Questions tagged [enthalpy]

A thermodynamic state function describing the total energy content of a system.

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Calculate volume-dependence of enthalpy in terms of quantities that can be obtained in the laboratory

I want to calculate $\displaystyle \left(\frac{\partial H}{\partial V}\right)_{T,N}$. First, we know that the enthalpy is given by the differential exact equation $$\displaystyle dH = T dS + V dp + \...
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Change in Enthalpy in Free Expansion of Ideal and Real gas against Vacuum

By free expansion, I am referring to gas allowed to expand freely against vacuum in a Joule Expansion. If gas is ideal then change in Internal Energy '∆U' and change in Enthalpy '∆H' is zero. (By ∆H = ...
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Calorimetry Question - determine temperature change without knowing enthalpy change?

I have a lab that I honestly cannot figure out. Basically, we have a video where we watch 200 ml of NaOH (2M) and HCl (1M) combined in a calorimeter with a magnetic mixer. Pretty standard. This video ...
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Find delta G, delta H, and Delta S for the following reaction at 25 degrees celcius [closed]

Calculate the ΔGo, ΔHo, and the ΔSo for the following reaction at 25 degrees C. 4 Fe (s) + 3 O2 (g) --> 2 Fe2O3 (s) Substance ΔGo ΔHo ΔSo (kJ/K mol) (kJ/K mol) (J/K mol) Fe (s)...
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Verify heat produced by flame using enthalpy of fomation

I'm simulating using a CFD solver a pure oxygen/methane flame. However, I doubt that the combustion model does not produce enough heat. The heat from reaction is defined by $S_h=-\sum_{j} \frac{h_j^0}{...
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43 views

Do most chemical reactions happen at constant pressure?

While studying enthalpy $H=U+PV$ and its changes, I realized I am not clear on the following: chemical reactions happen with the external pressure being constant and equal to the atmospheric pressure $...
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Definition of enthalpy of vaporization

The amount of heat required to vapourize one mole of a liquid at constant temperature and under standard pressure (1 bar) is called its standard enthalpy of vapourization. The enthalpy of vaporization ...
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What is the bond energy of H-O? Wikipedia disagrees with itself

What is the bond energy of H-O? Wikipedia disagrees with itself: On English Wikipedia: 467 kJ/mol On French Wikipedia: 366 kJ/mol What is the right value?
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A lot of confusion in 1st law of thermodynamics [closed]

My sir told me that Total energy of system = K.E + P.E in starting. Then Change in energy = F(External force on body ) * displacement of walls. Then from here , change in energy = W+q. (Don’t ...
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57 views

Why does enthalpy change differ in a chemical reaction because of temperature?

If enthalpy is a state function, shouldn't the change in enthalpy be the same between a given initial and final state, regardless of how the change took place? Kirchoff's equation describes this ...
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How can we tell if a substitution reaction will be thermodynamically favoured

My understadning was like in the case of a cyclohexane we would see a thermodynamically favoured reaction if an axial substituent became equitorial in the case of a inverted Sn2 reaction but I would ...
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Why is dilution exothermic?

The breaking of bonds usually absorbs energy and forming bonds usually releases energy. Then, why is the dilution of acids and bases exothermic?
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Why can the standard enthalpy of formation be calculated for different states of matter when the temperature should be constant?

If the standard enthalpy of formation needs to be determined at $25ºC$ and 1 atm of pressure then why is it possible for the enthalpy of formation to be calculated for the liquid, solid, and/or gas ...
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Why does the C−H bond dissociation energy vary in a homologous series of primary alcohols?

Specifically, for the primary carbon atom in the alcohol. Here is bond dissociation energy (BDE) data from chapter three of Luo's Comprehensive handbook of chemical bond energies [1] (boldface refers ...
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Question to find increment in total energy of a sample of He gas? [closed]

If a closed rigid wall container filled with 20 gram of Helium gas moving at a constant speed of 10 m/sec is stopped abruptly to stand still, find the increment in temperature of this He sample. Way I ...
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Why is enthalpy change at constant pressure not only internal energy change? [closed]

Okay so this might seem a bit Weird but the formula for change in enthalpy has been given the following way- ∆H=∆U+∆PV so in this case when we say ∆V is zero we get that ∆PV is also zero and thus ∆H=∆...
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What makes the enthalpy of hydration of calcium chloride dihydrate unique? [closed]

I have recently done an experiment in an attempt to find the relationship between the water ratio in a hydrate and the enthalpy of hydration, and my results showed a correlation, but an anomaly for ...
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29 views

Why is the change in enthalpy computation for a change of state done like this?

