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
votes
0answers
7 views

Where did the bomb calorimeter get its name from?

Is it because it is designed to contain combustions reaction with large pressure increases?
-2
votes
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
vote
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
votes
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?
1
vote
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 ...
0
votes
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 ...
0
votes
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 ...
-1
votes
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 ...
0
votes
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{...
-1
votes
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
vote
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 ...
0
votes
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, ...
1
vote
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 ...
0
votes
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
vote
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
votes
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 ...
0
votes
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
votes
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
votes
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
votes
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)...
0
votes
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
votes
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
votes
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
votes
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
votes
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 ...
0
votes
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 ...
0
votes
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-...
2
votes
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 ...
1
vote
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 ...
-5
votes
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
votes
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'...
3
votes
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
votes
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 ...
0
votes
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
votes
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
votes
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 ...
1
vote
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
votes
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
vote
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
votes
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 ...
0
votes
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?
4
votes
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
votes
0answers
38 views

Gibbs free energy of activation and activation energy [duplicate]

What the difference between Gibbs free energy of activation and activation energy
0
votes
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 ...
5
votes
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
votes
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
vote
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
votes
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
votes
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 ...