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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6
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0answers
66 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 (...
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2answers
48 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 = $x\ \mathrm{kJ/mol}$) $\ce{C (graphite) + 2H2 (g) -> CH4 (g)}$ ...
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0answers
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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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2answers
131 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 ...
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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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3answers
98 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 ...
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1answer
28 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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0answers
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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
26 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{...
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2answers
2k views

Calculating entropy change: reversible vs irreversible process

Since the change in internal energy and enthalpy, which are equal to the heats for a constant-volume and constant-pressure process, respectively, are state functions, the heats for a reversible v.s. ...
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1answer
89 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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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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1answer
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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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0answers
25 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 $\mathrm dw = -P_\mathrm{ex}\,\mathrm dV$ ...
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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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0answers
79 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 $\...
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1answer
7k views

What is behind of the attraction between a burned matchstick with a magnet?

I do not know how to understand the reasons of why a burned matchstick is attracted by a magnet (most probably) of Neodymium. Here are some screenshots of a video in which this natural phenomenon is ...
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1answer
59 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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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
38 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 ...
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1answer
9k views

Standard reduction potential of MnO4-/MnO2 couple

This is the data given: $$\ce{MnO4- /Mn^{2+}} = 1.5~\mathrm{V}$$ $$\ce{MnO2 /Mn^{2+}}= 1.23~\mathrm{V}$$ I know that : $$\ce{MnO4- +5e- + 8H+ ->Mn^{2+}}$$ $$\ce{MnO2 + 4H+ +2e- -> Mn^{2+}}$$ ...
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1answer
41 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, ...
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2answers
117 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 ...
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0answers
118 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 -...
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2answers
59 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. ...
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2answers
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Why is slush hydrogen at the triple point?

Slush hydrogen is a mixture of liquid and solid hydrogen at the triple point considered as a possible vehicle fuel. What is the need of having it at the triple point? Couldn't any other set of ...
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1answer
97 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 + ...
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1answer
591 views

Hydrogen Bonding in Water Phase Changes

How is the disparity between the heat of fusion and the heat of vaporization for $\ce{H2O}$ best explained? A) It takes more hydrogen bonds for water to fuse than it does to vaporize. B) Water ...
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0answers
79 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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1answer
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Does the potential energy increase when temperature is raised?

The potential energy of a material is its ability to do work. So I would suspect the potential energy to increase during phase changes that increase entropy (solid $\rightarrow$ liquid and liquid $\...
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2answers
1k views

Are non-exothermic explosions possible?

Explosives like picric acid and trinitrotoluene are unstable due to the close packing of so many $\ce{-NO2}$ groups onto a benzene ring. These highly strained bonds require only a relatively small ...
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2answers
45 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 ...
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2answers
79 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 ...
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2answers
165 views

Why is it necessary to consider infinitesimal changes in p,V,T for H,U and G given that they're state functions?

State functions such as $G$ only depend on the state of the system and are not dependent on the "path" that took the system to that state (which would be the case for work, for example, which is not a ...
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1answer
132 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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3answers
114 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,...
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2answers
63 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 \...
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1answer
125 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, ...
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1answer
53 views

Why work done Is 0 and why not a negative value?

If a gas expand in vacuum, $P_{ex.} =0$. Then why is the work done 0? Since the gas is expanding, it is doing some work on the piston so the work done must have some negative value.
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2answers
466 views

When boiling water, how is evaporation rate affected by the surface area of the container?

Imagine two pots boiling on a stove. One is tall, while one is wide. Both contain the same volume of water. What will be the difference in the rate of evaporation between the two containers?
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4answers
40k views

Difference between reversible and irreversible processes

On a quiz, I was asked for the difference between reversible and irreversible processes. I said: For a reversible process, you can change direction at any time without disturbing equilibrium. For an ...
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1answer
222 views

Why does silver (I) have a larger lattice enthalpy and hydration enthalpy than sodium?

The ionic radius of the $\ce{Ag^+}$ ion is $129$ pm, and that of the $\ce{Na^+}$ ion is $116$ pm. Since the sodium ion is smaller than the silver ion, it makes sense that it has a stronger polarizing ...
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2answers
7k views

Why does a hard-boiled egg have more entropy than a normal egg?

The question "Why does a hard-boiled egg have more entropy than a normal egg?" was recently asked on one of my tests. I said that entropy will decrease as a hard boiled egg has molecules in a more ...
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2answers
7k views

Will NH3 decompose into N2 and H2?

In chemical kinetics , we learn that in a reversible reaction ,both the forward reaction and reverse reaction occur at the same time. At equilibrium , the rates are the same. For example:Production of ...
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0answers
52 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 ...
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1answer
107 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 ...
13
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3answers
1k views

Ellingham Diagram

My textbook says the following about the Ellingham diagram: Each plot is a straight line except when some change in phase ($\ce{s -> liq}$ or $\ce{liq -> g}$) takes place. The temperature at ...
5
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2answers
659 views

Is the equation dG = Vdp − SdT valid only for a reversible process? Can it be applied for an irreversible one too?

In the derivation, we write: $$ \begin{align} G &= H - TS &\quad&\to&\quad \mathrm dG &= \mathrm dH - T\,\mathrm dS - S\,\mathrm dT \\ H &= U + pV &\quad&\to&\quad ...
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3answers
897 views

Equlibrium for a non-spontaneous reaction

Consider a reversible reaction $\ce{A -> B}$, at standard state wher $\ce{A}$ and $\ce{B}$ are at $\pu{1 atm}$. If the free energy of products is greater than the free energy of reactants, $\Delta_{...

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