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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Adiabatic expansion of gas

If a real gas is adiabatically expanded against constant pressure, then which of the following will definitely increase? Compressibility factor (Z) or Entropy (S) Correct answer: Entropy My answer: ...
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Entropy paradox and Confusion with Gibbs energy

Define $G=H-TS_{sys}$. Then, $dG=dH-S_{sys}dT-TdS_{sys}= -T(-dH/T + dS_{sys}-dT/T)$ At conditions of constant Temperature and Pressure, $-dH/T=dS_{surr}$ and $dT=0$. Thus, we end up with: $$dG=-T(dS_{...
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Thermochemistry questions regarding enthalpy

I have a few questions about heat of combustion. I have two different scenarios that I was to present: Scenario 1: Given heat of combustion in the equation $\ce{CH4 + 2O2 -> CO2 + H2O}$ and $\Delta ...
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I calculated internal energy at various temperatures and pressures. My results differ from someone else's. Is this OK?

I would like to make sure that my understanding of internal energy is correct. I'm not a thermodynamicist, so apologies in advance for what is probably a basic question. I previously calculated ...
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Why is the overall change in entropy of Photosynthesis positive?

Net Reaction: $$\ce{6CO2(g) +6H2O(l) -> 6O2(g) + C6H12O6(aq)}$$ The net reaction shows that 12 moles become 7 moles; 6 of gas,and 1 complex molecule. This implies a decrease in entropy over the net ...
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42 views

Thermodynamics of chemical reactions “at constant pressure”

Chemical reactions occur at constant temperature and pressure. Consider a gaseous, equilibrium reaction: $\ce{2NO2(g) <=> N2O4(g)}$. Most questions/textbooks formulate such questions by stating:...
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2answers
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Analysing reversible processes

A reversible process is a process in which the system is in mechanical and Thermal equilibrium with its surroundings. Consider a simple thermodynamic system: a gas in a vessel, with a piston (free to ...
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Thermal equilibrium in reversible processes

In whatever resources I have consulted to study reversible thermodynamic processes, a common statement seems to be : The system and surroundings (if the boundary allows) are in mechanical ($P_{sys}=...
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1answer
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If we take a adiabatic closed container and allow CaO and water to mix. Will reaction take place? will entropy of system change?

my attempt. If reaction takes place then it releases energy (exothermic) which will increase temperature inside the container this implies internal energy of container increases but because heat ...
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Am I correct saying that any reaction that actually occurs are all spontaneous reactions while it is occuring? [closed]

I understand that spontaneous reactions are ones where the product is lower in energy than the reactant and the opposite for non-spontaneous reactions. However, as non-spontaneous reactions are ...
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1answer
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Change in internal energy for isothermal process

A gas expands isothermally against a constant external pressure of 1 atm from a volume of 10 $\ce{dm^{3}}$ to a volume of 20 $\ce{dm^{3}}$ . It absorbs 800J of thermal energy from its surrounding. The ...
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Change in water vapor with change in pressure and volume

Given a closed (air tight) container (Volume $V_0$) held at the room temperature ($T_0$) and pressure ($P_0$). The container contains 100% saturated water vapor with no air (at $T_0$ and $P_0$). Now ...
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Extrema in free energy of a solution?

I am a high school student reading about the second law of thermodynamics, and one of the equations given was the free energy change in dilution. $$\Delta G_{\text{dil}} = -RT \log(C_1/C_2)$$ This ...
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Why does a dilution refrigerator work?

I read the wiki https://en.wikipedia.org/wiki/Dilution_refrigerator And my interpretation is that you get one cold liquid(helium 3) and add it to another (helium 4). The molecules of the liquids ...
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Feasibility of a Reaction [closed]

I understand for a reaction to be feasible, $\Delta$H needs to be negative and $\Delta$S positive. But in terms of Gibbs free Energy Change, without considering kinectic factors, If $\Delta$H is ...
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2answers
64 views

Effect on mass when decreasing pressure in equilibrium reaction

For example, if we have the reaction: $$\ce{A + B <=> C}$$ And the volume is decreased, the pressure would increase. Hence, according to Le chatelier's principle, the system will partially ...
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Why does increasing availability of microstates compensate for increased energy of products in an endothermic spontaneous reaction?

