Questions tagged [thermodynamics]
Applicable to questions about heat, energy, work, and their interconversion in chemistry. See the tag wiki for a detailed list of topics. Questions tagged may also be tagged with [enthalpy], [energy], [free-energy] where appropriate.
1,854
questions
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How to calculate the molar entropy of different substances at different temperatures?
I want to calculate the Gibbs free energy of ammonia synthesis at around $\pu{700K}$, but I can't seem to find the entropy values at that particular temperature. Is there a formula to calculate the ...
0
votes
1answer
22 views
Can dissipative work be circumscribed as irreversible heat transfer
My question refers to an article I read yesterday in which the author states that for the calculation of the entropy change for a process (in that case friction, during an irreversible adiabatic gas ...
0
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0answers
37 views
Hydrogen disulfide heterogenous equilibrium
I have been struggling with the following problem for quite a while:
I was able to calculate the Gibbs free energy and the equilibrium constant, using the formula $$ΔG = -RT\ln K$$
and ...
6
votes
1answer
62 views
Heats of formation of neutral molecules and homolytic vs heterolytic bond dissociation in mass spectrometry
I am currently studying the textbook, Mass Spectrometry, third edition, by Jürgen H. Gross. Chapter 2.4.3 Bond Dissociation Energies and Heats of Formation says the following:
Great efforts have ...
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0answers
16 views
How to find out the vapour pressure dependence on temperature and what is meant by thermodynamically consistent? [closed]
The Antoine equation often is used to describe vapor pressures: lnPσ = A − B /(T + C). For temperatures near the normal boiling point: a. Derive an expression that gives the behavior of ∆Hvap as a ...
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2answers
49 views
Does dissociation of molecules into atoms change the volume of mixture?
For example, if $\ce{H2}$ is converted to two $\ce{H},$ does the volume of the mixture change?
If it doesn't, does it imply that the work done is $0$ because work is $P\,\mathrm dV?$
2
votes
1answer
65 views
Why doesn't nitrogen react with oxygen at room temperature?
I already know that nitrogen's triple bond requires a lot of energy to be broken, so the activation energy is very high, but the book also lists the endothermic energy change as a reason.
Does the ...
2
votes
3answers
69 views
How can Ellingham diagrams be linked to Gibbs energies of formation?
My textbook quotes the following as the limitation of Ellingham diagram:
The interpretation of $\Delta G_\mathrm{f}^\circ$(standard Gibbs free energy of formation) is based on $K$ ($\Delta G_\...
9
votes
2answers
92 views
Pressure turning out to be zero?
I was studying for my freshman chemistry course, and ran into a mind-boggling contradiction.
We know, at equilibrium, entropy is at a maximum:
$$\left(\frac{\partial S}{\partial V}\right)_U=0$$
From ...
7
votes
1answer
83 views
Proving a thermodynamic relation between $(\partial H/\partial T)_p$ and $(\partial U/\partial T)_V$
(This question is taken from Problem 1.1(b) of the book Chemical Thermodynamics: Principles and Applications.)$\require{begingroup} \begingroup \newcommand{\pd}[3]{\left(\frac{\partial #1}{\partial #2}...
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2answers
44 views
Delta G in an equilbrium reaction(forward and backward)
Consider a reaction say
$$\ce{A + B <=> C + D}$$
Now $\Delta G$ for the forward reaction is:
$$\Delta H - T\Delta S$$
For the backward reaction $\Delta H$ and $\Delta S$ are clearly the same ...
6
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2answers
134 views
+50
What is the precise definition of standard molar entropy?
I understand what the standard molar entropy is, and how to use it in calculations, but I'm interested in understanding exactly how it's defined and measured.
My recollection from a long time ago is ...
0
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2answers
49 views
Equation of water at triple point
I was thinking about what exactly happens at the triple point of water, and what could be the effect of adding solutes/solvents. I feel we can represent the triple point using the following ...
-2
votes
2answers
46 views
How to determine the formula for calculating the enthalpy change of a reaction? [closed]
$$\ce{4 HCl(g) + O2(g) -> 2 Cl2(g) + 2H2O(g)}$$
In the reaction above, knowing the enthalpy of formation of each individual compound, in order to calculate the standard enthalpy change of the ...
1
vote
1answer
39 views
Euler's Theorem and Extensive properties
I read in a textbook (Mathematics for Physical Chemistry by D. McQuarrie) an example of Euler's Theorem used in Thermodynamics. The author states "The [variables] are all extensive [quantities], and ...
1
vote
1answer
54 views
On the origin of Gibbs Free Energy
It is intuitive to me that for a spontaneous process involving any system i.e. one occurring without constant human interaction $$ \text{d}S \geq 0 $$ as I can observe increasing energy dispersal in ...
