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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23 views

Why does water splash if you put it into a hot pan?

I had kept my tea pan on the stove (At high)for about 30seconds . I was making tea and getting the water to put in the tea pan. There was nothing in it before that .After that , I quickly poured some ...
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Calorimetry Question - determine temperature change without knowing enthalpy change?

I have a lab that I honestly cannot figure out. Basically, we have a video where we watch 200 ml of NaOH (2M) and HCl (1M) combined in a calorimeter with a magnetic mixer. Pretty standard. This video ...
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1answer
46 views

Free Expansion - Ideal Gas vs Real Gas

By free expansion, I am referring to gas kept in a piston-cylinder arrangement freely allowed to expand against vacuum. It is clear to me that free expansion is an irreversible process because if it ...
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25 views

Calculating change in Gibbs Free Energy for a specific type of problem

Can anyone describe what is going on here? $$Si(liq.)=Si_{(1 weight percentFe)}$$ I have come across this statement quite a few times in Metallurgical Thermodynamics.All that they ask for is change in ...
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What is the difference between Electron Affinity and Electron gain enthalpy?

What is the difference between Electron Affinity and Electron Gain Enthalpy? I have read on an unreliable resource that Electron Affinity is the amount of energy provided to the surroundings, and the ...
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1answer
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Why is Specific heat of Ammonia higher than water?

The specific heat of water is lower than that of $\ce{NH3}$, but why? $\ce{H2O}$ forms more hydrogen bonds if I am right, besides there's more electronegativity for Oxygen. But still, $\ce{NH3}$ has ...
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22 views

Would the initial step of this mechanism be exothermic or endothermic? (Protonation)

My understanding is that since an O-H bond is being formed, the protonation step must be exothermic because energy is released upon bond formation. However, if I were to draw an energy/reaction ...
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1answer
51 views

Activation energy and degrees of freedom

According to the Arrhenius equation the rate of a reaction increases as at higher temperature a greater percentage of molecules have translational kinetic energy greater than or equal to activation ...
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1answer
39 views

How are these three things equal?

In this answer, I don't understand how this step worked. Using $\mu_i = \mu^\circ_i + RT\ln \frac{P_i}{\pu{1 bar}}$ \begin{align} \Delta G &= (c\mu^\circ_\ce{C} + d\mu^\circ_\ce{D} - a\mu^\circ_\...
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Verify heat produced by flame using enthalpy of fomation

I'm simulating using a CFD solver a pure oxygen/methane flame. However, I doubt that the combustion model does not produce enough heat. The heat from reaction is defined by $S_h=-\sum_{j} \frac{h_j^0}{...
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1answer
34 views

Can we dissolve salts with higher lattice enthalpy by providing heat?

It is well-known that A salt with a higher lattice energy will not dissolve at all, because then $\Delta_{\text{sol}}H^\ominus$ will be negative. So, the reaction should be endothermic, right? My ...
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Partial derivative problems [closed]

If I wanted to derive the partial derivative of a certain thermodynamic property. Can I make the assumption that the other thermodynamic property is constant although it does not present in the ...
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Calculation of Gibbs free energy of a chemical reaction

So, I'm trying to derive the equation: $$\Delta G = \Delta G^o + RT \ln Q\tag{1}$$ And trying to follow the steps detailed on this website here. I am having trouble deriving equation 7, which tries to ...
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Where can I find literature values for standard molar enthalpies of neutralization?

I'm looking for enthalpy of neutralization literature values for inorganic weak acids. NIST, PubChem, etc. don't list them. Is there somewhere else I should look?
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Energy is required to heat ice

How much energy is required to heat 9 ice cubes each with a mass of $\pu{44.7 g}$ from $\pu{-26.3 °C}$ to $\pu{-17.0 °C}?$ I used the formula $Q = mc\Delta T.$ $$\pu{-17.0 °C} - (\pu{-26.3 °C}) = \pu{...
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3answers
107 views

Work done in expanding a gas reversibly and irreversibly

So, my chemistry teacher gave the class following $P_{external}$ versus $Volume$ diagrams for reversible and irreversible expansion of a gas which are as follows. (Reversible expansion) (...
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1answer
131 views

Why is the boiling point of sugarcane juice lower than the boiling point of water?

