Chemical Energetics :
Chemical energetics is a very useful thing for every student by itself. And sometimes it uses in our everyday life. So I have provided all notes about chemical energetics for everyone.
REVIEW OF THERMODYNAMICS :
1. Thermodynamics . is defined as the branch of science which deals with changes in energy (usually heat) that accompany physical or chemical changes in matter.
2. Objectives of Thermodynamics . (i) To predict the feasibility of a process (ii) To predict the extent of reaction (iii) To deduce certain laws of physical chemistry.
3. Limitations of Thermodynamics . (i) Law of thermodynamics apply only to matter in bulk and not to individual atoms or molecules (ii) It predicts only the feasibility of process but not its success (iii) It doesn't predict the rate of reaction (iv) It doesn't tell anything about the mechanism of reaction.
4. System . is a part of universe selected for thermodynamic study.
5. Surroundings . is the part of the universe not selected for thermodynamic study.
6. Open system . is the system which can exchange matter as well as energy with the surroundings.
7. Closed system . is the system which may exchange energy but not matter with surroundings.
8. Isolated system . is the system which can neither exchange energy nor matter with the surroundings.
9. Macroscopic system . is the system which consists of a large number of atoms, ions or molecules.
10. Enthalpy (H) is the total heat content of the system at constant pressure. Mathematically,
H = U + PV
Enthalpy is also a state property and its absolute value can't be determined. However, enthalpy change, ∆H can be determined easily.
∆H = Hp - Hr
Change in enthalpy = Enthalpy of products - Enthalpy of reactants
∆H = (Up + PVp) - (Ur + PVr) or
∆H = ∆U + P ∆ V
In case of gases, p ∆ V = ∆n RT, where ∆n is the change in number of moles of gases when volume changes by ∆V . Therefore,
∆H = ∆U + ∆nRT
∆n = (Number of moles of gaseous products) - (Number of moles of gaseous reactants)
∆H = ∆U when ∆n = 0
i.e. Enthalpy change = heat evolved or absorbed at constant pressure.
11. Exothermic Reactions are the reactions which are accompanied by evolution of heat energy and have negative value of ∆H e.g.
C(s) + O2(g) → CO2(g) ; ∆H = - 393.5 kj
12. Endothermic Reactions are the reactions which are accompanied by absorption of heat energy and have positive value of ∆H e.g.
N2(g) + O2(g) → 2NO(g) ; ∆H = + 180.5 kj
13. Enthalpy (heat) of Reaction is the total amount of heat energy evolved or absorbed when the number of moles of reactants react completely to give the products as given in the balanced chemical equation. Since most of the reactions take place at constant pressure, therefore, enthalpy change (∆H) is the heat of reactions, e.g.
C(s) + O2(g) → CO2 (g) ; ∆rH^0 = -393.5 kj
14. Enthalpy (heat) of Solution is enthalpy (heat) change when one mole of solute is dissolved in large excess of solvent, so that on further dilution, no heat change occurs, e.g.
KCL (s) + aq → KCL (aq) ; ∆sol H^0 = +19.9 kj
H2SO4(l) + aq → H2SO4 (aq) ; ∆sol H^0 = -95.8 kj
15. Enthalpy (heat) of fusion is the enthalpy (heat) change when one mole of a solid changes into liquid or vice versa, e.g.
H2O (s) → H2O (l) ; ∆fus H^0 = 6.06 kj
H2O (l) → H2O (s) ; ∆fus H^0 = -6.06 kj
1. First Law of Thermodynamics :
(i) In a system of constant mass, energy can neither be created nor destroyed by any physical or chemical change.
(ii) The sum of all the forms of energy in an isolated system in constant.
(iii) When one form of energy disappears exactly the same amount of energy appears in some other form.
(iv) The total sum of mass and energy in the universe is constant.
This law is also known as law of conservation of energy.
