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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69 views

Why are enthalpies of reaction or Gibbs FE change given in kJ/mol

It makes no sense to me that these thermodynamic changes are given in kJ/mol. Reactions can have several compounds on both sides and its not clear what exactly the molar variable refers to… I've been ...
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Will all thermodynamic equations have a linear dependence with moles?

I'm taking chemical thermodynamics this semester, and currently seeing things like the Maxwell's Relations. My professor never uses explicitly the variable "n" and always works with molar quantities ...
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Conditions for spontaneity

So the two conditions for a reaction to be spontaneous are: tendency to achieve minimum energy tendency to achieve maximum randomness. Both these things are contrasting and the only way I see them ...
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condensation of tellurium on inside of ampoule?

After heating an ampoule of tellurium around its melting point in a furnace for about a day, I've noticed drops of the element on the upper inside surface of the ampoule, as if it condensed there. I'...
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1answer
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Do liquid O₂ and N₂ form ideal solutions?

I suspect they do, but could not find data on this. (I mean at temperatures near those used in the distillation of liquid air.)
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1answer
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Ideal gasses reaction under const temp and pressure

Conceptual problem: let $A + 2B \rightarrow C + 2D$ be a reaction under constant pressure (P) and temp (T), that goes all the way towards the products. A,B,C,D are ideal gases. let $\Delta H$ be the ...
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Why does N₂ react with O₂ to Form NO at high temperatures?

This also raises questions that I have about the Haber Process which produces ammonia ($\ce{NH3}$) from molecular nitrogen ($\ce{N2}$) and hydrogen ($\ce{H2}$). I have heard multiple times that bond ...
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1answer
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How to prove that entropy is a state function?

Consider the following steps: \begin{align} \mathrm{d}U &= \mathrm{d}q - p\,\mathrm{d}V\\ \mathrm{d}U &= C_V\,\mathrm{d}T\tag1\\ \end{align} Side question: Is equation $(1)$ only true for a ...
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1answer
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How to calculate the Gibbs free energy from the equilibrium constant for the decomposition of sodium bicarbonate?

Solid $\ce{NaHCO3}$ is heated to $90~^\circ\mathrm{C}$. At equilibrium the total pressure of the gases produced is $0.545~\mathrm{atm}$. Calculate $\Delta G^\circ$ at $90~^\circ\mathrm{C}$ for the ...
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1answer
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Pre heat coal before burning

So I was wondering if pre-heating coal will improve the efficiency of a steam boiler. I did find that intake air pre-heater and water pre-heater do increase efficiency, but I could not find anything ...
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How do I determine if a process is reversible without numeric information?

How do I determine if a process is reversible without numeric information? It would be nice to know how to determine whether a process is reversible in order to solve problems LIKE this one: A ...
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3answers
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Is delta H a state function?

My teacher says $H$ is a state function but $\Delta H$ is not. But I think both of them should be state function. Is he correct?
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How do things heat one another? [duplicate]

For example, how does a gas stove heat a metal pot? A combustion reaction between the gas and the air takes place, and this releases energy due to new chemical bonds being formed. Do the atoms in ...
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Which formula is correct for calculating the heat of dissolution?

I want to calculate the energy change when a solute is dissolved in water. I know that I can achieve this by using the equation $q=mc \Delta T$. My question is does the mass ($m$) change with the ...
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Why is the volume in this compress gas cylinder constant throughout?

I am stuck on this gas question and its really stressing me out. A compressed cylinder of gas contains $2.25 \cdot 10^3~\mathrm{g}$ of $\ce{N2}$ gas at a pressure of $4.25 \cdot 10^7~\mathrm{Pa}$...
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1answer
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Entropy change of system in adiabatic irreversible process [duplicate]

In an adiabatic irreversible process we know that q = 0. Now change in entropy is defined as q/T. So when q is zero, why is change in entropy not zero ?
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How are pressure vessels heated?

I'm working on a project in which I'll be dealing with pressure vessels. As of now I've drawn/conceptualized everything on theory basis. The diagrams are made on the basis of theory, but then I came ...
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3answers
608 views

Kinetics vs Thermodynamics

The way kinetics is taught at the undergraduate level (Arrhenius and collision theory) chemical equilibrium is determined governed immensely by activation energy of the reaction. According to ...
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1answer
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What happens to energy changes in non-reversible processes?

