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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Lowering the freezing point of a lipid by adding another lipid

I'm trying to stop bacon grease from congealing in the fridge by adding another lipid with a lower freeze point to it, but I'm not sure how much to add, as I'd like to keep the ratio as bacon-grease ...
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The glass transition temperature Tg of polymer increases when we have a longer aliphatic side chain group. What exactly happens?

I understand that thermodynamically, entropy increases when we have a longer side chain. But what about steric hindrance?
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Explanation for difference in slope of graph of relative lowering of vapour pressure

Can someone tell the reason for the difference in the slope of graph of vapour pressure vs temperature of solid solvent and liquid solvent both the physical (actual process taking place) and ...
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Thermodynamics Thermochemistry [closed]

In a chemical reaction, the reactants combine to form the final product. Before making the final product, they form an intermediate complex: C + D -> C – D -> E The reactants C and D form intermediate ...
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55 views

What is the heat needed for an isothermal expansion of gas. Why do physics and chemistry yield different answers? [duplicate]

One mole of a certain ideal gas is contained under a weightless piston of a vertical cylinder at a temperature T. The face of the piston opens into the atmosphere. What is the heat supplied in the ...
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2answers
27 views

Exothermic dissolution

When I was learning about salts that show exothermic dissolution, I noticed that the same salts used in Daniel Cell( due to similarity of size of anion and cation) were undergoing exothermic ...
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How much work is done for gas and solvent vapor formed? [closed]

I got 20.04KJ of heat is released. But then I am lost after that on how to calculate work done in KJ
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Finding enthalpy and entropy at a different condition than the given ones [closed]

Using the given formulas we find the enthalpy and entropy at different temperature My query is why did we have to use the S300=s373+40*ln(373/300) in second last line When we already have found ...
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1answer
40 views

Water evaporation from calorimeter giving less exothermic enthalpy value?

To find the enthalpy change of combustion for an alcohol, you use a calorimeter filled with water and heat it by burning an alcohol. I was told that usually experimental values for the enthalpy ...
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47 views

Why doesn't Delta S total = 0 for this reversible process?

My textbook (Engel Physical Chemistry 3rd Edition) says: For any irreversible process in an isolated system, there is a unique direction of spontaneous change: $\Delta S > 0$ for the ...
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1answer
39 views

Are phase transitions reversible when performed at supercooled temperatures?

For the state transition of supercooled water to ice, is the process reversible? I wonder this because I want to know if I can use entropy change of the universe is 0. If it isn't reversible, how do ...
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Why is maximum work done for reversible paths in isothermal expansion and not minimum work done?

Work done is given by $W = -P_{ext}dV $ according to convention. Which should mean that in cases of expansion, work done on the system would be negative. If work done is negative, isn't it wrong to ...
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How did the temperature in the solution with a lower concentration of CuCl2 increased in a faster rate?

My lab assignment is about the reaction between $\ce{CuCl2}$ and Aluminium and we tracked the temperature of the solution every 15 seconds and we needed to make our own experiment to check how ...
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1answer
38 views

Entropy change of surroundings and system

The entropy change of the surroundings can be calculated by the equation $dS_{sur}=\frac{dq}{T_{sur}}$ regardless of the path (irreversible or reversible). The argument is that because the ...
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2answers
78 views

Deriving the equation for reaction spontaneity

I'm trying to make sense of the derivation of the equation $$\Delta G = \Delta G^0_r + RT\ln(Q)$$ While searching for a derivation of this equation that made sense to me I came across a publication ...
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Calculating Gibbs free energy for the reaction of burning methanol

Calculate $Δ_\mathrm{r}G^\circ$ for the following reaction: $$\ce{2 CH3OH(l) + 3 O2(g) -> 2 CO2(g) + 4 H2O(l)}$$ using $Δ_\mathrm{r}H^\circ$ and $Δ_\mathrm{r}S^\circ$ at $\pu{298.13 K}$ $(...
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Why is the heat of combustion of sulphur equal to the heat of formation of SO2 and not SO3

By definition the heat of combustion for compound is equal to the energy that is released on complete oxidation of one mole that substance to its highest oxidation state. In case of sulphur, online I ...
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Why do we use reaction coefficients for equilibrium constant equations, but rate law reaction equations are determined mostly experimentally? [closed]

For my understanding, with the exception of all the steps of the reaction given, including the rate-determining (slowest) step, that would determine the rate law reaction, the rate law reaction (order ...
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620 views

Why do the first and second laws of thermodynamics not contradict each other?

