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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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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27 views
Simmons-Smith reaction, example reaction
Does this reaction work in theory? It's based on the Simmons–Smith reaction but with ZnCCl$_4$ instead of ZnCH$_2$Cl$_2$.
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1answer
24 views
What would happen if I increase the pressure and temperature of a gas present in a container
Assume the gas to be a real gas and the container to be made of diamond or anything stronger than that. Take the amount of gas to be 2mL and the pressure is increased with the help of strongest ...
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2answers
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Is the cell in an isothermal titration calorimeter open to the atmosphere? [closed]
I'm just trying to understand how they can model an isochoric process.
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Does a larger bond energy in reactants mean a more exothermic reaction?
In the book "Combustion Physics" for C.K Law, page 42, it is stated that:
The parameter that has the next strongest influence on [adiabatic flame temperature] $T_{ad}$ is the $C/H$ ratio of ...
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26 views
At equilibrium, which one of the following is always Zero? [duplicate]
At equilibrium, which one of the following is always zero?
(A)∆H
(B)∆S(Total)
(C)∆S(System)
(D)∆G°
I know that at equilibrium ∆G=0, and was facing difficulty in connecting the above mentioned ...
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34 views
What determines the coefficient B in the Bell-Evans-Polyani equation?
From the Bell Evans Polanyi principle, $E_a = A + B \Delta H$. How is the $B$ coefficient determined?
This is my thinking: For exothermic reactions (where the transition state resembles reactants more ...
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1answer
58 views
How switching values of $R$, $K_p$ and $K_c$ does not alter the value of change in standard Gibbs energy? [duplicate]
$\Delta_\text r G=RT\ln \frac{Q}{K}$
If we switch $K_p$ from $K_c$, accordingly $Q_p$ will change to $Q_c$. Therefore, value of $\Delta_\text r G$ remains the same.
$ΔG^\circ=−RT\ln K=ΔH^\circ−TΔS^\...
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2answers
88 views
How to derive the relation between entropy and heat described by the equation dS=dq/T?
I know that entropy is measure of disorder, and a book (Intro to Thermal Physics by David J. Schroeder) states that $\mathrm dS=\mathrm dq/T$, where $S$ is entropy, $q$ is heat flow, and $T$ is ...
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28 views
What happens when you dilute a surfactant solution at the critical micelle concentration?
Say if instance I have a 100 ml solution with micelles formed using some surfactant. Now if I pour that 100 ml solution into 500 ml of water, will those micelles break up into free surfactant again?
...
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81 views
Can endergonic reactions be exothermic?
Pretty much the title. I've heard it said that no (see, e.g here, answer by Lukas Schaedler). To quote his argumentation:
No. Exergonic reactions may be both exothermic or endothermic. Endergonic ...
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27 views
Why does steam store large amounts of energy?
We all know that steam can be used to perform mechanical work and steam has a high capacity for energy storage. But why does steam have such high energy capacity?
I tried searching for answers on the ...
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22 views
Vapor - Liquid Equilibrium (VLE)
Most fundamental equation for VLE is $$ \mu_i^L = \mu_i^V $$
It states that for every component chemical potential must be equal in both liquid and vapor phase at equilibrium. However, in my thermo ...
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1answer
83 views
Why does fcc lithium melt with increased pressure?
According to the phase diagram of lithium given here, the fcc phase formed at moderately high pressure melts at decreasing temperature upon adding more pressure, an effect better known with the ...
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2answers
76 views
Equilibrium chemical potential if the population is not conserved?
The chemical potential, $\mu$, is equal to the free energy $F$, per photon.
$$F = \mu = u-Ts$$
The energy per photon is $u=h\nu$ and $s$ is the entropy per photon. Say we have a process where an ...
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1answer
311 views
Isn't the change in Gibbs free energy for a reversible process zero?
In problem 6, part 6.5 of the 2020 IChO (PDF), a thermodynamic cycle is given for “one mole of monoatomic perfect gas”:
$\mathrm{A \to B}$; isothermal reversible expansion receiving $\pu{250 J}$ by ...
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1answer
63 views
Justifying assumptions about method to find equilibrium pressure for co-existence of graphite and diamond
The standard state Gibbs free energies of formation of $\ce{C(graphite)}$ and $\ce{C(diamond)}$ at $T = \pu{298 K}$ are $\pu{0 kJ mol-1}$ and $\pu{2.9 kJ mol-1}$, respectively.
The conversion of ...