6
votes
0answers
56 views
Why is concentration used as a proxy for activity instead of mole fraction?
I am trying to understand how the Gibbs energy change of a chemical reaction is computed and I do not understand why concentrations (mol/L) are used as a proxy for activity rather than mole fractions (...
0
votes
2answers
43 views
Which of the following reaction will have greater enthalpy and why? [closed]
Which of the following reaction will have greater enthalpy and why?
$\ce{C (g) + 4 H (g) -> CH4(g)}$ (reaction enthalpy = $x\ \mathrm{kJ/mol}$)
$\ce{C (graphite) + 2H2 (g) -> CH4 (g)}$ ...
0
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0answers
19 views
Internal energy of reversible, adiabatic expansion
I have come across something in our answer key that I don't agree with or perhaps have misunderstood and would appreciate if someone could tell me if I am correct or not.
For a reversible and ...
0
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1answer
49 views
Are there reactions operated at high temperature at industrial level? [closed]
Temperature is an important parameter when you deal with a chemical reaction, first of all $\Delta G$ and $\Delta G^\circ$ depend on temperature, and as a consequence the spontaneity of a reaction ...
5
votes
2answers
118 views
Kinetics of converting a water + heavy water mixture to HDO
Imagine an initial 50%-50% mixture of normal ($\rm H_2 \rm O$) and heavy water ($\rm D_2 \rm O$).
I think, simply mixing them, the result will exponentially converge to an equilibric state, where ...
0
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1answer
27 views
Relating Gibbs Free Energy to Equilibrium Constants [closed]
I am having some trouble relating these two concepts. I know that if K=1 then delta g is zero, and that we can interpret signs and magnitudes. However, i am having trouble determining required ...
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votes
3answers
82 views
Need help understanding Enthalpy
My chemistry textbook says that:
Enthalpy (Internal Energy) = chemical potential energy + thermal energy
Now, I know that Enthalpy and Internal Energy are different so I really don't understand ...
1
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0answers
34 views
Effect of drinking on internal energy and entropy of the universe
I have just been doing some questions on thermodynamics and have come across a strange question that I can't seem to grasp.
The question asks me to estimate the change in body temperature as well as ...
1
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1answer
21 views
How are cells rechargable?
I have learnt that you can test if a reaction is thermodynamically feasible by testing if the cell voltage $E_\mathrm{cell}$ is greater than $0$. Surely, if a reaction is feasible, then the $E_\mathrm{...
1
vote
1answer
75 views
Entropy as a state function independent of the path
When entropy is a state function and the entropy change for a reversible process is
$$\mathrm dS_\mathrm{rev} = \frac{\delta Q_\mathrm{rev}}{T} = 0,$$
how can an irreversible process be ...
1
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0answers
39 views
Enthalpy of reaction between ammonia and oxygen
I have to ask this here because I asked literally everybody I know and I still disagree with them.
Using the information in the table below, calculate the heat of released from the following ...
1
vote
0answers
24 views
To calculate expansion work for a process
[1 mark] To calculate the expansion work for a process,
1) the formula $w = -P_\mathrm{ex}(V_2 - V_1)$ always works under all conditions.
2) the formula $\mathrm dw = -P_\mathrm{ex}\,\mathrm dV$ ...
2
votes
5answers
113 views
Clarification on usage of dU =nCvdT
In many places I have come across the following derivation
Work done in isothermal expansion
By first law of thermodynamics
$\mathrm dU=Q+W$
As $\mathrm dU=nC_V\,\mathrm dT$, and in ...
0
votes
0answers
10 views
Enthalpy of formation for Lithium ion in ethylene carbonate
The following link has the enthalpy of formation for aqueous lithium http://nshs-science.net/chemistry/common/pdf/R-standard_enthalpy_of_formation.pdf
I am wondering how to get the enthalpy of ...
2
votes
1answer
55 views
Effect of impurities on Enthalpy of vapourization
How does the Enthalpy of vapourization change when impurities are added? I know that the boiling point of a substance increases on addition of impurities, but how does the Enthalpy of vapourization ...
0
votes
1answer
48 views
Why is volume inversely proportional to pressure?
If temprature is directly proportional to volume (Charles's law) and
temperature is directly proportional to pressure (Gay-Lussac's law), then why is pressure and volume are inversely proportional?
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0answers
73 views
Controlling rate of endothermic reaction
In an experiment, I have two endothermic reactions taking place simultaneously:
$$
\begin{align}
\ce{A + B &-> C + D}\tag{1}\\
\ce{E + G &-> Z + F}\tag{2}
\end{align}
$$
$\ce{C}$ and $\...
