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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Work done in an isothermal irreversible process
In calculating the work done in an isothermal irreversible process, why do we consider the external pressure only (using formula: $W_{\text{ext}} = P_{\text{ext}} \mathrm{d}V)$ but not the internal ...
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Reverse rate coefficient for thermolecular and thermal dissociation reactions
I am an astrophysicist working in exoplanetary atmosphere chemistry right now, in particular modelling the chemical kinetics taking place within the atmosphere. Based on this IOP article, we have that ...
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Finding Gibbs energy at different temperatures
How can I find $∆G$ at a higher/lower temperature if I am given $∆G°$ and $∆H°$ $\pu{298 K}$, and a reaction equation (solubility)?
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Calculating the enthalpy of polymerisation of ethylene given the bond strengths
Given the average bond dissociation enthalpies of a $\ce{C-C}$ bond (say $x$) and a $\ce{C=C}$ bond (say $y$), find the enthalpy of the following polymerisation reaction (per mole of ethylene):
$$\ce{...
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Does expansion of an ideal gas not always represent an increase in entropy of the system?
Expansion of an ideal gas not always represent an increase in entropy of the system. Statement is True or False?
What I thought is that whenever an ideal gas expands the randomness of gas molecules ...
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If the human body consists of 60% water why can't we put out fire with our body? [closed]
I have often heard of people being burned at the stake, but if the body is 60% water shouldn't the fire just be put out?
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Sign of work done by the system in first law of thermodynamics equation
In the equation, $\Delta E = q - W$, according to my book, $W$ is positive when it is done by the system and negative when it is done on the system.
But in this problem:
$\pu{3600 kJ}$ heat is ...
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What does ΔG° refer to exactly?
My interpretation of this image is that $\Delta G^\circ$ is equal to the difference between $G^\circ$ of the products and $G^\circ$ of the reactants.
But, I've also read $\Delta G^\circ$ as being ...
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The sign of enthalpy of formation of magnesium oxide
I'm currently doing a lab to calculate the enthalpy of formation for $\ce{MgO}$. However at the moment me and my lab partner are having a disagreement. We've both calculated and agreed upon the same ...
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Ideal gas Law best describes the properties of which of the following gases at 0 degrees Celsius and 1 atm? [closed]
a. $\ce{PH3}$
b. $\ce{HBr}$
c. $\ce{SO2}$
d. $\ce{N2}$
I believe that it is $\ce{N2}$, but I'm not too sure, since technically all of them would act the same at STP.
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Unsteady state process for first law of thermodynamics with changing boundries [closed]
Firstly , this is not an homework question .I tried to solve this question to understand the concept.
I made the following attempt to solve this question.However I couldn't solve after exit valve is ...
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Is Kirchoffs law valid at all pressures?
$$\Large H_{T_\mathrm{f}, p}=H_{T_\mathrm{i},p}+\int_{T_\mathrm{f}}^{T_\mathrm{i}}c_p(T)dT$$
This can be derived by integrating: $$C_v =\left(\frac{\mathrm{d}U}{\mathrm{d}T}\right)_V$$
Applying this ...
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Why isn't standard Gibbs free energy always zero?
I read in my textbook that one of the standard conditions we should use when calculating the standard Gibbs free energy is having a concentration of 1 M of all the reagents and the products in the ...
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Deriving heat capacity in terms of internal energy U and natural variables S & V
My lecture notes have the following equation regarding heat capacities:
$$
C_p = C_v + T\left(\frac{\partial p}{\partial T}\right)_v\left(\frac{\partial V}{\partial T}\right)_p
$$
(just to be clear, ...
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Apparent contradiction in the type of deviation of an ethanol-water mixture
For a mixture of $\ce{EtOH}$ and $\ce{H2O}$: $\Delta H_{\text{mix}} < 0$ (exothermic) and $\Delta V_{\text{mix}} < 0$ (volume contraction). This means that the unlike interactions are stronger ...
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Why are there two contradicting formulas for Enthalpy (Delta H): [duplicate]
1) Delta H = Bonds broken - Bonds Formed
2) Delta H = $H_{products} - H_{reactants}$
In the first formula, bonds break in the reactants and form in the products, so its basically saying "bonds ...
