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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Where did the bomb calorimeter get its name from?
Is it because it is designed to contain combustions reaction with large pressure increases?
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0answers
25 views
Predicting reaction products [on hold]
Suppose we have some reagents: A,B,C,D...
We know their exact formulas and we don't know the structures of molecules. But we know the full energy of each reagent(let's say, from experiment).
How can ...
1
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0answers
21 views
What makes determining entropy changes theoretically so hard compared to enthalpy?
It is really hard to determine the entropy change of a reaction accurately because of subtle solvent effects and a lack of standard entropies. On the other hand, enthalpy is relatively easy and ...
4
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0answers
42 views
Why doesn't beryllium, an elemental solid, obey Dulong and Petit's law?
Beryllium has a molar heat capacity of $\pu{17.7 J mol-1 K-1},$ which is quite far from the $\pu{25 J mol-1 K-1}$ as predicted by Dulong and Petit's law. What is the reason for this discrepancy?
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0answers
20 views
Are there any exceptions to Le Chatelier’s principle? [duplicate]
Peter Atkins says
In introductory chemistry, we meet the empirical rule of thumb known as Le Chatelier’s principle:
When a system at equilibrium is subjected to a disturbance, the ...
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0answers
22 views
Toxic residue on wood burned with ammonium chloride? [on hold]
I'm thinking of making children's alphabet blocks and burning the letters into the wood with ammonium chloride and a heat gun. Would this process leave any toxic residue on the blocks? Thank you for ...
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0answers
27 views
Calculate change in entropy when both temperature and volume change
Calculate the change in entropy when $\pu{23.5 g}$ of oxygen gas is heated from $\pu{240 K}$ to $\pu{360 K}$ in such a way that its pressure increases from $\pu{2.00 kPa}$ to $\pu{8.00 kPa}.$ Assume ...
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0answers
26 views
Thermodynamic quantities and equation in differential form [on hold]
Which of the following are true ??
(a) $dH=TdS +VdP$
(b) $dE=TdS-PdV$
(c) $dH=-SdT-PdV$
(d) $dH=dE+PdV$
So in this question, these all quantity are in differential form. I couldn't ...
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0answers
44 views
Finding the temperature at which the system reaches equilibrium
I’m really stuck on this problem. This involves dissolving Borax:
$$\ce{Na2B4O7•10H2O(s)<=>2Na^+(aq) + B4O7^2-(aq) + 10H2O}$$
$$\begin{align}\Delta H&=\pu{109 kJ/mol} \\ \Delta S&= \pu{...
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2answers
51 views
What are the system and surroundings when dissolving sodium nitrate in water?
When sodium nitrate ($\ce{NaNO3}$) dissolves in water at room temperature, the
solution cools down. $\Delta H$ is positive because heat flows into the system during the process.
I thought though, ...
1
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0answers
35 views
Issue with evaporation rate of water
The evaporation rate of water (kg/s) is given by:
$$g = \frac{P}{P_o} AΘ$$
Where $P$ is given in Pascals and $A$ is the surface area (in $m^2$) and $Θ$ is the empirical evaporation ...
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1answer
23 views
Regarding enthalpy of a reaction
Enthalpy of a reaction $$\ce{2H2 (g) + O2 (g)-> 2H2O(g)} \Delta H_1$$
$$\ce{2H2 (g) + O2 (g)-> 2H2O(l)} \Delta H_2$$
Now I was asked to compare the enthalpy.
If I subtract reaction 2 from 1, ...
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0answers
14 views
Reaction enthalpy using bond energy in resonating molecules
To find out and enthalpy of a reaction using bond energy we use the formula
$$\Delta H = \sum\Delta_\mathrm{bond}H_\mathrm{reactants} - \sum\Delta_\mathrm{bond}H_\mathrm{products}$$
since we use all ...
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0answers
26 views
Why doesn't standard Gibb's Free Energy decide spontaneity? [closed]
I know that Gibb's Free Energy decides the spontaneity of a reaction. But why doesn't standard Gibb's Free Energy decide spontaneity? Standard Gibb's Free Energy is basically Gibb's Free Energy at ...
1
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1answer
41 views
Temperature and pressure for three phase equilibrium of aqueous sodium chloride solutions
I am currently investigating the effect of adding Salt(NaCl) to water on its triple point. Let us assume the mass ratio of water to NaCl is $1:1$. I asked a similar question previously. But, I still ...
2
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1answer
78 views
Energy absorbed by a system in isothermic process in which the phase changes from liquid to gas
A system contains $x$ mole of material, with surface area $S$ is pressed by a mass $M_1$ (no fraction and no any other outer forces involves). Let $\Delta H_\mathrm{l\rightarrow g}$ be the transition ...
