# Is this correct Hess' law, thermodynamics

• Diamond101
In summary, ∆rH° for the reaction c2h50h+3o2-->2co2+3h20 is calculated to be -1386.4 kJ/mol using heats of formation and -1368.5 kJ/mol using the provided ∆rU° value at 298K. However, there may be discrepancies due to possible errors in the provided ∆rU° value or differences in values from different sources.
Diamond101

## Homework Statement

Calculate ∆rH° for the reaction
c2h50h+3o2-->2co2+3h20
Given that ∆rU° = - 1373 kJ mol-1 at 298K.

## Homework Equations

c2h50h+3o2-->2co2+3h20

## The Attempt at a Solution

delta H f CO2(g) = -393.5 kJ/mole
delta H f H2O(l) = -241.8 kJ
delta H f O2(g) = 0 these values are from my textbook delta H reaction = ((2 moles CO2)(-393.5 kJ/mole) + (3 moles H2O)(-241.8kJ/mole)) - ((1 mole C2H5OH)(delta H f C2H5OH(l))

-1373 kJ = (-787.0 kJ - 725.4 kJ) - (delta H f C2H5OH(l)
-139.4 kJ = delta H f C2H5OH(l)

You are supposed to do this problem without using heats of formation. Any thoughts on how?

Chestermiller said:
You are supposed to do this problem without using heats of formation. Any thoughts on how?
no clue.

What is the definition of ΔH in terms of ΔU?

Chestermiller said:
What is the definition of ΔH in terms of ΔU?
i do know DH=DU+PDV

Diamond101 said:
i do know DH=DU+PDV
ΔH=ΔU+Δ(PV)
At constant temperature, for ideal gases, Δ(PV)=RTΔn.

What is the change in the number of moles of gases Δn between reactants and products for you chemical reaction? If you neglect the volume of the liquids relative to the gases, what is the change in the PV in going from your reactants to going to your products?

Chestermiller said:
What is the definition of ΔH in terms of ΔU?
WAIT ITS A COMBUSTION REACTION

Chestermiller said:
ΔH=ΔU+Δ(PV)
At constant temperature, for ideal gases, Δ(PV)=RTΔn.

What is the change in the number of moles of gases Δn between reactants and products for you chemical reaction? If you neglect the volume of the liquids relative to the gases, what is the change in the PV in going from your reactants to going to your products?
Well 1 mole of c2h5OH and 3 molews of 02 , 2 Moles of CO2 and 3moles of h20 so the change is 1 mole 5- 4 moles

Diamond101 said:
Well 1 mole of c2h5OH and 3 molews of 02 , 2 Moles of CO2 and 3moles of h20 so the change is 1 mole 5- 4 moles
I asked for only the change in the number of moles of gases. The changes in PV resulting from the changes in the liquids is negligible.

Chestermiller said:
I asked for only the change in the number of moles of gases. The changes in PV resulting from the changes in the liquids is negligible.
Change will be 2. so the relationship between enthalpy and internal energy is DH=DU=DNgasRT

Diamond101 said:
Change will be 2. so the relationship between enthalpy and internal energy is DH=DU=DNgasRT
What if I told you that I get a change in the number of moles of gas as -1, not 2?

Chestermiller said:
What if I told you that I get a change in the number of moles of gas as -1, not 2?
how? you didn't cater for h2o as a gas?

Chestermiller said:
What if I told you that I get a change in the number of moles of gas as -1, not 2?
because water isn't gas in its standard state ?

Diamond101 said:
because water isn't gas in its standard state ?
I see (l)'s next to your H2O's for the reaction. Apparently, they are saying that the product water is in the liquid state.

Chestermiller said:
I see (l)'s next to your H2O's for the reaction. Apparently, they are saying that the product water is in the liquid state.
my mistake water is supposed to be a gas.
so dh= -1373* 2 mol*8.314*298 k

Chestermiller said:
I see (l)'s next to your H2O's for the reaction. Apparently, they are saying that the product water is in the liquid state.
i got -3582.14 KJ thank you for your assistance. (-1373+ (2 MOLES * 8.314* 298))

Diamond101 said:
i got -3582.14 KJ thank you for your assistance. (-1373+ (2 MOLES * 8.314* 298))
Not correct. Watch out for the units on that PV term.

Chestermiller said:
Not correct. Watch out for the units on that PV term.
OH IT WILL BE JOULES EVERYTHING ELSE CANCELS ?

Diamond101 said:
OH IT WILL BE JOULES EVERYTHING ELSE CANCELS ?
But the Delta U is in kJ. The units of the two terms have to be consistent.

Also, as a check, how does your answer compare with what you get using heats of formation? (I know you must have thought of doing this check)

Chestermiller said:
But the Delta U is in kJ. The units of the two terms have to be consistent.

Also, as a check, how does your answer compare with what you get using heats of formation? (I know you must have thought of doing this check)
I did compare the enthalpy of formation against the enthalpy of reaction . so the unit is Joule per kelvin?

The units of heats of formation and heats or reaction are J/mol or kJ/mol

ΔH=2(-241.8)+3(-393.5)-(-277.7)=-1386.4 kJ/mol (Heat of formation calculation)

ΔH=-1373+2(0.008314)(298)=-1368.5 kJ/mol (From ΔU provided)

They don't quite match. Something is not quite right. Maybe the ΔU they provided was wrong.

Chestermiller said:
ΔH=2(-241.8)+3(-393.5)-(-277.7)=-1386.4 kJ/mol (Heat of formation calculation)

ΔH=-1373+2(0.008314)(298)=-1368.5 kJ/mol (From ΔU provided)

They don't quite match. Something is not quite right. Maybe the ΔU they provided was wrong.
hey i actually adds up the formation value for ethanol is -277.6 according to my chem3 textbook by burrows

## 1. Is Hess' law always correct?

Hess' law is a fundamental principle in thermodynamics and is always correct as long as the reactions involved are at constant temperature and pressure.

## 2. How is Hess' law related to thermodynamics?

Hess' law is a consequence of the first law of thermodynamics, which states that energy cannot be created or destroyed, but can only be transferred between systems.

## 3. Can Hess' law be applied to all chemical reactions?

Yes, Hess' law can be applied to all chemical reactions as long as they are at constant temperature and pressure.

## 4. What is the significance of Hess' law in chemistry?

Hess' law allows for the calculation of the enthalpy change of a reaction by using known enthalpy values of other reactions. This is useful in determining the feasibility and energy requirements of a reaction.

## 5. Can Hess' law be used to predict the direction of a reaction?

No, Hess' law only provides information about the energetics of a reaction, not the direction. The direction of a reaction is determined by factors such as entropy and free energy.

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