Take internal energy for example: $$dU\leq TdS-PdV$$ Firstly, I would like to make clear that the following question/argument relies on the following statement being correct: the left hand side of the ...
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1answer
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Is there a difference between Joule coefficient and Joule-Thomson coefficient?

Is there a difference between the Joule coefficient and the Joule-Thomson coefficient? Also, I am having a hard time understanding what they mean.
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1answer
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LiAlH4 / H2O reaction energy

I'm dealing with a free-energy scam on another forum. It is well-known that $\ce{LiAlH4}$ reacts explosively with water. My chemistry days are long behind me, so I thought I'd come here to ask: $1~\...
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2answers
361 views

Gibbs free energy or reaction enthalphy

If I conduct a reaction say, $$\ce{CaO(s) + H2O(l) \rightarrow Ca(OH)2 (aq)}$$ let us say , I want to measure the energy generated from the reaction.Will this energy I calculate equal the change in ...
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What's the name of the problem of finding equilibrium concentrations?

Is there any specific name of the problem of finding equilibrium concentrations with known equilibrium constants and initial concentrations? Would it be appropriate to call it "direct equilibrium ...
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Magnetic stirrer with hot plate

I did not know there are products such as a magnetic stirrer with a hot plate, enabling one to apply heat and stirring to a solution with relative ease. Did I understand correctly, that device needs a ...
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1answer
413 views

Reversible thermodynamic machine

Assume two identical solid blocks of copper with molar heat capacity at constant volume $C_{V,m}=24.5 \text{ J$\cdot$K$^{-1}$mol$^{-1}$}$ . Assume the heat capacity to be constant in the ...
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Temperature of an atom

I read somewhere that the temperature of an atom is not defined. The definition of temperature is only for larger systems. Why is this so?
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2answers
373 views

If dS ≥ dq/T for a spontaneous change is dS ≤ dq/T for a non-spontaneous change and is that even possible?

I am struggling with the concept of entropy but might be on the verge of understanding it. The Clausius inequality, $$\mathrm{d}S \geq \mathrm{d}q/T,$$ is true for all spontaneous changes with the ...
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1answer
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Why does the Born equation give the Gibbs free energy of solvation rather than enthalpy of solvation?

The Born equation gives the difference in energy required to charge a particle in a vacuum and in solution which results in the work required to transfer an ion from a vacuum into solution. It is ...
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1answer
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If the standard state symbol means that the substance is pure (and at 1 bar) how is it possible to have a standard REACTION enthalpy?

For the reaction one mole of substance $A$ in equilibrium with one mole of substance $B$ the standard reaction enthalpy is defined: $$\triangle_rH^{\theta}(T)=H_m^{\theta}(B;T)-H_m^{\theta}(A;T)$$ ...
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1answer
366 views

How does the chemical potential allow for open systems to be considered?

Gibbs free energy can be defined as: $$dG=VdP-SdT+\sum_i\mu_idn_i$$ where $\mu=(\frac{\partial G}{\partial n})_{P,T}$. Last term, $\sum_i\mu_idn_i$ allows for open systems to be considered (where $dn$ ...
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Can state of matter from liquid to gas maintain the same molecular structure in PE or in polymers in general, like water does?

If I’ve got a reactor containing PE-Polyethylene $(\ce{{C2}$H_\text{4})n$}$ (solid) and the rest is filled with nitrogen gas. The system is closed. Supposing we have an initial temperature of $30~\...
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Thermochemistry problems?

Question 1: A compound A reacts according to the following hypothetical equation and has a molecular weight of 48.36 g/mol. 3 A (s) +B (aq) ––> 2 C (aq) ∆H° = ? A sample of A, weighing 0.152 g ...
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1answer
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Does the density of a liquid and gas phase of a substance(nitrogen or water) converge when reaching critical point

I know that the there is no difference in density in a liquid and a gas phase of a substance after the critical point. But I do want to know if this difference in density is more continuous or if it ...
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1answer
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Why does the equilibrium constant depend on the temperature but not on pressure and concentration?

