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

Question

General Chemistry II Workshop 7:Hess's Law and Entropy Part 1: Hess's Law Hess's law is a manifestation that enthalpy is a state function and states that regardless of the multiple stages or steps of a reaction the total enthalpy change for the reaction is the sum of all changes. 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. All steps have to proceed at the same temperature and the equations for the individualsteps mustbalance out. 1. Calculate AHnn for the following reaction based on the given data. Is this reaction endothermic or exothermic? C2H4 (g) 302 (g) 2CO2 (g) 2H20 AH1 1411 k/mole C2H6 (g) 7/202 (g) 2002 (g) 3H20 (l) Ahiz 1560. kl/mole AHa 285.8 kJ/mole H2 (g) 1/202 (g) H20 2. How much heat is transferred between the system and the surroundings when 3.5 moles of ethylene (CzH4) reacts with excess of hydrogen gas to produce ethane (CaHo)? Please specify ifenergy is release or absorbed by the system.

Explanation / Answer

Q1.

HRxn for

C2H4 + H2 = C2H6

clearly, we need:

C2H4, H2 in the left, and C2H6 in the right

so, knowing this

RXN1 can be left as it is

RXN2, must be inverted

2CO2 + 3H2O --> C2H6 + 7/2O2 HRxn = -(-1560) = 1560

RXN(3) can be left as it is

add all rxn

C2H4 + 3O2 = 2CO2 + 2H2O

2CO2 + 3H2O --> C2H6 + 7/2O2

H2 + 1/2O2 = H2O

C2H4 + 3O2 + 2CO2 + 3H2O+ H2 + 1/2O2 --> 2CO2 + 2H2O +C2H6 + 7/2O2 + H2O H = -1411 + 1560 -285.8

cancel common terms

C2H4 + 3O2 + 2CO2 + 3H2O+ H2 + 1/2O2 --> 2CO2 + 2H2O +C2H6 + 7/2O2 + H2O H = -1411 + 1560 -285.8

C2H4 + 7/2O2 + H2 --> C2H6 + 7/2 O2 HRxn = -136.8 kJ

so

C2H4 + H2 --> C2H6 HRxn =  = -136.8 kJ

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