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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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Calculating the amount of a product at the reaction equilibrium

$\ce{Cr2O3}$ can be reduced to the elements chromium with high temperature and carbon by this two reactions: $$ \begin{align} \ce{Cr2O3(s) + 3C(s) &-> 2Cr(s) + 3 CO(g)} \label{rxn:1}\tag{1}\\ \...
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1answer
28 views

Is there any difference between thermal energy at room temperature in fluids(water) vs air?

Context of the question: I was reading on hydrogen bonds, specifically how close they are to the thermal energy of the fluids. So I googled the value of thermal energy at room temperature, which was $\...
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1answer
88 views

Calculating temperature change of helium due to Joule-Thomson effect

We are trying to fill a rocket helium tank that is 78 liters with helium, final pressure of $\pu{10500 psi}$. We plan to pressurize high pressure tanks to $\pu{20000 psi}$ using a compressing pump ...
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56 views

What is Delta H for dissolution? [closed]

An ionic compound has a solubility of $1\ \mathrm M$ in water at $25\ ^\circ \mathrm C$ and its solubility increases as the temperature is raised. What are the signs of $\Delta H^\circ$ and $\Delta S^\...
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1answer
57 views

Reaction energy of oxygen and hydrogen

From the bonding energy of following bonds that I found in a Wikipedia article, I calculated the reaction energy that would be released when oxygen and hydrogen are reacted to produce water. The phase ...
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1answer
43 views

How is “the surrounding” and its temperature defined to calculate entropy change during a reaction?

Lets say supercooled liquid water at $263\ \mathrm K$ isobarically changes to solid ice at the same temperature. I wish to calculate the change in entropy of the surroundings and I happen to know the $...
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26 views

Deriving the entropy as a function of concentration

In a physical chemistry class, the professor derived entropy $S$ as a function of concentration starting from $S=k\ln(W)$. Somehow he made a mistake and ended up with the wrong sign. This remains ...
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2answers
62 views

Why enthalpy is defined at a constant pressure?

How absolute enthalpy and enthalpy change are defined? What is the clear difference between enthalpy and heat? Is it the constancy of pressure what makes the enthalpy a state function? My teacher ...
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1answer
227 views

adiabatic expansion vs Joule-Thomson Effect

Source: The1stLawofThermodynamicsLesson4 I am looking for difference between adiabatic expansion and Joule-Thomson effect. Looking at the setup demonstrating Joule-Thomson effect, I find no essential ...
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25 views

What drives the formation of salts?

Can someone explain why potassium and chlorine undergo an exothermic reactiom to form an ionic solid despite that potassium has an ionisation energy of 425 kJ mol-1 and chlorine has an electron ...
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2answers
65 views

Flow rate of the fuel consisting of ethanol and water burning in air of a given flow rate

I hope that someone could give me some hints or tips on how to approach this question, even though these types of questions are usually closed off as homework. A burner is run by a fuel mixture of $...
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35 views

Is it possible to convert a combustion reaction directly into an electrochemical one?

I'm fairly inexperienced in chemistry so forgive me if this question is silly but, why is it not possible to convert a combustion reaction to an electrochemical one directly and harvest the electrical ...
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1answer
108 views

Why are catabolic reactions exergonic and anabolic reactions endergonic?

Catabolism breaks apart large molecules and anabolism creates large molecules. Catabolism is supposed to be exergonic, while anabolism is supposed to be energonic. Id on't really understand this. By ...
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2answers
71 views

Why are reversible reactions spontaneous?

If a reversible chemical reaction is spontaneous in one direction, shouldn't it be non spontaneous $(ΔG > 0)$ in the other? In other words for the reaction: $$\ce{A + B ⇌ C + D}$$ if $$\ce{A + B ...
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1answer
308 views

Explanation of the strength of phosphorus-oxygen bond

When discussing the Wittig reaction, Clayden's Organic Chemistry cites the strength of the $\ce{P=O}$ formed in triphenylphosphine oxide as a driver of the reaction through enthalpy: The $\ce{P=O}$ ...
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2answers
60 views

Exact meaning of constant volume heat capacity

From Wikipedia: $$ \left(\frac{\partial U}{\partial T}\right)_V = \left(\frac{\partial Q}{\partial T}\right)_V = C_V, $$ $C_V$ is what known to be constant volume heat capacity. I don't really get ...
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1answer
84 views

Is delta S the same in reversible and irreversible processes?

