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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0answers
51 views

Why will A atoms diffuse from β phase to α phase and B atoms from α phase to β phase?

At the initial compositions 1 and 2 of $α$ and $β$ respectively the chemical potentials of A and B atoms in each phase can be found by extrapolation of the tangents to the free energy curves at 1 and ...
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1answer
37 views

Why does a system want to achieve randomness to be feasible? [closed]

According to entropy and gibbs free energy, randomness is one of the factors that determines reaction spontaneity, but why? Shouldn't it be the opposite?
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1answer
101 views

What is the reason for phase separation? [closed]

I have seen phase separation as a phenomenon taking place. However I do not understand why this phenomenon occurs as a counter example to the natural laws of diffusion along the concentration gradient....
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2answers
121 views

Does energy come from fuel or oxygen? (surprise?)

Question: Is it oxygen or fuel that releases energy during burning? The answer from this article: Why Combustions Are Always Exothermic, Yielding About 418 kJ per Mole of O2 (Journal of Chemical ...
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1answer
85 views

Lowering the melting point of water with salt

I was trying to calculate amount of salt needed to lower the melting point of water by 5 °C. According to my solutions, I need to make sure $$n\mu(\ce{H2O(s)}) =n\mu(\ce{H2O(l)})+ RT\log(x)$$where $ x=...
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1answer
191 views

Why enthalpy changes in phase transition when there is no change of temperature?

Why there is a change in enthalpy in phase transition? I'm asking because enthalpy depends only on temperature: $$\mathrm dH = C_p\,\mathrm dT,$$ and the change in temperature is $\mathrm dT = 0.$
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248 views

For a gas having molar mass M, express specific heat at constant pressure in terms of heat capacity ratio

For a gas having molar mass $M$, express specific heat at constant pressure in terms of heat capacity ratio. My attempt: using $ C_p - C_V = R $ and $ \dfrac{C_p}{C_V} = \gamma $ $ C_p = \dfrac{...
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1answer
67 views

Why do we use Ḡ (“G bar”) as a notation for partial molar free energy?

For partial molar entropy there is a bar that is positioned above the $G$ with a subscript I that is sometimes added to the $G~(\bar{G}).$ The bar I read sometimes denote the "partial nature of the $G$...
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Why is the y intercept of a G vs X curve treated as chemical potential?

Whenever I am to prove the fact that the $y$ intercept to the $G$ vs $X$ curve is basically my chemical potential, there are two equations that need to be assumed. One is $$G=\mu _{A}X_{A}+\mu _{B}X_{...
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3answers
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What do the horizontal lines in a P-V phase diagram mean?

I have been studying phase diagrams of pure substances, and for a substance such as carbon dioxide for example, it's diagram looks something like this: The horizontal lines in the liquid phase puzzle ...
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2answers
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When boiling water, how is evaporation rate affected by the surface area of the container?

Imagine two pots boiling on a stove. One is tall, while one is wide. Both contain the same volume of water. What will be the difference in the rate of evaporation between the two containers?
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1answer
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Why does silver (I) have a larger lattice enthalpy and hydration enthalpy than sodium?

The ionic radius of the $\ce{Ag^+}$ ion is $129$ pm, and that of the $\ce{Na^+}$ ion is $116$ pm. Since the sodium ion is smaller than the silver ion, it makes sense that it has a stronger polarizing ...
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Calculating ideal chemical work using extent of reaction and chemical potential [closed]

I have a electrochemical reactor assumed isothermal and isobaric with 4 reactions and I am trying to calculate the "ideal chemical work' exerted by each reaction using the extent of reactions for each ...
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122 views

Entropy in organic chemistry

Why do we not talk about entropy when deciding feasibility of an organic reaction?...We always seem to only be interested in stability of product and intermediate(essentially delta H)and draw their ...
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1answer
42 views

How to explain Gibbs free energy is a pressure-dependent state function?

I'm aware that through deriving Gibbs free energy to infinitesimal changes, we could get the formula: $\mathrm dG = V\,\mathrm dp - S\,\mathrm dT$, giving that Gibbs free energy is pressure-dependent. ...
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1answer
45 views

Question about calculating enthalpy change

$$\Delta H = \left( \begin{array}{c} \text{total enthalpy of}\\ \text{bonds broken}\end{array}\right)-\left( \begin{array}{c} \text{total enthalpy of}\\ \text{bonds made}\end{array}\right)$$ $$\Delta ...
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How to calculate the value of heat absorption/ heat release during water electrolysis?

The overall reactions with the enthalpies of water electrolysis at room temperature can be represented by the equation below: $$\ce{2H2O + \pu{237.2 kJ mol-1} + \pu{48.6 kJ mol-1} → 2 H2 + O2}$$ ...
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2answers
111 views

Exothermic: enthalpy or heat?

I am used to thinking of exothermic reactions as being defined as having negative $ΔH.$ However, I also am used to hearing/thinking of exothermic reactions as "releasing heat". Since change in ...
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28 views

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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32 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 ...
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3answers
183 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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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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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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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
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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, ...
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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
25 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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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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1answer
63 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 ...
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1answer
87 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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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 ...
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1answer
140 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 ...
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2answers
123 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?
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1answer
131 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
65 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?
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2answers
313 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
435 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 ...
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3answers
577 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,...
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3answers
302 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
30 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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70 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 ...
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1answer
303 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
51 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
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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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166 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 ...
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1answer
228 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
194 views

What actually happens when 'energy is released'? [duplicate]

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 ...
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1answer
786 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
1k 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 ...
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1answer
227 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? ...