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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52 views

Using salt and ice vs cold brine to rapidly cool an object

This a question regarding the practicalities of using salt+ice vs just cold brine to cool something in the home (inspired by this recent post: https://cooking.stackexchange.com/questions/116437/...
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What is the enthalpy of formation of benzene? [closed]

I was trying solve this question from my book: The combustion of one mole of benzene takes place at $298 \mathrm{~K}$ and 1 atm. After combustion, $\mathrm{CO}_{2}(\mathrm{~g})$ and $\mathrm{H}_{2} \...
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$\Delta \bar{h}_{vap}$ and $\Delta \bar{s}_{vap}$ from vapor pressure vs. temperature data

I'm trying to work on the following two problems: The answers are given below: 4.2: 57.7 kJ/mol, 122.6 J/(K-mol), 12 kJ/mol 4.3: -58.8 kJ per mole of dimer For 4.2, I used the equation $$\frac{\...
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1answer
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Linear algebra and Hess's law?

I am a high school student taking chemistry who was tasked to solve a large set of Hess's Law problems. Naturally I tried to find a way to get my computer to carry out this repetitive piece of work ...
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1answer
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If something is not hydrophilic, is it necessarily hydrophobic?

Is it possible to create hydrophobic water? Two species are phobic within some medium under some conditions when they don't here together (cohere if they're the same species; adhere otherwise). To be ...
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1answer
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Can we have an equilibrium constant for a reaction at constant volume?

Suppose we have a sealed container (fixed volume) and we introduce two gases $\ce{A}$ and $\ce{B}$. The two reactants $$\ce{A + B <=> C}$$ form another gas $\ce{C}$ with equilibrium constant $K$....
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Determining Boiling Point and Melting Point by Graph

This is the plot of $\Delta G^{^\circ}$ vs $T$ for $\ce{Pb-> PbO}$ I understand that the points where there is a sudden change in the slope of the graph, there is a phase change but exactly what ...
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Gibbs free energy of mixing in a control volume

I'm trying to model the amount of energy lost due to mixing of a $\mathrm{HCl}$ solution using the Gibbs free energy of mixing. However, I really like a second opinion on the model and the results, ...
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Predicting the solubility of sulfate salts [duplicate]

I came across a problem that asks to predict the solubility of alkaline earth metal sulfates: Magnesium, calcium, strontium and barium sulfates. I know that the spontaneity of dissolving can be ...
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Why does van der Waals' equation of state give only one P, V pair for a particular temperature here?

I came across this question which I initially ignored believing that the data was insufficient: Calculate the pressure exerted by one mole of $\ce{CO2}$ gas at $\pu{273 K}$ if the van der Waals' ...
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Entropy as a function of pressure, volume, and temperature

I know we can write entropy $S$ as $S(P,T)$ and $S(V,T)$ to derive different relations between heat capacities $C_V$ and $C_P$. I was wondering if it is technically correct to write $$dS(P,V,T)=\left (...
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Finding the molar ratio at equilibrium

Consider the reaction $\ce{SO_2_{(g)} + \frac{1}{2} O_2_{(g)}\longrightarrow SO_3_{(g)}}$. What effect is there on the molar ratio $\frac{n_{SO_3}}{n_{SO_2}}$ at equilibrium if the pressure is ...
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Is a change in delta-delta-G typically notated as delta-delta-delta-G?

I'm currently writing a research paper, and need to refer to a difference between $\Delta{\Delta}G$s. The naive way to notate this would be $\Delta{\Delta}{\Delta}G$, but that looks cumbersome and ...
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1answer
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Are sudden adiabatic expansions reversible or irreversible? (Tyre burst problems)

Encountered Question: The tube of the tyre of a car is filled with air at $\pu{27^\circ C}$ and $\pu{2 atm}$ pressure. If the tube suddenly bursts, the final temperature of the air will be ($γ_\...
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Equilibrium and endergonic reaction

If a reaction A <-> B has a value of dG°>0, then I know that A -> B is endergonic while A <- B is exergonic. Now if I start with 100% B I could see how equilibrium is reached, B reacts ...
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Effect of inert gas blanket on oxygen solubilized in wine

If applied to a volume of wine, could a blanket of argon become dense enough to remove oxygen from the wine? This is not a question of sparging the wine. Rather, could a blanket of argon over the top ...
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How to estimate temperature loss of a mixture when we know the heat loss due to vaporisation of one of the component?

