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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thermodynamics problem [closed]

The boiling point of a substance $X$ at 1 atm pressure is $500 k$ . The enthalpy of vapourisation at the boiling point of $X$ (l) is 80 kj/mol .The molar specific heat of $X(l)$ = $5 × 10^{-3}$ kj/mol ...
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Deciding the order of heat of combustion of isomeric alkanes

I'm asked which one of the alkane has the lowest heat of combustion (note: the 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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Why are high pressures used in cracking of long-chain hydrocarbons?

If we have a long-chain hydrocarbon, such as decane, and we split it through thermal cracking (say in an industrial plant), we use high temperatures, and high pressures. Cracking produces smaller ...
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How to calculate the temperature at which a reaction becomes spontaneous?

At $25~^\circ\mathrm{C}$, $298~\mathrm{K}$, the reduction of copper(I) oxide, $ \Delta H = 58.1~\mathrm{kJ}$, $\Delta S = 165~\mathrm{J/K}$, is nonspontaneous, $ \Delta G = 8.9~\mathrm{kJ}$. ...
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1answer
781 views

Effect of temperature on equilibrium constant in terms of entropy change

This question was asked in the JEE examination of 2017 : [A] option is incorrect since the system is losing heat and hence $$ \Delta S = -ve$$ [B] Lets assume we have an endothermic reaction $$\...
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2answers
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How do you manage to liquefy a gas by applying only pressure? [closed]

How can you liquefy a gas by applying only pressure? To increase the pressure of a gas you need a compressor, and when the compressor does work on the gas , it will increase his pressure but at the ...
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What are examples for state functions?

List some examples of state functions. Check all that apply: internal energy work volume pressure I know that a state function is independent of the path taken, so I think work and ...
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5answers
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What exactly is temperature?

I've read at many places that temperature is the average kinetic energy of particles present in an object. I just don't intuitively get how kinetic energy is connected with temperature. And how is ...
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5answers
487 views

How can we define a non-spontaneous reaction?

Consider a reaction:$$\ce{$aA + bB$ <=> $cC + dD$}$$ The value of reaction quotient at a certain time $t$, $${Q_c = \frac{[C]^c[D]^d}{[A]^a[B]^b}}$$ where the concentrations $[A], [B], [C]$ and ...
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Why is average bond enthalpy measured by homolytic fission?

Average bond enthalpy: the energy required for 1 mole of a specific gaseous bonds to undergo homolytic fission Why specifically homolytic fission? Is it to allow comparison, but if that were the case,...
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1answer
169 views

Solving a quadratic equation for pressure of real gas with unknown volume

Question Calculate the pressure exerted (in atm) by 1 mole of $\ce{CH4}$ gas at a temperature of $\displaystyle\pu{\frac{24}{0.0821} K}$ if volume occupied by $\ce{CH4}$ molecules is negligible. ...
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1answer
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Difference between chemical energy and potential energy

I know that chemical energy is the potential energy stored in the arrangement of atoms within molecules. Forming new chemical bonds releases energy. what does potential energy mean? Is it the energy ...
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1answer
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Entropy Change In Rusting of Iron [closed]

I presume that the entropy of the system should decrease during rusting of iron because the atmospheric oxygen gets converted to solid state from gaseous state which should result in the decrease of ...
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Meaning of chemical energy [duplicate]

I am trying to understand the meaning of chemical energy. I saw in my book that chemical energy is stored in the bonds of chemical compounds and when the bonds are untied the chemical energy increases....
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0answers
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Change in temperature if salt is dissolved in water with different initial temperatures

I will mix a salt with room-temperature water and 8 degree Celsius water. Then I will compare the heat of solution with each other. Does the initial temperature of the water affect how much the heat ...
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1answer
740 views

Hydrates and melting points

Basalt melting point: wet vs dry I had a student ask me this in class the other day; I thought about it, but I still can't work it out. Many mineral and rock hydrates (take basalt, for instance) ...
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3answers
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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 ...
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1answer
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Is this example considered liquifying a gas under pressure?