Suppose that a system whose initial state is Pressure = $P_1$, Volume = $V_1$; and it is taken through a process after which final pressure and volume are $P_2$ and $V_2$ respectively. Now change in ...
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What is the mechanism of endothermic dissolution?

My real question is in fact "How can dissolution happen if it's endothermic?" However, there have been many questions regarding this exact topic and the answers about Gibbs free energy ...
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Significance of enthalpy of hydration of salts [closed]

I've been researching for weeks but can't find anything on a real life process of hydration, as my experiment is determining the relationship between the enthalpy of hydration and different types of ...
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Bond dissociation energy for polar molecules

This formula relates to how Pauling determined the values for electronegativity for all the elements and how delta (if big) shows that the molecule is very polar. Delta being: $\Delta =\Delta H\left(...
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Calculating change in state function following different paths

in thermodynamics, we learn that change in state function is independent of the path/process, and thus we can conveniently pick any arbitrary path that allows easier calculation. I'm TA'ing a class ...
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58 views

What is the significance of enthalpy?

I an getting confused between enthalpy and internal energy. My textbook says that change in enthalpy is the sum of change in internal energy and the amount of work done in expansion or compression but ...
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Applicability of Enthalpy-Heat relation for a non-isobaric process

Consider a process in which state variables changes from $(p_1, V_1, T_1)$ to $(p_2, V_2, T_3)$ . I've learned that to calculate the change in enthalpy for any process, we consider a hypothetical ...
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Question on the formula to calculate enthalpy change from calometric data

I have a sample question here. "In an experiment, a student placed 20cm^3 of a solution containing 0.04 mol of HCl in a plastic cup and added 20cm^3 of solution containing 0.04 mol of Na2CO3. The ...
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Good Hess's Law experiment [closed]

I am doing an Internal Assessment for my IB chemistry class, and I want to do something on Hess's Law, and I'll be doing it simulation based, using this website. With the solutions and equipment on ...
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About spontaneous combustion of methane

I heard that spontaneous reaction happens if $\Delta G=\Delta H-T\Delta S$ is negative. For combustion of methane, according to Chemguide: $\Delta H=\pu{-891.1 kJ K^{-1} mol^{-1}}$ $\Delta S=\pu{-0....
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Why does increasing charge density matter more for the hydration enthalpy compared to the lattice energy?

For the solvation of ionic salts, if we look down a group, say Group II, and we keep the anion constant, we find that the solubility decreases going down the group. This is because the hydration ...
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Specific Heat Derivation

The specific heat capacity at constant volume and the specific heat capacity at constant pressure are intensive properties defined for pure, simple compressible substances as partial derivatives of ...
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How would I correct this Hess cycle?

Given $$\ce{2 SO2(g) + O2(g) <=> 2 SO3(g)} \quad \Delta_\mathrm{r}H = \pu{-196.0 kJ mol^-1}$$ and $\Delta_\mathrm{f}H(\ce{SO2}) = \pu{-296.0 kJ mol^-1},$ find $\Delta_\mathrm{f}H(\ce{SO3}).$ I ...
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rate of combustion of alcohols and hydrocarbons in relation to the number of carbons in the chain

I understand that the heat released when burning alcohols is greater as long as the number of carbons in the alcohol chain is increased (i.e the enthalpy of butanol is greater than propanol > ...
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407 views

Why is the first ionization enthalpy of molecular O2 less than atomic O?

A cousin of mine asked for my help to answer one of his homework questions. It stated- Explain why- The first ionization energy of molecular oxygen $\ce{O2}$ ($\pu{1175 kJ mol-1}$), is lesser than ...
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How does the calorimeter affect heat released by the system (q)? [closed]

Here are the 2 questions I am pondering about How would the calculated quantity of heat (q) released by a system differ if the calorimeter's heat capacity was taken into account? My thought process ...
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How can you calculate the tendency of one compound to become another?

