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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7 views

How does adding sucrose to water affect its specific heat capacity?

I conducted an experiment to determine whether adding sucrose to water affects its specific heat capacity. I have noticed that solutions containing the most amount of sucrose tend to have a lower ...
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Finding reaction equilibrium constant from Gibbs free energy and enthalpy

For the reaction $$\ce{3 Fe2O3(s) <=> 2 Fe3O4(s) + 1/2 O2(g)}$$ $\Delta_\mathrm{r}H^\circ(\pu{298 K}) = \pu{55.5 kcal mol^-1}$ and $\Delta_\mathrm{r}G^\circ(\pu{298 K}) = \pu{46.5 kcal mol^-1}.$ ...
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Degrees of freedom of a molecule of N atoms

Apparently a molecule of N atoms has 3N degrees of freedom. How? Shouldn't it be dependent on the structure of the molecule? How exactly do we derive this?
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What is the difference between enthalpy (H) and heat (q)?

It seems that in some cases it's the same (that is, $\Delta Q_p=\Delta H$) yet in others they differ. For example, \begin{align} \ce{2H2(g) + O2(g) &->2H2O(l)} & \Delta H &= -572~\...
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1answer
38 views

What is the difference between heat of reaction (q) and enthalpy of reaction ∆H?

I am confused as I can’t understand the difference between the thermodynamic terms q and ∆H. In my book,“general chemistry” by Ebbing and Gammon,it is stated that at constant pressure Qp=∆U+P∆V=∆H. ...
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1answer
129 views

At constant temperature, can the solid phase be more stable than the gas if pressure decreases?

In a phase diagram, the triple point is the point at which the three phases can coexist. If we consider, for example, CO2 If we go below the triple point by decreasing pressure and keeping ...
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1answer
95 views

Statistical entropy of perfect gas and indistinguishability

The weight, $W$ (the same $W$ as in Boltzmann formula $S=k\text{ln}W$) of a configuration {$N_0, N_1, N_2$, ...} is given by $$W=\frac{N!}{N_A!N_B!N_C!...} \tag{1}$$ where $N$ is the total number of ...
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Derivation of relationship between Gibbs free energy and electrochemical cell potential

Why is $\Delta G = -nFE$? I don't understand what the motivation is behind this definition. Was it derived or just given? The textbook provides no justification for this equation. In fact, much of ...
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What is an example of an exothermic reaction with negative ∆S?

In high school we learn that when a reaction has both negative $\Delta{}S$ (entropy change) and negative $\Delta{}H$ (enthalpy change) it occurs spontaneously at lower temperatures, but becomes non-...
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How does DeepHF (a CRISPR sgRNA design tool) compute binding free energy?

I am looking at the code of DeepHF[1] that computes $\Delta G_\text{binding}$, and I see that it breaks a sequence into overlapping dimers and compute a weighted sum according to a dictionary with 16 ...
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Calculation of dry air density without relative humidity

I would like to estimate the density of dry air at different temperatures and constant pressure (1 bar) just using the densities of nitrogen (79%) and oxygen (21%). I consider the amount of other ...
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Why does the internal energy increase when ice melts to give water?

Ice melts at 0 °C to give water at 0 °C. The internal energy apparently increases. Thermal energy is transferred to ice causing this to occur. Since the temperature doesn’t change, all the thermal ...
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How does water sublimate at normal atmospheric pressures/conditions (i.e., without a phase boundary between solid and gas)?

I remember my high school chemistry teacher stating that water could sublime under normal atmospheric conditions that would exist in e.g., Lancaster County, PA, where Amish would hang clothes to dry ...
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If the reaction is spontaneous, is it the ΔS surroundings that is always positive or ΔS universe? [closed]

If the reaction is spontaneous, is it the ΔS surroundings that are always positive or ΔS universe? I tried researching this on Google. However, the answers that I have found vary with different ...
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Why does eutectic point have three phases, but consolute point have only two phases?

At eutectic point, there are three phases: solid A, solid B and homogenous liquid phase containing both A and B. But at consolute point, we are told it has 2 phases: liquid A and liquid B. Why isn'...
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1answer
59 views

Calculating Enthalpy of formation versus Calculating Enthalpy of a reaction not occurring at standard conditions

My understanding of Hess's law and its use in determining the change of enthalpy of a reaction has been challenged recently. Up until recently, I thought that the only way to calculate a chemical ...
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How do the qualitative driving forces of a reaction make the reaction proceed?

