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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1answer
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Explanation of the strength of phosphorus-oxygen bond

When discussing the Wittig reaction, Clayden's Organic Chemistry cites the strength of the $\ce{P=O}$ formed in triphenylphosphine oxide as a driver of the reaction through enthalpy: The $\ce{P=O}$ ...
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Does the potential energy increase when temperature is raised?

The potential energy of a material is its ability to do work. So I would suspect the potential energy to increase during phase changes that increase entropy (solid $\rightarrow$ liquid and liquid $\...
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Are non-exothermic explosions possible?

Explosives like picric acid and trinitrotoluene are unstable due to the close packing of so many $\ce{-NO2}$ groups onto a benzene ring. These highly strained bonds require only a relatively small ...
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How is entropy change defined for a chemical reaction?

I know that: ds = dq/T for reversible processes. Can I write change in entropy of a system in which a chemical reaction is happening using this equation. Basically, my question is about whether a ...
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2answers
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Basic question about an expression for Gibbs free energy

I'm getting into thermodynamics, and I have a basic question about the following expression for Gibbs free energy: $G = \sum \mu_i N_i$ Is it correct that this equality only holds for constant ...
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Why is it necessary to consider infinitesimal changes in p,V,T for H,U and G given that they're state functions?

State functions such as $G$ only depend on the state of the system and are not dependent on the "path" that took the system to that state (which would be the case for work, for example, which is not a ...
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How to prove that absolute zero is unattainable using Third law of thermodynamics? [closed]

By using second law of thermodynamics and Quantum Mechanics ,we can prove that Absolute zero is unattainable. A question was asked to prove this by using the Third law.
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Current in a Lithium Ion Battery

I am trying to find the current transferred to the anode and cathode of a Lithium Ion Battery at different voltages. I know the temperature (approximately) of the battery, the battery's starting ...
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Basic thermodynamics question about work

I am getting into thermodynamics, and I have a basic fundamental question about the definition of the change in internal energy $\mathrm{d}U$: $\mathrm{d}U = T\,\mathrm{d}S - P\,\mathrm{d}V + \sum\...
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In an isothermal process, how can the change in internal energy be 0?

It was written in my textbook, $$ \mathrm{d}U = \left(\frac{\partial U}{\partial T}\right)_V \mathrm{d}T +\left(\frac{\partial U}{\partial V}\right)_T \mathrm{d}V $$ If the process is isothermal,...
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Clarification required on whether to use $C_v$ or $C_p$ in this question on finding entropy change

$1 \text{ mol}$ of an ideal monoatomic gas at $300 \text{ K}$ is subjected to a reversible isentropic compression process until final temperature reaches $600 \text{ K}$. If initial pressure is $1 \...
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If volatility depends on intermolecular forces, then why in Raoult's law, two substances have different volatility?

I have a doubt, Ideal solutions are solutions where intermolecular forces between solute-solute and solvent-solvent are nearly equal to solute-solvent. Since intermolecular forces between A-A = B-B, ...
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1answer
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Why work done Is 0 and why not a negative value?

If a gas expand in vacuum, $P_{ex.} =0$. Then why is the work done 0? Since the gas is expanding, it is doing some work on the piston so the work done must have some negative value.
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When boiling water, how is evaporation rate affected by the surface area of the container?

Imagine two pots boiling on a stove. One is tall, while one is wide. Both contain the same volume of water. What will be the difference in the rate of evaporation between the two containers?
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Difference between reversible and irreversible processes

On a quiz, I was asked for the difference between reversible and irreversible processes. I said: For a reversible process, you can change direction at any time without disturbing equilibrium. For an ...
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If the heat exchanged is different for reversible and irreversible processes, how can enthalpy be a state function?

From the definition of Gibbs free energy, we can write for an irreversible process: $dG=dH-TdS<0$ With $TdS=\pu{q_{rev}//T}$ this becomes: $dH-dq_{rev}<0$ Since G is defined for constant (p,...
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1answer
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Why does silver (I) have a larger lattice enthalpy and hydration enthalpy than sodium?

