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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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What units of K should we use in ΔG=-RTlnK?

I understand that $K$ will be dimensionless. Let's say an equilibrium involving $K_p$ and the reaction is $\ce{A -> B}$, $K_p$ will be dimensionless. If the reaction $\ce{A + B -> C}$, then ...
2
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3answers
341 views

Entropy, reversibility and equilibrium

Entropy is maximum at equilibrium, so if we have an equilbrium chemical reaction like: $$\ce{HI <=> H+ + I-}$$ does it mean that the container of $\ce{HI}$ has maximum entropy or the universe?...
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62 views

Contribution of vibrational degrees of freedom

The ratio $\frac{\mathrm{C_p}}{\mathrm{C_v}}$ for linear triatomic molecules including vibration contribution is given by: ? Attempt: We are aware that $C_v = \frac{fR}{2}$ and $C_p = C_v+ R$ ...
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0answers
20 views

Isobaric-isothermal ensemble with degeneracy

I've been studying statistical mechanics for a while, the topic is very far from what I've learned till now so my questions may be superficial. When trying to see what my entropy is in an isobaric-...
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0answers
504 views

Hydrazine vs 1,1-dimethylhydrazine for rocket fuel?

The question in my textbook is as follows: Hydrazine and 1,1-dimethylhydrazine both react spontaneously with O2 and can be used as rocket fuels. $$\ce{ N2H4(l) + O2(g) -> N2(g) + 2H2O(g)}...
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1answer
42 views

Are these calculations valid enough? [on hold]

I am not able to understand how these calculations are being done in this book. Up until this section, the book is going smoothly. However, how are these calculations accountable? If anyone can ...
2
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1answer
48 views

Proving a relation for fugacity of component i in term of independent variables (T,V)

In Molecular Thermodynamics of fluid phase equilibria by Prausnitz et al. [1] the authors recommend to use the following equation which gives the fugacity of component $i$ in terms of independent ...
7
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2answers
158 views

Cp vs Cv for liquid water @ < 4 °C

I'm looking at a textbook question (Engel & Reid, Thermodynamics, Statistical Thermodynamics & Kinetics, 4 ed, Q3.1) which states the following: The heat capacity $C_{\mathrm m,p}$ is less ...
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0answers
27 views

Equilibrium concentration of point defects in solids at sufficiently elevated temperature

Equilibrium concentration of point defects in solids is defined as $\chi=\exp(\frac{\Delta s}{k_B})\exp(\frac{-\Delta h}{k_BT})$. At high temperatures, may be even higher than melting temperature of ...
2
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2answers
41 views

Which substances have the highest/lowest enthalpy of formation per gram?

Which substances have the most extreme enthalpies of formation per gram at standard temperature and pressure?
10
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1answer
259 views

Why are isotopically pure diamonds 50% more thermally conductive than other diamonds?

A laboratory grown type-IIa diamond (no nitrogen defects) has a lambda of $\pu{1800-2200 W/mK}$, but an isotopically pure diamond of $\ce{^{12}C}$ can have up to $\pu{3320 W/mK}$. Why are $\ce{^{12}C}...
2
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1answer
63 views

Gibbs Free Energy and Maximum Work

I am a high school student and my professor mentioned that the Gibbs free energy is the maximum amount of work (or useful work) that a system can do, whereas entropy is a measure of the non-available ...
26
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2answers
6k views

When a candle burns, why does the top of wick glow if bottom of flame is hottest?

Totally an elementary question. Staring at a candle, it appears that the bottom of the wick is dark whereas the top glows. However the bottom of the flame (the blue) is the hottest. Is the reason ...
0
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1answer
466 views

Calculating Heat Release from Bomb calorimeter

When $0.400\ \mathrm g$ $\ce{CH4}$ is burned in excess oxygen in a bomb calorimeter that has a heat capacity of $3245\ \mathrm{J/^\circ C}$ a temperature increase of $6.795\ \mathrm{^\circ C}$ is ...
10
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1answer
308 views

Explanation of the strength of phosphorus-oxygen bond

When discussing the Wittig reaction, Clayden's Organic Chemistry cites the strength of the $\ce{P=O}$ formed in triphenylphosphine oxide as a driver of the reaction through enthalpy: The $\ce{P=O}$ ...
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2answers
257 views

Why is my conclusion inconsistent with the van't Hoff equation?

Let's say I hypothesize that a graph of $\ln K$ vs. $1/T$ has a slope of $-∆G^\circ/R$ and a $y$-intercept of $0$. I prove it simply: $$∆G^\circ = -RT\ln K \quad\to\quad \ln K = -\frac{∆G^\circ}{RT}$$...
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2answers
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When are Kp and Kc values equal?