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0answers
30 views
intuitive explanation of Wegscheider condition and its violation
The Wegscheider condition restricts the values that rate constants in a set of chemical reactions can take on. can someone please explain intuitively why the condition has to be true, and give a ...
13
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1answer
121 views
Why does ordinary water have a triple point?
Triple points and triple lines for pure substances and mixtures
My understanding is that pure substances can have triple points. But mixtures of substances have triple lines, i.e. there isn't a ...
2
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2answers
80 views
Does adding solutes to water alter its triple point? [closed]
Does adding solutes to water alter its triple point? If yes, how do we find the new temperature and pressure of this point?
4
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1answer
116 views
How to get the efficiency of a heat engine which undergoes an elliptical cycle?
An ideal diatomic gas undergoes an elliptic cyclic process characterized by the following points in a $PV$ diagram:
$$(3/2P_1, V1)$$
$$(2P_1, (V1+V2)/2)$$
$$(3/2P_1, V2)$$
$$(P_1, (V1+V2)/2)...
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2answers
53 views
Statistical thermodynamics [closed]
Entropy of indistinguishable particles $S$ is
$$S = k\ln W,$$
but in some places notation is different:
$$S = k\ln Q + \frac{U}{T}$$
Can you explain, please?
3
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2answers
81 views
What mixing ratio of ethanol and acetone has the lowest freezing point?
I am in the lab trying to make a sample for 1H NMR. It has to remain liquid at the lowest possible temperature.
I was recommended to use a mixture of ethanol and acetone because when they're mixed in ...
3
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1answer
42 views
Why does the solubility of LiCl increase with temperature if the dissolution is exothermic?
In an answer to another question, the following data on the solubility of $\ce{LiCl}$ at different temperatures is presented.
This suggests that dissolving LiCl is endothermic (according to the ...
2
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3answers
114 views
How to explain disagreement between Le Châtelier's principle and the simplified Gibbs free energy equation?
For example, for the dissolution of a salt in water that is exothermic, heating the solution would drive the reaction towards the solid form of the salt according to Le Châtelier's principle.
However,...
2
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3answers
75 views
Where does the energy come from to lower the temperature of a brine solution?
Suppose I have a densely packed bucket of ice chips / cubes made from pure $\ce{H2O}$. It has been at room temperature for days, so the interior temp of the ice is likely close to 0 °C / 32 °F. (I ...
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1answer
25 views
How would a temperature profile in wall change if the rate of heat transfer increases
Given the system below, how would the temperature profile in the wall change if the heat transfer in Fluid II was very fast?
If the heat transfer in Fluid II is very fast, then the T-profile ...
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0answers
61 views
How to calculate the variation of the standard reaction enthalpy for the CO, CO2, H2O, H2 equilibrium?
For the reaction
$$\ce{CO2 (g) + H2 (g) <=> CO (g) + H2O (g)}$$
the equilibrium constant $K_p$ has a value of $0.534$ at $\pu{960 K}$ and $1.571$ at $\pu{1260 K}$. Calculate the variation of ...
1
vote
1answer
60 views
Are there more endothermic or exothermic reactions? [closed]
I'm wondering whether there are more endothermic or exothermic reactions.
Of course the amount of mass will stay the same, but I thought that not every exothermic reaction has an endothermic counter-...
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0answers
49 views
Is it appropriate to approximate solute particles in a dilute solution as an ideal gas?
I was studying osmosis and I came across the formula for osmotic pressure. I decided to see the derivation for osmotic pressure in a dilute solution. The derivation involved an equation involving the ...
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2answers
71 views
Do maxima in Gibbs energy also correspond to equilibrium positions?
ΔrG=(∂G/∂ξ ) at constant p and constant T, where ξ is the extent of reaction.
By Second Law implication, ΔrG= 0= (∂G/∂ξ ) at equilibrium. If we consider a graph of G against ξ, this would mean we ...
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2answers
135 views
Why is Avagadro's number independent of substance? [closed]
In the book of Irey, Thermodynamics, in vol1, IA, page 4, it is given that
Molecular (atomic) weight $M$:
The ratio of the mass of a molecule (atom) of a substance to one twelfth of the mass of an ...
6
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1answer
175 views
How does increase in volume change the speed of reaction in production of NO2?
$$\ce{2 NO + O2 <=> 2 NO2}$$
I understand that increasing the volume $×2$ means lower pressure so production of $\ce{NO2}$ is slowed down, but when asked how to express this mathematicaly I can'...
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1answer
72 views
What actually happens when 'energy is released'?
Thermodynamics classes and equations always discuss a chemical reaction in the form of energy being released or absorbed. This is always hand-waved as just 'energy' from the surroundings.
