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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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AP Chem Question about Reaction Rate - Confused?

I found this question in the AP Chem book, and I understand why B is the answer (as the answer key says). However, I don't understand why D is not also correct. The question is: A multistep reaction ...
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1answer
36 views

Why do reactions happen at all? [closed]

What makes some atoms/compounds reactive and some noble? What is the fundamental truth behind a reaction? It seems that two species can undergo number of combination to give a compound. Say Sodium and ...
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2answers
198 views

What will be the sign of work done?

Consider the following question: The enthalpy of vapour is $10\ \mathrm{kcal}/\mathrm{mol}$ higher than liquid at $350\ \mathrm{K}$ and $2\ \mathrm{atm}$. Under given conditions, the difference ...
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2answers
54 views

Does the reaction become more or less favorable as temperature increases?

Suppose a reaction has $\Delta S > 0$ and $\Delta H < 0$. The equation $\Delta G = \Delta H - T\Delta S$ predicts that as the temperature increases, the reaction becomes more favorable because ...
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2answers
66 views

Why is heat capacity defined as a partial derivative?

I understand that the heat capacity is the gradient of the slope of q against T, which implies that C = $\mathrm{ \frac{dq}{dT}}$, so how do we arrive at C = $\mathrm{\frac{∂q}{∂T}}$? At constant ...
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1answer
46 views

Meaning of an ideal chemical system [closed]

What is meant by a an ideal chemical system? Moreover, is it true that only in an ideal chemical system the equation $$\mathrm{d}U = \mathrm{d}H - \mathrm{d}(pV),$$ i.e. $$\Delta U = \Delta H - \...
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1answer
44 views

What is p defined as for the expansion and compression of an ideal gas system in the equation ΔU = ΔH - pΔV?

$$\Delta U = \Delta H - p\,\Delta V$$ Consider a gaseous system in a container with a frictionless piston, and a pressure outside the container. Is the pressure outside the container assumed to ...
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2answers
454 views

Is the differential, dp, exact or not? [closed]

Consider the differential $$\mathrm dp=\frac RV\,\mathrm dT+\left(\frac{2a}{V^2}-\frac{RT}{V^2}\right)\,\mathrm dV$$ (where $a$ is a constant value) (a) Determine whether the above differential, ...
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2answers
59 views

Why is change in entropy negative and change in enthalpy negative for the reaction of Magnesium and Hydrochloric acid?

So $$\ce{Mg(s) + 2 HCl(aq) -> H2(g) + MgCl2(aq) + Heat}$$ From a conceptual standpoint, shouldn't the change in entropy be positive for this reaction because there are more moles of gas in the ...
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1answer
50 views

How does the proticity of an acid affect the enthalpy change of its reaction with a base?

For an experiment I've carried out a series of reactions between monoprotic bases (ammonia and potassium hydroxide) and several acids. Two of these acids were monoprotic (hydrochlori acid and ethanoic ...
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2answers
105 views

Derivation of work done by real gas

According to the Wikipedia on real gases, it is stated that the expansion work done by a real gas differs from the ideal gas by $\int\left(V-\frac{RT}p\right)\,\mathrm dp$. Firstly, does this mean $...
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1answer
67 views

Is ΔV in ΔU = ΔH - pΔV always a positive number? [closed]

I know that $ΔU$ and $ΔH$ can be both positive and negative, but what about $ΔV$? I presume $ΔV$ can be both positive and negative as $w=-pΔV$, pressure always positive, and $w$ can be positive or ...
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1answer
81 views

How does refrigerator actually work?

In refrigerator, heat energy got extracted from cold sink and dumped into hot source. This is not spontaneous so work must be done. I came across the following while reading a textbook: For ...
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2answers
211 views

Calculating the strength of an ionic bond that contains poly-atomic ions

So the bond association enthalpy for ionic compounds like $\ce{NaCl}$ and $\ce{NaBr}$ can be easily calculated from a Born-Haber cycle. But the way a Born-Haber cycle is constructed it uses info that ...
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1answer
105 views

Find the minimum temperature at which the reaction will become spontaneous [closed]

This question came in my entrance exam and I think 273 K should be the answer because the reaction is already spontaneous. I just want to know if I've attempted it correctly. For a certain ...
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1answer
77 views

Pressure at which graphite and diamond are in equilibrium

I came across this problem : The standard state Gibbs free energies of formation of C(graphite) and C(diamond) at $T = \pu{298 K}$ are $\pu{0 kJ mol-1}$ and $\pu{2.9 kJ mol-1}$, respectively. ...
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0answers
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Is there a website or book that contains accepted values for the enthalpy change of the reactions between acids and bases?