0
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0answers
50 views
Modifications and alterations in the state of chemical equilibrium
Let us consider that we have a mixture of gases, considered ideal, that is in balance. Like, for example,
$$3\ce{NO} (g) \rightleftharpoons \ce{N2O} (g) + \ce{NO2}(g)$$
If we consult the ...
0
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1answer
35 views
How do bonds form without violating the second law of thermodynamics [closed]
It might be obvious that entropy related to reactions always increases and I understand (somewhat) why this happens. My question is why if you have two hydrogen atoms in a vacuum without being bonded ...
4
votes
2answers
112 views
Regarding units of water potential and chemical potential
If water potential is nothing but the chemical potential of a given solution, then why do their units differ?
If the unit for the latter is taken as 'Energy per molecule' (which makes sense to me ...
1
vote
1answer
39 views
Ground state elevation for phase transition
I have a question about the illustration from chem.libretexts.org:
the ground states of the liquid and gas are offset from that of the previous state by the heats of fusion and vaporization, ...
1
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0answers
74 views
When Cp = Cv + R
So in a physics test question, there was this question when is $C_p$ not $C_V + R$?
And I wrote: when number of molecules change. Because the derivation of above formula comes from $nC_p\,\mathrm dT -...
1
vote
2answers
54 views
Why is the entropy of container A the greatest?
Question
9.35 Container A is filled with 1.0 mol of the atoms of an ideal monatomic gas. Container B has 1.0 mol of atoms bound together as diatomic molecules that are not vibrationally active. ...
3
votes
1answer
77 views
Specific heat for tetraatomic gas molecules
I came across a question which involved non linear tetra atomic gasses.
In it, the value of specific heat capacity at constant volume was given as
$9R,$ added up as
$$\frac 3 2 R + \frac 3 2 R + ...
4
votes
0answers
66 views
Has a concept of temperature ever been defined in the context of a single atom?
I wonder if this answer to What would happen if we supercool and then superheat an atom very abruptly? goes far enough.
I almost wrote the comment:
I don't think we can even talk about the ...
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votes
1answer
19 views
What is the difference between stoichiometric coefficients and number of moles? [closed]
Please clear my doubt,
Thanks!
2
votes
2answers
60 views
Basic question about an expression for Gibbs free energy
I'm getting into thermodynamics, and I have a basic question about the following expression for Gibbs free energy:
$G = \sum \mu_i N_i$
Is it correct that this equality only holds for constant ...
0
votes
2answers
44 views
Clarification required on whether to use $C_v$ or $C_p$ in this question on finding entropy change
$1 \text{ mol}$ of an ideal monoatomic gas at $300 \text{ K}$ is subjected to a reversible isentropic compression process until final temperature reaches $600 \text{ K}$. If initial pressure is $1 \...
0
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2answers
42 views
How is entropy change defined for a chemical reaction?
I know that:
ds = dq/T for reversible processes.
Can I write change in entropy of a system in which a chemical reaction is happening using this equation. Basically, my question is about whether a ...
6
votes
1answer
125 views
Basic thermodynamics question about work
I am getting into thermodynamics, and I have a basic fundamental question about the definition of the change in internal energy $\mathrm{d}U$:
$\mathrm{d}U = T\,\mathrm{d}S - P\,\mathrm{d}V + \sum\...
1
vote
3answers
108 views
In an isothermal process, how can the change in internal energy be 0?
It was written in my textbook,
$$
\mathrm{d}U =
\left(\frac{\partial U}{\partial T}\right)_V \mathrm{d}T
+\left(\frac{\partial U}{\partial V}\right)_T \mathrm{d}V
$$
If the process is isothermal,...
1
vote
0answers
32 views
Methylhydrazine Calorimetry
Below is a chemistry practice problem. I am curious why the change in heat of the oxygen molecules doesn't affect the calculations.
Methylhydrazine ($\ce{CH6N2}$) is commonly used as a liquid ...
-1
votes
1answer
46 views
Gibbs free energy for an ideal gas
You know that for a generic system which during a process exchanges heat with a reservoir at a constant temperature $T_{a}$ it holds:
$$\Delta G=\Delta H-T_{a}\Delta S$$
Where:
$\star)$$\Delta G$ is ...
5
votes
1answer
50 views
If volatility depends on intermolecular forces, then why in Raoult's law, two substances have different volatility?
I have a doubt, Ideal solutions are solutions where intermolecular forces between solute-solute and solvent-solvent are nearly equal to solute-solvent. Since intermolecular forces between A-A = B-B, ...