What's ...
3
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1answer
45 views
Relation between equilibrium constant and entropy change
Like the Van't Hoff equation, which relates change in enthalpy to equilibrium constant, is there a similar equation for the relation between change in entropy and equilibrium constant?
Consider the ...
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2answers
41 views
Reaction quotient and Gibbs free energy at the start of a reaction
According to the equation 33 above, shouldn't be all the reactions be spontaneous initially? Because the reaction quotient Q, is zero at the start of the reaction and logarithm of Q makes it a very ...
3
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1answer
75 views
Why is nitroglycerin so unstable?
I know that nitroglycerin is an incredibly powerful explosive due to its three nitro groups, but why is it so unstable?
What in its chemical structure makes it so sensitive to shock and temperature? ...
2
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1answer
62 views
K in Gibbs free energy
When we write for an equilibrium that $$\Delta G^\circ = -RT \ln(K)$$ Which type of $K$ , ie is it $K_\text{concentration}, K_\text{pressure}$ or $K_{\chi}$ because all these have different numeric ...
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1answer
60 views
Thermodynamics: ΔU = Q + W or U = Q?
Heat is the total kinetic energy of all atoms of the system. When work is done on the system it means that a part of system kinetic energy is used to do the work, and this work makes the surrounding ...
2
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0answers
43 views
Hydrogen + Fluorine Adiabatic Flame Temperature
The theoretical adiabatic flame temperature of Hydrogen and Fluorine in their stoichiometric mixture is often quoted at around $\pu{4300K}$, such as in this presentation on page 2, or this paper which ...
1
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1answer
67 views
Why does the rate of the reaction decrease with an increase in temperature?
Why does the following reaction
$$\ce{2NO + O_2 -> 2NO_2}$$
have negative reaction coefficient, i.e., why does the rate of the reaction decrease with an increase in temperature ?
I have been ...
4
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3answers
134 views
Calculating ΔG at the extremes of reaction extent
This question follows on from a great post on comparing $\Delta_\mathrm{r} G^{\circ}$ to $\Delta_\mathrm{r} G$ answered here
The G vs ξ graph that is used in the answer is an excellent tool to ...
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1answer
23 views
Can somebody explain the low temperature Schottky anomaly for two level systems?
Statistical thermodynamics question. Can somebody explain the low temperature Schottky anomaly for two level systems? Why is this anomaly not seen in all materials?
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3answers
321 views
If change in free energy (G) is positive, how do those reactions still occur?
I was doing a couple of problems for homework:
Calculate $K_\mathrm{sp}$ of $\ce{AgI}$ at $55.0\ \mathrm{^\circ C}$
Calculate $K_\mathrm{b}$ of $\ce{NH3}$ at $36.0\ \mathrm{^\circ C}$
I have to use ...
2
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0answers
38 views
Gibbs free energy of activation and activation energy [duplicate]
What the difference between Gibbs free energy of activation and activation energy
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0answers
31 views
Calculating melting temperature of water with thermodynamical data
So at zero degrees celsius($\pu{273 K}$), for clean water, we have the equilibrium eq.:
$\ce{H2O(s) = H2O(l)}$
As it is at equilibrium, we know that $\Delta G = 0$. As we are dealing with clean ...
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2answers
63 views
Is temperature double-counted in the Gibbs free energy equation?
Complete amateur here. When looking at one form of the equation deriving the change in Gibbs free energy for a given chemical reaction, the terms change in enthalpy, and the negative product of ...
2
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1answer
42 views
Thermodynamics behind organic chemical reactions? [closed]
I took organic chemistry a few years ago at my university, and although I went through the motions to pass the exams, I didn't understand the thermodynamic reasoning behind why the reactions actually ...
1
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1answer
63 views
Why do we use external pressure when calculating work done by gas? [duplicate]
For irreversible expansion and compression, why is dw= -pextdV but not dw= -pdV ?
Many of the texts and related questions on this site I encountered give a same general idea: because work done= ...
0
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1answer
49 views
Entropy and total differential
In an exercise, I have derived that
$$\mathrm dS_\mathrm{m} = \frac{3R}{2T}\mathrm dT + \frac{R}{V}\mathrm dV$$
using the 1st and 2nd law of thermodynamics (1 mol ideal gas).
Now, I want to write $\...
2
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0answers
48 views
Given Kd find equilibrium concentrations of the reactants
$$\ce{Biotin + Avidin → Complex}$$
Biotin is a vitamin derivative that binds extremely tightly to avidin (a protein found in egg white) with a $K_\mathrm{d}$ of $\pu{1e-15 M}$.
If you make a ...
