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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8 views

Does temperature change if change in internal energy is zero?

I needed to find ΔU for the combustion of methane in a sealed rigid adiabatic container. In the solution given : as q = 0 ; w = 0 ,So ΔU = 0 (by FLOT). First Doubt:- that heat is released during ...
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12 views

Effect of specific heat capacity on heat loss

Assume we want to calculate the heat capacities of two substances with different heat capacity. We heat them both up with a immersion heater delivering a constant rate of energy (constant J/sec), and ...
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Calculating temperature of partial condenser, flash calculations

The iterative method I have learned to make flash calculations for a binary system where I know the inlet feed (F [mol/s]), total composition of the two components ($z_1$ and $z_2$), the amount of the ...
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11 views

How to calculate the unknown concentration with delta H and delta S? [closed]

3-ethyl-4-methylhexa-1,3-diene undergoes a combustion reaction. For this reaction, the following concentrations were used: [O2] = 0.06 M, [CO2] = 0.09 M and [H2O] = 0.10 M. Determine the concentration ...
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2answers
32 views

Isobaric entropy relation from isothermal Gibbs free energy relation

I have been given a problem to derive a relation for change in entropy from change in Gibbs free energy: Starting with the isothermal equation $$G(p_2) = G(p_1) + nRT\ln\frac{p_2}{p_1}\label{eqn:1}\...
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1answer
82 views

How is Gibbs free energy related to stability?

I have been learning thermodynamics for quite some time and I fail to relate Gibbs free energy and stability. For example, how do I compare stability of metal oxides using the following data? Also, do ...
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0answers
34 views

Is it possible to solve for the equilibrium value given only the initial amounts of reactants? [closed]

At $\pu{90 °C}$ following equilibrium is established: $$\ce{H2(g) + S(s) <=> H2S(g)}$$ If $\pu{0.20 mol}$ of hydrogen and $\pu{0.1 mol}$ of sulfur are heated to $\pu{90 °C}$ in a $\pu{1.0 L}$ ...
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47 views

What is the effect on Kp on doubling the volume?

Following reaction takes place at $\pu{127 ^\circ C}$ and $\pu{5 L}$ vessel, $$\ce{SO2Cl2(g) <=> SO2(g) + Cl2(g)} \qquad K_p = \pu{1 atm}$$ (a) If initially only $\ce{SO2Cl2(g)}$ was present at $...
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21 views

Standard heat of reaction [closed]

Nitrous acid ($\ce{HNO2}$)​ is used to solidify olive oil for shipping or for making margarine. It is highly unstable. What is the standard heat of reaction, or $\ce{ΔH}$, in $\pu{kJ/mole}$, for the ...
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25 views

How much energy is transferred [closed]

Aluminum has a specific heat capacity of $\pu{0.90 J g-1 ^\circ C-1}$ and water has a specific heat capacity of $\pu{4.184 J g-1 ^\circ C-1}$. How much energy is transferred from a piece of hot ...
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1answer
27 views

Derivation of Enthalpy Equation

Why are they taking delta H in equation (iv) to be at constant pressure while in the same equation they are considering delta E to be at constant volume to arrive at equation (v)? If they are arriving ...
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1answer
31 views

Can endergonic reactions be driven by heat?

I am having trouble conceptualizing endergonic reactions and their difference from endothermic reactions. An endergonic reaction is one that has a positive Gibbs free energy change, such that energy ...
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2answers
35 views

what is the change in enthalpy during constant volume

I have been studying thermodynamics for quite a time now, and recently learned about enthalpy but I have been facing a lot of problems So,which is the correct equation of enthalpy $$\Delta H=\Delta U+\...
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0answers
22 views

How to learn the concept behind Thermodynamics, Equilibrium, Electrochemistry, Chemical Kinetics [closed]

I am currently reading Thermodynamics, Chemical Equilibrium, Ionic Equilibrium, Electrochemistry, Chemical Kinetics for JEE Advanced exam in India. Now when I am solving problems I can not inter ...
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25 views

Why do we use nCv∆T=∆U almost universally(for ideal gases)

In my understanding $C_V$ which is defined when the volume is constant is written as $$C_V=\frac{q_V}{\Delta T}$$ Thus, $$q_V={nC_V\,\Delta T}$$ since for an isochoric process work done is zero $$\...
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2answers
107 views

What is the definition of thermodynamic stability?