The ionic radius of the $\ce{Ag^+}$ ion is $129$ pm, and that of the $\ce{Na^+}$ ion is $116$ pm. Since the sodium ion is smaller than the silver ion, it makes sense that it has a stronger polarizing ...
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Why does a hard-boiled egg have more entropy than a normal egg?

The question "Why does a hard-boiled egg have more entropy than a normal egg?" was recently asked on one of my tests. I said that entropy will decrease as a hard boiled egg has molecules in a more ...
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2answers
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Will NH3 decompose into N2 and H2?

In chemical kinetics , we learn that in a reversible reaction ,both the forward reaction and reverse reaction occur at the same time. At equilibrium , the rates are the same. For example:Production of ...
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Methylhydrazine Calorimetry

Below is a chemistry practice problem. I am curious why the change in heat of the oxygen molecules doesn't affect the calculations. Methylhydrazine ($\ce{CH6N2}$) is commonly used as a liquid ...
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1answer
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Gibbs free energy for an ideal gas

You know that for a generic system which during a process exchanges heat with a reservoir at a constant temperature $T_{a}$ it holds: $$\Delta G=\Delta H-T_{a}\Delta S$$ Where: $\star)$$\Delta G$ is ...
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Ellingham Diagram

My textbook says the following about the Ellingham diagram: Each plot is a straight line except when some change in phase ($\ce{s -> liq}$ or $\ce{liq -> g}$) takes place. The temperature at ...
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Is the equation dG = Vdp − SdT valid only for a reversible process? Can it be applied for an irreversible one too?

In the derivation, we write: $$ \begin{align} G &= H - TS &\quad&\to&\quad \mathrm dG &= \mathrm dH - T\,\mathrm dS - S\,\mathrm dT \\ H &= U + pV &\quad&\to&\quad ...
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Equlibrium for a non-spontaneous reaction

Consider a reversible reaction $\ce{A -> B}$, at standard state wher $\ce{A}$ and $\ce{B}$ are at $\pu{1 atm}$. If the free energy of products is greater than the free energy of reactants, $\Delta_{...
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1answer
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Whats the difference between reversible process in thermodynamics and reversible chemical reactions?

We know that in thermodynamics, reversible process never occur since entropy should be conserved. However, in chemistry we do have a lot of reversed chemical reactions that forms reactants from ...
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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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Formula of entropy change

In my 12th standard book, the formula for entropy change is given as $\Delta S = \frac{q_\text{reversible}}{T}$. What is the importance of absorbing heat reversibly and not irreversibly? What does ...
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1answer
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How can I calculate the percentage of dissociated dinitrogen tetroxide?

When $\pu{36.8 g}$ $\ce{N2O4(g)}$ is introduced into a 1.0-litre flask at $\pu{27 °C}$ the following equilibrium reaction occurs: $\ce{N2O4(g) <=> 2NO2(g)}$; $K_p = 0.1642~\mathrm{atm}$. ...
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563 views

Effects of pressure on equilibrium [closed]

Which system at equilibrium will not be influenced by a change in pressure? $$ \begin{align} \ce{3 O2(g) &<=> 2 O3(g)}\tag{A}\\ \ce{N2(g) + 3 H2(g) &<=> 2 NH3(g)}\tag{B}\\ \ce{...
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What is wrong in this argument that dG must always be zero?

Under constant temperature and pressure, the change of the Gibbs free energy can be written as $$\mathrm dG_\textrm{sys} = \mathrm dH_\mathrm{sys} - T\,\mathrm dS_\mathrm{sys}$$ And in the textbook ...
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1answer
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What is p defined as for the expansion and compression of an ideal gas system in the equation ΔU = ΔH - pΔV?

$$\Delta U = \Delta H - p\,\Delta V$$ Consider a gaseous system in a container with a frictionless piston, and a pressure outside the container. Is the pressure outside the container assumed to ...
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1answer
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Changes in entropy of the universe in a reversible reaction

Does entropy of the universe always increase in one of the forward or backward reactions of a reversible reaction and decrease in the other? If no then how do you explain the entropy of the universe ...
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Can we break double bonds by using laser and heat?

