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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49
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3answers
4k views

Unit of the equilibrium constant: contradiction of Bridgman's theorem?

The following equation is standard in thermodynamics: $$ \Delta G^\circ=-RT\log(K) $$ where $K$ is the equilibrium constant. In dimensional analysis, Bridgman's theorem tells us that the argument ...
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What is the difference between ∆G and ∆G°?

In Brady's Molecular Nature of Matter, I read that $\Delta_\mathrm{r} G^{\circ}$ is $\Delta_\mathrm{r} G$ at $25~^\circ\mathrm{C}$. But later, it gives a value for $\Delta_\mathrm{r} G^\circ$ at ...
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Why does the reaction quotient use the products (multiplications) of reactants and products, rather than their respective sums?

As you may know, the reaction quotient $Q_c$ is defined by the equation $$ Q_c = \frac{[C]^\gamma [D]^\delta}{[A]^\alpha [B]^\beta} $$ for the chemical reaction $$ \alpha A + \beta B \rightarrow \...
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Le Châtelier's Principle and heat

Consider the following reaction at equilibrium. $$\ce{A->B}, \Delta H < 0 $$ Suppose I increase the temperature. Now, quite a few people would invoke Le Châtelier's Principle and say that since ...
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Why do salts such as NaCl dissolve?

If we look at solubility of salts in water, we are told that they disassociate, because the positive $\ce{Na}$ ion is attracted to the partially negative oxygen in water and the negative $\ce{Cl}$ is ...
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3answers
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Can the change in internal energy be nonzero if temperature is constant?

I have learnt that internal energy, $U$, is a state function and it only depends on temperature... So if $\Delta T = 0$ then $\Delta U = 0$. However when I was studying exothermic and endothermic ...
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4answers
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Reason for inability to find the absolute value of internal energy

Today in my thermodynamics lecture, my teacher told me that it is not possible to find the absolute value of internal energy and so we have to calculate the change in internal energy. So, my question ...
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Is there a reason for the mathematical form of the equilibrium constant? [duplicate]

Why are the two molarities multiplied and not added, and why is each raised to the power of the coefficient rather than multiplied by it? What is the reasoning behind this form? Was it simply ...
21
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Why must both the critical temperature and pressure be exceeded to achieve the supercritical phase?

The characteristics and properties of supercritical fluids and the associated vapor-liquid critical point are well established. One thing that I've always been curious about, though: what is the ...
110
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1answer
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Is there a general consensus on the causes of the alpha-effect?

There have been various explanations posited for the α-effect. The α-effect refers to a phenomenon wherein nucleophiles with lone pairs on atoms adjacent (i.e., in the α- position) to the atom bearing ...
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Why do chemistry and physics have different sign convention in thermodynamics?

In physics class, we write the first law of thermodynamics as $\mathrm dU =\mathrm dQ - \mathrm dW$ and in the physical chemistry class, we write the same law as $\mathrm dU =\mathrm dQ + \mathrm dW$. ...
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Hypothetical: What happens to water as pressure increases to infinity? [duplicate]

I've asked a similar question here but the answer given shows the behaviour of water under general conditions. I'd like to know what the behaviour of water is like as pressures increase towards ...
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1answer
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Why can't helium be solidified at 'ordinary' pressures?

According to the UC Davis ChemWiki Chemistry of Helium, helium has a comparatively unusual property, specifically: Helium is the only element that cannot be solidified by lowering the temperature ...
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2answers
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What is wrong in this argument that dG must always be zero?

Under constant temperature and pressure, the change of the Gibbs free energy can be written as $$\mathrm dG_\textrm{sys} = \mathrm dH_\mathrm{sys} - T\,\mathrm dS_\mathrm{sys}$$ And in the textbook ...
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How can decreasing in temperature indicate an endothermic reaction?

I’m really wondering how can decreasing the temperature refer to an endothermic reaction. I faced this when I was calculating the heat of a reaction Q in a constant-pressure calorimeter. What I ...
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Difference between thermodynamic and kinetic stability

What is the difference between thermodynamic and kinetic stability? I'd like a basic explanation, but not too simple. For example, methane does not burn until lit -- why?
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Why is dU an exact differential and dq an inexact differential?

dU, dG, dH etc are all exact differentials and the variables themselves are known as state functions because they only depend on the state of the system. However, dq and dw for example, are inexact ...
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What is known about liquid carbon?

The wiki tells me that if you heat carbon at atmospheric pressure it eventually turns directly into a gas without being liquid first. At what pressure can you make liquid carbon? Has anyone actually ...
9
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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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3answers
826 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 ...
20
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2answers
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Formula of entropy change

In my 12th standard book, the formula for entropy change is given as $\Delta S = \frac{q_\text{reversible}}{T}$. What is the importance of absorbing heat reversibly and not irreversibly? What does ...
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4answers
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Spontaneous/Non-Spontaneous Reactions and Reversible Reactions

My chemistry textbook states "Reversible reactions constitute a limiting case between spontaneous and non-spontaneous processes." Does this mean that some of the reversible reactions are spontaneous ...
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1answer
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Derivation of relationship between equilibrium constant and Gibbs free energy change

Why is $\Delta G=\Delta G^o+RT\ln Q?$ It feels like all online sources were written for introductory Chemistry students! Where do I find a rigorous proof of this identity? Greatly appreciate it!
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Why does equilibrium exist?

I've read an article that equilibrium exists when $G$ (Gibbs free energy) becomes minimum. If formation of products decreases Gibbs free energy, why won't the reaction proceed to completion so that ...
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1answer
314 views

How is heat physically released in an exothermic process?

