Atom economy = (grams desired product / grams of all products and byproducts) *

Question

Atom economy = (grams desired product / grams of all products and byproducts) * 100 The combustion of ethanol can be used to fuel a car. 1) Write a balanced chemical equation for the combustion of ethanol. Be sure to specify the phases for each compound. 2) What is the Delta H rxn in kJ? 3) Calculate the atom economy for the reaction using the following scenarios. a. CO2 is considered the desired product b. H2O is considered the desired product c. Both CO2 and H2O are considered waste. 4) Which number most accurately reports atom economy for the reaction? Explain your reasoning.

Explanation / Answer

Ethanol is CH3-CH2-OH. So when you add the oxygen for combustion you know you have to have the products of CO2 and H2O. Then balance it: C2H6O + 3O2 ==> 2CO2 + 3H2O The combustion of C2H5OH is as follows: C2H5OH (l) + 3O2 (g) -----> 2CO2 (g) + 3H2O (l) The heat of combustion of ethanol is -1368 kJ/mol. Heat of formation of CO2 (g) = -393.509 kJ/mol Heat of formation of H2O (l) = -285.83 kJ/mol Total enthalpy change = total heat of formation of CO2 reaction + total heat of formation of H2O in reaction Therefore, total enthalpy change = 2(-393.509) + 3(-285.83) = -787.018 + (-857.49) = -1644.508 kJ/mol Obviously, there is a difference between the total enthalpy change of the reaction and the heat of combustion of ethanol. This difference is the enthalpy of formation of the ethanol.

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