I've read that change in entropy of a system is the same for a system in reversible and irreversible processes since it is a state function and does not depend on the pathway. Does this only apply to ...
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1answer
42 views

Inexact differential-path function

From Wikipedia: an inexact differential cannot be expressed in terms of its antiderivative for the purpose of integral calculations What's the mathematical reason that inexact differential cannot ...
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1answer
84 views

Confusion in change in enthalpy relation

In the equation $$\Delta H = \Delta U + \Delta PV,$$ under what circumstances can we simplify it into $$\Delta H = \Delta U + P\,\Delta V + V\,\Delta P$$ by product rule? In this article (pdf ...
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1answer
99 views

As dQ=nCdT, why not dQ is zero in isothermal process? [closed]

Explain in detail about the terms used in first law.
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1answer
197 views

When an inert gas is added to a system of constant volume, equilibrium is not effected?

But pressure of the system increases. Because of this increase in pressure, wouldn't (according to Le Chatelier's principal) equilibrium shift towards the side with less moles of gas?
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1answer
114 views

Calculating enthalpy change for a real gas

I know this question would be closed citing it to be a homework question. I posted it earlier also. But believe me I have tried it for myself before and that too many times but then couldn't arrive at ...
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1answer
168 views

Reversible and irreversible cyclic processes

The answer in Arihant's book is given as (b), but wouldn't (c) be correct as well? This is a question of IIT JAM Chemistry. A system undergoes two cyclic processes 1 and 2. Process 1 is reversible ...
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1answer
40 views

How do you calculate the delta H for multiple reactions?

I am having difficulty in a question: Given that 1. $\ce{N_2(g) \rightarrow 2N(g), \Delta H = 941 kJ/mol}$ 2. $\ce{N_2(g) \rightarrow N_2^+ + e^-, \Delta H = 1501 kJ/mol}$ 3. $\ce{N(...
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1answer
108 views

NaOH or lye. Would it work to remove ice snow or water from a paved surface? [closed]

Thinking of ways to clear ice using chemical reaction.
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2answers
89 views

Thermodynamics of a spontaneous isothermal chemical reaction. [duplicate]

Which of the following is/are correct for spontaneous isothermal chemical reaction? (A) $\Delta H = 0$, because $\Delta T = 0$ (B) $\Delta S = 0$ (C) $\Delta U = 0$, because $\Delta ...
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2answers
184 views

When to use internal energy and enthalpy in balances

I am confused as to when you use ΔU and when to use ΔH, I don’t really understand the difference. Is it that ΔU is used for closed systems whilst ΔH is used for open systems?
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37 views

Constraints on rate coefficients in chiral catalytic system

Consider some reversible chemical reaction in a closed system: $$\ce{A <=> B + D} \tag{1}$$ where $\ce{A}$ is some input reagent and $\ce{B}$, $\ce{D}$ are some output reagents. Let's denote ...
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1answer
69 views

Oxygen Solubility in Water

This is probably really obvious, but I really don't understand. Why does oxygen dissolve in water when oxygen is a nonpolar molecule and water is a polar molecule? It's obvious that oxygen does ...
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1answer
38 views

Is the water in an aqueous solution undergoing a reaction considered part of the system or surroundings?

System: In chemistry, a system is a chemical reaction. A system operates within its surroundings. Energy can move between the two. Surroundings: The environment around a particular chemical reaction....
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40 views

Experimental measurement on the ring-strain

for my thesis i calculated the ring-strain of a molecule by MP2 on the computer. I was wondering if there is an easy possibility to get the experimental value of the ring strain. I guess I have to ...
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2answers
109 views

If the heat exchanged is different for reversible and irreversible processes, how can enthalpy be a state function?

From the definition of Gibbs free energy, we can write for an irreversible process: $dG=dH-TdS<0$ With $TdS=\pu{q_{rev}//T}$ this becomes: $dH-dq_{rev}<0$ Since G is defined for constant (p,...
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1answer
27 views

How to derive the molar fundamental relation

We have the fundamental relation: $$dS = \frac{1}{T}dU+\frac{P}{T}dV-\frac{\mu}{T}dN$$ how do we get now the following: $$ds = \frac{1}{T}du+\frac{P}{T}dv$$ where $s = \frac{S}{N}, u = \frac{U}{N}, v ...
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52 views

Why does standard reduction potential get multiplied by the change in oxidation state and not by the number of electrons transferred?

