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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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1answer
80 views

What happens when Ar and HCl are mixed at 77 K? [on hold]

So our teacher asked us a simple question in the classroom: What happens when Ar and HCl are mixed at 77 K? and no other information was given. We replied various possible answers for that such ...
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0answers
33 views

Is it possible for the vapour pressure to exceed the boiling point? [on hold]

An unknown molecule has a normal boiling point of $\pu{339.15 K}$ and a vapour pressure of $\pu{2 atm}$ at $\pu{329 K}.$ Determine the pressure, in mmHg, at which this unknown liquid will boil at $\...
1
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0answers
30 views

Thermochemical data for thiocyanatoiron(III) ion

I am currently writing a lab report about the formation constant of $\ce{FeSCN^2+}.$ All I could find online was a formation constant at $\pu{25 °C}$ $(890),$ which was 70% higher than the ...
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0answers
17 views

Thermodynamics and equilibrium chemistry question [on hold]

Consider the reaction A ⥫⥬ B For this reaction, ΔrH = – 92 kJ and K = 106 at 1023 K. The value of the equilibrium constant, K at 1218 K is? I'm not sure how to go about this one.
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0answers
18 views

freezing of soft drinks [on hold]

There is a vending machine outside your classroom that has soft drinks. Two students purchase soft drinks, one buys a regular Coke, and the other buys a Diet Coke. They both return to the classroom ...
-4
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0answers
16 views

calculate the enthalpy of solution for ammonnium chloride [closed]

What is the change in temperature when 3.00 grams of ammonium chloride is dissolved in 100 mL of water? change is temp was -1.99 Given the change in the temperature from adding 3.00 grams of ...
2
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0answers
21 views

Estimation of Molar Entropies

There are plenty of compounds out there with unknown standard molar entropies. I know that molar entropy has a strong dependance on the physical phase, molecular weight, and number of atoms. So is it ...
-2
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1answer
47 views

Why does ice get colder when you add salt? [duplicate]

I know that when you put salt on ice it may turn to a slush and it does feel cold. What is the reason for this?
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2answers
76 views

Freezing Point Depression and Temperature Decrease

I don't have a problem understanding the chemical mechanisms underlying freezing point depression or colligative properties in general, but what I cannot seem to grasp is how if you add salt to ice ...
0
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0answers
31 views

Why do statistical effects impact stepwise stability constants?

I have a question regarding the stepwise stability constants of metal complex formation/substitution. My lecture notes say that if we have a metal complex, say $\ce{[MX_{6}]}$, and a substitution ...
0
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0answers
50 views

Is it possible to use the laws of termodynamics for non-ideal systems?

For the reaction $$\ce{2CO(g) + O2(g) -> 2CO2(g)}\quad\Delta H = \pu{−560 kJ}$$ it is given that the reaction occurs at constant volume of $\pu{1L}$ and the initial and final pressures are $\pu{...
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1answer
40 views

How do I know which alkane has the lowest combustion heat?

I'm asked which one of the alkane has the lowest heat of combustion (note: they question says heat, not enthalpy) and according to the answers, the right one is d. I'm trying to understand why d ...
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0answers
35 views

Energy of mercury(II) oxide decomposition

Study check 9.11 from [1, p. 298]: Mercury(II) oxide decomposes to mercury and oxygen. $$\ce{2 HgO -> 2Hg + O2}\quad ΔH = \pu{+182 kJ}$$ […] b. How many kilojoules are needed when ...
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1answer
29 views

Internal energy change for reactions at constant volume vs constant pressure

The reaction of cyanamide, $\ce{NH2CN(s)},$ with dioxygen was carried out in a bomb calorimeter, and $∆U$ was found to be $\pu{–742.7 kJ mol-1}$ at $\pu{298 K}.$ Calculate enthalpy change for the ...
4
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1answer
56 views

How is a phase equilibrium defined for a one-component system?

A question on this site asked whether a one-component system is at equilibrium when melting or boiling, and the disparate answers were somewhat dependent on the definition of phase equilibrium. ...
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0answers
21 views

Measurement of the lattice energy

The lattice energy of a solid ionic compound is the energy released when one mole of the solid compound is formed from its constituent gaseous ions at $\ce {298 K}$ and $\ce {1 bar}$. However, gaseous ...
2
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3answers
155 views

melting and boiling …really equilibrium? [closed]

Why are melting and boiling considered equilibrium processes even though the amount (concentration) of both phases keep changing i.e from solid to liquid and so on?
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0answers
21 views

Does putting a reversible reaction out of equilibrium affect temperature?

Lets say we have a reversible reaction in equilibrium A + B <--> C. It's exothermic towards the right. Would increasing the concentration of A or B cause a temporary increase in temperature from ...
1
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0answers
30 views

During solid-solid phase transition, if both solid phases are incompressible, is ΔG independent of p?

