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Thermochemistry

Definition of Thermochemistry- it is the branch of chemistry which deals with the quantitative relationship between heat and other forms of energy.


Thermochemistry Helps to predict the feasibility of the chemical reaction and provides the explanation of the macroscopic properties of the matter in terms of the concepts which are supported by the microscopic views of our material world, and also predict the extent to which the chemical reaction can occur before the equilibrium is attained. 


How to Write Thermochemical Equations- There are some rules to write a thermo-chemical equation which are described as below;1. An enthalpy change is written as dH and the enthalpy change for reverse process of same chemical reaction is represented as (-dH). 


The thermodynamic changes depend on the reactants and product phase in a chemical equation so the phase should be specified carefully.H2O (s) ?H2O (l); dH = 6.00 kJ Or the above reaction (melting one mole of ice to liquid water) needs the enthalpy of 6.00 kJ and for the reverse process (freezing the water to ice) water loses the enthalpy which is equal to -6.00 kJ. 


For the endothermic and exothermic process, the dH is positive and negative respectively and both process are reversible to each other. 2. For simplification, the thermochemical equation and thermodynamic quantity is multiplied by a constant. For example in the above reaction if 6.00 kJ of enthalpy is required to melt one mole of ice so 12.0 kJ (2x6) is required to melt two moles of ice.3. 



A Stoichiometric ratio is used to write the balanced thermochemical equation. For example, the reaction is written as: H2 (g) + Cl2 (g) ?2 HCl (g); dHº = -183 kJ Thermochemical Equations Examples- For example, the thermo chemical equation for the chemical change of N2 and H2 into NH3 in which 46.1kJ of heat is released for each mole of ammonia so the thermochemical equation for the reaction is, Since this is a exothermic process as heat is released so ?H < 0Balanced equation; N2 (g) + 3H2 (g) ? 2NH3 (g) the thermochemical equation is; N2 (g) + 3H2 (g) ? 2NH3 (g), ?H = - 92.2 kJ 


Thermochemistry Practice Problems-(1)2.5 mol of ideal gas at 2 atm and 27ºC expand up to 2.5 times isothermally against 1 atm external pressure. What will be the work done.Ans-w=-pexp (V2-V1)The initial volume V1="nRT" /P= 2.5(0.082)(300)/2 = 30.75LFinal volume V2 = 30.75x 2.5 = 76.87LSo work = -1 (76.87- 30.75) = -46.37L atm = (-46.37 x 101.325) = -4698.44 J(2) In the above question, calculate the work done for the isothermal reversible expansion.Ans-wrev= -2.303 nRT [log (T2/T1)]Or -2.303x 2.5 x 8.314 x 300 log (76.87/ 30.75)= - 5714.15 J