Calculating the Heat of Combustion for C2H6 Using Hess's Law

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Discussion Overview

The discussion revolves around calculating the heat of combustion for ethane (C2H6) using Hess's Law, with participants examining provided chemical equations and their validity. The scope includes theoretical reasoning and mathematical manipulation of thermochemical equations.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant requests assistance in calculating the heat of combustion for C2H6 using given equations, expressing difficulty in applying Hess's Law.
  • Another participant points out that the equations provided are not balanced, questioning whether the product in the second equation should be C2H6 instead of C2H2.
  • A participant suggests that the original equations may contain a typo, as they do not seem to balance correctly.
  • Further clarification is provided by another participant, reiterating that the equation C2H4 + H2 should likely produce C2H6, indicating a probable error in the source material.
  • One participant expresses understanding of the balancing issue and inquires about the methodology for calculating heat of combustion, questioning if it is based on formulas or general knowledge.
  • Another participant advises treating the equations like a puzzle, suggesting that participants can cancel out species that appear on both sides of the reaction arrow and reminding them that reversing an equation changes the sign of the enthalpy.

Areas of Agreement / Disagreement

Participants generally agree that there may be a typo in the provided equations, but there is no consensus on how to proceed with the calculation due to the unresolved nature of the equations.

Contextual Notes

The discussion highlights limitations in the provided equations, including potential typographical errors and the need for balancing, which affect the ability to apply Hess's Law effectively.

123456
Calculate the heat of combustion for C2H6 from the following information:
C2H4 + 3 CO2 --> 2 CO2 + 2 H2O /\H f -1409.5 kJ
C2H4 + H2 --> C2H2 /\H f - 136.7 kJ
H2 + 1/2 O2 --> H2O /\H f - 285.5 kJ

i saw someone else ask this question and they were referred to look at Hess's law.. i have done that but i am still unable to solve this i am so stuck.
could someone walk through the steps please?
 
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C2H4 + H2 --> C2H2 /\H f - 136.7 kJ
C2H4 + 3 CO2 --> 2 CO2 + 2 H2O /\H f -1409.5 kJ

these makes no sense(they are not balanced eqs)...did you mean for the product to be C2H6 in the 2nd eq.?
 
hmm

no, what i wrote was exactly how its written in the book... is it not able to be solved, possibly they made a typo in the book?
 
This C2H4 + H2 --> C2H2 does not make sense.

Try C2H4 + H2 --> C2H6 perhaps. If this is in the book, there is a probably a typo.
 
Last edited:
I see, must be a typo...

Ohhh I see what you mean now, when you said it doesn't balance. It must be a typo then. Is there a formula or anything you use to calculate heat of combustion.. or is it just knowledge...?
 
Last edited by a moderator:
play around w/ the equations like a puzzle and treat them like math equations. if two of the same things appear on the left and right side of the arrow you can cancel them out. use those equations to come up w/ the equation for C6H6+O2-->CO2 +O2 (i'm too lazy to balance). Remember, if you flip an equation around you change the sign of the Enthalpy of the rxn...
 

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