The picture shows the correct answers to this question. I'm not sure why these answers are correct. $K_p$ only counts with gases, while $K_c$ only counts with aqueous solution + gases. I assumed that ...
14
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3answers
2k views

Is it possible to boil a liquid by just mixing many immiscible liquids together?

In open air, when vapour pressure reaches 1 atm, boiling takes place. I read that if we add two immiscible liquids together, the total vapour pressure of the 'mixture' is close to $p = p^*_A + p^*_b$,...
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0answers
19 views

Hydrogen peroxide and carbon as hybrid rocket propellant?

I was thinking that a hybrid rocket could be made using hydrogen peroxide and activated charcoal or some other carbon source. Step 1: Decompose hydrogen peroxide to steam and oxygen, as in a ...
4
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2answers
1k views

Chemical Equilibrium - Why does changing the temperature shift the equilibrium?

I know that raising the temperature in a reversible chemical reaction causes the equilibrium to shift to the endothermic side. I know that $\Delta G = \Delta H − T\Delta S $ but I don't know how to ...
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4answers
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How to calculate the temperature at which a reaction becomes spontaneous?

At $25~^\circ\mathrm{C}$, $298~\mathrm{K}$, the reduction of copper(I) oxide, $ \Delta H = 58.1~\mathrm{kJ}$, $\Delta S = 165~\mathrm{J/K}$, is nonspontaneous, $ \Delta G = 8.9~\mathrm{kJ}$. ...
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1answer
68 views

Extrapolating from a calorimetry lab to find a new delta T with different volumes and grams of substance

So, we just finished an AP chem lab where the question was basically: Given the results of the lab were that $\pu{5 grams}$ of $\ce{CaCl2}$ and $\pu{45ml}$ of water produced a $\pu{13.33 ^\circ C}$...
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1answer
34 views

Calorimetry: Temperature Increase due to stirring of reactants

I am not yet in university, but I hear a common first-year chemistry lab/practical is to do calorimetry taking into consideration the small (and usually negligible) rise in temperature due to the ...
2
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1answer
31 views

Change in Gibbs free energy vs. change in enthalpy

When learning about other thermodynamic quantities, like $\Delta H$, I was taught that this represents the energy absorbed or released by a system after the reaction has gone 100% to completion. For ...
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1answer
319 views

Devitrification vs glass transition temperature

This graph comes from a so-called "differential thermal analysis". It is studying the effects of temperature on glass. I don't understand what the difference between "glass transition" and "...
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0answers
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Salt dissolves while absorbing heat, but is already soluble at room temp. Why? [closed]

Solid lithium carbonate dissolves in water with the absorption of heat. Nevertheless, the salt is quite soluble even at room temperature. How would I explain these occurrences using thermodynamic ...
0
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1answer
50 views

Current in a Lithium Ion Battery

I am trying to find the current transferred to the anode and cathode of a Lithium Ion Battery at different voltages. I know the temperature (approximately) of the battery, the battery's starting ...
6
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1answer
164 views

What is the Gibbs energy of dissolving a solid in pure water?

I am calculating the Gibbs energy of dissolving one mole of solid glucose in pure water such that the final solution has a volume of one liter and a concentration of one mole per liter. I have three ...
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0answers
32 views

Behaviours of endothermic reactions in an insulated isolated system

Are Endothermic reactions which occur in an insulated isolated system unable to give off heat to surroundings, if so is that because it is in a closed system, also why does the final temperature drop?...
2
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3answers
479 views

What happens to a greenhouse gas after it absorbs infrared radiation?

We say that a greenhouse gas increases the temperature of the surface of the earth but how does it do that ? Only by re emitting the absorbed IR towards the earth ?
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1answer
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What is the temperature of heat generated from LPG gas?

I'm working on a project and I require around 300-400 degree Celsius of temperature. As of now I'm taking the normal LPG gas that is available in our house for cooking purposes for my project, but I'm ...
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1answer
45 views

Enthalpy change of gas expansion against constant external pressure

A certain mass of gas is expanded from $\pu{1 L}$ at $\pu{10 atm}$ to $\pu{4L}$ at $\pu{5 atm}$ against a constant external pressure of $\pu{1 atm}$. If initial temperature of gas is $\pu{300 K}$ and ...
3
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3answers
4k views

Gibbs Free Energy

My teacher said that The decrease in Gibbs free energy is the maximum non-mechanical work that can be obtained when the process occurs reversibly at constant temperature and pressure But for a ...
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0answers
128 views

What is the ideal temperature and pressure to crack water?

