Question

QUESTION:
1 (a) Calculate the overall energy change in the combustion of ethane, C2H6. Is it an exothermic reaction or an endothermic reaction?

(b) Compare the energy released per gram from ethane with the energy released from glucose and butanoic acid.

(c) Based on these examples, predict which compounds contain more energy per gram: hydrocarbons, fats, or carbohydrates

Answer 1

(a) the overall energy change in the combustion of ethane is 3119.44 KJ/mol.

2C2H6 + 7O2 → 4CO2 + 6H2O

• energy change = energy of products - energy of reactants.

                                  = [ 4 * (-393.5) + 6 * (-285.8) ] - [ 2 * (-84.68) + 0 ]

                                  = -3288.8 + 169.36 KJ/mol

                                  = -3119.44 KJ/mol

                                  negative sign indicates that the reaction is exothermic.

(b)  energy released from combustion of glucose = -2803 KJ/mol.

• C6H12O6 + 6O2 → 6CO2 + 6H2O

energy released from combustion of butanoic acid is -1800.2 KJ/mol.

• C3H7COOH + 5O2 → 4CO2 + 4H2O

(c)  Hence, it is clear that hydrocarbons (ethane) contain more energy than fats (butanoic acid) and carbohydrates (glucose).