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.

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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 ...
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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 ...
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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 ...
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1answer
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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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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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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 $P\,\mathrm dV?$
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1answer
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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 ...
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3answers
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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_\...
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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 ...
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1answer
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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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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 ...
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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 ...
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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 ...
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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 ...
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1answer
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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 ...
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1answer
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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 ...
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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 (...
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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 = $x\ \mathrm{kJ/mol}$) $\ce{C (graphite) + 2H2 (g) -> CH4 (g)}$ ...
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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 ...
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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 ...
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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 ...
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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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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 ...
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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 ...
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1answer
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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{...
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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 ...
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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 ...
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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 $\mathrm dw = -P_\mathrm{ex}\,\mathrm dV$ ...
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5answers
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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 ...
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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 ...
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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 ...
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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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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 $\...
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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 ...
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1answer
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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 ...
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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 ...
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1answer
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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, ...
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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 -...
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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. ...
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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 + ...
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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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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 ...
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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 \...
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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 ...
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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\...
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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,...
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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 ...
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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 ...
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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, ...