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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0answers
19 views
Endothermic Reaction between Ammonium Chloride and Water
Why is the reaction between Ammonium Chloride and water endothermic and what exactly is the reaction? I already know that Ammonium Chloride dissociates in water as water is a polar molecule, but does ...
1
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1answer
58 views
Why do gases coexist as liquid and vapour?
I have having difficulty in understanding the liquefaction of gases.In the graph
of isotherms of $\ce{CO2}$ I cannot understand how can gas coexist as liquid and vapour under area of $\mathrm{XCEBY}$....
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0answers
33 views
Is equilibrium ever possible in an open system?
I have read in a few places (Wikipedia, school textbook) that "An open system can never reach equilibrium." There is the argument that if one of the products of a reaction is gaseous, it may escape, ...
1
vote
2answers
53 views
Help understanding the pressure's effect on equilibrium and Le Chatelier's principle
So let's say we have a closed system in the following reaction:
$\ce{N2O4 -> 2 NO2}$
The only way to increase the system's pressure without adding any mole of gas and keeping temperature constant ...
-1
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1answer
39 views
Why are combustion reactions spontaneous? [duplicate]
I learned that spontaneous reactions are reactions whose activation energies can already be overcome with the temperature they're already in. Since the temperature they're in already provides enough ...
0
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0answers
28 views
Thermodynamics of exo/endothermic reactions
Complete chemistry newbie here, hi. Sorry if I ask a stupid question, but I wonder what does exo/endothermic means from a thermodynamical point of view.
Considering that temperature (a macrostate) ...
0
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0answers
30 views
Freezing water droplets with liquid nitrogen vapor
I have an idea for a device but I'm not sure about the thermodynamics.
Would liquid nitrogen vapor (not liquid) be cold enough to instantly freeze a spray or mist of water? I know water freezes at 0 °...
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0answers
13 views
Creating empty space by controlling energy [closed]
This question is for a fantasy book that I want to write but I like to use physics and chemistry as a base for magic.
Can a person who can control energy (like move it from an object to another) ...
0
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1answer
45 views
Expansion work by a gas when pressure is not explicitly constant?
How is it possible to calculate the work done by a gas when pressure is not explicitly stated to be constant? The question states:
Calculate $\Delta H$, $Q$, and $W$ when $1\ \mathrm{mol}$ of He ...
0
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1answer
42 views
Entropy change during flow of heat
If heat $Q$ flows from the surrounding nature temperature at $T_\text{surr}$ to the system at temperature $T_\text{system}$, then $$\Delta S_\text{total}=\frac{+Q}{T_\text{system}}+\frac{-Q}{T_\text{...
0
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1answer
32 views
Current in a Lithium Ion Battery
I am trying to find the current transferred to the anode and cathode of a Lithium Ion Battery at different voltages. I know the temperature (approximately) of the battery, the battery's starting ...
3
votes
1answer
81 views
Why does ozone have higher entropy than oxygen?
$$\ce{3O2(g) -> 2O3(g)} \qquad \Delta H^\circ (\pu{298 K}) = \pu{+142 kJ mol^-1}$$
Since the RHS of the equation contains less number of gaseous molecules, naturally I expected the $\Delta S$ to ...
3
votes
1answer
77 views
Why is the Kelvin Temperature Scale Based on the Triple Point of Water?
Why would the Kelvin temperature scale not just be based on the freezing point of water, which is only .01 degrees Celsius off from it? Is this just a more accurate way of measuring and quantifying ...
1
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1answer
48 views
How many watts would I need to get CO2 from 30 °C to 4000 °C in 1 hour
Scenario: After reading some articles on new sorts of ways to deal with $\ce{CO2}$ in the atmosphere, a question came to mind: If I had 100 kg of $\ce{CO2}$, and I would like to heat it up from 30 °C ...
0
votes
1answer
86 views
should charge balance be based on activities?
We have a fluid with $\pu{0.01 M}~\ce{HCl}$ and $\pu{0.02 M}~\ce{K2HPO4}$, so if we assume activity coefficients are $1$ throughout we can find pH as $6.800012$ and confirm that at that pH there is ...
0
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2answers
55 views
Doesn't the isothermal irreversible expansion/compression of an ideal gas law violate the ideal gas law? [closed]
Let's say a cylinder has some ideal gas. We use a piston to suddenly expand the gas in isothermal condition from $1\ \mathrm{atm}$, $20\ \mathrm L$ to $0.5\ \mathrm{atm}$, $50\ \mathrm L$.
Since the ...