I'm wrestling with the idea right now of processes that are both endothermic and spontaneous.I understand that there must have been an associated increase in entropy. How does an increase in "...
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Can a reaction be both exothermic and non spontaneous? [closed]

Can a reaction be both exothermic and nonspontaneous?
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60 views

Is there an example of a bond with negative bond enthalpy? [duplicate]

My textbook says bond breakage is "generally endothermic." This implies there are some bonds out there that release energy when broken. Is there a classic example? Perhaps diatomic Helium?
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What is happening at the various boundaries of a phase diagram?

Hoping to clarify a few questions I have that the textbook doesn't address. Lets just say I have a piston (at equilibrium) with 100 moles of gaseous compound that looks like this: And here is the ...
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Solubility of salts in water

I came across the formula for enthalpy of solution, which is equal to the sum of lattice enthalpy and hydration enthalpy. Therefore, for salts that have endothermic dissolution, the lattice enthalpy ...
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65 views

Integration of chemical potential

I want to ask a question about chemical potential, $\mu$. I dislike the use of integrals to describe quantities in thermodynamics. If we observe the definition of chemical potential for constant ...
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1answer
46 views

What is the difference between Cyclic Process and Reversible Process? [closed]

I want to ask the difference between Cyclic Process and Reversible Process. Consider this graph: If we have a process that goes from A to B then go back to A through the same path, then will it be ...
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1answer
81 views

Sign of change in enthalpy and change in entropy

Why is it wrong to assert that the change in entropy and the change in enthalpy must always have the same sign? What makes me think that they must have the same sign is the fact that every reaction ...
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The relation between q and T when the volume is constant [closed]

$$ \begin{align} q_V &= ΔU \tag{1}\\ q_p &= ΔH \tag{2}\\ C_V &= \frac{\mathrm dq_V}{\mathrm dT} = \left(\frac{\partial U}{\partial T}\right)_V \tag{3}\\ C_p &= \frac{\mathrm dq_p}{\...
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Role of intermediate in determining the exothermic or endothermic nature of reaction

I was given a question to answer: Which of the following reactions is “not” exothermic? $\ce{CH3-Cl + CH3-CH3 -> CH4 + CH3-CH2-Cl}$ $\ce{CH3-Cl + (CH3)3-CH -> CH4 + (CH3)3C-Cl}$ $\ce{CH3-Cl + ...
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Relationship between reaction quotient and potential energy [closed]

I am trying to calculate the Gibbs free energy in an NVT simulation. Although $$\Delta G = -RT\ln Q,$$ in my simulation I can't calculate $$Q = \frac{[\text{Products}]}{[\text{Reactants}]}$$ because I ...
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2answers
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What does it mean for chemical reactions to release energy?

My Physics textbook gives the following definition of energy : The capacity of a body to do work is defined as the energy possessed by it This was the definition that was used to derive $U_G = mgh$ ...
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1answer
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Significance of pV work in enthalpy

If we already have $pV$ work included in the internal energy $U = q + w,$ then what is the $pV$ work in the equation for enthalpy $H = E + pV?$
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3answers
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Work done in Irreversible Isothermal Expansion

This is the graph for the Isothermal Expansion of an Ideal Gas. Sorry for the extremely basic doubt, but I just could not understand how the expansion was Isothermal if the graph was a straight line. ...
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450 views

Why does spontaneity of a reaction depend on temperature?

As we know, $$\Delta G=\Delta H - T\,\Delta S.$$ Both in my textbook and on the internet, it was given that temperature was a determining factor in the spontaneity of a reaction. For example, when $\...
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Calculation of free energy from potential and kinetic energy

I am having trouble understanding how (and even whether it's possible) to get free energy from kinetic and potential energy. I have run molecular dynamics simulations (NPT ensemble, CP2K software), ...
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A Thermodynamics question involving the molar solubility and Ksp of CaCO3

This thermodynamics question is from my textbook, I am not sure how to solve it correctly and I do not have the answer. The first thing I did was to calculate the number of moles from the Ca(OH)2 ...
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3answers
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Can work be done on a body when temperature and pressure are changed without changing its volume?