I am looking for a website/book/journal/etc. which contains the accepted values for the enthalpy change of the follwoing reactions: Hydrochloric acid with potassium hydroxide Hydrochloric acid with ...
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1answer
107 views

Effect of temperature on equilibrium constant in terms of entropy change

This question was asked in the JEE examination of 2017 : [A] option is incorrect since the system is losing heat and hence $$ \Delta S = -ve$$ [B] Lets assume we have an endothermic reaction $$\...
2
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1answer
50 views

Simple alternative to UNIFAC to prove that a mixture containing 1ml of x and 5 litres of propylene glycol is not flammable?

Section 2.6.1 of the REGULATION (EC) No 1272/2008 (CLP) defines a flammable liquid as: Flammable liquid means a liquid having a flash point of not more than $60 ^{\circ}$C. So data on flash ...
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3answers
117 views

Estimating the absolute entropy of water at room temperature

I thought it would be fun to get an idea of how enormously large $W$ in $S = k\ln W$ is. I'd specifically like to estimate this for a mole of water at $\pu{1 atm}$ and $\pu{298 K}$. To do this, I ...
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2answers
332 views

Entropy change of isothermal irreversible expansion of ideal gas [duplicate]

Two mole of an ideal gas is subjected to isothermal expansion from $\pu{2 atm}$ to $\pu{1 atm}$ at $\pu{300 K}$. Calculate entropy change of the system, surrounding and total, if the process is ...
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0answers
45 views

Adiabatic flame temperature of a blast furnace gas

Calculate the adiabatic flame temperature of combustion of a blast furnace gas analyzing $25\%$ $\ce{CO}$, $10\%$ $\ce{CO2}$, and rest nitrogen. Use theoretical amount of preheated air at $\pu{1000 °C}...
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3answers
69 views

Solubility of solids in liquids

How does crystallisation happen and why? I have been told that when a solid is dissolved in a liquid solvent, an equilibrium is set up between processes of dissolution and crystallisation. But if ...
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1answer
45 views

Quasi-static implies that T aprox equals Tsurr?

A few days ago I re-visited my undergraduate thermodynamics notes and (as you may have seen from the amount of questions I have been posting) I have encountered many difficulties that I overlooked at ...
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1answer
97 views

Work done in an irreversible process

I was given a problem to identify whether the given statement is true or false. And the statement was: "Consider an ideal gas which is expanded from $(P_{1},V_{1},T_{1})$ to $(P_{2},V_{2},T_{2}).$ ...
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1answer
89 views

Determine surface area of heat exchange tube

A heat exchanger is used to cool down a stream of $\pu{20 kg s-1}$ of water from $\pu{80 °C}$ to $\pu{60 °C}$. The cooling water enters at a rate of $\pu{12 kg s-1}$ and $\ce{20 °C}$. The overall heat ...
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1answer
39 views

When a piston cylinder system that evolves isothermally and quasistatically is irreversible?

Suppose we have two states: $\ce{A}$ and $\ce{B}$. Considering a quasi-static isothermal process of an ideal gas, (and another condition, that is $T = T_\mathrm{surr}$) that join those states. Then ...
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1answer
44 views

Equation for overall heat transfer coefficient

The heat flow (in $[\pu{J/s}]$) through the wall of a tube of length $L$ and inner radius $R_0$ is $$Q = (2πR_0L)U(T_{1\infty} - T_{2\infty}),$$ where $T_{1\infty}$ is the bulk temperature ...
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0answers
40 views

Existence of single ion activities

I have used Davies equation to find what I believed was single ion activities and multiplied them on concentrations to define the thermodynamic dissociation constants for any kind of buffer to find ...
2
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1answer
59 views

Freezing trend of water

In this experiment, I used a digital temperature probe to measure the freezing point of water and some other salt solutions. I was just wondering if there was any explanations for 1. The first ...
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2answers
78 views

Concept of enthalpy at a given temperature

In thermodynamics, we always quote a fixed temperature, whenever we mention enthalpy of a reaction. For instance, one can determine the enthalpy of combustion of methane at 25 °C. Now almost all ...
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1answer
327 views

Need some help proving the following statements in Thermodyamics?

So in my second year thermodynamics class we are currently talking about entropy and deriving its definition from the 1st law. $$\mathrm{d}q_{rev} = dU - dw_{rev}$$ $$= C_{V}(T)dT + PdV$$ $$= C_{V}(T)...
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1answer
63 views

How to determine if a mixing process is spontaneous at constant temperature but variable pressure

This is a conceptual question. Many times we work with the gibbs energy of mixing under conditions of constant temperature but not pressure. In a problem like this: Suppose that 2.0 mol H2 at 2.0 ...
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1answer
43 views

Will the motion of particles always increase with the rise of temperature?