1
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1answer
36 views
Expression for fugacity coefficient derived from a pressure-explicite EOS
In the book, Thermodynamics for Process Simulation, the authors propose to derive an expression for the fugacity coefficient from a pressure-explicit equation of state, as an example among many others....
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2answers
33 views
Is water vapour formation possible at -10 degree celcius?
Under any pressure conditions is it possible ? If not then is there no water vapour present in places where temperature drops to -10 degree celcius ?
0
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0answers
42 views
Is the concentration of water included in the acid disscociation constant? - Wikipedia confuses me
I refer to the following wikipedia article
https://en.wikipedia.org/wiki/Acid_dissociation_constant
Header paragraph: Equilibrium constant.
First the equilibrium constant is given in terms of ...
1
vote
0answers
47 views
Gibbs Free Energy: Exothermic, low temperature & reduced system entropy - such reaction is spontaneous or not?
According to $\ce{\Delta G^\mathrm{o}= \Delta H^\mathrm{o} -T\Delta S^\mathrm{o}}$, if a reaction
a) is exothermic,
b) reduces system entropy, and
c) the temperature is low,
then it is spontaneous....
1
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2answers
44 views
Equilibrium for very small amounts of reactants
Is there any difference in the concept of equilibrium when it comes down to dealing with extremely small amounts of reactants?
Say we have $$\ce{A + B <=> C + D}$$
and $K_c$ is $100,000$. If ...
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votes
2answers
67 views
Shouldn't all isothermal processes be adiabatic?
We know, $\mathrm{d}H = nC_p\mathrm{d}T$ which implies $\mathrm{d}H = f(\mathrm{d}T)$ using $\mathrm{d}H = \mathrm{d}U + \mathrm{d}PV$ and $\mathrm{d}q = \mathrm{d}U + \mathrm{d}w$ we get $\mathrm{d}...
1
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1answer
66 views
Excluded Volume in van Der Waals equation
I was just reading over the EOS topic when where I stumbled upon excluded volume.
Although the pictorial representation of the hard sphere model clearly shows that the excluded volume is 4 times the ...
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2answers
29 views
Thermochemistry: What is the difference between the calculation of bond enthalpy and Hess's Law
so I know that hess's law is a indirect way to find the heat of reaction using the known heat of reaction of 2+ thermochemical equation, thus the formula is delta heat of reaction = enthalpy of ...
0
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0answers
60 views
Molar Enthalpy change
A Gas ($C_{\mathrm m,V}=\frac52R$) behaving ideally was allowed to expand reversibly and adiabatically from 1 litre to 32 litres. Its intial temperature was 327 °C. The molar heat enthalpy change (in ...
1
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0answers
24 views
Limiting factor on Belousov-Zhabotinsky reaction
In the thin film Belousov-Zhabotinsky reactions (popular on Youtube/in chemistry demonstrations), concentric rings of different colors propagate across a petri dish for minutes to hours as the ...
1
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1answer
64 views
Change in internal energy for the heating and vaporisation of water
What is the value of change in internal energy at $\pu{1atm}$ in the process $$\ce{H2O(l,$323\ \mathrm K$)->H2O(g,$423\ \mathrm K$)}$$
given $C_{\mathrm m,V}(\ce{H2O,l})=\pu{75.0 J K^{-1} mol^{-1}...
0
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0answers
53 views
Reason for formation of bubbles at boiling point [duplicate]
As I understand it, boiling takes place when the saturated vapour pressure equals to the atmospheric pressure.
But, why does the vapour pressure need to be equal to the atmospheric pressure for ...
-1
votes
2answers
41 views
Isothermal expansion and compression for n steps [closed]
In my book, the graph for isothermal expansion for 2 steps and n steps is shown as this
I can't understand the graph for both compression and expansion. Why is external pressure changing in going ...
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1answer
34 views
How is an equivalence of a state and path function possible? [duplicate]
At constant pressure,∆H=q
Where ∆H=enthalpy
q=energy transferred by heating
∆H doesn't depend on how we get from one state to another, but q does. How can they be equal in any case?
Is ...
4
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1answer
92 views
“Proof” that the chemical potential is less than zero for irreversible material changes
From Levine's Physical Chemistry 6th edition:
In a closed system, $\mathrm dG = -S\,\mathrm dT + V\,\mathrm dP$ not only applies to constant composition but also reversible changes in composition (...
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1answer
58 views
When is a reaction said to be under kinetic control, is it not easily reversible?
If we consider this reaction: Compound 1 + Compound 2 --> Compound 3
I understand that if a reaction is under kinetic control, the less stable (e.g. a mono-substituted diene) is formed and if the ...
0
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1answer
44 views
How to optimise battery usage on modern electronics?