In my work place, we have a drum that has propene inside at a pressure of around 2,5 bar and at a temperature of around -15ºc and the boil-off of that drum is aspirate by a compressor and is ...
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3answers
333 views

Which substances have the highest/lowest enthalpy of formation per gram?

Which substances have the most extreme enthalpies of formation per gram at standard temperature and pressure?
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25 views

Combining enthalpies of formation and combustion in the context of Hess's law

I know that you can use enthalpy of combustion to work out formation, and Formation to work out enthalpy change. But can you use enthalpy of formation and combustion data is the same Hess's cycle to ...
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1answer
2k views

Thermodynamics of photochemical reactions

Suppose I have a photochemical reaction in gas phase, such as $$ \ce{CO2 + {$h\nu$} -> CO + O}. $$ I would like to work through the thermodynamics of such a reaction and understand the meaning of ...
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2answers
54 views

Why does electrode potential depend upon enthalpy of hydration and sublimation?

It's given in text that "Electrode potential among other things depends upon: enthalpy of sublimation,ionization enthalpy and hydration enthalpy." I could understand why it would depend upon ...
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0answers
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What is the lattice enthalpy of zinc chloride?

I am trying to find a literature value for the enthalpy of solution of zinc chloride to compare with my experimental value but I can't seem to find it. So I'm trying to use the formula: enthalpy of ...
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1answer
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How do I distinguish between internal energy and enthalpy?

I am finding it hard to distinguish between the two. Can someone explain the two terms and the difference between them? I tried learning from Wikipedia, but it said for both the above terms that it ...
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1answer
562 views

Is the flame temperature of a burning fuel affected by pre-heating the fuel?

This page on Adiabatic temperatures helpfully lists the theoretical maximum temperatures that flames will reach under certain conditions for a number of fuels. They assume initial atmospheric ...
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1answer
580 views

Entropy and enthalpy at the melting point of benzene

For the melting of Benzene, since it is melting, it feels intuitive to think that the change in entropy will be positive (more disorder) and enthalpy will be positive (exothermic). However, when I ...
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1answer
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Heat transfer to an ideal gas [closed]

In thermodynamics, why is the heat that is transferred to an ideal gas considered as work that was done on the system for an isothermal process?
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Entropy - “Wiggle”?

The title is not a reference to a Jason Derulo song. In any case: 1) How is change in entropy measured, experimentally? I've Googled this for a bit and I've found all sorts of mathematical ...
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2answers
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Most endothermic room-temperature reaction [closed]

In terms of energy per gram of reactants, the evaporation of water at -2.2 kJ/g, is much higher than any chemical endothermic reaction that I have seen demonstrated. At extreme temperatures there is ...
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1answer
226 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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2answers
153 views

How are reaction enthalpy and entropy affected by temperature

The equation for Gibbs Free Energy demonstrates how the standard change in Gibbs Free Energy for a reaction is affected by temperature. $$\Delta G^\circ = \Delta H^\circ - T\,\Delta S^\circ$$ ...
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1answer
72 views

Temperature and pressure for three phase equilibrium of aqueous sodium chloride solutions

I am currently investigating the effect of adding Salt(NaCl) to water on its triple point. Let us assume the mass ratio of water to NaCl is $1:1$. I asked a similar question previously. But, I still ...
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1answer
59 views

Intermolecular Forces of Attraction [closed]

Imagine a bottle 85% filled with water and the rest is just air, and the cap is closed. Here are now my questions. Since the cap is closed completely, the air bubble is formed. Is that air bubble ...
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1answer
74 views

Calculate pKa for lipids

I would like to calculate $\mathrm{p}K_\mathrm{a}$ of lipid molecules using some software tools. I have tried ChemAxon and compared with ChemDraw professional 16.0 version they are mostly similar. I ...
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0answers
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Calculating volume of a vessel using idea gas law [closed]