Can work be done on a gaseous system when temperature and pressure are changed without changing it's volume? In chemistry the formula of work is always $p\,\mathrm dV,$ whereas we can also do work on ...
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3answers
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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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Can there still exist equilibrium with a very high equilibrium constant?

Problem Consider the reaction translated by: $$\ce{2 CO(g) + O2(g) <=> 2 CO2(g)} \quad K_c = 2 \times 10^{22}~\text{at } \pu{1000 K}$$ The order of magnitude of the equilibrium constant, $K_c$,...
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2answers
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Difference between ΔQ and dQ [closed]

What is the difference between $$\Delta Q$$ and $$\mathrm dQ$$ in thermodynamics? I think one is used when the change is very small and the other with bigger changes, but I am not sure.
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What is the enthalpy change of neutralisation of vinegar (diluted acetic acid) and calcium carbonate?

If anyone has conducted an experiment that determined this value or knows reliable sources that show the literature/thereotical value of this enthalpy change, please share your answer. Thank you very ...
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Change in Entropy and Spontaneity [closed]

I was studying thermodynamics and got stuck with some questions. I am pointing them down. I have studied that $\Delta S_{\text{total}}$ accompanied by a spontaneous process is greater than 0. But is ...
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1answer
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Rigorous reason behind internal energy change being zero while mixing

A container is divided in two parts: one part contains oxygen gas $(n_1$ moles, at temperature $T_1)$ and the other part contains helium gas $(n_2$ moles, at temperature $T_2).$ The partition ...
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3answers
131 views

Why water cools from outside to inside?

It is a well-known fact that Water kept in a container, on cooling, results in freezing with initially at the circumference of surface (1D), then on the surface (2D) and then throughout the volume (...
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When, if ever, can standard enthalphy be subsituted for standard chemical potiential?

I'm reading through a combustion modeling paper by Gordon & McBride which is a bit scant on some of the algebra & calculus used to derive their equations. My advisor and I have derived nearly ...
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1answer
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Apparent paradox regarding spontaneity

Consider the following analysis ( rev and irrev respectively refer to a reversible and an irreversible path, between the same initial and final states): $dU_{rev}=dq_{rev}+dw_{rev}$,and,$dU_{irrev}=...
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Reversibiliy and equilibrium

My textbook states "A system is in thermodynamic equilibrium if it is in Thermal, Mechanical, and chemical equilibrium simultaneously". The Wikipedia article on "reversible processes&...
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Positive work in the Carnot cycle

In the Carnot cycle the overall work done on the piston is positive. I am having trouble with the real meaning of that. Does positive work mean the piston is moved up and has, over the course of the ...
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Crossing at critical temperature of a gas

This is written in the book 'Treatise on Thermodynamics' by Max Plank- the crossing of the critical temperature at any pressure other than the critical pressure differs in no way from the crossing of ...
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Can more than one thermodynamic process occur at the same time? [closed]

Thermodynamic processes I know about are, Isothermal, Adiabatic, Isochoric and Isobaric (if you know any other, please mention). Is it possible that any 2 or more of these four processes occur ...
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Chemical Potential in the Fluorescent Molecule

I'm reading this paper, which states that the chemical potential $\mu$ is determined by the steady-state balance of up and down transitions in a fluorescent molecule. I am happy with this ...
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32 views

Hydrophobic interactions and Gibbs Free Energy [duplicate]

I want to understand the basis of the hydrophobic effect. In biochemistry, protein structures are often explained in terms of hydrophilic and hydrophobic effects. The hydrophilic interaction is ...
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1answer
64 views

Heat capacity of a calorimeter

Why do we mix water with different temperatures to calculated the heat capacity of the calorimeter? Why can't we calculate it directly using just warm water? Asked differently, would the following be ...

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