Help solving this thermo problem.

  • Thread starter Thread starter ipitydatfu
  • Start date Start date
  • Tags Tags
    Thermo
Click For Summary

Homework Help Overview

The discussion revolves around a thermodynamics problem involving the combustion of liquid ethanol (C2H5OH) to produce carbon dioxide (CO2) and water (H2O) under specific conditions. Participants are tasked with calculating changes in enthalpy (ΔH), internal energy (ΔE), heat (q), and work (w) for various scenarios related to the combustion process, while considering ideal gas behavior and constant temperature and pressure conditions.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning, Problem interpretation, Assumption checking

Approaches and Questions Raised

  • Participants discuss the use of enthalpy of formation and the relationship between ΔH, ΔE, and work. There are attempts to relate the changes in internal energy to the work done and heat flow, with some questioning the relevance of combustion heat in certain scenarios. Others express confusion about the implications of constant volume and constant pressure on work and heat calculations.

Discussion Status

The discussion is ongoing, with participants exploring different interpretations of the problem and attempting to clarify their understanding of the relationships between internal energy, enthalpy, and work. Some guidance has been offered regarding the calculations and relationships involved, but there is no explicit consensus on the correct approach or final answers.

Contextual Notes

Participants are working under the constraints of a homework assignment, which may limit the information available and the methods they can use. There are indications of confusion regarding the assumptions made about heat exchange and the state of the gases involved in the combustion process.

ipitydatfu
Messages
13
Reaction score
0
this problem is quite long, all I am asking is some for some help by clarifying and hints so i can try to solve this question. Thanks in advance!
Consider the combustion of liquid ethanol, C2H5OH(l), to form
CO2(g) and H2O(g). Suppose we have 1 mole of ethanol in the presence of 4 moles of O2
gas in balloon at a temperature of T = 298K and a pressure of 1 atm (i.e., pi = pext = 1atm).
The ethanol combusts to completion, and assume we can neglect any changes in volume due to
the liquid ethanol. All of the gases can be treated as ideal gases.

a) If the combustion takes place such that the internal temperature of the gas remains constant, and the external pressuredoesn’t change, calculate ΔH, ΔE, q, w (hint: even though ΔT = 0, ΔE and ΔH are nonzero since the chemical energy and number of moles of gas have changed).

b) Suppose instead that the balloon’s volume was fixed, and that that the balloon could
not exchange heat with the surroundings (i.e., q = 0). Instead, the heat from combustion was
used to only increase the temperature of the gases in the balloon. Question: what is ΔE, ΔH,
w, and ΔT. What is the final temperature? What’s the final pressure? Hint: remember that
ΔH = Ef + pfVf- ΔEi -ΔpiVi.

c) Now suppose instead that the balloon was thermally isolated from the environment
(i:e:; q = 0) and the balloon was allowed to expand the volume of the balloon against a constant
pressure of 1 atm. What is the final temperature and volume of the balloon? What is ΔE, ΔH, w,
and ΔT?
 
Physics news on Phys.org
sorry for double post:

can you see if this is correct:

a)
using enthalpy of formation (info found from a table of values) of the reaction, i can say, enthalpy = delta(U) + delta (pv)
and that delta (pv) = RT* delta n

with that, i solve for delta U

work is p delta v, (vfinal found from using info given using gas ideal gas law)

therefore delta q = delta u - work.


b was tricky for me:

if q = 0, then delta u = delta work

using heat of formation from part A:
delta H = delta u + delta (pv)

now I am stuck. I am left with too many unknowns, a final temperature, and a final volume.

what I am truly confused about is while Work = 0 under constant volume, i don't think it applies to molecules that have combusted. ugh
 
ipitydatfu said:
this problem is quite long, all I am asking is some for some help by clarifying and hints so i can try to solve this question. Thanks in advance!
Consider the combustion of liquid ethanol, C2H5OH(l), to form
CO2(g) and H2O(g). Suppose we have 1 mole of ethanol in the presence of 4 moles of O2
gas in balloon at a temperature of T = 298K and a pressure of 1 atm (i.e., pi = pext = 1atm).
The ethanol combusts to completion, and assume we can neglect any changes in volume due to
the liquid ethanol. All of the gases can be treated as ideal gases.

a) If the combustion takes place such that the internal temperature of the gas remains constant, and the external pressuredoesn’t change, calculate ΔH, ΔE, q, w (hint: even though ΔT = 0, ΔE and ΔH are nonzero since the chemical energy and number of moles of gas have changed).
How many moles of gas are produced? How many did we start with? What volume do/did they occupy? What work is done (on the external atmosphere) in the reaction? How is the work done related to the heat flow? to the change in internal energy?