The term thermodynamic stability is used on this site, but I can't find a good definition. Is is a quantitative or a qualitative concept? Does it apply to a single compound, or a pair of compounds? ...
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1answer
46 views

Equilibrium for the synthesis of methanol

Liquid methanol is obtained with carbon monoxide gas and hydrogen gas in a reactor at $\pu{300 °C}$ and $\pu{250 atm}.$ $K_p = \pu{9.28E-3}.$ Find $K_c.$ With only these data, do I have to use the ...
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11 views

Quasi Static Process and Reversibility of Engines

So I understand that In a quasi static process, the temperature of the system is changed ever so slightly and slowly until it reaches its final temperature,so that at each stage of temperature change, ...
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0answers
67 views

Why is U the symbol for lattice energy?

I often see $U$ or even $\Delta U$ being the symbol for lattice energy. Why is that? Does it refer to internal energy?
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1answer
122 views

Choice of lower bound of curve for PV work

I am told that the area under the PV curve gives the magnitude of work done by a gas. Let $V_i$ be the initial volume, $V_f$ be the final volume, and the pressure as a function of volume be given by $...
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0answers
33 views

Entropy of free expansion and isothermal process

Let me tell what I understand of the concepts on which the question is based first. Heat is a flow of energy that takes place due to a temperature gradient. When doing reversible isothermal expansion ...
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1answer
86 views

Total entropy change for a spontaneous reaction

For the oxidation of iron, $$\ce{4 Fe(s) + 3 H2O(g) -> 2 Fe2O3(s) + 3 H2(g)}$$ the entropy change is $ \pu{–549.4 J K^{-1} mol^{-1}}$ at $\pu{298 K}.$ In spite of the negative entropy change of ...
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2answers
52 views

How can change in entropy be the same for all processes if the entropy production $\sigma$ is present for irreversible processes?

From the definition of entropy change, $$S_2-S_1=\left ( \int_{1}^{2} \frac{\delta Q}{T}\right )_{int.rev}$$ From the closed system entropy balance, we have $$S_2-S_1=\left ( \int_{1}^{2} \frac{\delta ...
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1answer
47 views

Compound with pressure sensitive melting point at moderate temperature and pressure

I'm in need for a compound with ideally all of the following properties: A melting point of 5 °C or less at 1 atmosphere (or slightly lower pressure) A melting point of 12 °C or more at less than 20 ...
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26 views

Why does chemisorption decrease with increasing temperatures?

Why is it that chemisorption decreases with higher temperatures? The explanations I found online were quite vague and one of them cited Le Chatelier's principle, but I thought chemisorption is an ...
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2answers
63 views

Is it valid to calculate concentrations given equilibrium constant and initial concentration?

Suppose we have the following reaction: $$\ce{2A <=> B + C}\tag{1}$$ which can be thought the sum of the following reactions: $$\ce{A <=> B}\tag{2}$$ $$\ce{A <=> C}\tag{3}$$ with ...
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3answers
67 views

What is non toxic to fish, has a low freezing point, and floats on water?

I am looking for a liquid that floats on water that will prevent the water from freezing (or at least slow it down). The application would be a small area (less than a square foot). It has to be non-...
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0answers
13 views

Is standard change in activation Gibbs free energy indepedent of pressure?

For a chemical reaction the activation volume is defined as: $$Δ^{\ddagger}V={V^\ddagger}-V$$ where both $V^\ddagger$ and $V$ refer to standard states (I have omitted the symbol for simplicity). One ...
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1answer
350 views

Can someone clear this confusion I have about the first law of thermodynamics

In our school we were taught that the first law of thermodynamics represented mathematically is $\Delta U=Q+W$. Also we were taught that for an isothermal process $\Delta U=0$ and therefore $Q=-W$, ...
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25 views

Are all exergonic reactions spontaneous?

Exergonic reactions have a negative $\Delta G$: the system loses free energy. Spontaneous reactions are also defined in the same way, as far as I know. Does this mean the two are synonymous? There are ...
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adiabatic reversible process problem

A compressor recieved air at To= 300 K and at pressure Po= 1 atm. At the exit, P1 = 2 atm. The process inside the compressor is (A) adiabatic and reversible (B) gas is an ideal gas (3) gamma = Cp/Cv = ...
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1answer
74 views

What is the general phenomenon taking place here?

For many cases while calculating the activity of a particular species I find them calculating like this: $$a=\frac{p_{Mn}}{p^{\circ}_{Mn}}$$ What I really do not understand is what is evaporating into ...
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3answers
863 views

Why is the sum of two inexact differentials exact?