To an evacuated vessel with movable piston under external pressure of $\pu{1 atm},$ $\ce{0.1 mol}$ of $\ce{He}$ and $\pu{1 mol}$ of unknown compound (vapour pressure $\pu{0.68 atm}$ at $\pu{273 K})$ ...
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1answer
657 views

Gibbs Free Energy and Maximum Work

I am a high school student and my professor mentioned that the Gibbs free energy is the maximum amount of work (or useful work) that a system can do, whereas entropy is a measure of the non-available ...
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1answer
48 views

For bond dissociation energies (BDEs) of double and triple bonds, is number shown in tables for that second or third bond? Or all of them?

Simple, 'dumb' question.... Is the kJ/mol number given in tables of bond strengths or bond dissociation energies , when it comes to double and triple bonds, just for the 'last' bond or for both (or ...
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1answer
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How hot is elephant toothpaste?

So, I remember in high school chemistry when my teacher combined hydrogen peroxide with potassium iodide and it exploded into a yellowish foam. He said it was an exothermic reaction so it's really hot....
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1answer
318 views

Enthalpy change of gas expansion against constant external pressure

A certain mass of gas is expanded from $\pu{1 L}$ at $\pu{10 atm}$ to $\pu{4L}$ at $\pu{5 atm}$ against a constant external pressure of $\pu{1 atm}$. If initial temperature of gas is $\pu{300 K}$ and ...
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2answers
120 views

Endothermic reaction absorbs heat from environment

I read on my school book about chemistry the reaction: $$\ce{H2O + NH4Cl}$$ with $T(\ce{H2O}) = \pu{25 °C}.$ The temperature of the final solution is $\pu{20 °C}$. I know that it is an endothermic ...
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2answers
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Is irreversible work a path function? [closed]

In $P/V$ plots of irreversible processes only initial and final state is defined due to sudden compression or rarefaction then how is work done a path function in irreversible processes. Why won't ...
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1answer
40 views

Mechanistic explanation of salt lowering temperature of ice slush

As we all know, adding salt to ice water lowers its temperature. I've read plenty of system-level accounts of energy balances, enthalpies, vapor pressures, phase equilibria and freezing points—I ...
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1answer
55 views

Endothermic or Exothermic [closed]

Is an endothermic reaction always endothermic at all conditions of pressure, temperature etc.? Similarly for exothermic... I mean is sign of ∆H always the same as sign of ∆H°? This might seem to be a ...
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Are there any reactions that are endothermic and exothermic? [closed]

As it says: are there any reactions that require heat and produce heat simultaneously?
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536 views

K°P vs KP in equilibrium thermodynamics

I'm having a little trouble with definitions. Here's a copy and paste from my hwk: ...K°P, and K°c have no units...Please enter the KP in 1/bar in 3 significant figures... So these values even ...
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1answer
101 views

Heats of formation of neutral molecules and homolytic vs heterolytic bond dissociation in mass spectrometry

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: Great efforts have ...
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2answers
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Is equilibrium constant at a given temperature derived from Gibbs free energy of reaction valid for doing simple kinetic modelling?

I intend to do a kinetic study of simple alcohol catalytic dehydrogenation reactions in the gas phase. I want to start with simple power law kinetics using $K_\mathrm{eq}$ to account for the ...
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1answer
57 views

Can someone explain the article below? [closed]

This is the article: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5611670/ I'm not a Chemist, so I can't understand the article very well. The conclusion states that the molecules tested should not ...
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1answer
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Why does the combustion of graphite have a positive Delta S?

The following reaction is exothermic and entropically favorable: $$\ce{C (s) + O2 (g) -> CO2 (g)}$$ My question: Why is this reaction entropically favorable? Clearly there are more ways ...
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1answer
51 views

Regarding the infinitesimal form of the First Law of Thermodynamics

First Law of Thermodynamics is expressed as[1] The internal energy of an isolated system is constant. If $w$ denotes the work done on a system, $q$ for the energy transferred as heat to the system,...

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