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Chapter 3

Kinetics and Thermodynamics of Reactions

Enthalpy (∆H)
Enthalpy can be regarded as thermodynamic potential. It is a state function and an extensive quantity. It also refers to the difference in bond energies between the reactant and product. In short enthalpy is ‘ the heat absorbed (or released) by a chemical reaction’.

Calculation of Enthalpy
Enthalpy is a measure of difference in bond energy between reactants and products.
E.g. Combustion of methane.
	CH_4(g) + 2 O_2(g) = CO_2(g) + 2 H₂O_(l)
	ΔH_reaction = Σn_p H(reactants) – Σn_r H(products )
	ΔH^o_comb = ΔH^o_f,methane +2 ΔH^o_f,oxygen-2 ΔH^o_f,water - ΔH^o_{f,carbon dioxide}
	= 75+0-(2*286+394) (ΔH^o_f,oxygen = 0 as oxygen is a pure element.)
	= -891 KJ mol^-1.
ΔH^o is +ve when reaction is endothermic (endergonic).
ΔH^o is –ve when reaction is exothermic (exergonic).