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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8
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1answer
2k views

Why do we use dG < 0 to describe a spontaneous process?

With something like a reaction or phase change, all sources use the criterion that $dG < 0$ for the reaction to be spontaneous and then substitute an appropriate expression for $dG$ to the specific ...
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SI Units for Isothermal Incompressibility and Specific heat capacity at constant volume

I am looking for the SI units of isothermal incompressibility and specific heat capacity at constant volume. What I found was that isothermal compression was measured in " $\mathrm{m^2}\ \mathrm{N^{-...
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331 views

Proof of kinetic energy equation

Is there a way to prove that $\frac{1}{2} mv^2 $$= \frac{3}{2}RT$ The first one is kinetic energy and the second is the average kinetic energy. Can this be proved by component of velocity.
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A thermos bottle containing milk is shaken vigorously

I have a conceptual physical chemistry question here. A thermos bottle containing milk is shaken vigorously. Consider the milk as the system. Will the temperature rise as a result of the shaking? ...
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1answer
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Finding the equilibrium temperature between two substances in a constant pressure calorimeter

I need to find the temperature two things equilibrate to in a constant pressure calorimeter given only their specific heats, masses, and starting temps. The only thing I know for specific heat is ...
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2answers
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Relation between thermodynamic reversible process and reversible reaction

I know it seems to be a weird question. But for long I have been thinking whether there is any relation between thermodynamic reversible process and reversible reaction. Do they have any connection ...
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1answer
198 views

A contradiction, probably from confusion

All things in nature tells us that high entropy is easier to happen. But then why does it takes energy to break bonds, in order to achieve high entropy. Any thing in gaseous state has higher ...
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878 views

How can a protein folding transition state have zero lifetime?

I'm doing a module on my Biochemistry course looking at protein folding, and in a discussion of [folding] transition states I was a little confused at the thought of a zero-lifetime transition state - ...
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1answer
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Heat of combustion for octane

I'm a Mechanical Engineering student and I have been working on a school project but I want to go a bit beyond that and it has led me here. I'm designing an engine cylinder, and I figured I should be ...
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Thermodynamically Reversible Processes: How does an infinitesimal step make a process reversible and how is maximum work derived from the process?

My book says 'Maximum work can be obtained only from thermodynamically reversible processes,' but why is it so? What is the cause? Actually to me the definition of reversible process is confusing. ...
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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 ...
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1answer
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Dry adiabatic lifting

If an unsaturated air parcel ascends adiabatically in the atmosphere with an initial temperature, would the final temperature of the air parcel, when it reaches it's destination, be the same at the ...
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Specific volume of an air parcel

So I have an unsaturated air parcel in the atmosphere that ascends adiabatically from the $1000\:\mathrm{mbar}$ level to the $700\:\mathrm{mbar}$ level with its initial temperature being $10\:°\mathrm{...
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Gibbs free energy of chemical compounds

How the Gibbs free energy of a chemical compound is theoretically calculated from scratch? When saying that the Gibbs free energy of NaCl is -384.1 kJ/mol, this value is related to the chemical ...
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How do we know that hydrogen gas consists of two hydrogen atoms?

Of course I know that both 1s orbitals will combine and that the electrons will occupy the bonding MO and that the two atoms will reach a lower energy state – but how do we really know that is the ...
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1answer
52 views

Can we determine apriori the volume of solvent in an electrolyte

Suppose I would like to create a one molar solution of NaCl in water. The molar concentration is defined as $c_i=\frac{n_i}{V} $ $n_i=1$ in this case. I need to take 58.5 grams of NaCl (atomic ...
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Nature of spontaneous reactions

The definition of spontaneous process states that "A spontaneous process is an irreversible process and maybe reversed by some external agency" So, can we say that all spontaneous reactions are ...
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Derivation of the relation between temperature and pressure for an irreversible adiabatic expansion

I have a formula for finding the final temperature in an irreversible adiabatic expansion: $$T_2=\left[\frac{C_V+\frac{P_2}{P_1}}{C_p}\right]T_1,$$ where $T_1$ is the initial temperature. $C_V$ and $...
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2answers
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Can lattice enthalpy only be determined by the Born-Haber cycle?

