Thermochemistry and Thermochemical Law

Exothermic And Endothermic Reaction Total energy of the products of a chemical reaction may be different from the total energy of the reactants, so it is evident that the process may be accompanied by an absorption or liberation of energy in the form of heat. A. Exothermic reaction: If heat is liberated in a chemical reaction the process is said to be exothermic reaction. C(s)+O2(g)=CO2; H=-94.03kcal B. Endothermic reaction: If heat is absorbed in a chemical reaction the process is said to be endothermic reaction. H2(g)+I2(s)=2HI(g); H=11.90Kcal

Some Definitions HEAT OF FORMATION: ‘The change in enthalpy that takes place when one mole of the compound is formed from its elements.’ HEAT OF COMBUSTION: The change in enthalpy of a system when one mole of the substance is completely burnt in excess of air or oxygen.’ THE HEAT OF SOLUTION: ‘The change in enthalpy when one mole of a substance is dissolved in a specified quantity of solvent at a given temperature.’ THE HEAT OF NEUTRALISATION: ‘The change in heat content (enthalpy) of the system when one gram equivalent of an acid is neutralised by one gram equivalent of a base or vice versa in dilute solution’.

Some other definitions HEAT OF FUSION: It is defined as : the heat change (or enthalpy change) when one mole of a solid substance is converted into the liquid state at its melting point. HEAT OF VAPOURISATION: The heat of vaporisation is defined as : the heat change (or enthalpy change) when one mole of liquid is converted into vapor or gaseous state at its boiling point. HEAT OF SUBLIMATION: Heat of sublimation is defined as : the heat change (or enthalpy change) when one mole of a solid is directly converted into the gaseous state at a temperature below its melting point. HEAT OF TRANSITION: The heat of transition is defined as : the change in enthalpy which occurs when one mole of an element changes from one allotropic form to another. BOND ENERGY: When a bond between two atoms is formed, there is a release of energy. The same amount of energy is absorbed when the bond is broken. The bond energy is defined as the average amount of energy required to break all bonds of a particular type in one mole of the substance.

HESS’S LAW Hess’s law states that heat change in a chemical reaction is the same whether it takes place in one or several stages. This means that the net heat of reaction depends only on the initial and final states and not on the intermediate states through which the system may pass. This generalization is known as Hess’s Law and it may be stated as : ‘ If a chemical change can be made to take place in two or more different ways whether in one step or two or more steps, the amount of total heat change is same no matter by which method the change is brought about’.

0 KJ C(graphite)+O KJ CO+0.5O KJ -283KJ CO 2 For example, in the diagram, we look at the oxidation of carbon into CO and CO 2. The direct oxidation of carbon (graphite) into CO 2 yields an enthalpy of -393 kJ/mol. When carbon is oxidized into CO and then CO is oxidized to CO 2, the enthalpies are -110 and -283 kJ/mol respectively. The sum of enthalpy in the two steps is exactly -393 kJ/mol, same as the one-step reaction.

The second law of thermodynamics The failure in practice to convert heat completely into work leads to another way of expressing the second law of thermodynamics. It states that “It is impossible to construct a machine functioning in cycles which can convert heat completely into the equivalent amount of work without producing changes elsewhere.” The term ‘functioning in cycles’ is inserted to indicate that the machine must return exactly to its original state at regular stages, as explained below, so that it can operate continuously. For better understanding the form of the second law as enunciated by R. Clausius (1854): ‘ Heat cannot of itself, without the intervention of any external agency can pass from a colder body to hotter body.’

Cyclic Process When a system undergoes a series of changes and in the end returns to its original state, it is said to have completed as cycle. The whole process comprising the various changes is termed a cyclic process. Since the internal energy of a system depends upon its state, it stands to reason that in cyclic process the net change of energy is zero. Or, we can say that the work done by the system during all these changes should be equal to the heat absorbed by the system. ΔE = 0 = q – w or q = w

Carnot’s cycle In 1824 Sadi Carnot proposed a theoretical heat engine to show that the efficiency was based upon the temperatures between which it operated. Carnot’s imaginary engine could perform a series of operations between temperatures T 1 and T 2, so that at the end of these operations the system was restored to the original state. This cycle of processes which occurred under reversible conditions is referred to as the Carnot cycle. The medium employed in operating Carnot’s engine was one mole of an ideal gas which could be imagined to be contained in a cylinder fitted with a frictionless piston. The Carnot cycle comprises four operations or processes: (1) Isothermal reversible expansion (2) Adiabatic reversible expansion (3) Isothermal reversible compression (4) Adiabatic reversible compression

The above four processes are shown in the indicator diagram of Carnot cycle: Continued…..

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