Question

Calculate the standard enthalpy of formation of potassium nitrate (KNO3) given the following information:- Heat of combustion of potassium is -468 kJ/mol- Heat of combustion of nitrogen is -940 kJ/mol- Heat of combustion of oxygen is -283.8 kJ/mol- Heat of formation of KNO3 is -383 kJ/mol.

Answer 1

To calculate the standard enthalpy of formation of potassium nitrate  KNO3 , we need to consider the balanced chemical equation for the formation of KNO3 from its elements in their standard states:K  s  + 1/2 N2  g  + 3/2 O2  g   KNO3  s Now, we will use Hess's Law, which states that the enthalpy change for a reaction is the same, whether it occurs in one step or several steps. We will use the given information to find the enthalpy change for each step and then sum them up to find the standard enthalpy of formation of KNO3.Step 1: Combustion of potassiumK  s  + 1/2 O2  g   K2O  s     H1 = -468 kJ/molStep 2: Combustion of nitrogen1/2 N2  g  + 1/2 O2  g   NO  g     H2 = -940 kJ/molStep 3: Combustion of oxygen3/2 O2  g   3/2 O2  g     H3 = -283.8 kJ/molStep 4: Formation of KNO3K2O  s  + NO  g  + 1/2 O2  g   KNO3  s     H4 = -383 kJ/molNow, we can use Hess's Law to find the standard enthalpy of formation of KNO3:Hf  KNO3  = H1 + H2 + H3 + H4Hf  KNO3  = -468 kJ/mol +  -940 kJ/mol  +  -283.8 kJ/mol  +  -383 kJ/mol Hf  KNO3  = -2074.8 kJ/molThe standard enthalpy of formation of potassium nitrate  KNO3  is -2074.8 kJ/mol.