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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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48 views

Sum of Bond Enthalpies in an Exothermic Reaction

The following reaction is exothermic. In an exothermic reaction, heat is given out to the surroundings so the enthalpy of the reaction mixture should decrease. Hence $\sum$ Reactant Bond Enthalpies ...
0
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1answer
37 views

How can change in Gibbs free energy ever be positive?

I have a question regarding this formula for Gibbs free energy of a system: $\Delta$G = $\Delta$H - T$\Delta$S. According to the second law of thermodynamics: T$\Delta$S $\ge$ $\Delta$Q and to my ...
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2answers
267 views

Could a non-spontaneous reaction occur “on its own”?

Does spontaneity really only apply to the net free energy change of a reaction? I used to be taught that spontaneous reactions could occur on their own BUT without enzymes or a spark they might take ...
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1answer
59 views

Where can I find the standard molar entropy for ammonium bicarbonate?

I'd like to know the standard molar entropy S0solid for ammonium bicarbonate to use in an example in class, to calculate ΔS0 for the reaction, NH4HCO3(s) → NH3(g) + H2O(g) + CO2(g). Already checked: ...
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1answer
65 views

Entropy change of evaporating NaCl [closed]

What is $\Delta S$ for this process? $$\ce{NaCl(s) -> NaCl(g)}$$ (A reference that I could quote would be ideal.)
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0answers
28 views

Boltzmann Transport equation: Is df/dt=o for collisionless only?

$$\frac{\mathrm df}{\mathrm dt} = \left(\frac{\partial f}{\partial t}\right)_\text{force}+\left(\frac{\partial f}{\partial t}\right)_\text{diff}+\left(\frac{\partial f}{\partial t}\right)_\text{coll}$$...
5
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1answer
91 views

What exactly is entropy? [closed]

I have looked at other questions, videos and articles but I kept reading disorganization, messiness or energy distribution (probability) or a thermodynamical equilibrium and stuff. What do we mean by ...
0
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1answer
121 views

Energy at constant Entropy and Volume

The system of constant entropy and constant volume will attain the equilibrium in a state of minimum energy. Why? I know that: $\Delta G= \Delta H-T\Delta S$ And at constant volume and entropy: $\...
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0answers
28 views

Diamond surface graphitization

Wikipedia says that diamond converts to graphite at ~700 °C. But then, in the same article we can find an information: "But diamonds (sp3C) are unstable against high temperature (above about 400 °C (...
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1answer
342 views

Calculate Heat of Atomisation [closed]

What is heat of atomisation of P4O6(s) Given heat of sublimation of P4O6 is x kJ/mol & P–O bond energy is y kJ/mol. What is heat of atomisation of P4O6(s) Given heat of sublimation of P4O6 is x ...
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0answers
35 views

A problem on Gas bubble column

It is known sulphite to sulhate oxidation can be achieved by using H2O2 or oxygen as oxidising agent. If we flow a liquid mixture of sulphite with H2O2 along with air bubble through the solution in a ...
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3answers
3k views

Work done in an isothermal irreversible process

In calculating the work done in an isothermal irreversible process, why do we consider the external pressure only (using formula: $W_{\text{ext}} = P_{\text{ext}} \mathrm{d}V)$ but not the internal ...
3
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0answers
41 views

Reverse rate coefficient for thermolecular and thermal dissociation reactions

I am an astrophysicist working in exoplanetary atmosphere chemistry right now, in particular modelling the chemical kinetics taking place within the atmosphere. Based on this IOP article, we have that ...
0
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1answer
93 views

Finding Gibbs energy at different temperatures

How can I find $∆G$ at a higher/lower temperature if I am given $∆G°$ and $∆H°$ $\pu{298 K}$, and a reaction equation (solubility)?
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1answer
513 views

Calculating the enthalpy of polymerisation of ethylene given the bond strengths

Given the average bond dissociation enthalpies of a $\ce{C-C}$ bond (say $x$) and a $\ce{C=C}$ bond (say $y$), find the enthalpy of the following polymerisation reaction (per mole of ethylene): $$...
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1answer
172 views

Does expansion of an ideal gas not always represent an increase in entropy of the system?

