Questions tagged [enthalpy]

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

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Calculating Enthalpy of formation versus Calculating Enthalpy of a reaction not occurring at standard conditions

My understanding of Hess's law and its use in determining the change of enthalpy of a reaction has been challenged recently. Up until recently, I thought that the only way to calculate a chemical ...
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Can reverse micelle's exist outside solution?

As in reverse micelle's, the polar liquid remains inside the micelle while the exterior is one of lower dielectric constant; if the liquid were to be carefully removed, would the micelle structure ...
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Combustion Temperature Calculation Using C_p and Heat of Formation

I am simulating the combustion temperature for various compositions of syngas and finding the product temperature. Part one is varying the ratio of CO and H2 from 3:1 to 1:3 in the syngas, burning it ...
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Why is my ethanol–water mixture showing negative change in enthalpy of mixing? [duplicate]

I am conducting an experiment where I mix two solutions $\text{A}$ and $\text{B}$ and then I am measuring the temperature change (for real solutions, i.e., $\Delta_\mathrm{mix}H\neq 0).$ I took the ...
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How strongly affected is the buffering capacity of sodium carbonate - sodium bicarbonate by temperature changes?

I would like to do a lab regarding the affects of temperature on the buffering capacity of sodium carbonate - sodium bicarbonate in particular for sulfuric acid. Currently I have two competing ...
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Enthalpy/Temperature change in a chemical reaction

I was wondering how exactly absolute temperature change in a chemical reaction could be calculated. For example, consider this reaction: 3H₂+N₂→2NH₃, ΔH=-46kJ/mol (298.15K) Assuming the given reaction ...
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Cooling down from −78 °C to −88 °C without cryogenics

Say I have a gas cylinder with nitrous oxide and cool it down to −78 °C with dry ice. Is there an endothermic process/reaction that would extract the remaining heat from $\ce{N2O}$ and bring it down ...
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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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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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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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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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Is enthalpy affected by isotopes?

Enthalpy is a sum of $U$ and $PV$, and I thought that for different isotopes, internal energy values may differ. Is it true? There are three stable isotopes of $\textrm{Mg}$, mass number 24, 25, and ...
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Enthalpy change of dissolving air in water?

I'm looking for a value of the enthalpy change of dissolving air in water but can't find it. In other words, if I pull a vacuum over some water to remove all the air, per Henry's Law, how much energy ...
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Why is average bond enthalpy measured by homolytic fission?

Average bond enthalpy: the energy required for 1 mole of a specific gaseous bonds to undergo homolytic fission Why specifically homolytic fission? Is it to allow comparison, but if that were the case,...
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Is the kinetic energy change of a system (q) equal to the enthalpy change of a system (∆H)?

When silver nitrate and hydrochloric acid are put together in a container, they form silver chloride. If you're given information about the reactants (including temperature change, heat capacity, and ...
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How to get the enthalpy of a reaction from the enthalpy of formations with Hess' law?

Let's consider a generic reaction: $$\sum_{i=0}^{i=I}n_{i}A_{i} \rightarrow \sum_{j=0}^{j=J}m_{j}B_{j} $$ where $n$ and $m$ are the stoichiometric coefficients, while $A$ and $B$ are reagents and ...
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Thermochemical data for thiocyanatoiron(III) ion

I am currently writing a lab report about the formation constant of $\ce{FeSCN^2+}.$ All I could find online was a formation constant at $\pu{25 °C}$ $(890),$ which was 70% higher than the ...
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Which pressure is used for calculation of standard enthalpy change of the reaction with two or more gases?

Standard state conditions for standard Gibbs free energy change stipulate (among other conditions) that the partial pressure of each gas is $\pu{1 bar},$ e.g. for the reaction $$\ce{2 A(g) + 3 B(g) &...
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Spontaneity of an exothermic chemical reaction

When the process is exothermic (ΔHsystem​<0), and the entropy of the system increases (ΔSsystem​>0), the sign of ΔGsystem​ is negative at all temperatures. Thus, the process is always spontaneous. ...
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Computing enthalpy of formation of dissolved gas from its enthalpy of dissolution

Provided that I know the enthalpy of formation $\Delta H_f^0(X_{g})$ of a substance $X$ in gas phase and its enthalpy of dissolution $\Delta H^0_{sol}(X)$ in the same conditions, considering that the ...
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What makes determining entropy changes theoretically so hard compared to enthalpy?

