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Chemical Energetics

Internal energy: The energy stored in a fixed amount of a substance (atom or molecule) is known as its internal energy. It is denoted by the sign E.

Change in internal energy: It is the difference in the internal energies of the product and the reactant species taking part in a chemical reaction at constant volume. It is denoted by ΔE.
ΔE = Ep – Er
Where,      Ep =Internal energy of the product  Er = Internal energy of the reactants.
We can also say the change in internal energy in a chemical reaction is the measurement of the heat evolved or absorbed in a chemical reaction carried out at constant volume and constant temperature.

Enthalpy of a system: is the sum of internal" energy of the various species present and the product of its pressure and volume. It is also known as the heat content of the system and is denoted by H.

H=E+pV

Where,  E is the internal energy of species, P is Pressure of the system and V is Volume of the system.
Change in enthalpy: in a chemical reaction carried at constant pressure is the heat energy that is exchanged between the system and the surroundings. It is denoted by ΔH.

ΔH = Hp – Hr

Where,      Hr = enthalpy of the product  Ha = enthalpy of the reactants.
Relation between  ΔH and ΔE:

ΔH=ΔE+PΔV

Exothermic reaction: If in a chemical reaction carried at constant pressure, a certain amount of heat energy is released from the system to the surrounding at a given temperature, the reaction is known as the exothermic reaction.

C(s)+O2(g) → CO2(g);           ΔH= -343.3 kJ

 Endothermic reaction: If in a chemical reaction carried at constant pressure, a certain amount of the heat energy is absorbed from the surroundings by the system at a given temperature, reaction is known as the endothermic reaction.

N2(g) + O2(g) → 2NO(g) ; ΔH= +180.5 kJ

Principle of conservation of energy: The sum total of mass and energy of the universe (system and surroundings) always remains constant although energy may be exchanged between the system and the surroundings in different forms.

Enthalpy of formation: (Heat of formation). It may be given as the enthalpy change that accompanies the formation of one mole of a substance from its elements. It is measured under standard conditions of temperature and pressure. It is known as standard enthalpy of formation and is denoted as ΔHof.

Enthalpy of combustion: (Heat of combustion). It may be given as the enthalpy change that accompanies die combustion of one mole of a substance in excess of air or oxygen. Since combustion is an exothermic process, the value of ΔH in this case is always negative.

Enthalpy of neutralization: (Heat of neutralization) It may be given as the enthalpy change that accompanies the neutralization of one gram equivalent of an acid in dilute solution by a base and vice-versa. For example, when one gram of equivalent of HCI in dilute solution is neutralized by NaOH solution, the enthalpy change ΔH is -57 kJ.

HCI (aq) + NaOH (aq) → NaCl (aq) +H2O,   ΔH = -57 kJ.

It is worth noting that the enthalpy of neutralization of all reactions involving strong acid and strong base is the same i.e., -57 kJ, whatever may be the strong acid or base.

Enthalpy of fusion: (Heat of fusion). It is defined as the enthalpy change that accompanies the conversion of one mole of a solid substance into the liquid stale at .its melting point. For example, when one mole of Ice melts at 0oC or 273 k. the enthalpy of fusion is 6 kJ indicating that it is endothermic change.
H2O(s) → H2O(l) ; ΔH =+ 6 kJ.
Enthalpy of vaporization: (Heat of vaporization) It is defined as the enthalpy change that accompanies the convers­ion of one mole of a liquid into gas at its boiling point. For example, the enthalpy change accompanying the vaporization of one mole of water (liquid) into gas (vapors) at its boiling point i.e.,  100°C or 373 k is 40.6 kJ.

H2O(l) → H2O(g) ; ΔH = + 40.6 kJ.

Enthalpy of sublimation: (Heat of sublimation). It is defined as the change that accompanies the conversion of the mole of a solid directly into its gaseous state at a temperature below its melting point. For example, the enthalpy of iodine is 62.39 kJ.

I2(s) → I2(g);  ΔH= +62.39 kJ.

Hess's law of constant heat summation: The enthalpy change in a process, wether physical or chemical depends upon the enthalpies of the reactants and the products and not upon the manner in which the process is carried.

Mathematically, If          Q1 cal be heat evolved when A → D
                                        q1 cal be heat evolved when A → B
                                        q2 cal be heat evolved when B → C
                                       q3 cal be heat evolved when C → D
According to Hess's law Q1 = q1 + q2 + q3 = Q2

Calorific value of food stuff: The calorific value of a fuel or food is the energy released in the combustion of one gram of that substance (food or fuel).

Function of food in the body: Human body is a machine and it requires some fuel to keep it running or working. The food acts as a fuel and supplies the body the required energy to keep it warm and working. During the process of digestion and assimilation, the carbon contents of the food stuff oxidized to carbon dioxide while hydrogen is oxidized to water.
Both are pushed out of the body when we breath out. Nitrogen is converted to urea which is excreted out in urine.

Chemical energetics: It deals with the energy changes in a chemical reaction.

Thermochemistry. It deals with the heat energy change in chemical reaction.

Difference between ΔE and ΔH:
ΔE it gives a measure of heat of a chemical reaction carried at constant volume. Δ H, it gives a measure of the heat of reaction carried at constant pressure.

Bond energy: is the energy required to break a bond or released when a bond is formed. Thus bond breaking is an endothermic process and bond formation is an exothermic process. For example, the bond .energy of the H-H bond (in H2) is 433 kJ mol-1.

Standard state of a substance: 1 atmosphere and 298 k.

Standard enthalpy change: The change in enthalpy occur­ring at 298 k and under 1 atmospheric pressure for a process is called standard enthalpy change, and is designated by ΔHo.