FSc Notes Chemistry Part 1 Chapter 7 Thermochemistry Lecture 3

FSc Notes Chemistry Part 1 Chapter 7 Thermochemistry Lecture 3

Thermodynamics:

The branch of science which deals with the study of transformation of heat from a region of high temp to a region of low temp in known as thermodynamics.

1st Law of Thermodynamics (Law of Conservation of Energy):

This law states that energy can neither be created nor destroyed, however it can be converted from one form to another.

To provide various mathematical forms of 1st law of thermodynamics:

Some quantity of heat at a constant pressure is provided. Some of this heat is used up in increasing ) the internal energy by an amount while some is used by the system to do work on the surrounding. The system performs work by moving the piston up.

Thus mathematically, 1st law of thermodynamics will be,

• It is important to note that : work done by the system on surrounding has negative value.
• It work is done on the system by the surrounding, then it has a positive value.
• If heat is provided to system at a constant volume then no work will be done and the heat (qv) only increases the internal energy of the system.

In this case the mathematical form of 1st law of thermodynamics will be

Similarly when work is done on the system and no heat transformation occurs, then the internal energy of the system increases by an amount which is equal to the work done on system.

In this case the mathematical form of 1st law of thermodynamics is

In General, if heat “q” is supplied to the system and at the same time work is done on the system,

then the increase in internal energy (DE) will be,

In case of eq iii we use same set of units for “E” “q” an d”w”. The most frequently used units

are” Joule “ J and “Calories” (Cal) where

I Cal = 4.18 J

Pressure – Volume Work:

The work ( done by the system or on the system) which appears as a result of compression or expansion of a gas against a constant appointing pressure in known as pressure- volume work.
Lets consider a gas enclosed in cylinder of cross- sectional area (A) fitted with a friction-less piston.

If a constant pressure is applied then, the volume of the gas decreases.

P = F/A

Or

F = P*A----------1

Here work is done on the system during which the position moves by a distance “Ol”, thus the work done will be,

W = F * l--------------2

Putting the value of F from eq 1:

W = P*A* l------------3

But as A*Dl = so eq iii becomes

W = P V

Enthalpy ( Heat content ) (H)

The total heat content of a system in known as enthalpy of the system. 
It can be properly defined as:

Sum o f the internal energy of system and its capacity of doing work is known as enthalpy of the system.

According to 1st law of thermodynamics

But as we know that enthalpy is the sum of “E” and “w” then,

H = E + W

 Or H =E + PV--------------ii

Enthalpy depends upon the initial and final states of the system, there fore it is a state function.

In case of a chemical reaction the amount of heat lost or gained during the reaction is known as enthalpy of reaction. It is denoted by “OH”.

• In case of exothermic reactions, enthalpy of reactants is greater than enthalpy of the products: therefore enthalpy of exothermic reaction has negative value.
• In case of endothermic reactions,m the enthalpy of reactants is lower than the enthalpy of the products; therefore enthalpy of endothermic reactions has a positive value.

Measurement of Enthalpy:

There are three main methods for measurement of enthalpy of a reaction : which are

•  Direct calorimetry
• Indirect calorimetry
• van't Hoff equation.

We are going to discuss the 1st two methods

Direct calorimetry:

The enthalpy of those reactions, which go to completion without reaction, is measured by direct calorimetry with the help of calorimeter. Examples of such reactions are neutralization of a strong acid strong base and combustion of a carbon containing compound etc.

A calorimeter consists of an insulated container which is filled with water. A thermometer is placed in the water and waited reaction mixture is placed in water.

• If the reaction is exothermic, then heat is evolved into water and the temp. increases thus the change in temperature is noted.
• If the reaction is endothermic, then heat is absorbed from the water and thus the temp decreases and the change in temperature is noted.

Then the specific heat of the substances with respect to which the enthalpy is measured ie reactants is noted and by the following equation, the heat lost or gained (Q) or enthalpy is measured.

Q = MC T

Q = heat lost or gained ( enthalpy )

M = mass of substances

OT = change in temp.

C= specific heat of reacting mixture

Specific Heat:

The amount of heat required to raise the temp of one mole of a substance through 1degree Celsius, is known as Specific Heat ( C ) of that substance.

Written by: Asad Hussain