Since we are able to determine the enthapy of formation experimentally, why it is impossible to determine the lattice enthalpy directly by experiment?
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1answer
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Relation between the virial coefficients and van der Waals constants

I don't know where to begin with this question. A function $f(x)$ can be expressed as a special power series called the Taylor expansion. The common expansion is $(1-x)^{-1}=1+x+x^2$ Note that the ...
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How fast will hydrazine boil out of a tube into a vacuum?

For context I'm talking about ISEE-3 and how long it would take to dump the now useless and hazardous fuel it still carries. Given a heated tank of hydrazine (somewhere near $0~^\circ\mathrm{C}$) ...
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1answer
5k views

Is sign convention for delta H and enthapy change of a solution different?

In the Ncert Chemistry Tb, it is given that $\Delta H$ is negative for exothermic reactions which evolve heat during the reaction and $\Delta H$ is positive for endothermic reactions which absorb ...
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2answers
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Why is pressure negative in force and work

The equation to measure work is $$w=F\cdot D$$ However, the equation for force is $$F=(-P \cdot A)$$ Plugging this into the work equation we have: $$w=-P\cdot A\cdot D$$ My question is, why is ...
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1answer
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Can the thermodynamic information of a compound be calculated or must it be found emperically?

Do attributes such as heat of formation have to be found through experimentation or can they be calculated based on the attributes of the bonds and the atoms involved?
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Why is carbon monoxide the product in reactions that produce high flame temperatures?

I was looking at substances that produce the highest flame temperatures upon combustion. I note that the equations for combustion are producing carbon monoxide rather than carbon dioxide. $$\ce{(CN)2 ...
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1answer
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Calculating amount of ice required for heat loss

Question: Given $400.0\ \mathrm{g}$ of hot tea at $80.0 °\mathrm{C}$, what mass of ice at $0 °\mathrm{C}$ must be added to obtain iced tea at $10.0 °\mathrm{C}$? The specific heat of the tea is ...
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Why is mercury used in thermometers?

I know it expands when hot and grows compact when cool (that's about all I know), but I suppose many other substances do, too. Also, and excuses for my ignorance, what the term used to describe the ...
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Why does hand sanitizer leave your skin feeling cool?

I noticed, and perhaps many others have too, that the application of hand sanitizer (mainly ethanol), leaves one's hands feeling rather chilly after application. What is responsible for this ...
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1answer
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What reaction is expectable from the atomized reactants?

Let we have a reaction that starts from some reactants to produce some products. When the activation energy is provided to the reactants they will be atomized and produce a gas of individual atoms. ...
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1answer
379 views

Implicit dependence of reaction rate through reduction potential and activation energy

On one hand, the larger the reduction potential of a chemical reaction and the smaller its activation energy, the larger the reaction reaction. on the other hand the rate of a reaction depends on ...
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1answer
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Is 4-nitrobenzaldehyde or 4-aminobenzaldehyde more likely to be hydrated?

Of 4-nitrobenzaldehyde and 4-aminobenzaldehyde, which has the higher $\ce{K_{eq}}$ of hydration? $\qquad \qquad$ Steric factors can likely be ignored in this problem since we are dealing with ...
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2answers
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Determining if reaction is endothermic or exothermic using equilbrium constant

For the following reaction $$\ce{W(s) + 4 Br(g) <=> WBr4(g)}$$ at $\pu{900 K},$ $K_p = 100$ and at $\pu{2800 K}$, $K_\mathrm{p} = 5.$ Determine if the reaction is endothermic or exothermic. At ...
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Why are metal oxides generally more stable than their corresponding elemental forms?

Why is the oxidation of certain metals such as iron favorable from a thermodynamic standpoint? What is it about these metal oxides that makes them inherently more thermodynamically stable than their ...
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1answer
326 views

Moist Chlorine & Hydrogen Psychrometric or Mollier Chart

In our electrolysis of brine, $\ce{Cl2}$ and $\ce{H2}$ are produced as gaseous products along with aqueous $\ce{NaOH}$. In this scenario I require what amount of maximum moisture can $\ce{Cl2}$ and $\...
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1answer
9k views

The difference between heat of reaction at constant pressure and that at constant volume

The $K_\mathrm{p}$ value of a reaction is $\pu{10 atm}$ at $\pu{300K}$ and $\pu{4atm}$ at $\pu{400K}$. The incorrect statement about the reaction is- a. The reaction is exothermic b. The ...
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What is the cause for thermodynamic reversible and irreversible process?