Excuse my inexperience with chemistry here. I've been poring over papers with terms like "Standard Gibbs free energy of formation", "Hess's Law", "standard enthalpy of ...
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Thermochemistry: On Units of Enthalpy Change and Implicit Given of Initial Temperature of Calorimeter

I would like to ask two separate questions. But first, so that we are on the same page, the experiment used a constant-pressure calorimeter, and it is a neutralization reaction of 1M of HCl and 1M of ...
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55 views

Why does lattice enthalpy decrease with increasing ionic size?

Lattice enthalpy decreases as ions get larger, but I have found two explanations: The charge density is greater in smaller ions, so greater attraction The ions are themselves able to get closer ...
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163 views

Why are branched alkanes used as fuels instead of straight chain alkanes?

My textbook states that branched alkanes are thermodynamically more stable than straight chain alkanes. Of course, iso-octane has an enthalpy of formation of $\Delta H_\mathrm f$= $\pu{-259.3.40 kJ ...
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323 views

Kinetics Vs Thermodynamics : Apparent contradiction in the definition of Enthalpy in the two disciplines

My book gives the following curve: It gives the following relation :$$ΔH= E_{\mathrm{activation,forward}}-E_{\mathrm{activation,backward}} \tag{1}$$ But I suspect that $ E_{\mathrm{activation,forward}...
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278 views

Why is the zero of standard enthalpy of formation a convention?

The standard enthalpy of formation $\Delta H_f^°$ of pure elements is zero by definition. Why is that a convention? It is true that enthalpy is defined unless a constant (like energy and entropy), but ...
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Predicting sign of enthalpy of allotropic transition of phosphorus

How to predict the sign of enthalpy during the transition $$\ce{P(s, white) -> P(s, red)}?$$ $\ce{P(s, red)}$ is more stable than $\ce{P(s, white)}$ due to release in angle strain. Since during ...
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Why do internal energy/heat and enthalphy differ? What is the physical significance of the $PV$ term for enthalphy of an ideal gas? [duplicate]

Let it be noted that this is not a duplicate of this downvoted-to-hell question, simply because I hope to ask it better! I've been reading a thermo textbook and I've got a simple question. The ...
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Why isn't the mass of the things dissolved in water not added to the mass of the water in an enthalpy calc?

Take a change in enthalpy question. When $\pu{5 g}$ of powder is dissolved in $\ce{100 ml}$ of water, the added substance raises the temperature of the water by $\ce{5 ^\circ C}$ and you are told to ...
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Feasibility of a Reaction [closed]

I understand for a reaction to be feasible, $\Delta$H needs to be negative and $\Delta$S positive. But in terms of Gibbs free Energy Change, without considering kinectic factors, If $\Delta$H is ...
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What is the enthalpy change of solution of oxygen and water? [closed]

What is the enthalpy change of solution of oxygen dissolved into water?
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Sign of change in enthalpy and change in entropy

Why is it wrong to assert that the change in entropy and the change in enthalpy must always have the same sign? What makes me think that they must have the same sign is the fact that every reaction ...
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The relation between q and T when the volume is constant [closed]

$$ \begin{align} q_V &= ΔU \tag{1}\\ q_p &= ΔH \tag{2}\\ C_V &= \frac{\mathrm dq_V}{\mathrm dT} = \left(\frac{\partial U}{\partial T}\right)_V \tag{3}\\ C_p &= \frac{\mathrm dq_p}{\...
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Significance of pV work in enthalpy [closed]

If we already have $pV$ work included in the internal energy $U = q + w,$ then what is the $pV$ work in the equation for enthalpy $H = E + pV?$
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491 views

Why does spontaneity of a reaction depend on temperature?

As we know, $$\Delta G=\Delta H - T\,\Delta S.$$ Both in my textbook and on the internet, it was given that temperature was a determining factor in the spontaneity of a reaction. For example, when $\...
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What is the enthalpy change of neutralisation of vinegar (diluted acetic acid) and calcium carbonate?

If anyone has conducted an experiment that determined this value or knows reliable sources that show the literature/thereotical value of this enthalpy change, please share your answer. Thank you very ...
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When, if ever, can standard enthalphy be subsituted for standard chemical potiential?

I'm reading through a combustion modeling paper by Gordon & McBride which is a bit scant on some of the algebra & calculus used to derive their equations. My advisor and I have derived nearly ...

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