Using sun mirrors and solar cells for electricity. What temperature, pressure and voltage ideal for cracking water. Source: NPR Source Would it take more or less electricity to crack water under ...
5
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1answer
847 views

Calculating internal energy of methane

I've recently found out my calculated value of internal energy of methane largely deviates from the ab initio output (at $\pu{1000 K}$ $0.058~\text{Hartree} = \pu{152.3 kJ mol-1}$). I used HF method, ...
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3answers
143 views

Gibbs energy and Entropy

In Gibbs energy equation: $$\Delta G=\Delta H-T\,\Delta S$$ At constant pressure, $$Q=\Delta H\tag{1}$$ and we know that $T \Delta S=Q$ (reversible) so finally $$T\,\Delta S=Q\tag 2$$ Putting ...
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0answers
5k views

dU/dV in van der Waals equation of state derivation

Previous, I proved that $$\frac{T_2}{T_1}^{C_V/(C_p-C_V)}=\frac{V_1}{V_2}$$ I've been asked to derive a similar expression for a gas that obeys the equation of state $$p=\frac{RT}{(\overline{V}-b)}$...
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2answers
24 views

Thermal conductivity and enthalphy data for nitrogen gas

I am struggling to find the data for thermal conductivity and ethalphy of nitrogen gas at reduced pressure (500 mbar). Most of the information is for nitrogen gas at 1 bar or above. Does anyone know ...
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2answers
51 views

Is heating water endothermic?

So I got a problem recently which asked to calculate the amount of energy needed to increase the temperature of 200 g of water from 26 to 51 degrees Celsius. The value I got was 20900 J. So far I ...
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0answers
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Why a Differential Scanning Calorimeter (DSC), to measure thermodynamic parameters, has problems?

I want to use a Differential Scanning Calorimeter (DSC) to determine when my protein unfolds as you increase the temperature in a DSC machine. The machine is pressurised (to 3A) during the run, so ...
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2answers
64 views

Is Gibbs energy change during osmosis positive? If so then why is it spontaneous?

For ideal solution, μA=μA* +RT lnxA ln function is an increasing function, means that the higher the xA the higher the μA......(1) Now consider osmosis. From Atkins' Physical Chemistry Ninth ...
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0answers
26 views

Calculate chemical potentials to find electrode Fermi level

I'm running some simple simulations of the semiconductor-electrolyte interface and I'm having a hard time figuring out the equilibrium condition. Can someone please help me with my electrochemical ...
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0answers
19 views

Magnesium burning in carbon dioxide

Why does $\ce{Mg}$ keep burning in $\ce{CO_2}$ and why does the fire generated by the exothermic production of $\ce{MgO}$ not get extinguished?
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2answers
33 views

Why enthalpy of adsorption is negative?

I have already read Is adsorption exothermic, and if so, why? but it didn't answered my query. To illustrate, let's take a hypothetical vacuum space. In that space, there is a solid and a gas, the ...
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2answers
40 views

For Hess' law type questions, when do you take the inverse of the constant and when do you switch the sign?

For example, when you're given the enthalpy $\Delta H$ of a reaction and you want to switch that reaction around in order to add it to something else, you switch the sign. Furthermore, when adding ...
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2answers
2k views

Is the Haber Process here proceeding at positive Gibbs free energy change?

$\ce{3H2 + N2 -> 2NH3}$ is the forward reaction used in the Haber process, the industrial production of ammonia. After studying Gibbs energy and how it is just another way of saying that the 2nd ...
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1answer
42 views

Mixture of water, methanol and dichloromethane

If I have a mixture of 25 vol% methanol in water and I mix it with an equal volume of dichloromethane (which is miscible with methanol but not water) can I roughly say that both phases (i.e. water and ...
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4answers
87 views

How does the room gets heated by leaving a fridge open?

Question: If the door of the refrigerator is kept open, the room in which the refrigerator is kept: gets cooled gets heated neither gets cooled nor gets heated gets cooled or heated ...
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1answer
473 views

Relation between enthalpy and Cv

MY ATTEMPT We already know that $\Delta U=nC_V\,\Delta T$ which is for constant volume Now we know that $\Delta H = \Delta U + p\,\Delta V$ so $\Delta H = nC_V\,\Delta T + 0$ (as at constant volume ...
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1answer
26 views

Is temperature of crystallisation same as melting point temperature for polymers?

Melting point tends to be a temperature range. Is it true that the temperature of crystallisation is the highest temperature at which the polymer melts and thus there is some distinction between ...