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Thermochemistry

Hesse's law Related Problems 
Question By applying, Hess’s law calculates the enthalpy change for the formation of an aqueous solution of NH4Cl from NH3 gas and HCl gas. The results for the various reactions and pressures are as follows. 
Solution

Question Calculate the heat of formation of ethyl alcohol from the following information. 
(i) Heat of combustion of ethyl alcohol is -1367 kJmol-1. 
(ii) Heat of formation of carbon dioxide is -393.7 kJmol-1.
(iii) Heat of formation of water is -285.8k Jmol-1. 
Solution

Question If the heat of combustion of C2H4, H2 and C2H6 are -337.2, -68.3 and -372.8 k calories respectively, then calculate the heat of following reaction. C2H4(g) + H2(g) C2H6(g) 

Solution. According to Hess’s law above reaction can be broken into three steps


Question Graphite and diamond are two forms of Carbon. The enthalpy of combustion of graphite at 25oC is -393.51 kJmol-1 and that of diamond is -395.41 k jmol-1. What is the enthalpy change of the process? Graphite diamond at the same temperature? 

Solution. According to Hess’s law above reaction can be broken into two steps 


Question: What is the meaning of the term enthalpy of ionization? If the heat of neutralization of HCl and NaOH is -57.3 kJmol-1 and heat of neutralization of CH3COOH with NaOH is -55.2 kJmol-1. Calculate the enthalpy of ionization of CH3COOH. 

Solution Enthalpy of ionization: Enthalpy of ionization is amount of energy required to convert the one mole of weak electrolyte into its ions. As strong acids and bases are completely ionized in water so we can write heat of neutralization for strong acids and base as 

Question: Explain what is meant by following terms. (i) Atomization energy (ii) Lattice energy 
Answer.
(i) Atomization energy: Amount of energy absorbed, when one mole a gaseous atom is formed from an element. Its units are kJmol-1. 
       Na(s)-------------->           Na(ℓ)                    ∆H0 = 108 kJmol-1 
      1/2Cl2(g) ------------->      Cl(g)                    ∆H0 = 121 kJmol-1      (ii) 

Lattice energy Amount of energy released when one mole of the ionic compound is formed from gaseous ions. For example, Na(g) + Cl(g) -----------------> NaCl(s)     ∆

Born-Haber cycle Related Problems 
Question. Draw a complete, fully labeled Born-Haber cycle for the formation of potassium bromide.

Answer

 
Question Using the information given in table below, Calculate the lattice energy of  potassium   bromide.
Solution


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