The whole basis of Frost diagrams is to have a graphical representation of the Gibbs free energy of formation ($\Delta{G}_{f}$) of different oxidation states of an element using the proportional ...
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1answer
73 views

Why does Carnot Engine have the curves as isothermal and adiabatic? [closed]

Why exactly does the Carnot engine have two isothermal and two adiabatic curves? (My book says that “a process is quasi-static if the temperature difference between the system and the reservoir is ...
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1answer
116 views

Why does Gibbs free energy correspond sololey to non-expansion work?

First of all: I know this question has been asked before (Why does the Gibbs free energy only correspond to non-expansion work?), and although I understand the mathematical derivation given there, I ...
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0answers
32 views

Reference conditions for balances on nonreactive processes

In one of the examples in my book (Elementary Priniciples of Chemical Processes) where I am asked to calculate the rate of heat input required to heat a stream of methane and air from 20°C to 300°C, ...
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1answer
119 views

Using thermodynamics to predict the acid-base character of fluoride ion in water

I was working through the end-of-chapter exercises of the acid-base chapter in Shriver's Inorganic Chemistry when I came across the following problem: 4.7. The effective proton affinity $\ce{A^{'}_{...
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1answer
68 views

Getting the pressure of a system by series expansion

An average number, $N$, of bosons of spin $S = 0$ is conned to a two-dimensional domain with surface $A$. The gas is ultrarelativistic with a single particle energy $\epsilon = cp$, where $c$ is the ...
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Manganese (II) oxidation by air

I am studying the reaction of dissolved dioxygen with manganese(II) hydroxide for the Winkler method: 2Mn(OH)2(s)+O2(aq)⟶2MnO2(s)+2H2O(l) The ...
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2answers
704 views

How to derive the relation between gibbs energy and equilibrium constant?

I want to understand the derivation between gibbs energy and equillibrium constant $$\Delta G=\Delta G^o+RT\ln Q?$$ I have seen a similar post on CSE Derivation of relationship between equilibrium ...
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1answer
32 views

How does the theory of thermodynamics account for warm water freezing before cool water?

According to typical sources on thermodynamics, the warmer a liquid is, the longer it will take to freeze it. For example, I recently was reading a book on refrigeration and this book specifically ...
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1answer
69 views

Interpretation of H as T goes to absolute zero

Euler's identity for the enthalpy states that (for a single component and phase) $ H = TS + \mu N$ What are the implications of this expression in the context of the third law of thermodynamics (...
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3answers
178 views

Is there a thermodynamic driving force for racemisation?

Why does racemisation occur? My doubts are: Is it due to entropy? If it is due to entropy, how is the reduction of optical activity by racemisation a consequence of increased thermodynamic ...
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1answer
128 views

Why do we use pV term independent of U in the equation H = U + pV? [duplicate]

I have read the following definition of enthalpy in my textbook: A substance has to occupy some space in its surroundings depending upon its volume ($V$). It does against the compressing influence ...
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1answer
41 views

Why is the equilibrium constant being calculated differently here?

In my NMR textbook, it says the equilibrium constant for a dynamic process (e.g., A in equilibrium with B) is calculated by the concentration of A divided by the concentration of B. Is this a typo? ...
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1answer
206 views

Are specific heat capacity and boiling point of a substance related or proportional to each other?

That is, if the substance has a high specific heat, will it also have a high boiling point? ( and vice versa)
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50 views

Expression for the entropy of an ideal gas

I want to obtain the Sackur-Tetrode expression for the entropy of an ideal gas based on: $$S = K_B ln \Omega$$ We have an isolated system with a fixed volume V and fixed number of particles N and ...
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1answer
325 views

What is the substance with the highest specific heat?

I always learned that water has the highest specific heat, but I recently saw that Hydrogen has a specific heat as high as 14 cal/gC and helium has a specific heat of 5 cal/gC, which would be much ...
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1answer
110 views

Understanding the partial derivative of temperature with respect to thermodynamic beta (coldness)

I am trying to prove that the specific heat is related to the fluctuations in the energy: $$c_V = \frac{\langle E^2 \rangle - \langle E \rangle^2}{k_\mathrm BT^2}$$ Where: $$\beta = \frac{1}{k_\...