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What is the origin of the expression for the electron affinity at temperatures above absolute zero? [duplicate]
Please explain the formula mentioned in this paragraph.
"In many books, the negative of the enthalpy change for the process is defined as the ELECTRON AFFINITY (A) of the atom under ...
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1answer
72 views
How to compute the work Done in an adiabatic irreversible expansion?
The question is as follows
What is the magnitude of work performed by one mole of an ideal gas when its volume increases eight times in an irreversible adiabatic expansion if initial temperature of ...
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1answer
68 views
Statistical entropy of perfect gas and indistinguishability
The weight, $W$ (the same $W$ as in Boltzmann formula $S=k\text{ln}W$) of a configuration {$N_0, N_1, N_2$, ...} is given by
$$W=\frac{N!}{N_A!N_B!N_C!...} \tag{1}$$
where $N$ is the total number of ...
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2answers
196 views
Why is the enthalpy change of an ideal gas a function only of temperature?
While teaching us thermodynamics our chemistry teacher told us that enthalpy change of a real gas is a function of temperature, pressure etc. but the enthalpy change of an ideal gas is only a function ...
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3answers
128 views
Is $ Q $ dependent on the ϵ which is chosen in the expression for Gibbs free energy of a chemical reaction?
In this lecture by MIT open courseware (*), Moungi Bawendi states that we can have some change in our chemical system containing reactants and products scaled by some $ \epsilon$ amount. Using this ...
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1answer
84 views
What is the meaning of thermodynamic tendency?
While giving a test, I encountered a question which was as follows
Lead reduces $\ce{NO_3^-}$ into $\ce{NO}$ and $\ce{NO_2}$ depending on the concentration of $\ce{HNO_3}$ in solution. Assuming that $...
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Is there a easy way to estimate how quickly something will dissolve depending on its surface area?
How exactly does the surface area of a solid for example a cube of sugar, affect the speed of its solvation. I guess everyone has observed that a batch of fine grounded sugar or salt dissolves much ...
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What is the Gibbs free energy change of physisorption?
In most sites I find online, I see that the Gibbs free energy of adsorption is negative. But, it is also said that physisorption is a reversible adsorption, suggesting that $\delta G=0$ which ...
3
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0answers
81 views
Why is it that when dG°<0, dH°<0? [closed]
Let me preface this question by saying that I probably made a mistake somewhere. However, I can't figure out what it is.
I was messing around with the Gibbs free energy equation to determine how the ...
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2answers
50 views
Change in entropy in reversible and irreversible processes
The change in entropy for a reversible process is given as-
$\Delta$S = $\frac{q_{rev}}{T}$
Where qrev is the heat supplied isothermally and reversibly.
The change in entropy for a irreversible ...
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1answer
25 views
Expansion of a gas under isothermal and reversible conditions
What is the reason for the statement - "When the expansion of an ideal gas is carried out under isothermal and reversible conditions, the internal energy does not change, i.e., $\Delta$U=0?"
...
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1answer
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Why do we use external pressure to calculate work done by a gas in case of compression instead of using internal pressure? [closed]
According to formula work done is (Resisting pressure X dV)? Then in case of compression, isn't the resisting force is internal Pressure? Then why we the formula for compression is same as expansion ...
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1answer
71 views
Some salts lower the vapour pressure of water - do any raise it?
Plain old NaCl salt can slightly lower the vapour pressure of water for a given temperature. Other salts such as CaCl₂ lower the vapour pressure to a larger degree.
Do any salts, or compounds of any ...
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46 views
correcting Henry's Law coefficient for higher pressure
I have a bottle at room temperature that contains water, nitrogen, and a trace amount of vinyl chloride (VC). I'm trying to determine how much the Henry's law coefficient for VC (at 25 C and 1 bar, ...
3
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1answer
35 views
Energy required to break apart water molecules in dissociation?
I'm learning about the process of dissociation. I'm aware in dissolution that essentially three processes occur (I know I may be simplifying it, but I am in Year 11):
Solute particles break apart (...
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1answer
54 views
Does the vapor pressure of a substance depend on the presence of other gases?
The Clausius-Clapeyron equation (CC equation) can be used to find the (saturated) vapor pressure of a substance ie. the gas pressure at which the two phases (vapor + liquid or vapor + solid) reach ...
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Where is my non-expansive work in combustion?
Consider if I burn some octane.
The enthalpy released is $5430 \;\text{kJ/mol}$. The Gibbs free energy released is $5537 \;\text{kJ/mol}$.
