad.
c) Atomic number of any one element was
found to be approximately the mean of the
other two elements of a triad.
d) Atomic number of the middle element was
found t be approximately the mean of the
other two elements of a triad.
76. The law of octaves was given by _______
a) Dobreiner b) Al Razi
c) Newlands d) None of these
77. Law of octave states that ______
a) The properties of every 6th element from the
given one were similar to the first.
b) The properties of every 9th element from the
given one were similar to the first.
c) The properties of every 8th element from the
given one were similar to the first.
d) The properties of every 7th element from the
given one were similar to the second.
78. Mendeleevs Periodic Table was based on _______
a) Atomic number b) Atomic mass
c) Atomic volume d) Electronic configuration
79. Moseleys work led to the periodic law, which
states that _______
a) The number of the electrons in the 1st
energy level increases as the atomic
number increases.
b) The properties of the elements are a
periodic function of their atomic mass.
c) The x rays spectra of the elements are
more complex than the optical spectra.
d) The properties of elements are the periodic
function of their atomic number.
80. A pair of elements in the same family in the
periodic table classification is ________
a) Chlorine and carbon
b) Calcium and aluminum
c) Nitrogen and neon
d) Sodium and potassium
81. In the period, the elements are arranged in
strict sequence in order of _______
a) Increasing charges in the nucleus.
b) Increasing atomic weights.
c) Increasing number of electrons in valence
shell.
d) Increasing valency.
82. Uranium is a member of
a) s block b) p block
c) d block d) f block
83. How many ionization energies can carbon
have?
a) 1 b) 2
c) 4 d) 6
84. Which ion has the maximum polarization
power?
a) L+ b) Mg 2+
c) Al 3+ d) O 2-
85. Which of the following halides is not oxidized
by MnO2?
a) F b) Cl-
c) Br d) I
86. The process requiring absorption of energy is
a) F → F b) Cl → Cl
c) O → O2 d) H → H
87. Most of the known elements are metals of
______ of periodic table.
a) D – block b) P – block
c) III – group d) Zero block
88. The volume in cubic centimeters occupied by
one gram atom of the element is called
______
a) Atomic volume b) Atomic weight
c) Mass number d) None
89. The lowest ionization energies are found in the
_______
a) Inert gases b) Alkali metals
c) Transition elements d) Halogens
90. The unit of ionization energy is _______
a) Joule b) Calorie
c) Electron volt d) None
91. The electropositive elements form _____
a) Acidic oxides b) Basic oxides
c) Neutral oxides d) Amphoteric oxide
92. The electronegative elements form _____
a) Acidic oxides b) Basic oxides
c) Neutral oxides d) Amphoteric oxide
93. The ionization energy of nitrogen is more than
oxygen because of _______
a) More attraction of electrons by the nucleus
b) More penetration effect
c) The extra stability of half filled p orbital
d) The size of nitrogen atom is smaller.
94. _______ ion has the largest radius.
a) Al+3 b) Cl-1
c) F-1 d) O-2
95. Ionic hydrides are usually _______
a) Liquids at room temperature
b) Good reducing agents
c) Good electrical conductors in solid state
d) Easily reduced.
96. The hydronium ion is a/an _______
a) Ion with formula H2O+
b) Ion with the formula H3O+
c) Free radical rather than an ion
d) Ion formed by removal of H- form a water
molecule.
97. When steam is passed over red hot coke at
1000oC, a mixture of carbon monoxide and
hydrogen gas is produced. It is known as
______
a) Heavy water b) Water gas
c) Phosgen gas d) None

Tuesday, 19 July 2016

What volume does 22g of CO2 at STP occupy?

no. of mole of CO2 = Volume of CO2/22.4 in litres
One mole of CO2 = 12 + 16X2
= 44 g
Therefore, no. of mole of CO2 in 22 g = 22/44 = 0.5 mole
Therefore,
0.5 = V/22.4
V = 22.4 X 0.5 = 11.2 litres Ans.

