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.
2,847
questions
2
votes
1
answer
82
views
Why is the work done on the system not equal in frame of gas and surroundings?
Consider an isothermal irreversible process for an ideal gas in a cylindrical container closed with a piston (system). Initial and final states are $(P_1,V_1,T)$ and $(P_2,V_2,T)$ respectively. ...
- 208
-1
votes
1
answer
74
views
Why are the latent heat of vaporisation values unavailable for cooking oils?
Oil evaporates. We know this, it's called the "smoke point". So why is it then, that whenever I try to find reference values for the latent heat of vaporisation for cooking oil, such as ...
- 121
2
votes
1
answer
152
views
Does it make any sense to graph 1/T versus ln(Kc)?
I calculated $K_c$ experimentally for the reaction $$\ce{CO2 + H2O -> H2CO3}$$ at different temperatures. Because $\Delta G =-RT\ln(K)$, I thought that graphing 1/T against $\ln(K_c)$ would give me ...
0
votes
1
answer
84
views
Is ΔH = ΔU + ΔPV only valid for gases?
In this question, formula ΔH = ΔU + ΔPV is used. In the solution, ΔPV is replaced for RTΔng. As there are solid products also in this equation, don't we consider ΔPV for them. Also if there were ...
- 11
0
votes
1
answer
108
views
Sign of ∆S in conversion of diamond to graphite
It is said that ∆S is positive as randomness is increasing from diamond to graphite as in diamond molecules are tightly packed together but we know that ∆S=∆H/T for phase transition and that ...
- 33
0
votes
3
answers
133
views
Confusion regarding spontaneous and non-spontaneous reactions
According to Wikipedia: "A chemical reaction is endergonic when non spontaneous. Thus in this type of reaction the Gibbs free energy increases."
My problem with that statement is the wording ...
- 117
0
votes
0
answers
54
views
Which Reaction is More Thermodynamically Favourable?
I have a gaseous mixture containing $\ce{CH4}$, $\ce{CO2}$ and $\ce{H2O}$ that is being reformed via the Steam-Methane Reforming Reaction (SMR), the Dry-Reforming of Methane (DR) or a combination of ...
- 21
-3
votes
2
answers
103
views
A general doubt regarding extensive/intensive properties and units in thermodynamics [closed]
I know that for reversible processes: entropy change = dq/t
Here its unit should J/Kelvin but it is mentioned in my books that its unit is J/Mol-Kelvin. I do not understand why there is mole in ...
- 1
0
votes
1
answer
125
views
Criterion for Spontaneity for Closed Systems at Constant Volume and Pressure
For a closed system at constant temperature and volume, the criterion for spontaneity is $dA < 0$. However, for a system at constant composition, the total differential of $A$ is given by
$$dA = -...
- 271
2
votes
2
answers
143
views
What solvents can I use to dissolve Epichlorohydrin rubber and Chloroprene Rubber
I am working with Epichlorohydrin and Chloroprene Rubbers and I would like to cast films of 2 mm at most. The solvents I have in my lab are DMSO, THF, DMF, Acetone, Ethanol and Propylene Carbonate. ...
5
votes
1
answer
354
views
Total Differential of Chemical Potential
I have seen many sources (for example this libretext) cite that the total differential of chemical potential is
$$d\mu = V_m dP - S_m dT$$
where $V_m$ is the molar volume and $S_m$ is the molar ...
- 271
1
vote
1
answer
48
views
How to relate zero K to statistical analogue of entropy?
Here we have a common equation of entropy
$$ S = klnW $$
At 0 K we have only one microstate and W is 1, therefore S = 0
Now we can consider another representation of this equation
$$ S = klnQ + \frac{...
-1
votes
2
answers
83
views
Why does the second law of thermodynamics imply that processes with positive entropy change are spontaneous
My understanding of the second law is that for an isolated system, the entropy change accompanying any process is non-negative and is zero if and only if the process is reversible. I don't see why ...
- 271
1
vote
0
answers
48
views
How do I approach the problem if Cp is given as a function of volume?
Here is the problem I'm trying to solve:
A system undergoes Isothermal Expansion under the atmosphere. For the system, $ C_p = a + \frac{b}{V} \ $. The process is given by $ P_{ATM}, V_1, T_1 \...
- 11
1
vote
0
answers
43
views
How to treat enthalpy change and pressure in reactions involving a liquid and solid state
In physical chemistry, it is often stated that "at constant pressure, the change in enthalpy gives the amount of heat transferred into the system" and the justification for this claim is
$$...
- 31
1
vote
0
answers
31
views
Unusual Inconsistencies in Gibbs Free Energy Values at Specific Temperatures in Thermodynamic Simulation
I was using FactSageEdu software to simulate a metal with a composition of 67% titanium (Ti) and 33% aluminum (Al). During the simulation, I encountered an inconsistency in the calculated Gibbs free ...
- 21
0
votes
1
answer
70
views
Temperature change in isobaric processes with and without change in number of molecules
Hello I was thinking about two thermodynamics problems and I wanted to get some insights into how to solve them.