Source: Atkins Physical Chemistry, Ninth Edition, page 154, Problem 4.13: Show that, for a transition between two incompressible solid phases, ΔG is independent of the pressure. My attempt: ...
2
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0answers
34 views

Negative Pressures in Van der Waals Equation of State

This is the plot of the Van der Waals equations: As you can see, the Van der Waals equation predicts a "stretched liquid" area (MNS). Is this physically feasible and what exactly happens there? Are ...
0
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0answers
59 views

Gibbs free energy of phosphorus pentachloride decomposition reaction

The equilibrium constant at $\pu{227 °C}$ for the equation $$\ce{PCl5(g) <=> PCl3(g) + Cl2(g)}$$ is $K_p = \pu{4.50E3 bar}.$ Calculate the value of $Δ_\mathrm{rxn}G^\circ$ at $\pu{227 ...
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1answer
119 views

Heat involved in isenthalpic process

Question 28 from [1, p. 4.4]: An ideal gas is expanded irreversibly from $\pu{5 L}$ to $\pu{10 L}$ against a constant external pressure of $\pu{1 bar}.$ The value of heat involved $(q)$ in this ...
4
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2answers
78 views

Clarification on the Clausius inequality

In Atkins' Physical Chemistry the criteria for spontaneity using Gibbs energy was calculated using the Clausius inequality: $$\mathrm dS ≥ \frac{\mathrm dq}{T},$$ and at constant pressure, $\mathrm ...
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1answer
56 views

Thermodynamic statements [closed]

Question 13 from [1, p. 4.2]: Which of the following statements is true? (a) An ideal gas always obeys the equation: $PV^γ = \mathrm{constant}$ in adiabatic process. (b) An ideal gas ...
2
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3answers
70 views

Is enthalpy of combustion the 'heat released' or 'enthalpy change'?

In a constant volume calorimeter, $\pu{3.5 g}$ of a gas with molar weight $\pu{28 g}$ was burnt in excess $\ce{O2}$ at $\pu{298 K}.$ Temperature of calorimeter increased from $298$ to $\pu{298.45 K}$ ...
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1answer
27 views

Change in enthalpy equation [closed]

If the formula for change in enthalpy is ΔH=ΔU+pΔV, and ΔU = q - pΔV then shouldn't the formula be simplified to ΔH = q regardless of whether the volume or pressure is constant? Is this only true when ...
4
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1answer
86 views

Is it really possible for a liquid to stay afloat a gas (vapor)?

I have a book at home called Chemical Thermodynamics for Process Simulation (I have the first edition) and at the very beginning of chapter 2 of the book one may read the following: No reference is ...
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2answers
36 views

Work done in cyclic process [closed]

I have been told that when ∆V=0, work done will be zero (W=0). I know that, in cyclic process ∆V will be zero, since it's a path function and |W| i.e. magnitude of work done will be the area enclosed ...
0
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1answer
49 views

Heat exchange in adiabatic process

I know my teacher told me that In adiabatic process there is no exchange for heat between system and surrounding. Then he came to point. From first law of thermodynamics $$∆U = q + W \tag{1}$$ ...
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1answer
21 views

Why Gibbs energy change of a process can be negative at constant temperature and pressure? [closed]

This question has been asked before, but the answers are not clear and satisfactory. If the pressure and the temperature are constant then G would be always zero. How would then G have a value other ...
0
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1answer
37 views

Irreversible Adiabatic Process [closed]

While studying about the entropy change of surroundings in irreversible adiabatic process, I came across a sentence. For infinite reservoir, all processes are considered reversible And suddenly ...
0
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1answer
42 views

qualitatively: Why are thioesters more reactive than regular esters? [duplicate]

In ochem students are taught that shorter bonds (like alkynes) have more energy than longer bonds such as alkanes. However in biochem we are taught that thioesters are very energetic bonds and release ...
5
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1answer
147 views

What is the true meaning of Gibbs energy and chemical potential?

After doing some research and reading I found some problems which I will try to state as clearly as possible. The definition of Gibbs Free Energy says "the greatest amount of mechanical work which ...
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0answers
50 views

Is pH in a charged hydrogel and its supernatant solution constant?

Let us assume we deal with ideal systems without interactions. The gel phase and the supernatant solution phase are in thermodynamic equilibrium. The supernatant solution shall consist of different ...
7
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0answers
101 views

Why are IUPAC's definitions of exo- and endothermic disconnected from the direction of heat flow?

