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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Why does Chloride ion get oxidized in electrolysis?

Suppose I have an NaCl solution in a beaker and I put two electrodes in the jar, the ends of which are connected to a normal AA battery (1.5V). Now, my question is, why is the following reaction ...
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Pressure dependence of the equilibrium constant

The equilibrium constant of a chemical reaction can be calculated if the $\Delta_\mathrm rG^\circ$ is known which is a function of only temperature. So if a reaction happens inside a container ...
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Not able to match the number of components in a system with the "species-approach" to the Gibbs phase rule

The Gibbs' phase rule states that the number of degrees of freedom in a system $F = C - P + 2$ where $C$ are the number of components and $P$ are the number of phases. The term $C$ can be expanded to $...
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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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How to melt iron at home with simple materials? [closed]

I am pretty sure that this is a very complicated process which involves a lot of heat, due to the melting point of iron. However, is there a way to really do this at home with cheap materials? By ...
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Why does it seem that the heat of combustion is higher with substances of greater molecular weights?

Why does it seem that the heat of combustion is higher with substances of greater molecular weights? How do the intermolecular forces or bonds of the reactants affect the heat of combustion? I have ...
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Weird equilibrium problem [closed]

I'll try to approach the equilibrium from both sides, and see what happens. Stay with me for the ride guys! Case 1: $$\ce{NH3_{(aq)} + H2O_{(l)} <=> NH4^{+}_{(aq)} + OH^{-}_{(aq)}}$$ Let the ...
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Virial expansion of real gas equation and volume work

Hi I am currently working on a thermodynamics problem, and it asks me to first find the volume work done on a gas that behaves like the simplified Dieterici equation (see below) predicts on isothermal ...
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Sign of Proton Affinity

I was searching for reactions with hydrocarbon anions and hydrogen cation/proton and noticed that the enthalpy change of a reaction is always positive or likewise proton affinity associated with the ...
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Calculate the enthalpy change for each of the following cases : 1.00 g methane is burned in excess oxygen [closed]

Consider the following reaction : CH4(g) + 2O2(g) CO2(g) + 2H2O(L) ∆H=-891 KJ Calculate the enthalpy change for each of the following cases : a- 1.00 g methane is burned in excess oxygen . b- 1.00*103 ...
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Reversible process and reversible reaction [closed]

If reversible process are hypothetic and doesn't exists in real world then how the formation of ammonia is possible from combination of nitrogen and hydrogen as it is a reversible reaction?
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Relationship between thermodynamics and steric hindrance in reaction outcomes?

I am investigating reactions with alkyl-substituted amines ($\ce{R} = \ce{Me}, \ce{Et}, \ce{^iPr}, \ce{^tBu}$) and a second reagent, in which product Z is formed. $\Delta G$ for all of these ...
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Why is the Heat capacity change of reaction different for ionization and electron gain?

Recently I read this statement in my book(ncert) that Electron gain enthalpy change and ionization enthalpy changes have different formulas $$ΔH_{eg}(T) = ΔH_{eg}(0k)+\int_0^TΔC_pdt$$ and for ...
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What does “efficiency” refer to for a refrigerator?

A problem from my textbook mentions a refrigerator working with $35\,\%$ efficiency. I thought that in this case the coefficient of performance (COP) was used, specifically $Q_\mathrm{cold}/W,$ which ...
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Why do we assume that the successive deprotonation of a polyprotic acid happens when only the previous deprotonation has reached equilibrium?