The first problem was: Suppose we have one mole of ideal gas under constant external ...
- 117
0
votes
1
answer
88
views
Closed pressurized water vessel
Suppose we have a closed container not isolated from the external environment so that heat exchanges can occur on the lateral wall (mainly for convection). The water inside is pressurized by a gas (...
- 23
1
vote
2
answers
103
views
If E° of a Half Cell is based on the chosen reference Electrode (Generally SHE) then how is the equation ΔG°= -nFE° be valid for a Half Cell
For example let us take the reaction:
Zn(s) ---> Zn^2+(aq) + 2e^-(aq) ;ΔG°, which we are considering as our half cell
And now suppose, by taking the value of E° of SHE(Standard Hydrogen Electrode) ...
- 13
-2
votes
1
answer
83
views
Temperature change when mixing different ratios of NaOH and H2SO4 solutions [closed]
Under the same conditions, mixing how many ml of 1M $\ce{KOH}$ solution and 0.5M $\ce{H2SO4}$ solutions to form a total volume of 100ml produces a rise in temperature?
user138000
1
vote
1
answer
74
views
Why is melting spontaneous when the liquid phase is less stable and should have more free energy than the solid phase?
Imagine a 1kg block of ice that has completely melted at room temperature. If melting is a spontaneous reaction, then wouldn't that mean the products (1kg of water) would have less Gibbs free energy ...
3
votes
0
answers
57
views
Does real-world gas phase chemistry use "concentration reaction quotients" (K_c)?
As a theoretical chemist, my entire exposure to reaction quotients and equilibrium constants ($Q$ and $K$) is through thermodynamics ($\Delta G^\ominus = - RT \ln K_{eq}$ and so on).
So as I started ...
- 131
4
votes
3
answers
1k
views
Does the term "vapor pressure" even mean anything in an open system?
I know this type of question has been addressed before, but I believe mine is slightly different and hints at a potential misunderstanding I've had for a long time.
I understand the concept of ...
- 49
3
votes
2
answers
297
views
Derivation of "activity coefficient form of the Gibbs-Duhem equation"
I'm trying to differentiate the excess Gibbs energy ($G^{ex}$) to get the activity coefficient ($\gamma_k$), which writes:
$$
\frac{\partial (G^{ex}/RT)}{\partial n_k} = \ln\gamma_k \quad (1)
$$
in ...
- 73
-1
votes
1
answer
73
views
Is the comparison between absolute values of ∆S and ∆H for a given reaction meaningful? [closed]
How do you know if the reaction is dominated by enthalpic or entropic factor? Do you just compare the absolute values of them? If the absolute value of ∆S is bigger than the absolute value of ∆H, can ...
- 55
0
votes
1
answer
167
views
Molar heat capacity of water at equilibrium
Molar heat capacity is defined as the heat energy given to one mole of substance to raise its temperature by 1 degree celcius (or 1 Kelvin)
Mathematically, $$C_{m} = \frac {q}{n\Delta T}$$
Here's a ...
-1
votes
1
answer
234
views
Gibbs Free Energy and dependence on temperature and pressure
So, I have read that change in Gibbs free energy is defined under constant pressure and temperature, i.e
$$\Delta G= \Delta H -T \Delta S $$
but for an ideal gas change in entropy of system can be ...
- 25
0
votes
1
answer
75
views
How is temperature inside a bomb calorimeter constant?
I've looked around and have been unable to find an answer.
When determining molar reaction enthalpy, you first perform the reaction in the steel bomb then measure the temperature change of the water ...
1
vote
1
answer
77
views
Why does bond energy not vary with position?
Consider the formation of a hydrogen bond:
$$\ce{H + H -> H2}$$
The $\Delta H$ is $\pu{-436 kJ mol-1}$, so we can equally write the equation as: $$\ce{H + H -> H2 + \pu{436 kJ}}$$
But depending ...
- 77
5
votes
2
answers
730
views
How does a reduction in potential energy in a chemical bond release energy
This question has to do with the idea of stability and energy. The premise is that systems will tend towards lower energy states, that’s why bonding happens and electrons prefer single orbitals. And ...
- 77
0
votes
1
answer
151
views
What meaning does Gibbs energy acquire when applied to solutions?
I have been picturing functions like enthalpy and Gibbs free energy as mathematical tools, which were defined in such a way as to be useful for some specific cases. I understood enthalpy as a useful ...
- 87
1
vote
0
answers
38
views
Determining the molar mass of a substance by the osmotic pressure as a function of a mass concentration
Let's say that I have a solution of some substance in some solvent. The density of the solute is $\rho_\text{solute}$ and the density of the solvent is $\rho_\text{solvent}$. Suppose that the osmotic ...
- 61
-1
votes
2
answers
33
views
Clarifications on enthalpy and change in heat energy
From my understanding, an exothermic reaction has a negative change in enthalpy meaning that energy is released to the surroundings. However, is it correct to say that during this exothermic process, ...
- 27
-2
votes
2
answers
47
views
Explanation for Raoults Law
The vapour pressure of pure liquids is independent of quantity of substance.