We're learning about entropy right now in general chemistry, and I'm trying to understand something. From the fact that entropy can be directly compared with enthalpy, free energy, electromotive ...
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1answer
37 views

Gibbs–Helmholtz equation

The Gibbs–Helmholtz equation was developed in the 1870's, while Nernst's heat theorem was developed in the early $20^{th}$ century (according to google/wikipedia). Nernst's heat theorem tells us that ...
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Thermodynamic model of inclusion process of alpha-cyclodextrin

I have a question about thermodynamic model of inclusion process of CD, published by Tabushi (DOI: 10.1021/ja00471a043). Here, in 3rd step, action is described as: "An apolar solute (guest molecule) ...
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1answer
57 views

Clausius–Clapeyron equation: shape of phase diagrams makes no sense

I am trying to model the melting point of a substance at varying pressures (ranging from very small to very very large). All I am trying to do is make an equation that relates melting temperature to ...
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1answer
34 views

Gibbs free energy where T, V, and P change simultaneously

So $\Delta G = \Delta H - T\Delta S - S\Delta T$. Is there a way to express $\Delta G$ for a process where T, V, and P change simultaneously only in terms of the initial and final T, V, P? Assume ...
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Why do things want to lower their energy? [closed]

Like if we consider a container of gas molecules as a system, Why does it want to lower its energy? I have read that always more spontaneous events are favored. So Gibb's Free energy should have a ...
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Ideal Gas equation and its Assumptions [duplicate]

The two assumptions are that the gases are points of mass that move, they have no volume and that there is no interaction between other molecules. But even the points of mass can collide with each ...
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1answer
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Is electrical work equal to the change in enthalpy or internal energy?

A $\pu{0.5 A}$ current from a $\pu{20 V}$ supply is passed through a water sample. $\pu{0.9 g}$ of water is vapourised at $p = \pu{1 atm}$ and $T = \pu{373 K}$. Calculate $\Delta U$ and $\Delta H$ for ...
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What is the relationship between entropy and heat capacity?

The molar heat capacity of hydrogen gas and deuterium gas are nearly the same, 28.8 J/K mol and 29.2 J/K mol respectively, but the absolute entropy of deuterium (145.0 J/K mol) is significantly larger ...
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Enthalpy of reaction and Hard/Soft Acid/Base

The following reaction has an Enthalpy change of -1 kcal/mol: CsI + RbCl -> CsCl + RbI Is it possible to explain this in terms of Hard/Soft Acid/Base ?
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What is the entalphy change with respect to pressure at Isothermal Process for *Real Gases*?

How much enthalpy of argon changes when the pressure is isothermally increased by $\pu{1 atm}$? (Gas should be considered as real gas under specified conditions) ($\pu{1 mol}$ of Argon gas is ...
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Why molar volume takes range of values along triple line whereas the temperature and pressure are constant along this line?

In the P-V-T surface the triple line is the line parallel to P-V plane on which three states of matter are in equilibrium. Temperature and pressure are same at all points along the triple line. But we ...
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1answer
44 views

Assumption behind Gibbs energy and maximum work

I'm somewhat confused by this derivation (in Schroeder's Book of Thermal Physics) of the fact that in a closed system, the change in the Gibbs Energy is always less than the Non-Expansion work done on ...
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1answer
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Confusion on when to use mass of entire solution when performing q=mc∆T

I hope you are having a good day. I am studying for the AP Chemistry exam in May 2020. While studying, I encountered a doubt. In Problem 1 (the question on the top), the educator saw that the ...
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1answer
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Dry paint becoming lubricant with heat