If it is possible then give an example for it. That chemical reaction is in this process. step 1: {Reactant + heat(sunlight)} gives weakened bonded product1 step 2: {product1 + laser} gives {one solid ...
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Anomaly in van der Waals constants of noble gases

In van der Waals equation for a real gas, the equation is $$\left(P + a\frac{n^2}{V^2}\right)(V - nb) = nRT$$ where $P$ is the pressure of the real gas, $n$ is the number of moles of the gas, $R$ is ...
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1answer
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What all information can be gathered qualitatively from the radial distribution functions of a molecule?

Apart from knowing the radius of coordination shells and the probability of finding, is there any other thermodynamic information we can gather by looking at radial distribution functions of a ...
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1answer
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How do I know which alkane has the lowest combustion heat?

I'm asked which one of the alkane has the lowest heat of combustion (note: they 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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1answer
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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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Dynamic equilibrium in unsaturated solutions

One standard definition of equilibrium in beginner chemistry is that for a reversible reaction, (dynamic) equilibrium has been achieved when the rate of the forward and backward reactions are equal. ...
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0answers
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As phase transitions are considered reversible processes, shouldn't they be non-spontaneous? [closed]

On the one hand it is stated that reversible processes cannot occur, as they are infinitely slow. On the other hand phase transitions are said to be reversible processes?
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1answer
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Steam rises when the heat supply stops

When water boils and you suddenly shut the heat off, a lot of steam arises from it just after you shut the heat off. Why this happens, I thought that maybe water condenzes very fast in small droplets ...
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1answer
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Troubles with Gibbs free energy

The book I'm reading writes that for a system at constant pressure and temperature it holds: $$dG=-TdS_{tot}$$ where $dG=dH-TdS$. So if you want to see if a certain process is spontaneus, you just ...
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1answer
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Calculating ΔH with non-integer coefficients in the balanced equation

I am working a practice problem out Ebbing's General Chemistry and for part of the problem I needed to calculate $ΔH.$ The problem considers the burning of ethane in oxygen to yield carbon dioxide and ...
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How much time is need for a copper block at 0 °C to equilibrate with 75 °C water?

I have a copper cylinder that is at $0\ \mathrm{^\circ C}$, how much time will be needed for it to equilibrate with $10\ \mathrm{kg}$ of water at $75\ \mathrm{^\circ C}$? Copper cylinder: Mass: $105....
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1answer
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Enthalpy Changes [closed]

0.50 g of Mg is added to a 50 mL HCl solution. The temperature increased by 5.2 degree Celsius. Calculate the enthalpy change for the reaction per mole of Mg that reacted.
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Entropy as an intrinsic property of matter

I would like to ask how we can see entropy as an intrinsic property of molecules, namely as something depending also on their geometry (and not only as gross disorder). Is this what is called the "...
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5answers
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How can decreasing in temperature indicate an endothermic reaction?

I’m really wondering how can decreasing the temperature refer to an endothermic reaction. I faced this when I was calculating the heat of a reaction Q in a constant-pressure calorimeter. What I ...
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0answers
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Determining heat capacity of gasses

I need a little help on answering a question which popped into my head while I was reading about gas laws: Suppose that I have three canisters, each holding a different gas of unknown identity [...
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1answer
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Why does eutectic point has three phases, but consolute point has two phases?

At eutectic point, there are three phases: solid A, solid B and homogenous liquid phase containing both A and B. But at consolute point, we are told it has 2 phases: liquid A and liquid B. Why isn'...
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1answer
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Why could it be assumed that the standard enthalpy change is independent of temperature down to 25 °C?

For the reversible reaction of pyruvic acid to 2,2-dihydroxypropanoic acid, we calculated enthalpy using NMR and extrapolated from area (integration) and peaks etc. We then plotted $\ln K$ vs $1/T.$ ...
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2answers
60 views

Hess's Law and ΔH

Target equation: $\ce{Mg(s) + 1/2 O2 -> MgO(s)}$ $$ \begin{align} \ce{MgO(s) + 2 HCl(aq) &-> MgCl2(aq) + H2O(l)} &\quad ΔH &= \pu{-1300 kJ} \tag{1}\\ \ce{Mg(s) + 2 HCl(aq) &-&...