What is the best way to use an external power bank with a modern smart phone or similar device if the goal is to get as much usage out of the combination?
Premises:
The devices has a battery of 2 Ah ...
0
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0answers
22 views
How to calculate the enthalpy of fusion at a different temperature?
The heat capacity of liquid iodine is $80.7$ $\ce{J*K^{-1}*mol^{-1}}$, and its enthalpy of vaporization is $41.96$ kJ $\ce{*mol^{-1}}$ at its boiling point ($184.3$ $\ce{^oC}$). Furthermore, the ...
3
votes
1answer
62 views
Are Cubic Equation of states applicable to Liquids?
I was just going through a graph of isotherm given by cubic equation of state and noticed that isotherms passes trough the subcooled liquid region. Are these equation applicable to liquids?
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0answers
31 views
Why fire spreads out, depending on burning substance, with different speed?
In Action movies, I have seen a couple of times that if flammable gases or liquids like benzene are ignited, the fire spreads out within fraction of a second and sometimes there is also an explosion. ...
1
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2answers
100 views
Is there a widespread error/contradiction on whether to use internal or external pressure in computing work done by an expanding gas?
I've been looking at some sample problems involving work done by gas in an irreversible expansion in a piston. They all say $W = P\Delta V$, but some of them say to use internal pressure for $P$, ...
4
votes
2answers
77 views
Tunneling in chemical reactions
We know that quantum tunneling is the reason behind several natural phenomenon like alpha decay and thermonuclear fusion inside the stars. How can it influence chemical reactions by tunnelling a ...
1
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0answers
26 views
Is there a thermodynamic explanation for how a substance behaves based on its pKa and the solution’s pH?
If the pH of the solution is lower than the pKa of the compound, then the compound will be protonated, because pH is a measure of the concentration of protons, and low pH means a high proton ...
0
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0answers
25 views
Question regarding thermodynamic principles of metallurgy
According to NCERT (12), India:
Why is the reduction of a metal oxide easier if the metal formed is in
liquid state at the temperature of reduction?
Answer: The entropy is higher if the ...
2
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0answers
54 views
What factors contribute in making a solution more endothermic?
For example, a solution between potassium nitrate and water is more endothermic than sodium nitrate in water. Can someone explain?
0
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2answers
52 views
Why is the oxidation of sulfur dioxide not termed as heat of formation of sulfur trioxide?
According to me, the following reaction's heat should be termed as heat of formation of sulfur trioxide:
$$\ce{SO2 + 1/2 O2 -> SO3}$$
However, in my textbook, they have that given it is not the ...
2
votes
1answer
40 views
Double surface in the pressure temperature concentration curve for a binary mixture
In the 3 dimensional diagram given, when a constant pressure plane intersects with the double surface, it produces a looped curve without a maxima or minima. What is the double surface and how is the ...
0
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2answers
10 views
Why do some chemical reactions require many steps? [duplicate]
I was a college student of physics ten years ago, and recently started to learn biochemistry. I enjoy finding out that some familiar concepts in physics play important roles in biochemistry such as ...
0
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0answers
30 views
Is this the equation of state?
\begin{align}
pn^{-\lambda} &= \text{constant} \tag1\label{eq:one}\\
pV^\lambda &= \text{constant} \tag{2, adiabatic equation} \label{eq:two}\\
n\left(\frac{l}{2}\right)\delta T &=...
3
votes
0answers
75 views
If dG is a state function, why do reversible and irreversible paths differ?
For a reversible process at constant T,P in a closed system with no non-expansion work dG = 0. For an irreversible process under the same conditions, dG < 0. Since dG is a state function I would ...
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2answers
79 views
Absolute entropy
The third law of thermodynamics states that the absolute entropy of a pure crystalline substance is $0$ at $0\,\mathrm{K}$. In the analysis of chemical reactions this absolute scale is sued to ...
3
votes
1answer
39 views
Understanding elementary rate laws from a probability stand point
I am learning about chemical kinetics and dynamics and as I understand for a general chemical reaction
$$\ce{aA + bB -> cC + dD} $$ whose reaction rate, r, can be described by an elementary rate ...
1
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1answer
24 views
Reaction direction change based on phase of reactants and products
I am curious specifically how the entropy of a reaction will change if the phases of the reactants and products at different conditions change.
For instance, a reaction
$$\ce{O2(g) + C(s) -> CO2(...
0
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1answer
27 views
Would the change in enthalpy change of chemical reaction in space be considered as change in internel enrgy?
The formula of enthalpy change is
change in H = U + P*(change in V)
In space since there is no air, thus atmospheric pressure = 0 , P= 0 , Pv = 0 ,
So will H = U ?