The steps for finding the concentration of the species at the equilibrium of polyprotic acid are as given in this book in section $10.16$ and illustrated on the next page. I have three questions, but ...
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What quantity is one mole in the reaction in enthalpy change? [duplicate]

For example, I have a reaction: $H_2(g)+\frac{1}{2}O_2(g)→H_2O(l),𝛥H=-285.8 kJ mol^{-1}$ What does this "per mole" in the unit represent? What quantity is one mole?
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Saturated vapor and liquid densities (VLC curve) from Helmholtz free energy equation of state

I have been reading about the statistical associating fluid theory (SAFT) which computes the Helmholtz free energy ($A$) from the SAFT EoS (a hell lot of equations) for a molecule of interest, and is ...
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i came across this relation between ebullioscopic constant and latent heat of vapourisation while learning colligative properties [duplicate]

[ can someone give an intuitive explanation of why this hold goods , or a derivation ( I know some basics of thermodynamics )
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Heat Capacity at constant Volume and Gibb's free energy

I want to know if it is possible to derive heat capacities, in this case in constant volume from another thermodynamic Potential which is not the Helmholtz free energy $F$. I am aware of the ...
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1answer
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How can I calculate the density of a gas mixture using the ideal gas?

I am trying to calculate the density of the ideal gas mixture given the following data: The gases are helium and oxygen. There are $\pu{8 g}$ $\ce{He}$ and $\pu{16 g}$ $\ce{O2}$. The temperature is $\...
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Maxwell's Relations and Differentiating With Respect to Nᵢ

I was wondering if anyone could write out Maxwell's relations for partial derivatives with respect to particle count $N_i$. Starting from the fundamental thermodynamic relation, \begin{align} \...
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How does part of frozen liquid ( like soup) boil and still be frozen at same time if temperature stays same while melting?

I was reading the following basic question about temperature change during phase change (During phase change in matter, why doesn't the temperature change?), and the answers provided clearly ...
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Why is the the temperature change due to the combustion of paraffin wax so high (10000 Kelvin)? [closed]

I'm trying to work out the temperature change (I got something like 10k Kelvin,hotter than the Sun's core...) caused by the combustion of paraffin wax which has a molar enthalpy of combustion $E^{wax}...
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1answer
37 views

What causes water to condense when air pressure is reduced?

In this video: https://youtu.be/IyBRGhXBuGw The cabin pressure is reduced and subsequently a mist is formed. What exactly is the cause?
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Why is my ethanol–water mixture showing negative change in enthalpy of mixing? [duplicate]

I am conducting an experiment where I mix two solutions $\text{A}$ and $\text{B}$ and then I am measuring the temperature change (for real solutions, i.e., $\Delta_\mathrm{mix}H\neq 0).$ I took the ...
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1answer
49 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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1answer
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water to hydrogen to water [closed]

I'm trying to find out if there is any loss of water when converting water to hydrogen and then back into water and can't find an answer anywhere I've searched. I understand that the water produced is ...
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1answer
43 views

How is reversible expansion work a state function? [closed]

Work, as we know, is a path function. But between given initial and final states, there can be only be one reversible path. Is this true? If yes, then is reversible expansion work a state function? ...
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Reversible compression and proving relation with state function

I am currently working on a question I am having problems with. I tried to solve it, but to no avail; it states that this expression is always valid for any three-variable dependent function: It ...
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5answers
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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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Why one compartment in an osmotic container have different pressure?

I am reading about osmotic pressure and the derivation in Wikipedia, states: Consider the system at the point when it has reached equilibrium. The condition for this is that the chemical potential of ...
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How to calculate lower and higher calorific values using standard enthalpies?

Calculate the lower and higher calorific value of benzene, expressed in $\pu{kJ L^-1},$ if the standard enthalpy of formation is known: $$ \begin{array}{lc} \hline \text{Compound} & \Delta_\mathrm{...
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Why increasing working pressure increases heat engine efficiency

I recently came across this problem in the 2018 IChO (link): Contemporary means of transportation rely on burning fossil fuels, although the efficiency of real combustion engines is inherently ...
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1answer
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why do we need to assign ΔH =0 for the formation of aqueous hydrogen ions? [duplicate]

Many books state that the enthalpy of formation of aqueous hydrogen ions is 0 BY DEFINITION, and hydrogen is chosen just for convenience. Once we do that, all other reactions have their enthalpies ...
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1answer
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How much heat is lost to the calorimeter when hot water is mixed with cold? [closed]