Many non-covalent interactions (e.g. electrostatic interactions) result in decreased entropy and are driven by negative enthalpy. If the heat released in the reaction has its physical basis in an ...
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Difference between exothermic and exergonic

In High School I learned that an exothermic reactions releases energy, while an endothermic reaction needs energy to occur. Now I learned that there is a separate, somewhat similar classification ...
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Why does water evaporate spontaneously at room temperature despite ΔG > 0?

Standard Gibbs free energy of formation of liquid water at $\pu{298 K}$ is $\pu{−237.17 kJ mol-1}$ and that of water vapour is $\pu{−228.57 kJ mol-1}$ therefore, $$\ce{H2O (l) -> H2O (g)}\qquad\...
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1answer
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Why does the Gibbs free energy only correspond to non-expansion work?

It has been defined as the energy available for work other than expansion work. Why can't it be used for expansion work
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Why do impurities lower the melting point of an isolated substance?

It is known that impurities in a desired isolated product lower the melting point of the mixture, even if the impurities' melting point is much higher than the desired product. Why is that so?
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1answer
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Why is NaCl3 possible?

There. And there. Almost a year ago, a group of scientists claimed to have reached compounds of $\ce{Na}$ and $\ce{Cl}$ with weird stochiometries ($\ce{NaCl3, Na3Cl, NaCl7, Na3Cl2}$ and $\ce{Na2Cl}$). ...
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3answers
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What are the chemical reactions behind fire?

I've always wondered what the chemistry behind fire is. What are the basic chemical reactions behind a simple wood fire, and how do they manifest into this phenomenon?
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Does an irreversible reaction have an equilbrium between reactants and products?

Retrospective analysis 2/13/2017 -- The barium sulfate example is a poor choice. Equilibrium equations should really be defined using activities, and the activity of solid barium sulfate is by ...
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Reversible and Irreversible adiabatic expansion

A $\mathrm{200.0\ m^3}$ balloon at $253.0\ \mathrm K$ ascends to a higher altitude. If the initial pressure inside the balloon is $325$ millibar, and it ascends to a latitude with a pressure of $7.45$ ...
6
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1answer
678 views

Le Chatelier's principle with pure solids and liquids

In the following reaction: $$\ce{A(s) <=> B(g) + C(g)}$$ since adding a pure solid is not supposed to shift the equilibrium in any way, does that mean that I can essentially remove all of the ...
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2answers
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Enthalpy (H) formula: what is the pressure volume product

$$H=U+pV$$ Above is the formula for enthalpy or the sum of the internal energy and pressure volume product. $-p\Delta V$ is work done by or on the system. But $pV$, yet very similar to work, I think,...
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Comparing formula for enthalpy change with bond dissociation energy and formation enthalpy

I learnt that given a reaction: $$\ce{A -> B}$$ the enthalpy change is given by: $$\Delta H = \left( \begin{array}{c} \text{total enthalpy of}\\ \text{bonds broken}\end{array}\right)-\left( \begin{...
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Relation between thermodynamic reversible process and reversible reaction

I know it seems to be a weird question. But for long I have been thinking whether there is any relation between thermodynamic reversible process and reversible reaction. Do they have any connection ...
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1answer
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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 ...
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1answer
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What happens to water under high pressures without possibility of escape?

Knowing very little about the nature of water, wondering how it might behave at the centre of a planet or centre of an another massive gravitational body. Could water take such pressures or might it ...
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1answer
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Vaporized sodium hydroxide?

Some time ago, I was making a $\ce{NaOH}$ solution. The solution I had made was much too concentrated, and the heat released in the dissolution of the ions caused the water to begin steaming. ...
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5answers
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Deciding the order of heat of combustion of isomeric alkanes

I'm asked which one of the alkane has the lowest heat of combustion (note: the question says heat, not enthalpy) and according to the answers, the right one is d. I'm trying to understand why d ...
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What is a pure substance?

I found as I was preparing to teach "Introduction to thermodynamics" that there are varying definitions or examples of pure substances. For instance the text book I have to use "Thermodynamics an ...
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1answer
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Phosphorus standard state

Why is white phosphorus considered to be the standard state of phosphorus although it is only metastable? Red phosphorus is thermodynamically more stable than white phosphorus, yet it is not the ...
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Why can entropy be measured in joules per kelvin?

I'm a little confused as to what the measurement associated with entropy represents. It's obviously the joules of energy per kelvin, but how does this measure the disorder of a system?
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What is the difference between ΔG and ΔrG?

Consider the reaction $$\ce{A -> B}$$ The reaction Gibbs free energy, $\Delta_\mathrm{r} G$ is given by the following equation $$\Delta_\mathrm{r} G = \Delta_\mathrm{r} G^\circ + RT \ln Q$$ Now ...
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Justification for Freezing Point Depression & Boiling Point Elevation in Solutions?

I was wondering if the following justification for freezing point depression and boiling point elevation are conceptually correct. The reason why I ask this question is because I have been self ...
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What exactly is temperature?

I've read at many places that temperature is the average kinetic energy of particles present in an object. I just don't intuitively get how kinetic energy is connected with temperature. And how is ...
11
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2answers
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What exactly is a spontaneous reaction?

What I understand by a spontaneous reaction is that - A reaction whose G is negative and which occurs without the input of energy. Gasoline reacts with oxygen only if a spark is provided. So how can ...
14
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3answers
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Transition state and free energy

We have products $\ce{A + B}$ combining to form $\ce{C + D}$ through the transition state $\ce{X}$. Are all reactions at least virtually reversible? Is the difference between a reversible reaction ...
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4answers
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Why calcium chloride is used to melt ice over sodium chloride

I'm not quite sure I understand this. My question is: Calcium chloride is a salt used widely to melt ice on sidewalks and roads. Explain why one mole of $\ce{CaCl2}$ would be more effective than ...

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