I came across this following question while poring over my textbook. Predict the change in the motion of particles you would expect in the following if the temperature was increased: a) Solid b)...
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1answer
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I am looking for any substance that would be unstable or dangerous when it is cooler or not on heat, and is stable when it is being boiled

I am a first responder and one of our protocols when entering any unknown lab environment is to turn off any burners. Most of the time, we try to not disturb the environment as much as possible so a ...
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2answers
126 views

Hydrogen vs neon in terms of entropy

I know that both molecular size and complexity increase overall entropy, but between the two I'm not sure which increases it more. Would 1 mole of hydrogen have the greater entropy because it is two ...
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0answers
70 views

Freezing point depression of salt solutions

I have recently come across this particular equation which is used to estimate the freezing point depression of a salt solution: $$\theta_\mathrm{B} = \frac{\Delta H^\mathrm{fus}_{0,T_\mathrm{F}} - ...
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1answer
83 views

Free energy of solvation and Henry's law constant

I am trying to calculate the free energy of solvation of $\ce{CO2}$ from its Henry's law constant. As given on Wikipedia, the dimensionless Henry's law constant is $0.83$. If I try to calculate $\...
2
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1answer
78 views

How are octet-rule models supported by thermodynamics and quantum theory? [closed]

I understand that the four quantum numbers dictate how many electrons can "fit in a shell," but I don't understand how gaining a full octet is particularly energetically favorable. The example of ...
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1answer
37 views

Thermodynamics at absolute temperature [closed]

Assume I have kept a single molecule in vacuum and in absolute zero (0 K) temperature. There is no energy left in the particle, and hence any type of motion is absent. Now, what happens when I ...
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1answer
65 views

Work done by gas in isothermal process [closed]

A $20\ \mathrm l$ steel cylinder containing $\ce{N2(g)}$ at $20\ \mathrm{atm}$ and $300\ \mathrm K$ is kept in a big jar containing air at $1\ \mathrm{atm}$ and $300\ \mathrm K$ and sealed. $\ce{N2(g)}...
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1answer
611 views

Why was a Plimsoll symbol chosen to indicate standard state?

Historically, the Plimsoll symbol (aka Plimsoll line) was created as hull mark that would serve as a ready indicator of whether a ship was overloaded and thus running too low in the water. It was ...
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1answer
39 views

Whats the difference between reversible process in thermodynamics and reversible chemical reactions?

We know that in thermodynamics, reversible process never occur since entropy should be conserved. However, in chemistry we do have a lot of reversed chemical reactions that forms reactants from ...
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4answers
140 views

Is the titration of sodium carbonate with hydrochloric acid temperature dependent?

I am a chemistry teacher in a Cambridge school. In one of the titrations of sodium carbonate with HCl, there was one question asked: On heating the sodium carbonate solution before doing the ...
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1answer
52 views

Impact of Pressure on equilibrium of Tin allotropes

The pressure at the bottom of the Mariana Trench in the Pacific Ocean is $1090$ bar. What temperature will the two allotropes of tin be at equilibrium? Assume that the molar volume, energy, and ...
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1answer
113 views

What is the molar enthalpy of reaction

When 0.45 g of Zn is added to 50.0 ml of 0.95 M HCl solution, the solution inside the calorimeter heats up by 12 °C. What is the molar enthalpy of the reaction (in kJ/mol)? I know how to find the ...
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1answer
106 views

Calculating ΔH from bomb calorimetry

Suppose we carry out a reaction in a bomb calorimeter whose starting temperature is $298.15\ \mathrm K$. Here we assume $\Delta V$ is close enough to zero that we consider the process to be at ...
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1answer
46 views

Using Hess's law to find molar enthalpy of formation without an initial reaction [closed]

I'm having issues relating Hess's law to this question: The given chemical equation represent the combustion of ammonia and the combustion of hydrogen $$ \begin{align} \ce{4 NH3 + 3 O2 &-&...
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4answers
75 views

Adiabatic processes and the Ideal Gas Law [closed]

in my first year chemistry textbook it wrights that in adiabatic proccess when there is a system of canister filled with gas and a piston above(and wheights on it) if we let the piston move upward by ...
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3answers
52 views

Is it correct to say that “some degree of pollution is an inescapable consequence of the Second Law of Thermodynamics”?

The quote above is from ‘Enlightenment Now’ by Steven Pinker, I was just wondering if this was true or if it misrepresented the Second Law. The full quote below... ...some degree of pollution is ...