General ChemistryII Workshop 7: Hess\'s Law and Entropy Part 1: Hess\'s Law Hess

Question

General ChemistryII Workshop 7: Hess's Law and Entropy Part 1: Hess's Law Hess's law is a manifestation that enthalpy is a state function. It states that regardless of the multiple stages or steps of a reaction the total enthalpy change for the reaction is the sum of enthalpy changes in all steps. This law allows us to calculate the overall change in enthalpy by simply summing up the changes for each step of the way until product is formed 1. Calculate AHrn for the following reaction based on the given data. Is this reaction endothermic or exothermic? CaHe (8) C2H() H2 (8) C2H6(g)+3.502 (g)2C02 (g) 3H20 ) AH-1560. kl/mol CHs (8)+302)2C02g) 2H20 () 2H20 2H2 (g)+0z (g) AH-? :-14114/mol H-571.6 ki/mol 2. How much heat is transferred between the system and the surroundings when 25 grams of ethane (CzHo) decomposes to produce ethylene (CaHe) and hydrogen gas? Please specify if energy is release or absorbed by the system.

Explanation / Answer

Ans 1

Write the second equation as it is

C2H6 + + 3.5 O2 = 2CO2 + 3H2O

H1 = - 1560 kJ/mol

Reverse the third equation

2CO2 + 2H2O = C2H4 + 3O2

H2 = +1411 kJ/mol

Multiply the fourth equation by 0.5

H2O = H2 + 0.5 O2

H3 = 571.6*0.5 = 285.8 kJ/mol

Now add these above equation and we get the first equation

C2H6 = C2H4 + H2

H = H1 + H2 + H3

= - 1560 +1411 +285.8

= 136.8 kJ/mol

Ans 2

Mass of ethane C2H6 = 25 g

Moles of C2H6 = mass/molecular weight

= 25 g / 30 g/mol

= 0.8333 mol

Heat transferred = (136.8 kJ/mol) x 0.8333 mol

= +114 kJ

The sign is positive so the energy is absorbed by the system

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