Expansion of an ideal gas not always represent an increase in entropy of the system. Statement is True or False? What I thought is that whenever an ideal gas expands the randomness of gas molecules ...
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7answers
12k views

If the human body consists of 60% water why can't we put out fire with our body? [closed]

I have often heard of people being burned at the stake, but if the body is 60% water shouldn't the fire just be put out?
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2answers
60 views

Sign of work done by the system in first law of thermodynamics equation

In the equation, $\Delta E = q - W$, according to my book, $W$ is positive when it is done by the system and negative when it is done on the system. But in this problem: $\pu{3600 kJ}$ heat is ...
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0answers
43 views

About the approximation of reaction enthalpy by standard reaction enthalpy

It seems that almost all of the thermochemistry I know (which is admittedly not much) is based on the approximation of the actual reaction enthalpy by the standard one (at temperature set equal to 298 ...
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0answers
383 views

Calculating Condensation Temperature using Raoult's Law and Antoine's Equation

I'm having trouble solving this particular problem: A gaseous mixture which contains 80% mol of N2 and 20% mol styrene has a volume flow rate of 100 m^3/h and a Temperature and Pressure of 100°C and ...
0
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0answers
57 views

How do scientists calculate bond enthalpies?

I'm curious about how chemists managed to investigate the bond enthalpies of certain bonds, e.g. the $\ce{O=O}$ bond. Did they measure the bond enthalpies, or did they calculate them? If so, how did ...
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0answers
199 views

What does ΔG° refer to exactly?

My interpretation of this image is that $\Delta G^\circ$ is equal to the difference between $G^\circ$ of the products and $G^\circ$ of the reactants. But, I've also read $\Delta G^\circ$ as being ...
3
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1answer
152 views

The sign of enthalpy of formation of magnesium oxide

I'm currently doing a lab to calculate the enthalpy of formation for $\ce{MgO}$. However at the moment me and my lab partner are having a disagreement. We've both calculated and agreed upon the same ...
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2answers
1k views

Ideal gas Law best describes the properties of which of the following gases at 0 degrees Celsius and 1 atm? [closed]

a. $\ce{PH3}$ b. $\ce{HBr}$ c. $\ce{SO2}$ d. $\ce{N2}$ I believe that it is $\ce{N2}$, but I'm not too sure, since technically all of them would act the same at STP.
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1answer
111 views

Unsteady state process for first law of thermodynamics with changing boundries [closed]

Firstly , this is not an homework question .I tried to solve this question to understand the concept. I made the following attempt to solve this question.However I couldn't solve after exit valve is ...
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0answers
53 views

Is a system considered Stationary when a Stationary Flow goes through it?

In my Thermodynamics book, I've seen the word "stationary" used in two different senses: A) Stationary System: A system in which the change in both kinetic and potential energies is equal to 0. B) ...
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2answers
94 views

Is Kirchoffs law valid at all pressures?

$$\Large H_{T_\mathrm{f}, p}=H_{T_\mathrm{i},p}+\int_{T_\mathrm{f}}^{T_\mathrm{i}}c_p(T)dT$$ This can be derived by integrating: $$C_v =\left(\frac{\mathrm{d}U}{\mathrm{d}T}\right)_V$$ Applying this ...
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0answers
37 views

Should I use temperature or specific volume to perform a linear interpolation for obtaining an internal energy value?

If I have a state of a process in which I know the temperature (T) and specific volume (v) of a superheated vapor, and I use such values on steam tables to find out the internal energy (U) associated ...
3
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1answer
178 views

Why isn't standard Gibbs free energy always zero?

I read in my textbook that one of the standard conditions we should use when calculating the standard Gibbs free energy is having a concentration of 1 M of all the reagents and the products in the ...
7
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1answer
258 views

Deriving heat capacity in terms of internal energy U and natural variables S & V

My lecture notes have the following equation regarding heat capacities: $$ C_p = C_v + T\left(\frac{\partial p}{\partial T}\right)_v\left(\frac{\partial V}{\partial T}\right)_p $$ (just to be clear, ...
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0answers
106 views

Apparent contradiction in the type of deviation of an ethanol-water mixture

For a mixture of $\ce{EtOH}$ and $\ce{H2O}$: $\Delta H_{\text{mix}} < 0$ (exothermic) and $\Delta V_{\text{mix}} < 0$ (volume contraction). This means that the unlike interactions are stronger ...
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0answers
616 views

If equilibrium constant K is greater-than-1/less-than-1, can we automatically assume Delta G is going to be negative/positive, respectively?

One of my chemistry problems gives the following: $\ce{A(g) -> B(g) + C(g)} $ $K_p=2\times10^{-4}$ at $\pu{298 K}$ A mixture of the three gases at $\pu{298K}$ is placed in a reaction flask ...
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1answer
68 views

Why are there two contradicting formulas for Enthalpy (Delta H): [duplicate]

1) Delta H = Bonds broken - Bonds Formed 2) Delta H = $H_{products} - H_{reactants}$ In the first formula, bonds break in the reactants and form in the products, so its basically saying "bonds ...
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1answer
101 views

Is a person warmed by fire an example of endothermic reaction? [closed]

My chemistry textbook gives the example of a man being warmed by a fire as an endothermic reaction. If the man's body is the system and it is increasing in temperature, how can this be endothermic?
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3answers
490 views

Is heat an extensive or intensive property?