By sir waqar

Sunday, 17 July 2016

Fsc part 1 chapter chemical equlibrium lecture 4 5 6

Prediction of Direction of Reaction:

Let we have a reversible reaction for which value of Kc is 4 the reaction is

A+B ------------------ C+D

Let at a particular time interval, their molar concentrations are (a), (b), (c) and (d) respectively. Here we are not sure whether these concentrations are the equilibrium concentration. If the provided concentrations of reactants and products are b/f equilibrium, then the reaction may be in forward or in reverse direction.

The direction of reaction can decided easily by taking the ration of product of concentration of products and product of concentration of reactants

Now

If Qc = Kc

Then the reaction is at equilibrium

If Qc > Kc

Then the reaction is in the reverse direction.

If Q> Kc

Then the reaction is in the forward direction.

Prediction of Extent of Reaction.

If we have a reversible reaction i.e

A+B -----------------------C+D

Let their equilibrium concentrations be (a), (b), (c) and (d). Then equilibrium constant Kc will be

Kc = (c) (d)

Now there are several possibilities :

If the equilibrium concentration of C& D are very large as compared to those of A & B, them the value of Kc will be large enough i.e Kc >> 1. It shows that the reactants, react very well and thus produce large quantity of produces. But on the other hand, the products do not react well. Thus at equilibrium most of the concentration of reactants is converted into the products.
If the concentration of C & D is very small as compared to that of reactants A & B, at equilibrium, then the value of Kc will very very small, i.e Kc<< 1 If shows that the reactants do not react well to form the products in enough quantity but the products react well to reform the reactants.
If at equilibrium, the concentration of the reactants (A,B) and products (C,D) are almost equal, then value of kc will be nearly or exactly 1. If shows that both the reactants and products react well.

Calculation of Equilibrium Concentration:

Le-chatelier's Principle:

To the study disturbance of a system at equilibrium, by changing its conditions like temperature,

pressure , concentration or presence of catalyst, a French chemist Henry Le Chatelier (1888)

presented a principle which is known as Le-chatelier's . It states that : If a system at equilibrium is

subjected t disturbance by changing it conditions like concentration the system shifts in such a direction to re-establish a new equilibrium.

Applications of Le- Chatelier's Principle : (Effect of Change in Conditions upon Equilibrium)

Le – Chatelier's principle is used to predict, how the variable like concentration. Temp, pressure etc, affect the position of equilibrium.

This principle also has a valuable application in prediction the condition for maximum yield of a particular in a reversible reaction.

1) Effect of Concentration:

According to Le – Chatelier's principle an increase in the concentration of any the reactants, shifts the equilibrium to the right i.e forward reaction increases similarly an increase in the concentration of any of the products shifts the equilibrium to the left i.e reverse reaction increases Same examples are as follow

N2 + 3H2--------------------2NH3

The addition of H2 and N2 would shift the equilibrium to the right. Thus the use of excess of N2 yield of NH3

FeCl3+n3NH4CNS --------------------Fe ( CNS)3 + 3NH4C ff2

Yellow colorless blood red colorless The addition of Fecl3 , shifts the equilibrium to the right & thus more Fe (CNS)3 (blood red) is formed , forward reaction increases. Similarly the addition of NH4Cl , shift the equilibrium to the left resulting in the formation of more NH4CNs & FeCl3 i.e reverse reaction is increased.

2) Effect OF Pressure:

In case of reactions ( reversible ) in which the total volume of reactants is equal to the total volume of the products , the equilibrium is not affected by change in pressure.

e.g

1. CO(g) + H2O(g) ---------------- CO2(g) + H2(g)

1 vol 1 vol 1vol 1vol
Total 2 vol 2 vol

2. N2 (g) + O2(g) --------------- 2NO(g)

1vol 1 vol 2 vol

Total 2 vol 2 vol

Change of pressure has no effect upon the equilibrium in the above mentioned equilibrium system between total volume of reactants is equal to total volume of products. In case of reactions ( reversible ) where the total volume of reactants is not equal to the volume of the products, with an increase of pressure the equilibrium system shifts towards the smaller volume.