From the first law of thermodynamics, $\mathrm{d}U = đQ + đW$, where $\mathrm{d}$ represents an exact differential and $đ$ an inexact differential. Exact differentials correspond to state functions ...
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26 views

Relation between Entropy, Resonance and stability

Why is it so that the continuous Delocalisation of electrons in a molecule tends to lower the total internal energy of the molecule? I understand that there is a distribution of energy in multiple ...
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1answer
48 views

Confusion between various formulas of Gibbs free energy [closed]

According to the assumptions of Constant P and T and no non mechanical work, we get ΔG=ΔH-TΔS But there is another form of the Gibbs free energy equation which takes Reversible process and no non ...
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34 views

Elementary derivation relating Gibbs energy with equilibrium constant

Is there an elementary derivation of $\Delta G = \Delta G° + RT \ln K$ where pressure and temperature are assumed to be constant (typical biological conditions; particularly for aqueous reactions)? I ...
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1answer
92 views

Can G, H, and S be negative?

Consider Gibb's free energy of a reaction in a closed system $G = H - TS$. Can $G$ ever be negative? (note, I am not talking about $\Delta G$) I would suspect no because there is always some energy ...
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1answer
149 views

Can vapour pressure be used to 'generate' work for free?

Given a bottle of water, a closed system, some of the water molecules in the liquid phase will have enough energy to escape that phase, forming water vapour, contributing to vapour pressure. Now ...
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1answer
54 views

Does the enthalpy of formation include translational and rotational energy?

I can look up the enthalpy of formation of water vapor and get -241,826 KJ/mol. I want to use this to compute the total energy of water vapor at STP from the energies of O2 and H2 gas. What I don't ...
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1answer
52 views

New relationship between exponential factors and entropy of a reaction?

Consider the reaction $\ce{A <=> B}$. Let $k_f$, $E_A$, and $A$ denote the forwards rate constant, activation energy, and pre exponential factor, respectively. Let $k_r$, $E_A'$, and $A'$ denote ...
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1answer
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Why is the standard enthalpy of formation of aluminium oxide higher than magnesium oxide?

$$\begin{align} &\text{Aluminium oxide:} &\ce{4Al + 3O_2 &-> 2Al2O3} &(\Delta H_\mathrm f &= \pu{-1675 kJ/mol})\\[1.5em] &\text{Magnesium oxide:} &\ce{2Mg + O_2 &-&...
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1answer
47 views

Le Chateliers principle and increase in temperature [duplicate]

If you increase the temperature, the endothermic reaction is favored. The Le Chatelier's principle states that the endothermic reaction is favored in order to minimize the effect of an increase in ...
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36 views

Do external actions affect critical temperature?

Critical temperature $T_\mathrm{c}$ is the temperature when properties of both gas and liquid converge to the same values and the difference between the phases disappear: $$T_\mathrm{c} = \frac{8a}{...
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1answer
61 views

Change in enthalpy and entropy when sugar dissolves in water

We know that dissolution of sugar in water is a spontaneous process. So, change in Gibbs free energy $(ΔG)$ must be negative for the overall process: $$ΔG = ΔH - TΔS < 0$$ Hence either the enthalpy ...
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52 views

Enthalpy of Neutralisation for HF and strong base

Now I know that when a strong acid and strong base neutralise each other, enthalpy change is almost constant and equal to -57.3 kJ/mol. (This is because no heat is required to ionise the acid and base ...
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1answer
89 views

Any examples of reactions where simpler molecules are combined into more complex ones and are endothermic at the same time? [closed]

Can you name examples of reactions that are endothermic and simple molecules are combined into more complex ones at the same time? Like what plants do they take heat and sunlight and they synthetize ...
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36 views

Why does a wooden spoon creates bubbles when put in hot oil?

This question might be a bit weird, but I just asked myself why a wooden cooking spoon creates bubbles when put in oil at about 170°C. This is a common trick to find out, if Oil has the right ...
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3answers
485 views

What does heterogeneous mean?

We say that a homogeneous sample consists of the chemically same type of molecules whereas heterogeneous ones vary chemically, i.e. the molecules throughout are not same and also not uniform. Why do ...
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2answers
78 views

Ellingham Diagram for carbon [closed]

Consider the following reactions: $$\begin{align}\ce{C + O2&->CO2}\tag{1}\\[1em] \ce{2C + O2&->2CO}\tag{2}\\[1em] \ce{2CO + O2&->2CO2}\tag{3}\\[1.5em] [(2)+(3)]/2 &= (1)\end{...
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3answers
61 views

Specific Heats for Non-Constant Processes

Why can the specific heat at constant Pressure ($C_p$) be used to calculate the change in enthalpy as $\text{d}H = C_p\text{d}T\newcommand{\d}{\text{d}}$ for processes that are not even at constant ...

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