In high school chemistry, I learned that there are 4 “qualitative driving forces” that cause aqueous reactions to proceed: Forming a solid (precipitate) Forming a liquid Forming a gas Transferring ...
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Literature recommendation - non-stationary heat transfer in porous solids

I am trying to calculate a non-stationary heat transfer to a porous spheres by conduction. These spheres are placed in a thin metal tubes with a known heat capacity. Hot/cold air (or similar gaseous ...
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1answer
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Does this coupled reaction actually happen (decomposition of calcium carbonate)

I found the following example of a coupled reaction to drive the decomposition of calcium carbonate. I get the calculation part of it, that the changes in Gibbs energy sum to a negative amount. But ...
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True or false: "If a reaction has a large negative value of ∆G, then it will be a fast reaction."

I recently took a Gen. Chem. 2 exam that contained this question. I answered false, as I knew that thermodynamics and kinetics are separate matters. My professor said the answer is true. I pressed him ...
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Improving efficiency of absorption refrigerator (ammonia+hydrogen)

Apologies if this is not the right forum for this question, I checked the Physics side and there's no questions on this, whereas there's at least one other question here about absorption refrigeration....
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Does higher alkanes require higher temperature for cracking in comparison to lower alkanes? [closed]

For e.g. does decane or higher alkane need more temperature for alkane than nonane or other lower alkane. Is it true that higher the alkane, higher the temperature required for cracking?
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What does the phase diagram mean by "liquid" at NTP? (The liquid and vapor coexist even when the situation is not on the curve)

[Solved. For anyone who is also confused, see the comments under Poutnik's answer. As Poutnik reminds me, remember there is no air involved. I successfully picture that with the piston experiment in ...
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395 views

What does n(Na2CO3) mean?

In my book, $n(\ce{Na2CO3})$ was used to indicate the number of moles of $\ce{Na2CO3}$ without any clarification. Is it common knowledge among chemists that $n(\ce{Na2CO3})$ indicates the number of ...
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3answers
260 views

Total entropy change for a spontaneous reaction

For the oxidation of iron, $$\ce{4 Fe(s) + 3 H2O(g) -> 2 Fe2O3(s) + 3 H2(g)}$$ the entropy change is $ \pu{–549.4 J K^{-1} mol^{-1}}$ at $\pu{298 K}.$ In spite of the negative entropy change of ...
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1answer
47 views

Mechanism of successively melting ice with table salt [closed]

Hi I was reading about how regular sodium chloride in itself can melt ice outside, for example ice on roads during low temperatures. However, I am unsure about the exact details, as I have found some ...
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1answer
231 views

Effect of specific heat capacity on heat loss

Assume we want to calculate the heat capacities of two substances with different heat capacity. We heat them both up with a immersion heater delivering a constant rate of energy (constant J/sec), and ...
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1answer
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dU/dV in van der Waals equation of state derivation

Previous, I proved that $$\frac{T_2}{T_1}^{C_V/(C_p-C_V)}=\frac{V_1}{V_2}$$ I've been asked to derive a similar expression for a gas that obeys the equation of state $$p=\frac{RT}{(\overline{V}-b)}$...
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Can reverse micelle's exist outside solution?

As in reverse micelle's, the polar liquid remains inside the micelle while the exterior is one of lower dielectric constant; if the liquid were to be carefully removed, would the micelle structure ...
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1answer
77 views

Does the vapor pressure of a substance depend on the presence of other gases?

The Clausius-Clapeyron equation (CC equation) can be used to find the (saturated) vapor pressure of a substance ie. the gas pressure at which the two phases (vapor + liquid or vapor + solid) reach ...
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Feasibility of a chemistry experiment involving combustion vegetable oil and fuel [closed]

As a part of a chemistry experiment I would like to see the effect on the enthalpy of combustion of a mixture of vegetable oil and gasoline/diesel fuel when its percentage composition is changed (i.e ...
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1answer
50 views

Why does Raoult's Law have a weighted sum instead of a direct sum?

I'm fully aware of the various questions asked about Raoult's Law, and I'm asking this after having gone through all of them and not finding a suitable answer to my question. I have studied, that at a ...
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How can a system that has phases of different compositions be in equilibrium?

I understand determining conditions of equilibrium for a unary system; each of the separate phases can be treated as a one-phase system with its own set of extensive and intensive properties. However, ...
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Meaning of chemical potential in chemical reactions

In my thermodynamics course, we introduced the chemical potential as a modification of the first + second law of thermodynamics in the case of a system that can exchange particles with its ...
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1k views

Is the total entropy change of all isothermal processes 0?