In case of these reversible reactions in which the volume of reactants is greater than that of the products, the equilibrium shifts to the right side i.e more products are formed by increasing the pressure of the system.
In case of those reversible reactions in which the volume of products is greater than that of reactants increases with increase of pressure.

3) Effect of Temperature:

There are two types of reversible reactions i.e endothermic and exothermic.

In case of endothermic reversible reactions, heat is absorbed in the forward direction and the same is evolved in the reverse direction and the same is absorbed in reverse direction. With increase of temperature of an reversible reaction ( endothermic or exothermic ) at equilibrium, the equilibrium always shifts in that direction where the temperature decreases. This in case of endothermic reaction (reversible) the equilibrium with crease of temperature . shifts to the forward direction

Examples :

Reversible endothermic Reactions

N2+O2+Heat ………………..2NO

H2S+O2+Heat ………………….H2 + S

As these are endothermic breakfronts thus in decease of temperature Shifts The equilibrium in forward direction . As the reaction are exothermic thus an increase in temperature will shift the equilibrium in

reverse direction. Thus more amount of NH3& SO2 can be formed by lowering the temperature.

4) Effect of Catalyst:

According to Le – Chatelier's principle, catalyst has no effect upon the equilibrium except the equilibrium reaches earlier in the presence of a catalyst. Actually the catalyst increases the speed of both forward & reverse reactions of a reversible reaction to the some extent, therefore the equilibrium remains unchanged. However it is reached earlier.

Auto-Ionization or Self-Ionization of Water:

Pure water is a very weak conclusion of electricity but the fact is that , water can conduct electric current, so there should be changed particles ( ions) in water.

The question arises that where do these ions come from?

This problem can be solved with the help of Lowery-Brorsted concept. Actually a water molecule releases a hydrogen ion (4+) ie proton and acid as an acid while this H+ is accepted by another H2O ion and thus this H2O molecule acts as a base (proton acceptor) this it is clear that H2O acts as an acid as well as, as a base, therefore we say H2O is amphoteric in nature

PH Scale:

In 1909, Soreson constructed a scale. Which is known as PH scale with the help of ph Scale the strength an acid or base is determined.

The term ph is refered as power of the hydrogen ions. The values of ph are from "O" to "14" The ph of acids is below "7". The stronger the acid the smaller will be its ph values and vice versa. Similarly the larger the value of ph a base the stronger will be that base and vice versa. The strength of bases can be determined by the tern POH. The values of POH also range from O to 14 but are in the reverse direction of values of ph

PH = 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

POH 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

The strength of an acid is determined by determining its ph. The ph of an acid is the negative log of hydrogen ion (H+) concentration Ie

PH = -log [H+]

The strength of a base id determined by determining its POH. The POH of base is the negative log of hydroxide ion concentration ie

Ph = -log [H+]

It is important to note that sum of the pH and pOH of a solution is always equal to 14
Ie pH + pOH = 14

The above relationship can be proved as consider the auto- ionization of water ie

2H2O ---------------> H3O + OH(+)
Here, the ionization of H2O is smaller that no considerable change occurs in the concentration of water and it still remains constant.

Solubility Product:

The term solubility product is used for the sparingly soluble salts in water. It can also be called as solubility product constant.

It can be defined as:

The solubility product or solubility product constant of sparingly soluble salt is defined as the product of molar concentration of its ion raised to the power of their stoichiometric co-efficient in the equilibrium equation:

Buffer Solutions:

The solution which resists change in its PH, even with the addition of small amount of a strong acid or a strong base is known as buffer solution.

A buffer solution is made as

1). When a weak acid ( like CH3COOH ) is mixed with its salt made by it with a strong base ( ie: CH3COONa). They are mixed in a particular ration. This mixture is known as buffer and the solution of this mixture is known as buffer solution. Such a buffer is known as Acidic buffer. Ie CH3COOH/ CH3COONa buffer

2). When a weak base ( like NH4OH ) is mixed with its salt made by it with a strong acid ( ie NH4CL ) in affixed ratio and then dissolved in water. We get a buffer solution such a buffer is known as basic buffer

ie NH4OH / NH4CL . buffer The buffer solution has a particular PH range and this range remains constant even for long time or even after the addition of small amount of strong and or strong base.

