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.
0
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1answer
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What units of K should we use in ΔG=-RTlnK?
I understand that $K$ will be dimensionless. Let's say an equilibrium involving $K_p$ and the reaction is $\ce{A -> B}$, $K_p$ will be dimensionless. If the reaction $\ce{A + B -> C}$, then ...
1
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0answers
20 views
Isobaric-isothermal ensemble with degeneracy
I've been studying statistical mechanics for a while, the topic is very far from what I've learned till now so my questions may be superficial. When trying to see what my entropy is in an isobaric-...
-2
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1answer
42 views
Are these calculations valid enough? [on hold]
I am not able to understand how these calculations are being done in this book. Up until this section, the book is going smoothly. However, how are these calculations accountable? If anyone can ...
0
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0answers
27 views
Equilibrium concentration of point defects in solids at sufficiently elevated temperature
Equilibrium concentration of point defects in solids is defined as $\chi=\exp(\frac{\Delta s}{k_B})\exp(\frac{-\Delta h}{k_BT})$. At high temperatures, may be even higher than melting temperature of ...
2
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1answer
48 views
Proving a relation for fugacity of component i in term of independent variables (T,V)
In Molecular Thermodynamics of fluid phase equilibria by Prausnitz et al. [1] the authors recommend to use the following equation which gives the fugacity of component $i$ in terms of independent ...
5
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0answers
63 views
Contribution of vibrational degrees of freedom
The ratio $\frac{\mathrm{C_p}}{\mathrm{C_v}}$ for linear triatomic
molecules including vibration contribution is given by: ?
Attempt:
We are aware that $C_v = \frac{fR}{2}$ and $C_p = C_v+ R$
...
2
votes
1answer
63 views
Gibbs Free Energy and Maximum Work
I am a high school student and my professor mentioned that the Gibbs free energy is the maximum amount of work (or useful work) that a system can do, whereas entropy is a measure of the non-available ...
26
votes
2answers
6k views
When a candle burns, why does the top of wick glow if bottom of flame is hottest?
Totally an elementary question.
Staring at a candle, it appears that the bottom of the wick is dark whereas the top glows. However the bottom of the flame (the blue) is the hottest.
Is the reason ...
3
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2answers
257 views
Why is my conclusion inconsistent with the van't Hoff equation?
Let's say I hypothesize that a graph of $\ln K$ vs. $1/T$ has a slope of $-∆G^\circ/R$ and a $y$-intercept of $0$. I prove it simply:
$$∆G^\circ = -RT\ln K \quad\to\quad \ln K = -\frac{∆G^\circ}{RT}$$...
0
votes
0answers
19 views
Hydrogen peroxide and carbon as hybrid rocket propellant?
I was thinking that a hybrid rocket could be made using hydrogen peroxide and activated charcoal or some other carbon source.
Step 1: Decompose hydrogen peroxide to steam and oxygen, as in a ...
7
votes
2answers
158 views
Cp vs Cv for liquid water @ < 4 °C
I'm looking at a textbook question (Engel & Reid, Thermodynamics, Statistical Thermodynamics & Kinetics, 4 ed, Q3.1) which states the following:
The heat capacity $C_{\mathrm m,p}$ is less ...
0
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1answer
34 views
Calorimetry: Temperature Increase due to stirring of reactants
I am not yet in university, but I hear a common first-year chemistry lab/practical is to do calorimetry taking into consideration the small (and usually negligible) rise in temperature due to the ...
2
votes
1answer
31 views
Change in Gibbs free energy vs. change in enthalpy
When learning about other thermodynamic quantities, like $\Delta H$, I was taught that this represents the energy absorbed or released by a system after the reaction has gone 100% to completion. For ...
14
votes
3answers
2k views
Is it possible to boil a liquid by just mixing many immiscible liquids together?
In open air, when vapour pressure reaches 1 atm, boiling takes place.
I read that if we add two immiscible liquids together, the total vapour pressure of the 'mixture' is close to $p = p^*_A + p^*_b$,...
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0answers
24 views
Salt dissolves while absorbing heat, but is already soluble at room temp. Why? [closed]
Solid lithium carbonate dissolves in water with the absorption of heat. Nevertheless, the salt is quite soluble even at room temperature. How would I explain these occurrences using thermodynamic ...
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0answers
32 views
Behaviours of endothermic reactions in an insulated isolated system
Are Endothermic reactions which occur in an insulated isolated system unable to give off heat to surroundings, if so is that because it is in a closed system, also why does the final temperature drop?...