I'm wondering if anyone can provide a rationale for IUPAC's definitions of exothermic and endothermic, since they disconnect these terms from the direction of heat flow. Specifically, here are IUPAC'...
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1answer
44 views

Equilibrium pressure

Ammonium carbamate dissociates as follows: $$\ce{NH2COONH4(s)<=> 2NH3(g) + CO2(g)}$$ The value of $K_p$ for this reaction is found to be equal to $\pu{2.92 \times 10^-5 atm^3}$. If ...
1
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1answer
29 views

What are D_b and D_a in the Generalized Fugacity Coefficients table?

I've been referring the book: Chemical Process Principles: Part-II Thermodynamics. I found a table that gives me fugacity coefficients in a tabular form corresponding to various reduced pressures and ...
-1
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1answer
23 views

Bond energy and enthalpy of formation

Is the following equation true? $$\ce{∆_fH} = \ce{∆_{BE_{reactants}}H} - \ce{∆_{BE_{products}}H}$$ where $BE$ stands for Bond energy and $f$ stands for formation. I read that, $$\ce{∆_fH} = \ce{∆_{...
2
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1answer
51 views

Can someone walk me through this gas mixture question?

Argon $(\ce{Ar})$ and helium $(\ce{He})$ are initially in separate compartments of a container at $\pu{25 °C}.$ The $\ce{Ar}$ in compartment A which has a volume $V_\ce{A}$ of $\pu{9.00 L}$ and a ...
3
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1answer
41 views

Isothermal vs adiabatic compression of an ideal gas [closed]

Could someone please explain these specific questions physically (especially the first question!)? I understand the corresponding mathematical proofs, but not the physical reasoning? I have looked ...
0
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1answer
50 views

Enthalpy and heat relationship

They say that enthalpy of reaction is the heat when at constant temperature and pressure. So if enthalpy is $$\mathrm dH = \mathrm dU + p\,\mathrm dV = T\,\mathrm dS - p\,\mathrm dV + p\,\mathrm dV + ...
2
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3answers
93 views

Genuine confusion about the equilibrium constant [closed]

I gather that the equilibrium constant K = 1 implies that, at equilibrium, neither the forward nor the backward reactions are thermodynamically favoured. But if pressure of the system increases and ...
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1answer
39 views

Enthalpy change of water [closed]

A given amount of (liquid) water in an open vessel is heated from 20 degree celsius to 50 degree celsius. Suppose it absorbs x joule in the process. What is the enthalpy change in the process? Is it = ...
0
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0answers
31 views

Standard state conditions in thermodynamics

Standard state conditions for standard Gibbs energy change stipulate (among other conditions) that partial pressure of each gas = 1 bar. e.g. 2A + 3B = 4C + 5D (all gases) Then for standard Gibbs ...
2
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1answer
98 views

Mathematical justification for Le Chatelier's principle

At equilibrium $$ \begin{align} K &= \exp\left(\cfrac{TΔS^\circ - ΔH^\circ}{RT}\right)\\ ⇒ \frac{\mathrm d \ln K}{\mathrm dT} &= \frac{ΔH^\circ}{RT^2} \end{align} $$ If enthalpy change is ...
1
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1answer
63 views

How to find the Cp, difference of Cp and heat of fusion from the dependence of vapor pressure on temperature?

The vapor pressure of zinc varies with temperature as $$\log P(\pu{mm Hg}) = -6850/T - 0.755\log T + 11.24 \label{eqn:1}\tag{1}$$ and that of liquid zinc as $$\log P(\pu{mm Hg}) = -6620/...
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0answers
71 views

How to I find the equilibrium temperature in this problem?

From Castellan's Physical Chemistry, chapter 12 [1, p. 276]: 12.18 At $\pu{25 °C}$ we have for rhombic sulfur: $ΔG_\mathrm{f}^\circ = 0,$ $ΔS^\circ = \pu{31.88 ± 0.17 J K-1 mol-1};$ and for ...
1
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1answer
65 views

Constant Pressure and Temperature during Phase Change

One of the solution guides to a question I was working on said that pressure and temperature is constant for a phase change. I understand why temperature is always constant for a phase change, but don'...
2
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1answer
97 views

Application of Nernst Equation in Potentiometric titration of Fe2+ with Ce4+

It is well Known that to find $E_{cell}$ during titration of $Fe^{2+}$ with $Ce^{4+}$ we consider 3 domains: Before Equivalance point , $E_+ = E_{Fe^{3+}/Fe^{2+}}^o+\frac{RT}{nF}log\frac{[Fe^{3+}]}{...
0
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0answers
30 views

Software to predict equilibrium products in the ideal gas phase

I'm interested in software packages that can answer the following kind of question: If I mix 30mol% $N_2$ and 70mol% $H_2$ together at 2 bar and 2000C, what is the equilibrium composition after all ...