Buffer Action:

The ability of a buffer to resist change in its PH, even after the addition of small amount of strong acid or base is known as buffer action.

Now the question arises that how a buffer solution resists change in its PH, even with the addition of small amount of a strong acid or base.

It can be explained as: consider the CH3COOH / CH3COOna solution: if we acid small amount of a strong acid like HCL the PH doesn’t change. It is between of the fact that in CH3COOH / CH3COONa : CH3 COOH ionizes to large extent : so there are CH3COO- in large excess which come from CH3COONa . Here common ion effect of CH3COOH and HCL occurs because both produce H+. As a result the solubility of CH3COOH is further decreased. Thus most of H+ ion of HCL ( which are responsible for change of PH ) are converted into in weak acid, CH3COOH which is already in excess In buffer solution.

Thus the PH of buffer remains constant because H+ ions of HCL don’t remain in solution.

Buffer Capacity:

The extent till which a buffer can resist change in its PH is known as buffer capacity. A buffer can resist change in its Ph if we add particular amount of strong acid or base. If we exceed the amount of strong acid or base them the ph of ph will certainly change. So we can say that there is certain limit till which a buffer can resist change in its ph. This limit is known as buffer as buffer capacity.

Calculation of PH of Buffer Solution:

The ph of a buffer solution is calculated by an equation known as "Hander son Herelback equation" This equation is

Ph = Pka + log [ base ]-------------[acid ]

Application of Buffer Solutions:

Buffer solutions have tremendous applications. Some important applications of Buffer Solutions are .The human blood is a buffer whose then serious problems may happen. In spite of having so many acidic and basic substances: the ph doesn’t changes.
Buffer solutions are mostly used in Analytical laboratories for carrying out particular reactions at particular ph.
Buffer solutions are used are used in various industries.
Buffer solutions are used in Biological laboratories etc.

Fsc Part 1 Chapter 8 Chemical Equilibrium lectures 1,2,3

Reversible Chemical Reaction:

Any chemical reaction in which the reactants are converted into the products and the products are converted back into the reactants, is known as reversible chemical reaction.

For example when hydrogen gas reacts with iodine hydrochloric acid (HI ) is formed but HI immediately converts back into H2 and I2, so it is a reversible reaction , i.e
H2(g) + I2(g) 2HI(g)

In case of representing a reversible chemical reaction by means of a chemical equation, the reactants and products are separated by means two opposite half arrows (i.e. ) From the above discussion, it is clear that during a reversible chemical reaction, actually two reaction occurring. One is to the right side, converting the reactants into the products and is known as “forward reaction”

e.g

H2 + I2 ------------------ 2HI

The other is to the left side, converting the products back into the reactants and is known as “reverse or backward reaction”

e.g

H2 + I2 ---------------------2HI

Rate of Reaction :

The change in concentration of reactants or products per unit time is called as rate of reaction.

During a chemical reaction, with the passage of time , the concentration of reactants decreases and the of the products increases.

So in terms of reactants, the rate may be defined as the decrease in concentration of reactants per unit time.

In case of reversible reactions there are two reactions i.e forward reaction and reversible reaction,

so we can discuss the rate of such reactions as :

Rate of forward reaction & Rate of reverse reaction.

Rate of Forward Reaction

The change in concentration of reactants during a reversible reaction, per unit time is known as rate of forward reaction.

Rate of Reverse Reaction.

The change in concentration of products, during a reversible reaction , reaction ,per unit time is known as rate of reverse reaction.

Chemical Equilibrium:

That state of a reversible chemical reaction, at which the rate of forward reaction becomes equal to the rate of reverse reaction, is known as chemical equilibrium.