If a student adds $\pu{100.00 g}$ of hot water at $\pu{50.00 °C}$ to $\pu{100.00 g}$ of cold water at $\pu{25.00 °C}$ and measures a heat change of $\pu{10707.37 J}$ for the hot water and $\pu{4778.86 ...
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Changing Variables in Thermodynamics

I have a question about changing variables in the context of thermodynamics, but I suppose this would extend to any set of variables that have defined and nonzero partial derivatives on a given set of ...
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1answer
27 views

Physical meaning of enthalpy change when internal pressure Is not constant [closed]

My chemistry textbook states the following "general enthalpy" formula for the change in enthalpy when the internal pressure is not constant: $∆H= ∆U + ∆(PV)$ How is the internal pressure ...
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1answer
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Calculating the electrode potential from thermochemical data (part 2)

This question is in continuation of this question. I am taking an example of $\ce{Cu^2+}$ in aqueous medium. $$\begin{align} \ce{Cu (s) &-> Cu (g)} & \Delta_\text{a}H^\circ &...
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1answer
71 views

How does the rate equation account for solids and liquids?

The question is long because I wanted to include the whole thought process. Given the hypothetical reaction: $$\ce{ A(s) + B(aq) <=> C(aq) + D(aq)}$$ One would obtain the equilibrium constant: $$...
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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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Is the heat (or work) of the system always equal and opposite to that of the surroundings?

I know energy must always be conserved by the first law of thermodynamics, but is there ever a scenario in which a system say releases some amount of energy as work, and the surroundings gains equal ...
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1answer
103 views

Why is the Gibbs free energy of a reversible reaction not always 0? [duplicate]

From my understanding, reversible proceses are those where the expression: $$\Delta S_\mathrm{total} = \Delta S_\mathrm{system} + \Delta S_\mathrm{surroundings} = 0$$ is true for all for the entire ...
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What is the name of the physical quantity that is given in kJ/g?

The chemical potential $\mu$ of a pure phase corresponds to the Gibbs free energy $G$ divided by the amount of substance $n$: $${G\over n} = \mu,$$ so the units of $\mu$ are $\pu{kJ mol^-1}.$ What do ...
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Enthalpy/Temperature change in a chemical reaction

I was wondering how exactly absolute temperature change in a chemical reaction could be calculated. For example, consider this reaction: 3H₂+N₂→2NH₃, ΔH=-46kJ/mol (298.15K) Assuming the given reaction ...
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Is dehydration of cobalt(II) chloride hexahydrate dynamic equilibrium?

I know that the dehydration of cobalt(II) chloride hexahydrate is a reversible reaction, is it a dynamic equilibrium? $$ \begin{align} \ce{CoCl2·6H2O &<=> CoCl2·2H2O + 4 H2O}\tag{R1}\\ \ce{...
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Heat of Sublimation of GaAs and SiC

I am trying to find values for the heat of sublimation of GaAs and SiC, but can't find anything. I also tried searching for the heat of fusion and vaporization, to calculate the heat of sublimation ...
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Why does the trend of the enthalpy of reaction per atom of carbon in hydrocarbons approach a near-constant value?

I was looking at trends for enthalpies of hydrocarbons for a high school project and found an interesting paper regarding it, stating that the enthalpy per carbon atom approaches a constant. I'm ...
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1answer
33 views

Endothermic reaction with dV < 0 [closed]

I am looking for an endothermic reaction dH > 0 for which the net volume change is negative, e.g. gases reacting to form a liquid. Does anybody have an illustrative example?
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1answer
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Practical use of the partition function in molecular simulations

In the second chapter of Understanding Molecular Simulation, Frenkel and Smit derive an equation for the partition function and the thermal average of the generic observable A, stating that these ...
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What is meant by "analogous reactions" in the Bell-Evans Polanyi Principle?

The BEP principle says that for analogous reactions, enthalpy change and activation energy are linearly related. What is meant by analogous reactions? Eg. For electrophilic addition of HBr to pent-1-...

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