Mathematically, first law of thermodynamics can be expressed as :
∆U = q + w or q = ∆U - w
Where '∆U' is the increase in internal energy, 'q' is the heat absorbed and 'w' is the work done on the system.
Hess's law of Constant Heat Summation :
It states that the enthalpy change during a chemical reaction is the same. Whether the reaction takes place in one step or in several steps.
2. Second Law of Thermodynamics :
1. According to Kelvin . " It is impossible for a cyclic process to take heat from a cold reservoir and convert it into work without at the same time transferring heat from a hot to cold reservoir. "
2. According to Claussius . " It is impossible to construct a machine which is able to convey heat by a cyclic process from one reservoir at a lower temperature to another at higher temperature unless work is done on the machine by some outside agency.
It is important to note that a cyclic is a process in which a system returns to the original state after a succession of steps.
3. There exists a functions 'S' called entropy which is a state function. The entropy of the universe remains constant in a reversible process but it increases in an irreversible or spontaneous process. The entropy of the universe never decreases.
IMPORTANT PRINCIPLES AND DEFINITIONS OF THERMOCHEMISTRY :
Concepts of Standard State
The molar enthalpy of a substance is a function of temperature and pressure. In thermodynamics, the pressure dependence is removed by defining the standard state of substance as follows :
(i) For a pure gaseous substance, the standard state at a given temperature is the ideal gas at one bar pressure.
(ii) For a liquid substance, the standard state at a given temperature is the pure liquid at one bar pressure.
(iii) For a pure crystalline substance, the standard state at a given temperature is the pure crystalline substance at one bar pressure.
(iv) For a substance or ion in solution, the standard state at a given temperature is the unit molality of the species in ideal solution at one bar pressure.
The standard molar enthalpy of a substance is represented by placing the superscript degree to the symbol H. For example, the standard molar enthalpy of liquid water at 273 K is represented as H° (H2O, 1, 273K) or simply H°.
Standard Enthalpies Of Formation (∆f H°) :
Enthalpy of Formation : It is defined as the enthalpy change when one mole of substance is formed from its constituent elements.
It is denoted by ∆f H. For example, 2 moles of hydrogen gas react with one mole of oxygen gas to produce 2 moles of water (liquid). The reaction is exothermic and proceed with the evolution of 572kj of heat. If may be expressed as :
2H2(g) + O2(g) → 2H2O(l) ; ∆H = -572 kj
Hence, heat of formation of H2O(l) i.e.
∆f H = -572 kj/2 = -286 kj
Standard enthalpy of formation . It is defined as :
The enthalpy change when one mole of a substance is formed from its constituent elements in their standard states.
Third Law Of Thermodynamics :
According to third law of thermodynamics, " The entropy of a perfectly crystalline substance are absolutely zero is taken as zero ". This law is given by 'Nernst'. As the temperature is raised, the entropy of pure substance increases. The definition is vaild for perfectly crystalline solids.
Lewis and Randall has given modified definition as below :
" Every substance has a finite value of positive entropy but at absolute zero temperature, the entropy may become zero and it does become zero in case of perfectly crystalline solids".
It is known that is not possible to get absolute zero temperature, So that the third law of thermodynamics can be indirectly stated as below :
" It is not possible to reduce temperature of any system to absolute zero by any method involving finite number of operations, however, the ideal method may be,".
And who want to get complete notes of Chemical Energetics. Check here 👇👇https://drive.google.com/file/d/109e-e6gpy06I7th2VQ7_mZsCgkHw_u4m/view?usp=drivesdk
And I hope it will be helpful for for every student. Please share with your friends who want to get these notes.
__________________________________________________________________
Who want to know more about like"Atomic physics" Click here 👇👇
Who are interested to know more about like this topic "What is the Herbariums ?" Click here 👇👇
Comments
Post a Comment
[Attention!!!] If you have any questions and need anything,
Please comment here 👆👆 and get in minutes.