There are several important definitions of concepts and chemical laws used in chemistry, chemical studies and industrial manufacturing procedures that are very useful to students, researchers, scientists and engineers in industrial production and manufacturing processes where these vast arrays of knowledge, in the form of laws and theories are applied. These laws include the Hess Law of constant heat summation, Le Chaterliers principle, Boyles Law, Charles law and definitions such as standard heat of combustion, standard heat of solution, bond energy, lattice energy, standard heat of formation, standard heat of neutralization and standard heat of reaction.
- Hess’s Law of Constant Heat Summation:– The enthalpy change of a chemical reaction can be found by experiment and it can also be found from known values of enthalpy changes of related reactions and this is done using Hess’s law of constant heat summation. Hess’s law of constant heat summation states that the total enthalpy change of a chemical change is constant regardless of the route by which the chemical change occurs, provided that the conditions at the start of a reaction are the same as the final conditions.
- Le Chartlier’s Principle:– Le Chatelier’s principle states that if an external constraint such as a change in pressure, concentration or temperature, is imposed on a chemical system in equilibrium, the equilibrium will shift so as to cancel, annul or neutralize the constraint. Le Chaterlier’s principle is important because it helps us:–
(i) Reduce undesirable reversibility
(ii) Define the optimum conditions for the chemical process that is deployed in an industry and
(iii) Predict the effect of an altered factor on the equilibrium position of an untried reaction.
- Boyle’s Law:– Boyle’s law states that the volume of a given mass of gas is inversely proportional to its pressure, provided that the temperature remains constant. According to Boyle’s law, the volume of a gas increases as the pressure decreases and vice versa. That is, P1V1=P2V2, where V1=volume at P1 and V2= volume at P2.
- Charles’ Law:– Charles’ law states that the volume of a given mass of gas is directly proportional to its temperature in Kelvin, provided that the pressure remains constant. The law implies that the volume of the gas decreases as the temperature decreases and increases as the temperature increases.
That is, V1/T1=V2/T2
- Heat of Combustion:– The standard heat of combustion of a substance is the heat evolved when one mole of the substance is burned completely in oxygen under standard conditions.
- Heat of Solution:– The standard heat of solution is that amount of heat absorbed or evolved when one mole of a substance is dissolved in so much water that further dilution results in detectable heat change. The solubility of an ionic solid can be determined by its heat of solution. Therefore, a solid with a positive heat of solution value will not be as soluble as one with a negative value and since solubility varies with temperature, a substance which dissolves exothermically(with the release of heat and energy) would be more soluble at lower temperatures while substances which dissolves endothermically(with the absorption of heat and energy) would be more soluble at higher temperatures.
- Heat of Formation:– The standard heat of formation of a substance, is the heat evolved or absorbed, i.e. the enthalpy change, when one mole of that substance is formed from its elements under standard conditions. For instance, the standard heat change of formation of water is -285.58kJ/mol
- Heat of Reaction:– The characteristic internal energy possessed by a substance which is due to its structure and physical state. The standard heat of reaction is the heat evolved or absorbed when a chemical reaction occurs between molar quantities of the substance as represented in the equation of reaction under standard conditions.
- The Lattice Energy:– The lattice energy of an ionic crystal is the heat of formation of one mole of ionic compound from widely separated gaseous ions under standard conditions. Lattice energies enable us to understand the stability of ionic crystals but, just like bond energies, it cannot be measured experimentally. Lattice energies can only be assumed theoretically.
- Heat of Neutralization:– The standard heat of neutralization is the amount of heat evolved when one mole of hydrogen ions, from an acid reacts with one mole of hydroxide ion, from an alkali to form one mole of water, under standard conditions. Note that neutralization is an exothermic reaction.
Standard Heat of Neutralization of some Acid-Base Reactions
The table below outlines the standard figures of the heat of neutralization of some common acid-base reactions. These figures were derived from experimentations and they have become global standards with wide applications in chemistry and chemical industry applications.
Acid Alkali kJ/mol
HNOӡ NaOH -57.3
HCl NHӡ -51.5
CHӡCOOH NaOH -55.2
CHӡCOOH NHӡ -51.5