Questions tagged [enthalpy]

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

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59 views

What would be the physical interpretation of the equation dG = Vdp − SdT?

I was trying to understand the derivation of the formula $G=G^\circ+RT\ln (p/p^\circ)$ starting with the following relationships: $$ \begin{align} \mathrm dU&=T\,\mathrm dS-p\,\mathrm dV\tag{1}\\ \...
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Calculating compound phases

Is there a pragmatic approach to estimating phases for compounds? There is not much connection between the heat-capacity of a compound and the heat-capacity of the elements it is made of. Heat ...
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55 views

Meaning of the symbol “$\Delta_\mathrm{sub}H^\circ[\ce{I2,s}]$”

One of the back exercises (13(C)) from the book for practice provided at my school asked whether the following relation is true: $\Delta_\mathrm{f}H^\circ(\ce{I2,g}) = \Delta_\mathrm{sub}H^\circ[\ce{...
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Enthalpy, heat of solution test: Calcium chloride vs magnesium chloride in water

I did a school experiment to measure the heat of solution (enthalpy) of various salts dissolved in water. My results are completely different to the teacher's answer. Specifically my test showed ...
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Calculating Enthalpy

I’m typing this from my phone...sorry if the quality is sub par. I have a very involved spreadsheet from a colleague of mine that was put together in 1994. The spreadsheet calculates the excess air ...
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55 views

Does the enthalpy of formation include translational and rotational energy?

I can look up the enthalpy of formation of water vapor and get -241,826 KJ/mol. I want to use this to compute the total energy of water vapor at STP from the energies of O2 and H2 gas. What I don't ...
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Why use enthalpy? [closed]

I learned that Enthalpy is useful because it can be used to see the heat flow. But base on the formula, enthalpy change only equals to heat flow when in constant pressure. If the pressure is non-...
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Lattice enthalpy of sodium chloride formation

So my text book says : "The ionization enthalpy of sodium chloride formation is 495 kJ/mol While the electron gain enthalpy of chlorine is -348 kJ/mol. The sum of the two is 147 kJ/mol which is ...
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Why is the bond energy for a C=O bond higher in CO2?

I was looking at the table of bond energies, and what caught my eye was how the bond energy for a $\ce{C=O}$ bond is $\pu{736 kJ/mol}$ with the exception of the $\ce{C=O}$ bonds in $\ce{CO2}$ being $\...
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Enthalpy of vaporization meaning

I already know that ΔH in a vaporization is the energy needed to bring water(l) to water(g) (constant P) but... from 1st law: $$dU = dQ - dW$$ (I know I can't write dQ and dW like that but you ...
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Why is the standard enthalpy of formation of aluminium oxide higher than magnesium oxide?

$$\begin{align} &\text{Aluminium oxide:} &\ce{4Al + 3O_2 &-> 2Al2O3} &(\Delta H_\mathrm f &= \pu{-1675 kJ/mol})\\[1.5em] &\text{Magnesium oxide:} &\ce{2Mg + O_2 &-&...
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Change in enthalpy and entropy when sugar dissolves in water

We know that dissolution of sugar in water is a spontaneous process. So, change in Gibbs free energy $(ΔG)$ must be negative for the overall process: $$ΔG = ΔH - TΔS < 0$$ Hence either the enthalpy ...
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How do I go about calculating the enthalpy of a solution with an unknown salt?

I have the $\Delta T$, I have the specific heat of the calorimeter, I have known weights/mass. I know the salt is lithium-based. What steps do I take to calculate the enthalpy of the solution so I can ...
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Why constant volume combustion gives higher energy than constant pressure?

I have noticed when I have done some combustion equilibrium that enthalpy is higher when doing a constant volume combustion than pressure constant combustion. Why is it so ? What are the reasons ...
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Does the enthalpy of solution formula (LE + Hyd) change depending on the question?

Hey guys. I have been working on enthalpies and literally every video and website I’ve seen, the formula for calculating enthalpy of solution = lattice enthalpy of dissociation + enthalpy of hydration,...
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Why does Delta H change when the coefficients change in a reaction?

So I understand that if twice as much of the reactants are present, then twice as much energy is released. But isn’t the energy released per mole of reactant still the same? You are just scaling up ...
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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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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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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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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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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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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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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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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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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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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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