But when in a solution, the vapour pressure of the components are given by
$$P_A=P^°_AX_A$$$$P_B=P^°_BX_B$$
Since $A-A$, $A-...
- 1
2
votes
0
answers
99
views
Formula derivation for adiabatic process
$\pu{1 mol}$ of an ideal gas at $\pu{400 K}$ and $\pu{10 atm}$ is allowed to expand adiabatically against $\pu{2 atm}$ external pressure. Find the final temperature of the gas if it is diatomic.
...
-3
votes
1
answer
78
views
State functions and change in state functions
So, I have been studying thermodynamics, in it I read that state functions are those functions which depend only on one state of the system and are independent of the path taken, which is pretty easy ...
- 25
1
vote
2
answers
127
views
Understanding the Role of Gibbs Free energy and Enthalpy as the energy required to drive a reaction
I was looking at the enthalpy change for water-splitting reaction:
$$ \Delta H^o_R = [\Delta H^0_{H_2(g)} +\frac{1}{2}\Delta H^0_{O_2(g)}]-\Delta H^0_{H_2O(l)} = \pu{285.83 kJ/mol}$$
According to ...
- 193
3
votes
0
answers
293
views
Equilibrium constant of the formation of ferric thiocyanate
I was reading the paper Relaxation Kinetics of Ferric Thiocyanate (Goodall et. al, 1972) and I came across a passage which read
$\space$ Reaction (1) is the simplest representation of the equilibrium ...
- 173
3
votes
0
answers
299
views
Derivation of the Van 't Hoff equation
I was reading the paper Relaxation Kinetics of Ferric Thiocyanate (Goodall et. al, 1972) and I came across the passage
Reaction (1) is the simplest representation of the equilibrium between ferric ...
- 173
0
votes
1
answer
339
views
Why is there internal energy change in a bomb calorimeter?
In a bomb calorimeter, the reaction occurs at a constant volume, hence we say that the heat absorbed by the water in the surroundings is equal to the change in internal energy for the reaction. But, ...
3
votes
1
answer
336
views
Enthalpy change of a solution of 1-pentanol in 1-butanol
I came across a question asking to indicate the enthalpy of solution for a solution of 1-pentanol in 1-butanol.
The options were the following:
$\Delta H < 0$
$\Delta H \approx 0$
$\Delta H >...
- 173
1
vote
2
answers
215
views
How exactly does the Nernst equation account for temperature? [duplicate]
I am doing an experiment that involves changing the temperature of the electrolytes in a voltaic cell and seeing the effect on the initial voltage produced. I'm a bit confused because if I use the ...
- 11
0
votes
0
answers
68
views
What exactly is meant by standard state in thermodynamics? [duplicate]
I was told that the standard enthalpy of a reaction is the enthalpy change when the reaction occurs with all reactants and products being in there standard states and under standard conditions(1 bar ...
1
vote
0
answers
51
views
Enthalpy change in an isothermal experiment at constant pressure
I read, in a passage regarding isothermal and adiabatic calorimetry for determining heat of solution, that
In a truly isothermal experiment at constant pressure, $\Delta H$ of the system in the ...
- 173
1
vote
2
answers
174
views
Interpretation of the Gibbs free energy
Gibbs free energy is defined as the total non-expansion work (i.e. electrical work) that a system can perform. However, it also indicates whether or not a reaction is spontaneous. I am therefore ...
- 173
4
votes
1
answer
122
views
What is correct sign convention for Helmholtz free energy change?
I have been studying about the Helmholtz free energy function ($A$) that
$$A=U-TS$$
Evidently the change in $A$ is given by (generally for isothermal reversible processes)
$$\Delta A = \Delta U - T\...
- 742
2
votes
0
answers
89
views
Discrepancy between calculated and experimental enthalpy change of formation? Hydrogen Iodide
Afterthought: See edits below!
$$\ce{1/2 H2(g) + 1/2 I2(g) -> HI(g)} \qquad \Delta_\mathrm f H^\circ=26.5\ \mathrm{kJ\ mol^{-1}}$$
source: https://atct.anl.gov/Thermochemical%20Data/version%201....
- 494
-3
votes
1
answer
77
views
Liquification of ideal solution of two liquids from vapor phase [closed]
A and B form an ideal solution. In a cylinder piston arrangement, $\pu{2.0 mol}$ of vapor of liquid A and $\pu{3 mol}$ of vapor of liquid B are taken at $\pu{300 torr}$ and $T~\pu{K}$. At what ...
0
votes
0
answers
70
views
The boiling point of carbon monoxide (CO) at 1 Torr
The boiling point of carbon monoxide (CO) at one atmosphere (760 Torr) is -191.5 degrees Centigrade or 81.65 degrees Kelvin. What is its boiling point at 1 Torr ? Or perhaps there is no liquid phase ...
- 21
1
vote
1
answer
110
views
Why are source and system at same temperature in a Carnot cycle
I am learning Carnot cycle and Carnot theorem from my textbook. I am also familiar with the graph of Carnot cycle featuring a pair of adiabatic and isothermal processes.
Say that the source is at a ...
- 742