FSc Notes Chemistry Part 1 Chapter 7 Thermochemistry Lecture 4

FSc Notes Chemistry Part 1 Chapter 7 Thermochemistry Lecture 4

Indirect Calorimetry:

In this method enthalpy change is measured indirectly with Hess’s law. If a chemical reaction is too slow or too fast, then its enthalpy changes of such reactions are measured indirectly by using Hess' law.

Enthalpy of a Standard State:

The numerical value enthalpy of reaction depends on conditions like pressure and temp. Under which the reaction occurs. Thus the idea of reaction ( reactants & products ) is reported at a set of standard conditions of pressure and temperature.
The standard state for a substance is the state at which it is stable. Such conditions are 1 atm pressure and 298k, the st. enthalpy of H2(g) is zero. When the temp increases ( above 298k ) then the enthalpy of H2w ill have some negative value.

Consider the following reaction:

C(s) + O2(g) ----------------------CO2(g) = - 393.5k

Here both C and O2 are in their standard state, therefore their enthalpy are taken as OKJ/ mol

Now as we know that:

Enthalpy change of reaction = sum of enthalpy products- sum of enthalpy of reactants.

Some Common Forms of Enthalpy at Standard. States:

1. Standard Molar enthalpy  of Formation:

The amount of heat evolved or absorbed during the formation of one mole of a substance from its reactants at standard condition ( I e 1 atm. Pressure & 298 K Temp ) is known as standard molar enthalpy of formation of that substance. It is represented by Hf.

2. Standard Molar Enthalpy of Combustion:

The amount of heat evolved, when one mole of a substance in standard state O2 to produce CO2 and H2O is known as standard molar enthalpy of combustion of that substance. It is represented by Hc.

3. Standard Molar Enthalpy of Hydration:

The amount of heat evolved or absorbed when one mole of a substance( in its stand state ) is completely dissolved in water at 1 atm & 298 k, to form a solution is known as st. molar enthalpy of hydration of that substance. It is responsible by OHf.

4. Standard Enthalpy of Neutralization:

The amount of heat evolved, when one mole of H+ ions of an acid reacts with one mole of OH ions of a base to form one mole of water at standard conditions is known as Standard enthalpy of neutralization. It is represented by Hn.

Lattice Energy:

The amount of heat evolved when one mole of an ionic crystal lattice is formed form its ions ( in gaseous state ) is known as lattice energy of that ionic compound.

For example, when one mole of gaseous sodium ions ( Na+ (g) ) combine with one mole of gaseous chloride ions ( Cl- (g) ) to form one mole of sodium chloride ( in solid state ), is 770KJ. Thus lattice energy of NaCl is 770 KJ/ mole

Ie.

Na+ (g) + Cl- (g) ---------------- NaCl(s) ?HL.E = - 770 KJ

Written by: Asad Hussain