Question

How to determine the heat of formation of sucrose after a bomb calorimeter experiment?

Answer 1

Bomb calorimeter gives the calorific value i.e. amount of energy produced per gram, of a substance.

The bomb calorimeter can be used to determine the heat of formation of sucrose as follow-

1. Determine the calorific value of sucrose.

Using bomb calorimeter, determine the calorific value of sucrose.

Suppose, the value obtained is X kJ/ gram.

2. Determine heat of formation.

Part A: Bomb calorimeter carries out combustion of the specified substance in excess of O₂. For sucrose, the reaction can be summarized as follow-

            C₁₂H₂₂O₁₁ + O₂ (excess) ------> 12CO₂ + 11H₂O   ; equation 1

The calorific value indicates that X kJ energy is released when 1 gram of sucrose is combusted to yield CO₂ and H₂O. So, the amount of energy needed for form sucrose from CO₂ and H₂O is equal to amount of energy released during its combustion- both the reactions are opposite to each other-

            12CO₂ + 11H₂O --------> C₁₂H₂₂O₁₁ + 12O₂ ; equation 2

Part B: Bomb calorimeter gives energy produced per gram of sucrose.

Calculate the amount of energy released from combustion of 1 mole of sucrose. This energy is equal to (molecular weight of sucrose x amount of energy released from combustion of 1 gram of sucrose).

Thus, a mole (180 gram) of sucrose yields 180X kJ energy.

Since, equation 1 and 2 are same but in opposite direction.

Thus, the heat of formation of sucrose = +180X kJ mol^-1