I am undertaking a physics experiment and have come up with a chemistry problem. Could paint perhaps become a lubricant under mild temperature. (It would take a while to explain my experiment, but an ...
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Entropy and Equilibrium Constant: Chemical Thermodynamics

For a reaction taking place in a container in equilibrium with its surroundings, the effect of temperature on its equilibrium constant $K$ in terms of change in entropy is described by: (A) With ...
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1answer
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How can temperature be constant during a reaction? [closed]

So I was reading my textbook about the topic thermodynamics. Here the second paragraph States that the above equation describes the combustion of liquid ethanol at constant temperature and pressure. ...
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2answers
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How to calculate the half time of a unimolecular reaction given the Arrhenius coefficient and the activation energy?

Consider the following unimolecular reaction: $$\ce{H2O2 + M -> OH + OH + M}$$ The high pressure limit unimolecular rate coefficient for this reaction is $k_\mathrm{uni} = A\cdot \mathrm{e}^{...
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1answer
35 views

Vapour pressure of a liquid subjected to pressure

I am following the derivation by Atkins in section 4B(c) (Physical transformations of pure substances; thermodynamic aspects of phase transitions), in which the authors derive the equation ${p = p_* * ...
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1answer
62 views

How to calculate the variation of enthalpy in a reaction that does not occur in standard conditions?

Standard enthalpy of formation is a value that represents the enthalpy variation in the formation of compounds with the purpose of using this values to know the enthalpy of a reaction where these ...
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1answer
58 views

Adiabatic isobaric irreversible process [closed]

I cannot understand that in an irreversible adiabatic isobaric process the enthalpy change is $ \Delta H <0$. Could anyone please help me how to prove it. Any help is appreciated.
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1answer
49 views

Intutive understanding of internal energy [closed]

Our high school teacher told us that once we calculate the formula of internal energy, it is valid everywhere. I cannot understand how? I asked him, he simply answered because it is state function. It ...
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Finding vibrational frequencies and heat capacity from bond enthalpies?

So I have heat of formation data for H2, HD, and D2, and I'm being asked to find the difference in vibrational frequency and zero point energies between H-H and H-D. I know that i need to use the ...
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1answer
287 views

Why is the Standard Entropy of a Proton Zero?

It was my assumption that the standard entropy was the absolute entropy of a substance at standard state; however, my textbook states various standard entropies ranging from $-144.77\ \mathrm{J/(mol\ ...
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1answer
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Pressure of gas increases with temperature at constant volume due to

At constant volume, for a fixed number of moles of gas the pressure increases with rise of temperature due to: I think with rise of temperature, entropy of system will increase because $\Delta S=nc_V\...
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2answers
912 views

What happens in an exothermic reaction at the atomic level? [closed]

Exothermic Reactions are those chemical reactions in which heat is released. How does this happen? What I mean is where does the heat energy come from? Which form of energy is converted to heat ...
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1answer
22 views

Effect of pressure on transition temperature

I was wondering if someone can explain why the answer to the following question is volume instead of the others. If pressure has no effect on the transition temperature between two crystalline forms ...
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1answer
132 views

Is this a counterexample to ΔU = 0 J for isothermal processes? [closed]

In class, I have learned that for isothermal processes the following is true at all times. $$\Delta U=q+w=0\text{ J}$$ But suppose I lift an object against the force of gravity. Then I (the ...
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2answers
213 views

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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1answer
56 views

How is a reaction with more products than reactants spontaneous?

If I have a reaction that has $\Delta G^\circ = 0,$ where there are more products than reactants (for example, $\ce{A <=> 2B + 2C}),$ then if I change the concentration of all the reactants and ...
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1answer
49 views

heat capacity in kelvin vs celsius [closed]

I understand Fact 1) Average kinetic energy of an atom in a noble gas is proportional to the temperature in Kelvin. Fact 2) Average kinetic energy of an atom is not linearly proportional to ...
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2answers
62 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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