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 * deltaH_f value )]- [ ∑reactants( coefficent * deltaH_f 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 H_f ) 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 H_f values for each substance in the reaction.