So I am modelling in Chemkin and want to include phase change phenomenon in my model. But there's no capability of phase change in Chemkin so I have to think of some out of the box method for this. So ...
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Cause of reversible reactions in terms of Gibbs free energy

Heading I recently learned about the standard Gibbs free energy change of reaction, ΔG=ΔH-TΔS, and how its sign indicates whether the conversion of (ALL) reactants and products is spontaneous or not. ...
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What is 'non-expansion work' and how does ΔG represent the maximum non-expansion work that can be extracted in a reversible process? [duplicate]

I have just learned about Gibbs free energy and how its value indicates the change in entropy of the universe and can thus be used to find the spontaneity of a reaction. However, I found this other ...
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Are spontaneous exothermic reactions spontaneous due to more stable products or to an increase in the entropy of the surroundings? [closed]

The lattice formation of an ionic compound from its infinitely separated gaseous ions is spontaneous under standard conditions, but I cannot figure out the 'driving force' behind this. One way to look ...
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Is it possible to increase the internal energy of an ideal gas in an isothermal process?

Ideal gas is the one in which there are no attractive or repulsive forces acting and hence its internal energy is due to kinetic energy of its molecules. We may then say, for an ideal gas internal ...
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Do superoxide salts of heavier alkali metals contradict the principle that lattice energy depends inversely on cation-anion distance?

I have a very basic understanding of quantitative lattice energy calculations: I only know that the lattice energy is directly proportional to the product of charges on the ions and inversely ...
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Entropy change during electrolysis of water

Consider the electrolysis of water reaction: $$\ce{H_2O -> H_2 + 1/2O_2}$$ At room temperature and atmospheric pressure, the change in enthalpy of this reaction is $\Delta H = \pu{+286 kJ mol-1}$. ...
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Hydrogen permeation from low to high absolute pressure area due to difference in partial pressure?

I need to get help with a question regarding hydrogen permeation which I do not really how to get around. The statement is as following: A steel container contains helium gas at high pressure (100 ...
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Do the differential expressions of second law of thermodynamics imply a smooth increase in entropy? [closed]

Another statement of second law of thermodynamics can be formulated in terms of system properties and not properties + surroundings (isolated system). For a closed system at constant temperature and ...
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Effect of addition of inert gas on concentration in an equilibrium [duplicate]

We have the following equilibrium established in a container with movable piston at atmospheric pressure. $$\ce{N_2 (g) + 3 H2 (g) ⇌ 2NH_3 (g)}$$ Now how does the concentration of the reacting species ...
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Why is standard state useful?

In order to calculate the equilibrium constant we must know the value of $\Delta_\mathrm{r}G^{\circ}$ (which is a function of $T$) so we can calculate $K$ as: $$K = \exp\left(-\frac{\Delta_\mathrm{r}G^...
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Will addition of inert gas change the vapour pressure of a liquid?

From this question on Physics SE Thermodynamics of evaporation: Now imagine the experiment is repeated but instead of vacuum conditions, the water is pressurized with nitrogen at 1 atm. According to ...
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Does Raoult's law hold for immiscible liquids?

I know that Raoult's law holds true only for a non volatile solute in a volatile solvent mixture wherein the vapour pressure of the solution gets lowered due to the addition of solute. Now, first of ...
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How change in potential energy corresponds to change in enthalpy?

So while reading about activation energy , I came across this graph : Here, change in potential energy of reactant and product is depicted as ${ΔH}$, but at constant pressure and temperature , ...
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Is ΔG the maximum non expansion work for reversible processes only?

The derivation of $$\mathrm{d}G=\delta w_{non.exp. max.}$$ $$G=H-TS$$ $$ \mathrm{d}G=\mathrm{d}H-T\mathrm{d}S-S\mathrm{d}T$$ $$\mathrm{d}G=\mathrm{d}U+P\mathrm{d}V+V\mathrm{d}P-T\mathrm{d} S-S\mathrm{...
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Examples of endothermic reactions occuring spontaneously at constant temperature and pressure [closed]

Typical spontaneous endothermic processes I can think of (e.g. melting of ice, isothermal gas expansion) involve either an initial temperature difference between the system and the surroundings, or ...
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Error in the IUPAC Gold Book definitions of exergonic, endergonic, exothermic and endothermic?