We define total entropy as: $$\Delta S_\mathrm{total}=S_\mathrm{system}+S_\mathrm{surroundings}$$ I know that for an isothermal AND reversible expansion: $S_\mathrm{total}= 0$ But what about an ...
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Validity of the adiabatic equation in irreversible processes

I read here that the ideal gas equation and all state equations are valid only at thermodynamic equilibrium. In irreversible processes, the system is not at thermodynamic equilibrium. Also, for ...
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Why is my P-T gradient decreasing for increasing radius of spheres when it should be increasing? [closed]

Why is my P-T gradient decreasing for increasing radius of spheres when it should be increasing? I'm simulating some hard sphere collisions with $Radius_{container}=10,$ and varying pressure and ...
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1answer
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What happens when $V=Nb$ in the Van der Waals equation (divergent)?

The VdW equation: $$\left(P+a\left(\frac{N^2}{V^2}\right) \right)\left(V-Nb\right)=Nk_BT$$ when the intermolecular forces are zero $a=0$, so $P=\frac{Nk_BT}{V-Nb}$ which diverges at $V=Nb$ for fixed ...
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Is the value for the Boltzmann Constant different in 2D?

So I'm simulating some hard sphere collisions and want to determine the Boltzmann constant in two dimension, using the equation $$PA=Nk_BT$$ where $A=\pi R^2_{container}$, $P=(averageImpulse/time)/(2\...
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1answer
59 views

Converse of ΔU = 0 for isothermal processes: is it true?

For an ideal gas undergoing an isothermal process $\Delta U = 0.$ But is the converse true? Let's say, $\Delta U$ for an ideal gas is zero. Is its temperature constant then? Can we say that it's ...
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602 views

Under what circumstances would azeotrope exist in non ideal solutions? (when would the dew point curve and the bubble point curve meet?)

[solved. For anyone who's interested in this topic, please view MaxW's comment under his/her answer. A thousand thanks to Poutnik and MaxW.] I looked up several books on thermodynamics, and they all ...
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1answer
144 views

Why does the heat of formation of organic radicals and positive ions decrease with their size and degree of branching at the radical or ionic site?

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: The heat of formation ...
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Why is it that the enthalpy of hydration equals the difference of solvation enthalpies of anhydrous and hydrous salts

Taking an example from my textbook At finite dilution of CuSO4, heat is liberated $$\ce{CuSO4(s) -> CuSO4 (aq)}.$$ ΔH of solvation = -15.8 kcal (anhydrous CuSO4 solvation reaction) and when it is ...
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Combustion Temperature Calculation Using C_p and Heat of Formation

I am simulating the combustion temperature for various compositions of syngas and finding the product temperature. Part one is varying the ratio of CO and H2 from 3:1 to 1:3 in the syngas, burning it ...
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456 views

Extrapolating from a calorimetry lab to find a new delta T with different volumes and grams of substance

So, we just finished an AP chem lab where the question was basically: Given the results of the lab were that $\pu{5 grams}$ of $\ce{CaCl2}$ and $\pu{45ml}$ of water produced a $\pu{13.33 ^\circ C}$ ...
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Can a gas expand and change its volume at STP by doing work on its surrounding?

I came across this question: 1 g of H2 gas expands at STP to occupy double of its original volume. The work done during the process is? So can a gas expand and change its volume at STP by doing work ...
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1answer
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Why is it difficult to calculate ΔS of the surroundings (said a teacher)?

My chemistry teacher told me that the Gibbs Free Energy (G) function was created because determination of ΔSsurroundings, and thus ΔStotal was difficult, and we needed something better to explain ...
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Reversible and Irreversible processes

In an ideal world, is moving a piston quickly, a reversible process as there is no heat loss due to friction? (as opposed to a quasi static process)
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What do we define as "surroundings" in thermodynamics? [duplicate]

I was thinking about the 2nd law of thermodynamics, and how ΔStotal=ΔSsystem+ΔSsurrounding. You could potentially have a reaction with negative ΔStotal occurring- a non-spontaneous reaction. But by ...
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Is the Haber Process here proceeding at positive Gibbs free energy change?

$\ce{3H2 + N2 -> 2NH3}$ is the forward reaction used in the Haber process, the industrial production of ammonia. After studying Gibbs energy and how it is just another way of saying that the 2nd ...

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