Is heat extensive or intensive property? I surfed the Internet for the same and found quite different answers. Like, heat is not a property, so it is neither. One said that since heat depends on the ...
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0answers
15 views

Why don't we add liquid or solid in our equilibrium constant? [duplicate]

I kinda want to understand this aspect intuitively, but I can't find a good way to understand this. Any clarification would be great. Thank you.
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0answers
75 views

How do the pressure acting on the reactants and the initial temperature of the reactants affect the enthalpy of reaction?

In an exothermic reaction that occurs under constant pressure, does the pressure acting on the reactants affect the enthalpy of reaction? Does the initial temperature of the reactants affect the ...
0
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1answer
295 views

Chemical potential of mixtures of ideal gases and the Gibbs-Duhem equation

As is well known for a constant composition system, $\mu$ (symbolizing chemical potential) is equal to the molar Gibbs energy. Using $$ \mathrm{d}G = V\mathrm{d}p-S\mathrm{d}T, $$ we can write $$ \mu =...
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2answers
203 views

Derivation of the Temperature Dependence of the Boltzmann Distribution

In lecture notes, Atkin's Physical Chemistry, etc. the Boltzmann Distribution is derived by first claiming that the system takes on properties of its weightiest configuration. The weight is therefore ...
0
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1answer
101 views

Isothermal compression at critical temperature

Suppose I am compressing a gas isothermally at its critical temperature. Initially, it has a large volume. I start compressing it and reach the critical point. At this point, the gas will start to ...
0
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0answers
92 views

Why do we neglect the zero point energy in statistical mechanics?

Quantisation associated with translational modes can be modelled like a particle in a box (PiB). $$E_n = \frac{n^2h^2}{8mL^2} \qquad n \in \mathbb{Z} \, | \, n \ge 1$$ By the equation above, the ...
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2answers
490 views

How can endothermic reactions be spontaneous?

If endothermic reactions absorb heat from the surroundings, how can they be spontaneous, if heat only flows from high temp to low temp objects?
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0answers
60 views

What would happen to a diamond in the presence of oxygen and elevated temperature and how fast?

Let's say I have a $\pu{1cm^3}$ diamond and I put it into an oven where the temperature is about $\pu{ 400°C}$. We all know that diamond oxidizes. Would it evaporate or turn into graphite? If so, ...
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1answer
45 views

Finding direction of the reaction using Affinity of the reaction

I encountered following reaction: $$\ce{CO + 1/2 O2 -> CO2}$$ I am provided with the composition of the various gases at $\pu{1000K}$ and am asked the direction of the reaction by finding the ...
5
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1answer
72 views

slow down ice melting in water

Having to separate freshwater crabs for commercial fisherman while on the boat, we dump the crabs in ice water, as crabs aren't as aggressive and bite less when cold. There's only so much room on the ...
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2answers
91 views

Why don't we consider VΔP when we define Q?

We only define $Q = \Delta U + W_\text{exp}$ (expansion work = $-P\Delta V$). If heat can cause $\Delta U$ and work, why work is defined only as expansion work in the first place where there are other ...
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2answers
256 views

Why does increasing the pressure cause ice to melt more?

This question was given under chemical equilibrium. Why does increasing the pressure on this system cause ice to melt more? $$\ce{H2O(s) <=> H2O(l)}$$ I'm pretty sure this has to be ...
0
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1answer
108 views

Variation of equilibrium constant

In an exothermic reaction, the value of equilibrium constant increases with the rise of temperature because, $\Delta H < 0$ ( exothermic reactions) $\Delta G = \Delta H - T\Delta S$ Now if $\...
5
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1answer
256 views

The confusion of entropy change

The entropy is defined as randomness of a substance or a system. The randomness increases as we go from solids to liquids to gases. Similarly from hard diamond to soft graphite. Since graphite is soft ...
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3answers
165 views

Why is the enthalpy of a reaction equal to the difference between the enthalpies of combustion of the reactants and the products?

My textbook gives me the following formula for calculating the enthalpy change of any reaction: $$\Delta H_\mathrm{r}^\circ=\sum(\Delta H_\mathrm{c}^\circ)_\text{reactants}-\sum(\Delta H_\mathrm{c}^\...