All definitions in the IUPAC Gold Book refer to standard values: exergonic reaction (exoergic reaction) [link] This expression is often applied to a reaction for which the overall standard Gibbs ...
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1answer
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Why Does Electrical Work Cause Changes in Internal Energy of the System? [closed]

If we look at system at constant temperature and volume which is galvanic cell, first law of thermodynamics states: $$ dU = dQ + dW'$$ Where W' is electrical work exchanged with surroundings and Q is ...
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Thermodynamics Property Tables

Two rigid tanks are connected by a valve. Tank A contains $0.2\ \mathrm{m^3}$ of water at $400\ \mathrm{kPa}$ and $80\ \%$ quality. Tank B contains $0.5\ \mathrm{m^3}$ of water at $200\ \mathrm{kPa}$ ...
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Regarding Ellingham diagrams

In my textbook, the following information is given about Ellingham diagram: The interpretation of $\ce{\Delta_rG^\circ}$ is based on ${K}$ ($\ce{\Delta G^\circ = -RTlnK}$). Thus it is presumed that ...
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1answer
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Why is the formation free energy of carbon dioxide almost constant in Ellingham diagrams?

While reading about Ellingham diagram in my textbook, I noticed that for: $\ce{C_(_s_) + O2_(_g_) -> CO2_(_g_)}$, the standard Gibbs free energy ($\ce{\Delta_fG^\circ}$) doesn't seem to vary with ...
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What Does Electrical Work Represent in Context of Electrochemical Cells?

If cell operates reversibly than change in Gibbs energy of system equals electrical work of the cell. As far as I know electrical work is work done by electric field when charge moves between certain ...
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Different statements of second law of thermodynamics

This may sound trivial but I am having a hard time linking different statements of second law. What I get from second law is that heat cannot be completely converted into work and hence efficiency of ...
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1answer
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Does the critical temperature of a gas depend on the amount of the gas taken?

Does the critical temperature of a gas depend on the amount of the gas taken? Or is it independent of it? My book does not specify this point. I think according to the gas laws, the critical ...
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1answer
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Enthalpy of Reaction and Efficiency of Electrochemical Energy Devices

For electrochemical energy devices such as batteries or fuel cells working reversibly, efficiency is defined as: $$ \eta = \frac {\Delta_rG^⦵} {\Delta_rH^⦵} $$ Since cell operates reversibly Gibbs ...
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Is there a public database for enthalpy of solution values?

I'm looking for a public database for enthalpy of solution values. The CRC handbook only has selected compounds available. The NIST webbook doesn't appear to have any enthalpy of solution values. Any ...
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Maximum non-PV work and reversibility at constant temperature and pressure

When we talk about systems at constant temperature and pressure, maximum amount of non-PV work can be extracted if process is carried reversibly and in that case it is equal to change in Gibbs energy ...
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Are the derivations on Gibbs free energy consistent with the interpretation of entropy?

User Buck Thorn writes here that In chemistry we are most often interested in the effect of changes in $T$ or $P$ on $\Delta G$ for some process such as a phase change or chemical reaction which is ...
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385 views

Why does the dissolution of calcium hydroxide have a negative entropy?

We had an experiment on the dissolution of calcium hydroxide and we obtained the experimental entropy value $\pu{-203 J mol^-1 K^-1}$ with a percent error of $26.7\%.$ Doesn't dissolving a solid in a ...
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1answer
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When does a piston stop in an irreversible process?

Suppose we have an ideal gas filled inside a container fitted with a piston. The external pressure is equal to internal pressure. If we suddenly decrease the external pressure, we say that the gas ...
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Calculate Specific Heat given initial temp, final temp and equilibrium temp [closed]

I would appreciate if someone could enlighten me on this question. Question: A metallic object with a mass of 2 kg at 50ºC is placed in 10 L of water at 20ºC. When the thermal equilibrium the ...
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Why is the reverse aldol cleavage of fructose 1,6-bisphosphate highly endergonic under standard conditions?

I have another question similar to this one. I just don't have intuition for why reactions are thermodynamically favorable or unfavorable and I'd like to build it. This time I'd like to ask about ...
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1answer
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What is the difference between bond enthalpy and bond formation enthalpy?

I came across the following while going through my textbook The standard enthalpy of reaction is related to bond enthalpies of reactants and products in gas phase reactions as: $$\Delta H^{\circ}_r = ...
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Why is enthalpy change = heat change only for constant pressure?

Please help me, a mathematician doing his elective course on Physical Chemistry, out with this very simple question: why does the relation between enthalpy and heat $$\Delta H = \Delta U + p\,\Delta V ...

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