At the state of chemical equilibrium there is no more change in the concentration of reactants and products but becomes constant although the reaction does not stop but retains dynamically.

The chemical equilibrium can be explained by considering the following

Example :

H2(G) + I2(g) 2HI

At the beginning, the concentration of reactants (H2+I2) is maximum, hence the rate o9f forward reaction will be high and only for ward reaction will occur. After sometime, enough amount of products (HI) is formed and reverse reaction starts, the rate of forward reaction will be high than rate of reverse reaction. After sometime, the concentration of reactants decreases enough and thus, rate of reverse reaction increases enough.

After sometime, the rate of forward reaction is just equal to the rate of reverse reaction and the concentration of reactants and products becomes constant and no more change occur in them, although the reaction does not stop. This is, what is known as the equilibrium state.

It is important to note that it is not necessary, that at equilibrium state concentration of reactants will be equal to the concentration of the products. May be, the concentration of reactants is more than that of the products at equilibrium and undergoes no more change or nay be the concentration of the products is more than that of the reactants and undergoes no more change or the concentration of reactants may be equal t the concentration of products at equilibrium and undergo no more change.

Types of Equilibrium:

There are two types of equilibrium.

Homogeneous equilibrium.
Heterogeneous equilibrium.

Homogeneous equilibrium.

The reversible reactions in which all the reacting substances are in same physical state i.e all gases liquids or solids. e.g consider the following equilibrium system of a reversible reaction

N2O4 ---------------> 2NO2(g)

Here all the reacting substances are in gaseous state.

Heterogeneous equilibrium.

The reversible reactions in which the reacting substances (reactants & products) are in different physical states are known as heterogeneous equilibrium. e.g

CaCo3 -------------->CaO(S) + Co2(g)

In the above equilibrium system it is clear that the reacting substances are in different physical states.

Law of Mass Action & Equilibrium Constant Expression:

In 1864, two Norwegian chemist, Goldberg and Peter, presented a law for the rate of a chemical reaction which is known as law of Mass Action.

This law states that the rate of a chemical reaction is directly proportional to the product of the concentration (or active masses) of reading substances.

The term active mass represents the concentration (in mole* dm-3) of reactants and products. Let we have a reversible reaction, i.e

A+B----------> kr C+D

The equilibrium concentration of A, B, C and D are represented in square brackets i.e (A), (B), (C) and (D) are expressed in moles xdm-3.

According to law of Mass Action, the rate of forward reaction, is proportional to the product of molar concentration of A and B.

Rate of forward reaction (rf) * (A) (B)

Or rf = kf (A) (B) --------I

Here Kf is the constant of proportionality and is known as “rate constant for forward reaction”

Similarly the rate of reverse reaction is,

Rate of reverse reaction (*r)----- (C) (D)

Or Rr = Kr (C) (D)-------II

Here Kr is the constant of proportionality and is known as “rate constant for reverse reaction”

As the reaction , Under consideration is a reversible reaction, therefore at equilibrium Rf = Rr.

Kc is known as equilibrium constant in terms of concentration of reactants and products.

Equilibrium Content Expression. For Heterogeneous Equilibrium:

A heterogeneous equilibrium is an equilibrium in which the reactants and products are in more than one physical state. While writing the equilibrium constant expression for a heterogeneous equilibrium, the concentration for pure solids and liquids are neglected i.e. they are not indicated, It is because of the fact, that , the concentration of a pure solid or pure liquid is constant at a constant temperature and does not depend upon the quantity of the substance.

For example the molar concentration of copper (at 20 c) is the same, whether we have 1g or 1 ton of copper.

3Fe (s) + 4H2O(g)---------------> Fe3O4(s) + 4H2(g)

The equilibrium constant expression for the above system is ,

Kc = Concentration of Fe & Fe3 O4 are neglected because they are in their pure solid state and their concentrations do not undergo appreciable change.

Significance or Applications of Kc:

Kc ( equilibrium constant ) has the following applications,

Prediction of Direction of Reaction :
Prediction of extent of reaction.

Calculation of equilibrium concentration