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.

Thermodynamics is the study of various forms of energy in observable systems. As applied to chemical reactions, thermodynamics is used to study and explain the behaviour of chemical reactions in terms of the energy contained within chemical bonds of a substance, and which must be input into or is output from the reaction as a requisite or consequence of the reaction.

The different questions that come under this tag involve one or more of the following:

  • free energy, enthalpy, internal energy or entropy of a gas or their change
  • state and path functions
  • heat capacity
  • expansion and compression processes or their graphical representation
  • various standard enthalpies of reaction (sublimation/neutralization/etc.)
  • spontaneity of a reaction
  • Hess's law/Born-Haber cycle

and other related topics.

Questions about statistical mechanics or "microscopic derivation of thermodynamical laws" invoking thermodynamics are suited for Physics.SE instead.

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Why is dU an exact differential and dq an inexact differential?

dU, dG, dH etc are all exact differentials and the variables themselves are known as state functions because they only depend on the state of the system. However, dq and dw for example, are inexact differentials. My questions is, what does this…
RobChem
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Placed in equivalent freezers, would a liter of water or a liter of lava turn from liquid to solid first? (question from a 6 year old)

Placed in equivalent freezers, would a liter of water or a liter of lava turn from liquid to solid first? (question from a 6 year old) Based on this page in a “Blaze” book, my six year old asked “which would win?” between water and lava. On…
T.D. Smith
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Is the specific heat capacity of liquid iron constant?

Does liquid iron have a constant specific heat capacity? Everywhere I've looked (save one sourceless online reference to $c_p = 611\ \mathrm{J\ kg^{–1}\ K^{–1}}$ here), I find no value listed for liquid iron. Is this because the heat capacity is…
Excellll
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If the human body consists of 60% water why can't we put out fire with our body?

I have often heard of people being burned at the stake, but if the body is 60% water shouldn't the fire just be put out?
ArnavT
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standard reference for thermodynamic properties?

I'm a non-chemist attempting to put together a chemistry paper. Up to now I've got my thermodynamic data from various random pdf files found on the web, together with Wikipedia. Now that the paper is nearly finished I need to replace these with…
N. Virgo
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What exactly is a spontaneous reaction?

What I understand by a spontaneous reaction is that - A reaction whose G is negative and which occurs without the input of energy. Gasoline reacts with oxygen only if a spark is provided. So how can it be a spontaneous reaction? Do nonspontaneous…
biogirl
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What is the lower bound to the temperature at which a fire can burn?

To have a fire, you need 1) oxygen, 2) fuel and 3) heat. You can have different fuels that burn at different temperatures, but you always need some amount of heat to have a fire. If you can use any fuel, what is the lowest temperature at which you…
Nzall
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Is the equation dU= TdS - PdV always valid (even for irreversible processes?)

I really don't think it should be. $dU = dQ + dW$ is the statement of the first law of Thermodynamics. $dW$ can be put as $-pdV$ and $dQ $can be put as $TdS$ only if the process is reversible. (From the definition of entropy, the $dQ$ in $dS$ =…
Rick
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Why do laws of thermodynamics "outweigh" any other law ever formulated?

"It is the only physical theory of universal content concerning which I am convinced that within the framework of the applicability of its basic concepts, it will never be overthrown." - Albert Einstein "Nothing in life is certain except death,…
Arishta
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What happens in an exothermic reaction at the atomic level?

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 energy? What makes some reactions release heat? What…
Rajdeep Sindhu
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Calculating entropy: why consider a reversible path?

I am reading up on entropy in a textbook and I got confused by this: It says that to calculate the entropy for an irreversible process using heat flow, one must imagine the reversible process in which the initial and final states are the same as for…
user646
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The heat content of the products is more than that of the reactant in an ............. reaction

The heat content of the products is more than that of the reactant in an ............. reaction. In this question the answer given is "exothermic"; yet I thought the answer would be endothermic as the products have more heat content and energy…
Mr.HiggsBoson
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Deriving heat capacity in terms of internal energy U and natural variables S & V

My lecture notes have the following equation regarding heat capacities: $$ C_p = C_v + T\left(\frac{\partial p}{\partial T}\right)_v\left(\frac{\partial V}{\partial T}\right)_p $$ (just to be clear, all terms after the brackets in this question…
ZChemZ
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Mathematical explanation behind dq = TdS

Can somebody mathematically prove why $$\mathrm dq = T\,\mathrm dS$$ for a reversible process? I realize I am supposed to provide some research but I can't find anything. The best I got is $\mathrm dE = T\,\mathrm dS$ which is basically $\mathrm dG…
mathnoob123
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Why is the curve of an isothermal process above the that of a adiabatic process during compression?

For an adiabatic process $pV^{\gamma}=k$ and after differentiating the equation we get that the slope of an adiabatic process is $$\frac{\mathrm dp}{\mathrm dV}=-{\gamma}\frac{\mathrm p}V$$ For an isothermal process $pV=k$ and after differentiating…
Abhishek Mhatre
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