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.
1,818
questions
0
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2answers
28 views
Clarification required on whether to use $C_v$ or $C_p$ in this question on finding entropy change
$1 \text{ mol}$ of an ideal monoatomic gas at $300 \text{ K}$ is subjected to a reversible isentropic compression process until final temperature reaches $600 \text{ K}$. If initial pressure is $1 \...
0
votes
0answers
19 views
How is entropy change defined for a chemical reaction?
I know that:
ds = dq/T for reversible processes.
Can I write change in entropy of a system in which a chemical reaction is happening using this equation. Basically, my question is about whether a ...
5
votes
1answer
80 views
Basic thermodynamics question about work
I am getting into thermodynamics, and I have a basic fundamental question about the definition of the change in internal energy $\mathrm{d}U$:
$\mathrm{d}U = T\,\mathrm{d}S - P\,\mathrm{d}V + \sum\...
1
vote
4answers
89 views
In an isothermal process, how can the change in internal energy be 0?
It was written in my textbook,
$$
\mathrm{d}U =
\left(\frac{\partial U}{\partial T}\right)_V \mathrm{d}T
+\left(\frac{\partial U}{\partial V}\right)_T \mathrm{d}V
$$
If the process is isothermal,...
1
vote
0answers
28 views
Methylhydrazine Calorimetry
Below is a chemistry practice problem. I am curious why the change in heat of the oxygen molecules doesn't affect the calculations.
Methylhydrazine ($\ce{CH6N2}$) is commonly used as a liquid ...
-1
votes
1answer
33 views
Gibbs free energy for an ideal gas
You know that for a generic system which during a process exchanges heat with a reservoir at a constant temperature $T_{a}$ it holds:
$$\Delta G=\Delta H-T_{a}\Delta S$$
Where:
$\star)$$\Delta G$ is ...
5
votes
1answer
33 views
If volatility depends on intermolecular forces, then why in Raoult's law, two substances have different volatility?
I have a doubt, Ideal solutions are solutions where intermolecular forces between solute-solute and solvent-solvent are nearly equal to solute-solvent. Since intermolecular forces between A-A = B-B, ...
1
vote
1answer
50 views
Why work done Is 0 and why not a negative value?
If a gas expand in vacuum, $P_{ex.} =0$. Then why is the work done 0? Since the gas is expanding, it is doing some work on the piston so the work done must have some negative value.
-4
votes
0answers
20 views
answer the following question [closed]
The following is not true in case of
thermodynamic work :
(A) We JPav for a reversible process only |
(B) W=0ifdV=0
(C) W+ J PdV for a free-expansion process :
(D) W is an inexact differential
-1
votes
0answers
28 views
Reversibility of a chemical process based on a graph [closed]
By what parameters can I establish whether a chemical process is reversible or not, based on a graph of the following type (assuming this is an ideal gas and mono-atomic):
Thank you!
0
votes
1answer
60 views
Changes in entropy of the universe in a reversible reaction
Does entropy of the universe always increase in one of the forward or backward reactions of a reversible reaction and decrease in the other?
If no then how do you explain the entropy of the universe ...
0
votes
0answers
19 views
Causes of a reversible reaction without external energy input [closed]
I'm afraid that I've had a blank.
I understand from Causes of reversible reaction that all processes can be reversed with a certain input of external energy. I have been taught that for a process to ...
1
vote
2answers
66 views
What is wrong in this argument that dG must always be zero?
Under constant temperature and pressure, the change of the Gibbs free energy can be written as
$$\mathrm dG_\textrm{sys} = \mathrm dH_\mathrm{sys} - T\,\mathrm dS_\mathrm{sys}$$
And in the textbook ...
6
votes
0answers
72 views
Anomaly in van der Waals constants of noble gases
In van der Waals equation for a real gas, the equation is
$$\left(P + a\frac{n^2}{V^2}\right)(V - nb) = nRT$$
where $P$ is the pressure of the real gas, $n$ is the number of moles of the gas, $R$ is ...
1
vote
0answers
32 views
Can we break double bonds by using laser and heat?
If it is possible then give an example for it. That chemical reaction is in this process.
step 1: {Reactant + heat(sunlight)} gives weakened bonded product1
step 2: {product1 + laser} gives {one solid ...
1
vote
1answer
51 views
What all information can be gathered qualitatively from the radial distribution functions of a molecule?
Apart from knowing the radius of coordination shells and the probability of finding, is there any other thermodynamic information we can gather by looking at radial distribution functions of a ...
1
vote
0answers
26 views
As phase transitions are considered reversible processes, shouldn't they be non-spontaneous? [closed]
On the one hand it is stated that reversible processes cannot occur, as they are infinitely slow.
On the other hand phase transitions are said to be reversible processes?
