The Stoichiometry of the Enthalpy of Reaction 

Unit 9: Thermodynamics                                                                                 Chapter 17
WORK MODE:  Pairs                                                                                    GIA 9-1

KEY WORDS: Enthalpy, heat of reaction, stoichiometry

LEARNING OBJECTIVE:  To form and use the stoichiometric ratios between the moles of substances in a thermochemical equation and the enthalpy of the reaction.

INSTRUCTIONS:

Thermochemical equations are balanced chemical equations which show the enthalpy of a reaction as shown in the balanced equation.  An example is

2 H₂(g)   +   O₂(g) à 2 H₂O(g)                        DH^o = -241.826 kJ

This thermochemical equation states that for every 2 mol of dihydrogen gas consumed, the heat of reaction is -241.826 kJ.  One can write an equality relationship and two unit conversion factors for this relationship:

2 mol H₂(g) = -241.826 kJ                2 mol H₂(g)      and      -241.826 kJ
                                                        -241.826 kJ                   2 mol H₂(g)

Likewise, for every 2 mol of gaseous water produced, the heat of reaction is -241.826 kJ, and so forth.  Since the amount of energy in a chemical reaction is directly related to the amount of material involved in the reaction, it follows that if 4 mol of dihydrogen are consumed, the heat of reaction will be twice as much as that for the consumption of 2 mol of dihydrogen.  This is calculated by dimensional analysis as follows:

4 mol H₂(g)  x   -241.826 kJ  =  -483.652 kJ
                            2 mol H₂(g)

            In this assignment you will be given a balanced thermochemical equation.  You and your partner will use this equation to calculate the mole, mass, or energy quantities requested.

ACTIVITY:

            The combustion of ethane gas, C₂H₆(g), is given by

            2 C₂H₆(g)   +   7 O₂(g) à  4 CO₂(g)   +   6 H₂O(g)     DH^o = - 1723.6 kJ

Use this balanced thermochemical equation to do the following:

Student 1:  For the equation as written, write the equality relationship between ethane and the heat of the reaction.

Student 2:  From the equality relationship, write the two unit conversion factors between the mol of ethane and the heat of the reaction.

Student 1:  Calculate the observed heat of this reaction if  6.50 mol of ethane are consumed.

Student 2:  Calculate the number of moles of ethane needed to give an observed heat of reaction of -4655 kJ.

Student 1:  Calculate the number of moles of ethane needed to give an observed heat of reaction of –6345 kJ

Student 1:  Calculate the number of moles of carbon dioxide produced if the observed heat of the reaction is -2742 kJ.

Student 2:  Calculate the observed heat of this reaction if 15.5 mol of carbon dioxide are produced.

Student 1:  Calculate the observed heat of reaction if 25.5 mol of carbon dioxide are produced. 

Both students:  How is the heat of a reaction affected by the amount of reactants consumed and the amount of products produced?

SUPPLEMENTAL QUESTIONS: Do these also!

(The following questions can be solved outside of class time)

Calculate the enthalpy change if 150.0 g of dioxygen are consumed.

If 225.5 g of ethane burn in excess dioxygen, what is the heat of the reaction?

If 850.0 g of ethane burn in the presence of 850.0 g of dioxygen, how much heat could be released?