As the boiling point is a colligative property, one would expect that the boiling point of sugarcane juice should be higher than pure water because Sugarcane juice contains water (75–85%), non-...
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1answer
57 views

Entropy change during irreversible process

Entropy change is defined as $\Delta S = \int \frac{\delta Q_{rev}}{T} $, where $Q_{rev}$ is heat transferred through a reversible process. I’m a little confused how this works, so I have a couple of ...
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Why does entropy increase in reactions that make more molecules?

I used to understand this (more molecules ⇒ more degrees of freedom), but do no longer, because the total number of atoms is conserved, so translational d.o.f. should just be turned into internal d.o....
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1answer
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Question regarding $Z$ (Compressibility factor)

Oh! This problem has been bugging me for a long time. According to Wikipedia, the Compressibility factor $Z$ is defined as the ratio of the volume occupied by a real gas to the volume occupied by an ...
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Small ring's orbital characters

small rings have more p character in the ring and more s character outside the ring Is there any explanation? I don't understand why small rings have more p character in the ring and more s character ...
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5answers
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Is light a reactant in photochemical reactions?

According to IUPAC a photochemical reaction is a reaction caused by absorption of light. I still can't understand how we should consider light as a part of a reaction. Is a catalyst or a reactant? In ...
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1answer
33 views

What is change in internal energy of a system in which combustion occurs at constant temperature?

We got a question in a test, in which we were asked which system has zero change in internal energy and it had an option which was combustion of methane at constant temperature. I imagined this to be ...
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0answers
76 views

What causes little spikes in a flame? [closed]

I have a lighter at home which everytime I use it, generates little spikes (see picture below) which eventually disappear. The Spikes are not an effect generated by the camera. I can confirm that ...
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1answer
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Confusion in calculating $\Delta U$ from a bomb calorimeter

In the book, it is mentioned the formula for $\Delta U$ in a bomb calorimeter without any derivation: $$\Delta U = q_v = \frac{Q\times M\times \Delta T}{m}$$ where $$Q=\textrm{heat capacity of ...
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A thermodynamics problem with an interesting equilibrium condition

Usually, when we analyze reactions, we assume that the reaction takes place at constant pressure or at constant volume. But there is no reason to assume this is generally the case, and so my friend ...
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Hampson-Linde cycle: why does expansion need to be isenthalpic

If I understand correctly, the fluid in the Hampson-Linde cycle goes through an isenthalpic (basically a free?) expansion. For ideal gases, T would remain the same. But, depending on the substance and ...
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0answers
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What favors the active transport in a membrane?

I was reading about active transport in membranes where ATP is used. ATP "reacts" with the protein pump and converts into ADP and also make a conformational change to the pump. Now this ...
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0answers
32 views

Why does sodium carbonate heat up when mixed with water? [closed]

I have a sample of sodium carbonate anhydrous (from heating sodium bicarbonate above 100c). When I mix in water, it heats up. Why does this happen? Where is the energy coming from? What form is the ...
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1answer
34 views

When to include the mass of the dissolved substance in calorimetry problems?

In some calorimetry problems, in which a substance is dissolved in water and we want the heat of the reaction, the mass of the dissolved substance is included in the mass plugged into mcat but other ...
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1answer
87 views

Are all exothermic reactions necessarily spontaneous? [closed]

I was told that the statement "all exothermic reactions have negative free energy change" is false. But I learnt Gibbs free energy as the "energy stored inside a system capable of doing ...
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0answers
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when is a system of mass-action reactions guaranteed to satisfy detailed balance?

when does a system of mass-action reactions satisfy detailed balance for any non-zero choice of the rate constants? consider a trivial example: $X + Y \ce{<-->[k1][k2]} XY$ satisfies detailed ...
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1answer
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A lot of confusion in 1st law of thermodynamics [closed]

My sir told me that Total energy of system = K.E + P.E in starting. Then Change in energy = F(External force on body ) * displacement of walls. Then from here , change in energy = W+q. (Don’t ...
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Help in understanding first way of changing the state of system

There is a statement in my book: THERMODYNAMICS One way: We do some mechanical work, say $\pu{1 kJ},$ by rotating a set of small paddles and thereby churning water. Let the new state be called $\...
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What is the standard state for a solid, liquid, gas and a solute?