2
votes
1answer
19 views
Steam rises when the heat supply stops
When water boils and you suddenly shut the heat off, a lot of steam arises from it just after you shut the heat off. Why this happens, I thought that maybe water condenzes very fast in small droplets ...
0
votes
1answer
47 views
Troubles with Gibbs free energy
The book I'm reading writes that for a system at constant pressure and temperature it holds:
$$dG=-TdS_{tot}$$
where $dG=dH-TdS$.
So if you want to see if a certain process is spontaneus, you just ...
4
votes
3answers
92 views
Dynamic equilibrium in unsaturated solutions
One standard definition of equilibrium in beginner chemistry is that for a reversible reaction, (dynamic) equilibrium has been achieved when the rate of the forward and backward reactions are equal. ...
2
votes
1answer
30 views
Calculating ΔH with non-integer coefficients in the balanced equation
I am working a practice problem out Ebbing's General Chemistry and for part of the problem I needed to calculate $ΔH.$ The problem considers the burning of ethane in oxygen to yield carbon dioxide and ...
-1
votes
1answer
22 views
Enthalpy Changes [closed]
0.50 g of Mg is added to a 50 mL HCl solution. The temperature increased by 5.2 degree Celsius. Calculate the enthalpy change for the reaction per mole of Mg that reacted.
2
votes
2answers
63 views
Entropy as an intrinsic property of matter
I would like to ask how we can see entropy as an intrinsic property of molecules, namely as something depending also on their geometry (and not only as gross disorder). Is this what is called the "...
0
votes
0answers
53 views
Determining heat capacity of gasses
I need a little help on answering a question which popped into my head while I was reading about gas laws:
Suppose that I have three canisters, each holding a different gas of
unknown identity [...
-2
votes
0answers
83 views
How much time is need for a copper block at 0 °C to equilibrate with 75 °C water?
I have a copper cylinder that is at $0\ \mathrm{^\circ C}$, how much time will be needed for it to equilibrate with $10\ \mathrm{kg}$ of water at $75\ \mathrm{^\circ C}$?
Copper cylinder:
Mass: $105....
1
vote
1answer
29 views
Why does eutectic point has three phases, but consolute point has two phases?
At eutectic point, there are three phases: solid A, solid B and homogenous liquid phase containing both A and B.
But at consolute point, we are told it has 2 phases: liquid A and liquid B.
Why isn'...
0
votes
2answers
60 views
Hess's Law and ΔH
Target equation: $\ce{Mg(s) + 1/2 O2 -> MgO(s)}$
$$
\begin{align}
\ce{MgO(s) + 2 HCl(aq) &-> MgCl2(aq) + H2O(l)} &\quad ΔH &= \pu{-1300 kJ} \tag{1}\\
\ce{Mg(s) + 2 HCl(aq) &-&...
1
vote
0answers
25 views
How to get the enthalpy of a reaction from the enthalpy of formations with Hess' law?
Let's consider a generic reaction:
$$\sum_{i=0}^{i=I}n_{i}A_{i} \rightarrow \sum_{j=0}^{j=J}m_{j}B_{j} $$
where $n$ and $m$ are the
stoichiometric coefficients, while $A$ and $B$ are reagents and ...
0
votes
1answer
80 views
Why could it be assumed that the standard enthalpy change is independent of temperature down to 25 °C?
For the reversible reaction of pyruvic acid to 2,2-dihydroxypropanoic acid, we calculated enthalpy using NMR and extrapolated from area (integration) and peaks etc.
We then plotted $\ln K$ vs $1/T.$ ...
0
votes
0answers
25 views
What is the formula for vapor pressure of liquid related to surface tension of liquid?
What is the formula for vapor pressure of liquid related to surface tension of liquid? I know that if the surface tension increases, vapor pressure will decrease. Toluene evaporates faster than ...
1
vote
2answers
95 views
How can I find the Van der Waals constants (a and b) without any critical values?
I'm given a container of gas that is kept at constant temperature, and whose volume is decreased by applying pressure, from A to E, as the graph shows. I am given the temperature, the total moles in ...
-1
votes
1answer
98 views
What happens when Ar and HCl are mixed at 77 K? [closed]
So our teacher asked us a simple question in the classroom:
What happens when Ar and HCl are mixed at 77 K?
and no other information was given.
We replied various possible answers for that such ...
1
vote
0answers
32 views
Thermochemical data for thiocyanatoiron(III) ion
I am currently writing a lab report about the formation constant of $\ce{FeSCN^2+}.$ All I could find online was a formation constant at $\pu{25 °C}$ $(890),$ which was 70% higher than the ...
