ENERGETICS OF CHEMICAL REACTIONS:
The branch of science which deals with study of Principle of conservation of energy is called Thermodynamics. Thermo means heat and dynamics means motion (or) movement. The branch of science which deals with the energy changes is also called thermodynamics. Those reactions in which energy is changes (or) heat changes takes place are called thermo-chemical Reactions. The most important Examples of thermodynamics is “heat Engine” which converts heat energy into work. Exo = Release/emit/liberate + therm = heat / energy. Those reactions in which energy is released are called Exothermic Reactions. All combustion reactions are examples of Exothermic Reactions. Endo = to absorb /gain + therm = heat / energy. Endothermic reactions are those in which energy is gained (or) absorbed (or) added. When value of (∆H) is +ve Reaction is Endothermic and if (∆H) –ve the reaction is Exothermic reaction. Anything which is under observations and experiments called SYSTEM. The part if universe which is under thermodynamic study is also called system. All the remaining portion of the universe which act and effect on system is called SURROUNDING. The real (or) imaginary surface separating system from surrounding is called BOUNDARY. When system is uniform throughout is called HOMOGENEOUS SYSTEM. When a system consists of two (or) more phase or “not uniforms throughout” is called HETEROGENEOUS. There are three types of systems. When a system doesn’t exchange matter and energy with surrounding are called “ISOLATED SYSTEM”. When a system doesn’t exchange matter but must exchange energy with surrounding called “closed system”. When a system exchanges both matter and energy with surrounding called “OPEN SYSTEM”. Thermos flask is an example of isolated system. Hot water and beaker is an example of open system. Hot water in sealed tube is an example of closed system. The description of properties of system at any change is called STATE OF SYSTEM. The change is system before and after is called state functions.
eg ∆P = P2 –P1, ∆T = T2 – T1, ∆E = E2 –E1
Temp, pressure, volume and Enthalpy are state functions while work and entropy are not state functions. The properties of system which can be easily measured are known as Macroscopic properties. Such as Temperature, pressure, volume composition etc. The properties of system which doesn’t depend upon amount of material called “INTENSIVE” and which depends called “EXTENSIVE” properties. Density, Temperature, pressure, viscosity, surface tension, Refraction index, melting point and boiling point are Intensive properties. Mass, , mole, enthalpy, entropy and all kinds of energies are example s of extensive properties.
FIRST LAW OF THERMODYNAMICS:
First law of thermodynamic is also called law conversion of energy and was given by “Helmholtz” in 1847. Energy can neither be created nor be destroyed but it can be changed from one form to another form is called first law of thermodynamic. The total energy of system and surrounding must remain constant is also called first law of thermodynamic.
∆𝑬 = 𝒒 − 𝒘 (Or) q = ∆𝑬 + 𝒘 is first law of thermodynamic.
∆𝑬 = 𝒒 − 𝑷∆𝑽 (Or) q = ∆𝑬 + 𝑷∆𝑽 is pressure, volume and work equation.
There are two conditions of first law of thermodynamic. At constant volume ∆𝐸 = 𝑞, At constant pressure ∆𝐸 = ∆𝐻 − 𝑃∆𝑉 (or) ∆𝐻 = ∆𝐸 + 𝑃∆𝑉. Enthalpy (heat constant) expresses the thermal changes at constant pressure. For all endothermic reactions ∆𝐻 will be +ve. Enthalpy is indicated by H where H = E + PV. For all exothermic reactions ∆𝐻 will be –ve. For all endothermic reactions the amount of heat is less towards reactant and greater toward product. Work is +ve when work is done on system. Work is +ve when work is done by surrounding. Work is –ve when work is done by system. Work is –ve when work is done on surrounding. Heat (q) is +ve when heat is absorbed by system. Heat (q) is +ve when heat is added by system. Heat (q) is +ve when heat is gained by system. Heat (q) is +ve when heat is released by surrounding. Heat (q) is –ve when heat is released by system. Heat (q) is –ve when heat is absorbed by surrounding. 1 calories = 4.184 joules (or) 1j = 0.239 cal. Study of chemical reactions on the basis of heat or temp. is called THERMO-CHEMISTRY.
HESS’S LAW OF CONSTANT HEAT SUMMATION
In 1840 G.H HESS proposed Hess’s law of constant heat summation. The reaction takes place (or) completes in one (or) more than one steps the total change of enthalpy (heat) will remain always same is called HESS’S LAW. Hess’s law helps to calculate HEAT OF REACTION or HEAT OF FORMATION or HEAT OF ATOMIZATION. Hess’s law is used to calculate very fast and very slow reactions. Hess’s law is another form of first law of thermodynamic.
HEAT OF FORMATION:
Change of enthalpy when one gram mole of substance is formed from its element is called HEAT OF FORMATION . Heat of formation is represented by∆HF. Heat of formation at 𝟐𝟓℃ and 1 atm pressure called STANDARD HEAT OF FORMATION. Heat of formation is representedby ∆HF. Heat of formation is of CO2 is -394 kj/mole and H2O = -286 kj/mole. Heat of formation is of CH4 (Methane) is -75.66 kj/mole and acetic acid (CH3COOH) is -489.2kj/mole. Heat of formation is of Ethane (C2H6) is -85.368 kj/mole. There are four thermodynamic processes. Iso-choric process: when volume of system is constant. Iso-baric process: when pressure of system is constant. Iso-thermal process: when temperature is constant.
Adiabatic process: when there is no change of heat energy between system and surrounding. The total sum of potential and kinetic energy is called internal energy. When energy occurs itself at constant temperature and pressure is called Gibb’s free energy. The ability to do work is called FREE ENERGY. The force per unit area is called PRESSURE. The pressure applied by system in unit area is called FORCE. P = 𝑭. 𝑨 then force =PA
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