I am getting very confused about the standard state of a substance. Some definitions says that standard state for a gas is when gas is at 1atm pressure, some says it is when it's fugacity is unity. ...
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1answer
35 views

What is the implication of having the internal energy equal the Gibbs free energy?

I came up with a hypothetical equation of state, where one of the consequences is that $U=G$, where $U$ is the internal energy and $G$ is the Gibbs free energy. I wanted to ask the chemists, is there ...
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0answers
17 views

How can we tell if a substitution reaction will be thermodynamically favoured

My understadning was like in the case of a cyclohexane we would see a thermodynamically favoured reaction if an axial substituent became equitorial in the case of a inverted Sn2 reaction but I would ...
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1answer
43 views

How to calculate ∆rGº from entropy and ∆fGº in different temperatures? [closed]

"The total oxidation of glucose occurs according to the following chemical equation: C6H12O6 (s) + 6O2 (g) -> 6CO2 (g) + 6H2O (l) The following table gives us the free energies of standard ...
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42 views

Is it possible to have three stable liquid phases in a binary mixture?

With this system, it would be possible to have three liquid phases and two liquid-liquid coexistence regions at various compositions. Similar behaviour is common for solids such as alloys, sometimes ...
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1answer
39 views

Change of entropy of a thermal reservoir [closed]

How to calculate/derive the change in entropy of a heat reservoir as function of its internal energy? If the constant temperature of heat reservoir is T, and $$ S(U_0)=S_0,$$ what is $$ S(U)? $$
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(Probabilistic) entropy associated with many equally probable microstates

This is a pretty naive question. The probabilistic definition of entropy is (for a mole of particles) $$-R\sum_i p_i\ln p_i $$ in which $p_i$ is the probability of a given microstate and $\sum p_i =1, ...
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0answers
40 views

Gibbs free energy as a difference of irreversible and reversible heats

I read in a book that ∆G=∆H-T∆S and then ∆H and T∆S were substituted with Q and Qrev respectively(where Q and Qrev are heats added to the system in any process and reversible process respectively) and ...
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0answers
49 views

Why does the plot of volume and temperature have a kink in a second order phase transition?

The $(b)$ part of the figure shows how thermodynamic variables change in a second order phase transformation. We observe there is a kink in the plot of volume with temperature. Why is that so? The ...
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1answer
38 views

Enthalpy of mixing of a non-ideal solution [closed]

I have this equation of Flory–Huggins model: $$\Delta_\mathrm{mix}G = RT(n_1\ln\phi_1 + n_2\ln\phi_2) + \chi n_1\phi_2,$$ where $\phi$ is the volume fraction, $n$ is the amount of substance and $\chi$ ...
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1answer
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Why is adiabatic process isentropic?

Adiabatic process is isentropic, but I can't get it why is it so. If we go by statistical method $$S = K\ln W,$$ where $W$ is the thermodynamic probability, and if we look into adiabatic expansion the ...
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1answer
35 views

Work done by system of ideal gas in isobaric expansion

Consider a system of ideal gas in a container with piston and the isobaric expansion of gas takes place. As the process is isobaric, so initially the pressure of gas is equal to atmospheric pressure ...
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1answer
67 views

Finding work done for a chemical reaction at non-constant pressure and temperature

I have studied the expression for work done for a chemical reaction which is basically $W=-\Delta n_{g}RT$. Along with this, the assumption that has been made is that chemical reactions take place at ...
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0answers
21 views

Why can the standard enthalpy of formation be calculated for different states of matter when the temperature should be constant?

If the standard enthalpy of formation needs to be determined at $25ºC$ and 1 atm of pressure then why is it possible for the enthalpy of formation to be calculated for the liquid, solid, and/or gas ...
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1answer
45 views

Water Gas Shift Reaction [closed]

Considering the water-gas shift reaction $$\ce{CO(g) + H2O(g) <=> CO2(g) + H2(g)},$$ I would like to understand how evolves $K^\circ$ the equilibrium constant of this reaction when I add water ...
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1answer
56 views

Exergonic vs Endergonic Reactions

Problem Problem 3, concept check 8.2 from Campbell Biology [1, p. 150]: Some partygoers wear glow-in-the-dark necklaces that start glowing once they are “activated” by snapping the necklace. This ...

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