2
votes
0answers
22 views
Estimation of Molar Entropies
There are plenty of compounds out there with unknown standard molar entropies. I know that molar entropy has a strong dependance on the physical phase, molecular weight, and number of atoms. So is it ...
-2
votes
1answer
53 views
Why does ice get colder when you add salt? [duplicate]
I know that when you put salt on ice it may turn to a slush and it does feel cold. What is the reason for this?
1
vote
2answers
88 views
Freezing Point Depression and Temperature Decrease
I don't have a problem understanding the chemical mechanisms underlying freezing point depression or colligative properties in general, but what I cannot seem to grasp is how if you add salt to ice ...
0
votes
0answers
32 views
Why do statistical effects impact stepwise stability constants?
I have a question regarding the stepwise stability constants of metal complex formation/substitution.
My lecture notes say that if we have a metal complex, say $\ce{[MX_{6}]}$, and a substitution ...
0
votes
0answers
52 views
Is it possible to use the laws of termodynamics for non-ideal systems?
For the reaction
$$\ce{2CO(g) + O2(g) -> 2CO2(g)}\quad\Delta H = \pu{−560 kJ}$$
it is given that the reaction occurs at constant volume of $\pu{1L}$ and the initial and final pressures are $\pu{...
2
votes
1answer
55 views
How do I know which alkane has the lowest combustion heat?
I'm asked which one of the alkane has the lowest heat of combustion (note: they question says heat, not enthalpy) and according to the answers, the right one is d.
I'm trying to understand why d ...
0
votes
0answers
42 views
Energy of mercury(II) oxide decomposition
Study check 9.11 from [1, p. 298]:
Mercury(II) oxide decomposes to mercury and oxygen.
$$\ce{2 HgO -> 2Hg + O2}\quad ΔH = \pu{+182 kJ}$$
[…]
b. How many kilojoules are needed when ...
1
vote
1answer
35 views
Internal energy change for reactions at constant volume vs constant pressure
The reaction of cyanamide, $\ce{NH2CN(s)},$ with dioxygen was carried
out in a bomb calorimeter, and $∆U$ was found to be $\pu{–742.7 kJ mol-1}$
at $\pu{298 K}.$ Calculate enthalpy change for the ...
4
votes
1answer
60 views
How is a phase equilibrium defined for a one-component system?
A question on this site asked whether a one-component system is at equilibrium when melting or boiling, and the disparate answers were somewhat dependent on the definition of phase equilibrium. ...
1
vote
0answers
27 views
Measurement of the lattice energy
The lattice energy of a solid ionic compound is the energy released when one mole of the solid compound is formed from its constituent gaseous ions at $\ce {298 K}$ and $\ce {1 bar}$. However, gaseous ...
2
votes
3answers
167 views
melting and boiling …really equilibrium? [closed]
Why are melting and boiling considered equilibrium processes even though the amount (concentration) of both phases keep changing i.e from solid to liquid and so on?
1
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0answers
39 views
During solid-solid phase transition, if both solid phases are incompressible, is ΔG independent of p?
Source: Atkins Physical Chemistry, Ninth Edition, page 154, Problem 4.13:
Show that, for a transition between two incompressible solid phases, ΔG is independent of the pressure.
My attempt:
...
2
votes
0answers
46 views
Negative Pressures in Van der Waals Equation of State
This is the plot of the Van der Waals equations:
As you can see, the Van der Waals equation predicts a "stretched liquid" area (MNS). Is this physically feasible and what exactly happens there? Are ...
0
votes
0answers
67 views
Gibbs free energy of phosphorus pentachloride decomposition reaction
The equilibrium constant at $\pu{227 °C}$ for the equation
$$\ce{PCl5(g) <=> PCl3(g) + Cl2(g)}$$
is $K_p = \pu{4.50E3 bar}.$ Calculate the value of $Δ_\mathrm{rxn}G^\circ$ at $\pu{227 ...
6
votes
1answer
135 views
Heat involved in isenthalpic process
Question 28 from [1, p. 4.4]:
An ideal gas is expanded irreversibly from $\pu{5 L}$ to $\pu{10 L}$ against a constant external pressure of $\pu{1 bar}.$ The value of heat involved $(q)$ in this ...
4
votes
2answers
86 views
Clarification on the Clausius inequality
In Atkins' Physical Chemistry the criteria for spontaneity using Gibbs energy was calculated using the Clausius inequality:
$$\mathrm dS ≥ \frac{\mathrm dq}{T},$$
and at constant pressure, $\mathrm ...
0
votes
1answer
56 views
Thermodynamic statements [closed]
Question 13 from [1, p. 4.2]:
Which of the following statements is true?
(a) An ideal gas always obeys the equation: $PV^γ = \mathrm{constant}$ in adiabatic process.
(b) An ideal gas ...