According to my textbook (and intuitively) certain changes when the aforementioned 3 variables are altered occur in accordance with Le Chatelier's Principle. However, what I don't understand is what ...
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1answer
104 views

Why is a reaction more favorable if one of the products is not soluble/sparingly soluble?

I have heard people go so far as to say that the "driving force" of a reaction is the formation of the precipitate. Why is this so? Whilst the process is almost certainly exothermic (I think - please ...
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Finding the thermodynamics of protein unfolding from temperature and absorbance using fluorescence spectroscopy?

I recently did an experiment on protein unfolding using a Perkin Elmer fluorescence spectrometer. The protein was $\alpha$-chymotrypsin. I was given this paper for my procedure. I am now trying to ...
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1answer
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How to derive the pressure dependency for the Gibbs free energy?

$$\mathrm{d}G=\left(\frac {\partial G}{\partial p}\right)_T+\left(\frac {\partial G}{\partial T}\right)_p$$ The above is an exact differential. Firstly what does this mean physically? (I hear about ...
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1answer
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Phase diagrams (weird region)

On the left, there is a region for $\ce{MgO(ss)}$. I've been told such regions exist because stuff are not miscible. However, does that mean the composition axis on the bottom is a lie? Namely, ...
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What Causes there to be a Maximum Amount of Salt which can be Dissolved in Water?

When you add salt to water and stir, after a while the salt will no longer dissolve. Why is this?
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Thermodynamically Reversible Processes: How does an infinitesimal step make a process reversible and how is maximum work derived from the process?

My book says 'Maximum work can be obtained only from thermodynamically reversible processes,' but why is it so? What is the cause? Actually to me the definition of reversible process is confusing. ...
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1answer
407 views

calculating Kp with partial pressures

This is a question I solved in class: At 35 degrees, $\ce{N_2O_4}$ is 15% dissociated in a flask at a pressure of 1.2 atm. Calculate $K_p$ Equation: $$\ce{N_2O_4(g)<=>2NO_2 (g)}$$ This ...
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Derivation of the relation between temperature and pressure for an irreversible adiabatic expansion

I have a formula for finding the final temperature in an irreversible adiabatic expansion. $$T_2=\left[\frac{C_v+(\frac{P_2}{P_1})}{C_p}\right]T_1$$ where $T_1$ is the initial temperature. $C_v$ and $...
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What is the appropriate way of writing the Iron/thiocyanate equilibrium?

I understand the theory, but am struggling to answer the following discussion questions: (i) Why isn't this represented by: $\ce{Fe^{3+} (aq) + 3 SCN- (aq) <=> Fe(SCN)3 (aq)}$ Why do we dilute ...
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1answer
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Transfer of energy

After completing a lab for my chemistry class I am finding it difficult to find the initial temperature of an iron nail after being heated. Procedure of the lab went as follows: heat an iron nail, ...
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1answer
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How will liquid nitrogen affect a steel instrument's ability to function?

I want to retrieve cryotubes from the bottom of a liquid nitrogen freezer without taking all the freezer racks out and draining the liquid nitrogen. I think I can accomplish this using a tool like ...
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If the equilibrium constant in a reaction is greater than 1 is it possible that there are more moles of reactants than products?

I'm wondering if this is possible because could there not be a situation in which the reactants were pure solids/liquids and would therefore not contribute to the value of k but still contain more ...
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Thermodynamics: Is Isolated system possible?

I have just started learning thermodynamics. I have learned that The system which is not able to exchange energy and matter on its own with surrounding is called isolated system. I wanted to know ...
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Bomb calorimeter and heat capacity of calorimeter

Can anyone describe the process through which we determine the heat capacity of calorimeter, for me? The formula is $q_{calorimeter} = C_{calorimeter} \times \Delta T$ The heat of bomb calorimeter ...
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Concept of heat of reaction in a isolated system

One description of the concept is that heat of reaction (exothermic) is the quantity of heat that would given off to the surrounding by the product of a reaction if we allow the product to cool down. ...