My book defines Thermodynamic irreversible process is defined as the process in which the changes take place rapidly, instead of occurring in successive stages with infinitesimal changes in ...
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1answer
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Why is ΔE described as Q-W and Q+W in different contexts?

Depending on the textbooks looked at, the energy change is described as either Q+W or the signage is changed to Q-W. Which is correct? Is it solely context dependent? Could anyone explain the ...
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1answer
463 views

Can a solid and liquid be miscible?

To be specific, what I mean is: is the following situation possible? At given temperature T, there are two substances, X and Y, such that pure X is a solid, pure Y is a liquid, and X and Y mix to form ...
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1answer
867 views

How to calculate the heat absorbed by a reversible process?

Given a nonideal substance inside a box with diathermal walls, the pressure is increased reversibly. How do you calculate the heat absorbed? I assume we use dE = dq + dw = dq - pdV => dq = dE + pdV ...
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Problems with thermodynamic derivatives

How is the following true? $$\left ( \frac{∂S}{∂P} \right )_{T}=\frac{-C_{P}}{T}\left ( \frac{∂T}{∂P} \right )_{S}$$ I know that $$C_{P}=T\left ( \frac{∂S}{∂T} \right )_{P}$$ which means the right-...
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0answers
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Is superglue a good thermal insulator?

How good of a thermal insulator is superglue compared to other materials like ceramics, plastics, and metals? I know this might not seem like a chemistry question, but I was hoping someone here could ...
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What is the correct definition of the Gibbs free energy of activation?

Is the following correct? Gibbs free energy of activation is used in energy profiles where the stabilities of the species are expressed as changes in Gibbs energy, while the activation energy Ea is ...
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1answer
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Why is the equilibrium constant given in litre per mole?

At $800\:\mathrm{K}$ a reaction mixture contained $0.5\:\mathrm{mol}$ of $\ce{SO2}$, $0.12\:\mathrm{mol}$ of $\ce{O2}$ and $5\:\mathrm{mol}$ of $\ce{SO3}$ at equilibrium. $K_\mathrm c$ for the ...
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1answer
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Why is there sulfur in black powder / gun powder?

Black powder is composed of charcoal, sulfur and a nitrate salt. The charcoal and sulfur serve as fuel and the nitrate is the oxidizing agent. However, wouldn't it be cheaper to leave out the sulfur, ...
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0answers
630 views

What is the name for the equation ΔG = ΔG° + RT ln Q?

As the title states, what is the name of the equation $$\Delta G=\Delta G^{\circ}+RT \ln Q$$ My textbook does not explicitly state the name and I could not find anything on google.
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How do breaking chemical bonds turn into kinetic energy

I’m trying to understand how breaking chemical bonds turn into kinetic energy. If AB is a chemical which breaks into A and B which zip away from each other, is it because when the bond breaks they ...
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1answer
211 views

What can we say about the Gibbs free energy if it decreases or not?

[Sorry if the question looks trivial but I have thought about it for days and red a number of books all with no success.] In almost every thermodynamics book that has something to say about chemical ...
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1answer
179 views

Calorimetry | Heat produced VS Change of enthalpy

There are two questions on the same test paper. Calculate the amount of heat energy, $q$, produced in the reaction Find the change of heat ($-q$). I'm not sure whether or not they mean the same ...
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1answer
188 views

Synthetical formation of diamonds

As everyone knows a diamond won't last forever. It will eventually turn into a graphite over a long period of time, as the free energy of graphite is lower than that of diamond, hence making the ...
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2answers
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How can I find the heat that will be released in kJ per gram for the reaction of Al with Fe2O3?

Find the heat that will be released in kJ per gram when aluminium ($\ce{Al}$) reacts with $\ce{Fe2O3}$ as follows: $$\ce{2Al(s) + Fe2O3(s) → Al2O3(s) + 2Fe(l)}$$ The information given is: $\Delta ...

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