Thermodynamics Calculated Solution: Calculate Delta H and Delta S

Calculate H and S for the combustion

how would you calculate delta H and delta S for a combustion reaction of 1 mole of ethane (C2H6) at 25C

Answer by: dannimay

First Answer Best Answer


Step1: Set up a balanced equation for the combustion of 1 mole of ethane (C2H6) gas. 

1 C2H6 (g) + 7/2 O2(g) → 2 CO2 (g) + 3 H20 (l)
 -84.0 kj/mol           0                      -393.5 kj/mol         -285.8 kj/mol
Step 3: Use the formula [∑  products ( coefficent * deltaHf value )]- [ ∑reactants( coefficent * deltaHf value )]
The delta H of the combustion of 1 mole Ethane gas at 25 C is -1560.9 kj/mol
To find the delta S, follow the same steps. However, instead of using each substance’s standard molar enthalpies , you will use the standard molar entropy.
 ** Remember the standard entropy  values are given in Joules . The standard molar enthalpy values are given in kilojoules.**

Following the same steps but using the standard entropy values of the substance the solution for delta S = -309.8 j/mol

1 C2H6 (g) + 7/2 O2(g) → 2 CO2 (g) + 3 H20 (l)

Since the combustion reaction is at 25 ° C (298.15 K ), we can use the standard molar enthalpy formations  (delta Hf ) to find the delta H of combustioin.

* It is very important to include the physical phase(solid, liquid, gas, etc.)  for each 
substance in your balanced equation.

Step 2:  Using your Appendix ( most likely in the back of the book)under the Data Table for Thermodynamic Properties of Substances at 25°C (298.15 K)  find the delta Hf values for each substance in the reaction.


3 Responses

  1. If you ever want to hear a reader’s feedback 🙂 , I rate this article for 4/5. Decent info, but I just have to go to that damn msn to find the missed parts. Thanks, anyway!

  2. emm. bookmarked.

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