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Is a person warmed by fire an example of endothermic reaction? [closed]
My chemistry textbook gives the example of a man being warmed by a fire as an endothermic reaction. If the man's body is the system and it is increasing in temperature, how can this be endothermic?
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Is heat an extensive or intensive property?
Is heat extensive or intensive property?
I surfed the Internet for the same and found quite different answers. Like, heat is not a property, so it is neither. One said that since heat depends on the ...
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Does hot water freezes faster than cold water? [closed]
When we put cold water in the fridge it takes time to freeze but sometimes hot water is faster in freezing.Why?
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How do the pressure acting on the reactants and the initial temperature of the reactants affect the enthalpy of reaction?
In an exothermic reaction that occurs under constant pressure, does the pressure acting on the reactants affect the enthalpy of reaction? Does the initial temperature of the reactants affect the ...
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Chemical potential of mixtures of ideal gases and the Gibbs-Duhem equation
As is well known for a constant composition system, $\mu$ (symbolizing chemical potential) is equal to the molar Gibbs energy. Using
$$
\mathrm{d}G = V\mathrm{d}p-S\mathrm{d}T,
$$
we can write
$$
\mu =...
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Derivation of the Temperature Dependence of the Boltzmann Distribution
In lecture notes, Atkin's Physical Chemistry, etc. the Boltzmann Distribution is derived by first claiming that the system takes on properties of its weightiest configuration. The weight is therefore ...
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Isothermal compression at critical temperature
Suppose I am compressing a gas isothermally at its critical temperature. Initially, it has a large volume. I start compressing it and reach the critical point. At this point, the gas will start to ...
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Why do we neglect the zero point energy in statistical mechanics?
Quantisation associated with translational modes can be modelled like a particle in a box (PiB).
$$E_n = \frac{n^2h^2}{8mL^2} \qquad n \in \mathbb{Z} \, | \, n \ge 1$$
By the equation above, the ...
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How can endothermic reactions be spontaneous?
If endothermic reactions absorb heat from the surroundings, how can they be spontaneous, if heat only flows from high temp to low temp objects?
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What would happen to a diamond in the presence of oxygen and elevated temperature and how fast?
Let's say I have a $\pu{1cm^3}$ diamond and I put it into an oven where the temperature is about $\pu{ 400°C}$. We all know that diamond oxidizes. Would it evaporate or turn into graphite? If so, ...
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Finding direction of the reaction using Affinity of the reaction
I encountered following reaction:
$$\ce{CO + 1/2 O2 -> CO2}$$
I am provided with the composition of the various gases at $\pu{1000K}$ and am asked the direction of the reaction by finding the ...
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slow down ice melting in water
Having to separate freshwater crabs for commercial fisherman while on the boat, we dump the crabs in ice water, as crabs aren't as aggressive and bite less when cold. There's only so much room on the ...
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Why don't we consider VΔP when we define Q?
We only define $Q = \Delta U + W_\text{exp}$ (expansion work = $-P\Delta V$).
If heat can cause $\Delta U$ and work, why work is defined only as expansion work in the first place where there are other ...
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Why does increasing the pressure cause ice to melt more?
This question was given under chemical equilibrium.
Why does increasing the pressure on this system cause ice to melt more?
$$\ce{H2O(s) <=> H2O(l)}$$
I'm pretty sure this has to be ...
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Variation of equilibrium constant
In an exothermic reaction, the value of equilibrium constant increases with the rise of temperature because,
$\Delta H < 0$ ( exothermic reactions)
$\Delta G = \Delta H - T\Delta S$
Now if $\...
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The confusion of entropy change
The entropy is defined as randomness of a substance or a system.
The randomness increases as we go from solids to liquids to gases. Similarly from hard diamond to soft graphite. Since graphite is soft ...
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Why is the enthalpy of a reaction equal to the difference between the enthalpies of combustion of the reactants and the products?
My textbook gives me the following formula for calculating the enthalpy change of any reaction:
$$\Delta H_\mathrm{r}^\circ=\sum(\Delta H_\mathrm{c}^\circ)_\text{reactants}-\sum(\Delta H_\mathrm{c}^\...