1
vote
1answer
45 views
Enthalpy change of gas expansion against constant external pressure
A certain mass of gas is expanded from $\pu{1 L}$ at $\pu{10 atm}$ to $\pu{4L}$ at $\pu{5 atm}$ against a constant external pressure of $\pu{1 atm}$. If initial temperature of gas is $\pu{300 K}$ and ...
2
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2answers
43 views
Which substances have the highest/lowest enthalpy of formation per gram?
Which substances have the most extreme enthalpies of formation per gram at standard temperature and pressure?
0
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2answers
24 views
Thermal conductivity and enthalphy data for nitrogen gas
I am struggling to find the data for thermal conductivity and ethalphy of nitrogen gas at reduced pressure (500 mbar). Most of the information is for nitrogen gas at 1 bar or above. Does anyone know ...
0
votes
2answers
51 views
Is heating water endothermic?
So I got a problem recently which asked to calculate the amount of energy needed to increase the temperature of 200 g of water from 26 to 51 degrees Celsius. The value I got was 20900 J.
So far I ...
1
vote
2answers
64 views
Is Gibbs energy change during osmosis positive? If so then why is it spontaneous?
For ideal solution, μA=μA* +RT
lnxA
ln function is an increasing function, means that the higher the xA the higher the μA......(1)
Now consider osmosis. From Atkins' Physical Chemistry Ninth ...
1
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0answers
26 views
Calculate chemical potentials to find electrode Fermi level
I'm running some simple simulations of the semiconductor-electrolyte interface and I'm having a hard time figuring out the equilibrium condition. Can someone please help me with my electrochemical ...
1
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0answers
13 views
Why a Differential Scanning Calorimeter (DSC), to measure thermodynamic parameters, has problems?
I want to use a Differential Scanning Calorimeter (DSC) to determine when my protein unfolds as you increase the temperature in a DSC machine. The machine is pressurised (to 3A) during the run, so ...
-1
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0answers
19 views
Magnesium burning in carbon dioxide
Why does $\ce{Mg}$ keep burning in $\ce{CO_2}$ and why does the fire generated by the exothermic production of $\ce{MgO}$ not get extinguished?
2
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2answers
40 views
For Hess' law type questions, when do you take the inverse of the constant and when do you switch the sign?
For example, when you're given the enthalpy $\Delta H$ of a reaction and you want to switch that reaction around in order to add it to something else, you switch the sign. Furthermore, when adding ...
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vote
2answers
33 views
Why enthalpy of adsorption is negative?
I have already read Is adsorption exothermic, and if so, why? but it didn't answered my query.
To illustrate, let's take a hypothetical vacuum space. In that space, there is a solid and a gas, the ...
6
votes
1answer
164 views
What is the Gibbs energy of dissolving a solid in pure water?
I am calculating the Gibbs energy of dissolving one mole of solid glucose in pure water such that the final solution has a volume of one liter and a concentration of one mole per liter. I have three ...
0
votes
1answer
42 views
Mixture of water, methanol and dichloromethane
If I have a mixture of 25 vol% methanol in water and I mix it with an equal volume of dichloromethane (which is miscible with methanol but not water) can I roughly say that both phases (i.e. water and ...
1
vote
1answer
26 views
Is temperature of crystallisation same as melting point temperature for polymers?
Melting point tends to be a temperature range. Is it true that the temperature of crystallisation is the highest temperature at which the polymer melts and thus there is some distinction between ...
1
vote
1answer
36 views
Heat of formation of aqueous H⁺
In my book it is given that by convention heat of formation of aqueous $\ce{H+}$ is taken to be zero. Heat of formation is energy released or absorbed when 1 mole of a compound is formed from its ...
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votes
0answers
21 views
Can an object reach 0 heat? [duplicate]
The definition of heat in my textbook is: "Heat involves the transfer of energy between two objects due to temperature difference. Heat is not a substance contained by an object."
Knowing this, is it ...
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4answers
87 views
How does the room gets heated by leaving a fridge open?
Question:
If the door of the refrigerator is kept open, the room in which the refrigerator is kept:
gets cooled
gets heated
neither gets cooled nor gets heated
gets cooled or heated ...
0
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2answers
55 views
Derivation of Gibbs free energy formula
Okay, so I'm trying to derive an important formula which states that -
∆G(r) = ∆G° + RTln( RQ )
where ∆G(r) is the instantaneous rate of change of Gibbs energy with ...