AM
 
how much moles of gas was produced:

EtOH + 4 O2 -> 2CO2 + 3 H2O + O2 (left over from the reaction)
therefore N initial = 4 moles of gas, N final = 6 moles of gas

using pv = nrt with initial conditions would lead to roughly 97 L

messing around with the ideal gas law equation it leads to:
Vfinal = (#mol final/#mol initial) V initial

... i explained how to get the info in my second post, ok more specifcally
Work done on external = pext delta V = 1 atm ( (6mol/4 mol)(97 L) - (97L))
work done on external = 48.5 atm L = 4914.26 joules

change in enthalpy = delta (u) - delta (pv)
and that delta pv = (delta n) RT = (6-4)RT

enthalpy of formation (in this case combustion) = -1243.75 Kj
therefore

delta (u) = -1243.75 Kj - 2R(298K) = -1248.70 Kj

delta (u) = delta q + delta work
delta q = -1248.70 Kj - 4.91426 Kj = -1253.62

something feels wrong about it tho :-/
 
Last edited:
ipitydatfu said:
how much moles of gas was produced:

EtOH + 4 O2 -> 2CO2 + 3 H2O + O2 (left over from the reaction)
therefore N initial = 4 moles of gas, N final = 6 moles of gas

using pv = nrt with initial conditions would lead to roughly 97 L

messing around with the ideal gas law equation it leads to:
Vfinal = (#mol final/#mol initial) V initial

... i explained how to get the info in my second post, ok more specifcally
Work done on external = pext delta V = 1 atm ( (6mol/4 mol)(97 L) - (97L))
work done on external = 48.5 atm L = 4914.26 joules

change in enthalpy = delta (u) - delta (pv)
and that delta pv = (delta n) RT = (6-4)RT

enthalpy of formation (in this case combustion) = -1243.75 Kj
therefore

delta (u) = -1243.75 Kj - 2R(298K) = -1248.70 Kj

delta (u) = delta q + delta work
delta q = -1248.70 Kj - 4.91426 Kj = -1253.62

something feels wrong about it tho :-/
Why are you using the enthalpy of combustion? It seems to me that the heat from combustion is irrelevant, since we know the initial and final state of the gases. The change in internal energy minus the work done gives [itex]\Delta Q[/itex], which is the net heat flow. The net heat flow is all that you are concerned with in part a.

AM
 
i thought i could use that to work around the Delta(H) = Delta(U) + Delta(pV) so i could solve for Delta(U), and that knowing work, would just give me q.

i guess i am wrong. how am i supposed to approach it?
 
ipitydatfu said:
i thought i could use that to work around the Delta(H) = Delta(U) + Delta(pV) so i could solve for Delta(U), and that knowing work, would just give me q.

i guess i am wrong. how am i supposed to approach it?
You start with 4 moles of O2 and you end up with 2 moles of CO2, 3 moles of H2O and one mole of O2 at the same temperature and pressure. What is the difference in internal energy? What is the work done? How is that related to the net heat flow according to the first law?

AM
 
yeah man, I am at a complete halt. please help
 
ipitydatfu said:
yeah man, I am at a complete halt. please help
You have essentially converted 3 moles of O2 into 3 moles of H2O + 2 moles of CO2, all at the same T and P. What is the internal energy change? [itex]\Delta U[/itex] would be the internal energy of 3 moles of H2O + 2 moles of CO2 minus the internal energy of 3 moles of O2. Can you work that out? What is Cv for each gas? How is Cv, n, and T related to U?

There is a slightly easier approach using Cp, since this is all at constant pressure. What is the relationship between Q (or H) and Cp, n, and T?

AM
 
Last edited:

Similar threads

Why Might Statement (b) Be Incorrect in Ideal Gas Processes?
  • · Replies 2 ·
Replies
2
Views
2K
Thermodynamics: calculating work question
  • · Replies 6 ·
Replies
6
Views
2K
Enthelpy & Internal Energy Change relation with Cp & Cv
  • · Replies 4 ·
Replies
4
Views
3K
Internal energy of an ideal gas as a function of pressure?
  • · Replies 5 ·
Replies
5
Views
3K
Determine all of the Physical Chemistry Variables of the System
  • · Replies 14 ·
Replies
14
Views
2K
Bizarre Atmospheric Physics Problem
  • · Replies 7 ·
Replies
7
Views
2K
Thermo cycles with only T and mol
  • · Replies 1 ·
Replies
1
Views
2K
Thermo: isothermal, reversible expansion of ideal gas
  • · Replies 4 ·
Replies
4
Views
3K
Is Work Meant to Be Positive and Heat Negative During Ethanol-Water Mixing?
  • · Replies 2 ·
Replies
2
Views
2K
Moles of gas helium balloon; buoyancy; PV=nRT
  • · Replies 3 ·
Replies
3
Views
8K