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How does pH change during the electrolysis of water? [closed]
How does the pH change during electrolysis of a water and magnesium sulphate solution? Also, will this pH change happen every time? Does the pH change differently when there is just water in the ...
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Thermodynamics energy balance question help
I made the following attempt to solve this question.However I don't know how to find final pressure.I don't know how to find final temperature for both sides.Can you give me a hint?
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In what approximations, if any, can we assume a constant temperature in a Bomb calorimeter?
In most, if not all, general chemistry books, you will find that in from constant volume calorimeters, $\Delta H = \Delta U + RT \Delta n_\mathrm{g}$, which is, of course derived from $H = U + PV$.
...
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What is the reason behind the phenomenon of Joule-Thomson effect?
For an ideal gas there is no heating or cooling during an adiabatic expansion or contraction, but for real gases, an adiabatic expansion or contraction is generally accompanied by a heating or cooling ...
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Is dH work dependent?
In the equation $\ce{d}H=\ce{d}U+\ce{d}(PV)$
Under constant pressure is $\ce{d}(PV)$ equal to the work done ($W$) on the system? (where $W= P_{\text{ext}}\mathrm{d}V$ (for irreversible process) and ...
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Deriving a relation between entropy, volume and particles through the chemical potential
I'm a teaching assistant on a first year course on Introduction to Thermodynamics based on the book "Molecular Driving Forces" by Dill & Bromberg.
There's a question which asks the students to ...
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Standard state of a substance - definition
In chemistry, the standard state of a material (pure substance, mixture or solution) is a reference point used to calculate its properties under different conditions. IUPAC recommends using a standard ...
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What is the difference between enthalpy of reaction and standard enthalpy?
What is the difference between enthalpy of reaction ($\Delta H$) and standard enthalpy ($\Delta H^\circ$)? I was told that the standard enthalpy of reaction is the change in heat when one mole of ...
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Why does increasing temperature for an endothermic reaction always increase equilibrium constant?
According to Le Chatelier's Principle, increasing temperature for an endothermic reaction shifts the reaction towards the products.
However, unless there are things that I am missing, I believe there ...
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Can a droplet of formalin evaporate in midair while falling?
I am designing an evaporator for use in an enclosed environment. Due to the hazardous nature of formaldehyde, I want my evaporator to be wick-free.
The environment in which the evaporator will be used ...
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Enthalpy of formaton [closed]
today I was doing a set of HW problems regarding enthalpy and stumble upon one problem that I found confusing.It was the standard enthalpy of formation for CaCO3 is -1207.6 kJ mol when 30.7 g of ...
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Is chemical equilibrium possible at any temperature and pressure?
Consider the chemical reaction ${\rm Na+Cl\to NaCl}$ by mixing ${\rm Na}$ with ${\rm Cl}$ (or by mixing ${\rm NaOH}$ with some acid). This process is exothermic while the reverse ${\rm NaCl\to Na+Cl}$ ...
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How would one calculate the entropy change for an adiabatic irreversible process?
If two states $A$ and $ B$ are connected by a reversible path, they can never be connected by an irreversible path during an adiabatic process because:
$\underbrace{\Delta U}_{\text{state function}} ...
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Can pressure remain constant in isothermal expansion?
An ideal gas undergoes isothermal expansion at constant pressure. During the process:
enthalpy increases but entropy decreases.
enthalpy remains constant but entropy increases.
enthalpy ...
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What do "degrees of freedom" refer to?
Can you explain simply what are the degrees of freedom in chemistry? I don't fathom it very intuitively from Wikipedia.
I have almost no background in physics. I understand that it refers to all the ...
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Is a low specific heat capacity for the material of the calorimeter good or bad?
I wish to ask if a low specific heat capacity of the material of the calorimeter means that the calorimeter would be more exact as it takes less heat for the material to reach equilibrium. I thought ...
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Why might copper have a lower heat capacity than lithium according to the Shomate Equation?
I've learned that according to Condensed phase thermochemistry data, according to NIST, the functions of heat capacity, enthalpy, and entropy can be predicted by the Shomate Equation:
$$\begin{align}
...
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