From my understanding, enthalpy is the heat change in the ...
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2answers
225 views
Does entropy contribute work?
I've seen that Gibbs free energy is defined as the maximum amount of work that a system can do. From this I gather, this is how much energy it will release - right?
But, I've also learnt that the heat ...
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How does volume change as a piston separating two compartments of a thermally insulated container moves to reach thermodynamic equilibrium?
The situation is a thermally insulated container divided into two compartments, which are separated by a diathermic piston. Initially, the left compartment has a volume (V1) of 2L, and a temperature ...
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Using salt and ice vs cold brine to rapidly cool an object
This a question regarding the practicalities of using salt+ice vs just cold brine to cool something in the home (inspired by this recent post: https://cooking.stackexchange.com/questions/116437/...
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What is the enthalpy of formation of benzene? [closed]
I was trying solve this question from my book:
The combustion of one mole of benzene takes place at $298 \mathrm{~K}$ and 1 atm. After combustion, $\mathrm{CO}_{2}(\mathrm{~g})$ and $\mathrm{H}_{2} \...
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1answer
104 views
$\Delta \bar{h}_{vap}$ and $\Delta \bar{s}_{vap}$ from vapor pressure vs. temperature data
I'm trying to work on the following two problems:
The answers are given below:
4.2: 57.7 kJ/mol, 122.6 J/(K-mol), 12 kJ/mol
4.3: -58.8 kJ per mole of dimer
For 4.2, I used the equation
$$\frac{\...
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1answer
1k views
Linear algebra and Hess's law?
I am a high school student taking chemistry who was tasked to solve a large set of Hess's Law problems. Naturally I tried to find a way to get my computer to carry out this repetitive piece of work ...
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1answer
89 views
If something is not hydrophilic, is it necessarily hydrophobic?
Is it possible to create hydrophobic water?
Two species are phobic within some medium under some conditions when they don't here together (cohere if they're the same species; adhere otherwise).
To be ...
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1answer
79 views
Can we have an equilibrium constant for a reaction at constant volume?
Suppose we have a sealed container (fixed volume) and we introduce two gases $\ce{A}$ and $\ce{B}$. The two reactants
$$\ce{A + B <=> C}$$
form another gas $\ce{C}$ with equilibrium constant $K$....
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Determining Boiling Point and Melting Point by Graph
This is the plot of $\Delta G^{^\circ}$ vs $T$ for $\ce{Pb-> PbO}$
I understand that the points where there is a sudden change in the slope of the graph, there is a phase change but exactly what ...
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Gibbs free energy of mixing in a control volume
I'm trying to model the amount of energy lost due to mixing of a $\mathrm{HCl}$ solution using the Gibbs free energy of mixing. However, I really like a second opinion on the model and the results, ...
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Predicting the solubility of sulfate salts [duplicate]
I came across a problem that asks to predict the solubility of alkaline earth metal sulfates: Magnesium, calcium, strontium and barium sulfates. I know that the spontaneity of dissolving can be ...
3
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2answers
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Why does van der Waals' equation of state give only one P, V pair for a particular temperature here?
I came across this question which I initially ignored believing that the data was insufficient:
Calculate the pressure exerted by one mole of $\ce{CO2}$ gas at $\pu{273 K}$ if the van der Waals' ...
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38 views
Entropy as a function of pressure, volume, and temperature
I know we can write entropy $S$ as $S(P,T)$ and $S(V,T)$ to derive different relations between heat capacities $C_V$ and $C_P$. I was wondering if it is technically correct to write
$$dS(P,V,T)=\left (...
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1answer
53 views
Finding the molar ratio at equilibrium
Consider the reaction $\ce{SO_2_{(g)} + \frac{1}{2} O_2_{(g)}\longrightarrow SO_3_{(g)}}$. What effect is there on the molar ratio $\frac{n_{SO_3}}{n_{SO_2}}$ at equilibrium if the pressure is ...
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Is a change in delta-delta-G typically notated as delta-delta-delta-G?
I'm currently writing a research paper, and need to refer to a difference between $\Delta{\Delta}G$s. The naive way to notate this would be $\Delta{\Delta}{\Delta}G$, but that looks cumbersome and ...
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1answer
101 views
Are sudden adiabatic expansions reversible or irreversible? (Tyre burst problems)
Encountered Question:
The tube of the tyre of a car is filled with air at $\pu{27^\circ C}$ and $\pu{2 atm}$ pressure. If the tube suddenly bursts, the final temperature of the air will be ($γ_\...