0
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0answers
25 views
Why is high pressure and high temperature favourable for the formation of diamond? [duplicate]
Graphite can be converted to diamond and this conversion is favourable under high pressure and high temperature.
Is there a reason for this from the point of view of "Le Chatelier's Principle"?
I ...
2
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0answers
128 views
What is the ideal temperature and pressure to crack water?
Using sun mirrors and solar cells for electricity. What temperature, pressure and voltage ideal for cracking water.
Source: NPR
Source
Would it take more or less electricity to crack water under ...
5
votes
1answer
142 views
Calculating entropy change: reversible vs irreversible process
Since the change in internal energy and enthalpy, which are equal to the heats for a constant-volume and constant-pressure process, respectively, are state functions, the heats for a reversible v.s. ...
4
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4answers
102 views
Is heat just a change in temperature?
According to my chemistry textbook, heat is defined as "Energy that is exchanged because of a difference in temperature or a change in phase." My textbook also says, "Heat is a form of energy. ...
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0answers
18 views
What are the activation energies for the combustion of RP-1 with liquid oxygen, and UDMH with Dinitrogen tetroxide?
I need to compare the chemical and physical properties of the two reactions for a school project. However, I can't find a source that explicitly states their activation energies! We haven't learned ...
0
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0answers
27 views
Regarding ΔG and equilibrium constant [duplicate]
We know that
$$ΔG = ΔG^\circ + RT\ln{Q} \tag{1}$$
where $Q$ is reaction quotient, and also that at equilibrium becomes
$$ΔG^\circ = -RT\ln{k} \tag{2}$$
I was trying to apply this equation to a ...
2
votes
1answer
29 views
Intensive and Extensive Properties--Resistance
Is resistance an intensive quantity or an extensive one? And why?
Let me present 2 scenarios.
Resistance of a wire:
The resistance of a wire depends on area and length. This implies it depends on the ...
3
votes
0answers
51 views
Whether the entropy change of the system is positive or negative
$$\ce{N2(g) + O2(g) → 2NO(g)}$$
My thinking: since gases are combined together, so there randomness or disorder gets decreased. Hence the sign of entropy change of the system should be negative.
...
0
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2answers
75 views
Question about liquid-phase in P-T Diagram
Currently, I have been learning about P-T diagrams and vapour pressure and I have a question that seems like it should have a simple explanation yet I am not able to understand it.
Taking the P-T ...
4
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2answers
132 views
Gibbs Free energy of a reversible reaction
From what I understand, it sounds like the Gibbs Free Energy change of a reversible reaction at equilibrium is zero. However, since I know that Gibbs Free energy change depends on temperature, does ...
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0answers
36 views
Conversion of graphite into diamond [duplicate]
The standard state Gibbs free energies of formation of C(graphite) and C(diamond) at $T = \pu{298 K}$ are $\pu{0 kJ mol-1}$ and $\pu{2.9 kJ mol-1}$, respectively.
The conversion of graphite [C(...
3
votes
2answers
75 views
Can an exothermic (endothermic) process increase (decrease) internal energy?
We classify exothermic and endothermic processes based on enthalpy change, $\Delta H < 0$ and $\Delta H > 0$ respectively.
I assume this translates to $\Delta U < 0$ and $\Delta U > 0$, ...
6
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2answers
389 views
What is a good source for large tables on the properties of water?
I'm currently doing an investigation for school on finding the Clausius-Clapeyron relation/gradient for boiling water and comparing it to the gradients along my line of best fit. It is given by the ...
1
vote
1answer
93 views
Are the vibrational modes of CO2 active at room temperature?
I was doing an experiment whereby I had to measure the specific heat ratios of certain gases. Carbon dioxide came out to be around 1.3, and checking with the accepted values this is close. My question ...
1
vote
1answer
32 views
Behavior beyond the critical pressure [closed]
I have seen some phase diagrams that look like this:
https://www.learner.org/courses/chemistry/images/text_img/phase_diagram_water.jpg
And also some like this:
https://en.wikipedia.org/wiki/...
0
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0answers
35 views
AP Chem Question about Reaction Rate - Confused?
I found this question in the AP Chem book, and I understand why B is the answer (as the answer key says). However, I don't understand why D is not also correct.
The question is:
A multistep reaction ...
1
vote
1answer
36 views
Why do reactions happen at all? [closed]
What makes some atoms/compounds reactive and some noble? What is the fundamental truth behind a reaction? It seems that two species can undergo number of combination to give a compound. Say Sodium and ...