It is really hard to determine the entropy change of a reaction accurately because of subtle solvent effects and a lack of standard entropies. On the other hand, enthalpy is relatively easy and ...
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How to describe the enthalpy of acid dissociation in aqueous solutions?

How to describe the enthalpy of acid dissociation in aqueous solutions? If we had a weak acid "PB" that dissociates to give a proton and a conjugate base. Classically it would be described like the ...
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Experimental measurement on the ring-strain

for my thesis i calculated the ring-strain of a molecule by MP2 on the computer. I was wondering if there is an easy possibility to get the experimental value of the ring strain. I guess I have to ...
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Why do we use pV term independent of U in the equation H = U + pV? [duplicate]

I have read the following definition of enthalpy in my textbook: A substance has to occupy some space in its surroundings depending upon its volume ($V$). It does against the compressing influence ...
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Predicting the temperature of an object using the heat of its surroundings after a certain amount of time?

how would you predict what the temperature of something would be if it spends a certain amount of time exposed to heat from its surroundings? is this possible? ex. what temperature would a hot ...
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Anomalous trend in formation enthalpy of alkali metal halides

I read that in alkali metal halides, the formation enthalpy for fluorides become less negative as we go down the group, whereas the reverse is true for chlorides, bromides and iodides. Why is it so? ...
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What really are the units of enthalpy change? [duplicate]

A reasonable definition of standard enthalpy change of formation would be: 'The enthalpy change when one mole of a compound is formed from its constituent elements under standard conditions' ...
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Application of the definition of enthalpy in a modified way

My book gave me this equation at constant temperature and constant volume: $$\ce{2NH3(g) -> N2(g) + 3H2(g); \Delta U = 92.3 kJ}$$ And it says here enthalpy and internal energy are the same as ...
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How to evaluate enthalpy variation of combustion of sucrose in a constant volume calorimeter?

Consider combustion reaction (with the formation of $H_2O$ and $CO_2$) of a mass $m$ of sucrose in a constant volume calorimeter with thermal capacity $C$. This reaction increases the temperature of $...
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Finding Enthalpy [closed]

Given Information $\ce{CH_4 + 2 O_2 -> CO_2 + 2 H_2O}$ "The Heat of reaction is $-802000\, \frac{J}{mol}$ of $\ce{CH_4}$ at $25\, ^\circ C$. Rate of Moles of $\ce{CH_4}$ used is $0.625\, \frac{...
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Hydration enthalpy of cations

Can someone provide reliable data on the solvation / hydration enthalpies of $\ce{Ti^{4+}}$ and $\ce{Cr^{3+}}$ ions, or explain which is more stable qualitatively? According to this article1 CFSE in $...
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Internal Energy Definition, Why is dU zero when dT is zero?

According to my Equation sheet, $$\delta U=\left(\frac{\delta U}{\delta T}\right)_V\,\delta T+\left(\frac{\delta U}{\delta V}\right)_T\,\delta V=C_V\,\delta T-C_V\left(\frac{\delta T}{\delta V}\right)...
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Why is the specific heat of ethanol not representing experiments

I am in my last year of high school completing an assignment hot and cold packs, and particular the enthalpy behind them. My major lack of understanding is why my calculations are working for heat of ...
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Where can I find the standard enthalpy of formation for Sodium Cyanamide (Na2NCN)?

I am trying to find the standard enthalpy of formation for sodium cyanamide. Ive looked on various databases that list data about specific compounds but I can't find anything on this particular one. ...
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How to calculate enthalpy change of neutralisation - what mass?

How to calculate enthalpy change of neutralisation - what mass? I made an experiment where I reacted 100ml HCL 2M with 100ml NaOH 2M in 1300ml Water, and got a temperature change dT. My question is ...
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Energy required to break salt bridges in peptides and proteins

Is there a way to calculate the energy required to break a salt bridge in a peptide or on the exposed surface of a protein in aqueous solution? Is there any useful literature on this topic? I'm ...
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Calculating energy change for calcium carbonate decomposition

The $\Delta H^0$ for the decomposition of $\ce{CaCO3(s)}$ is $178.3~\mathrm{kJ/mol}$ at $25~\mathrm{^\circ C}$ and $1.000~\mathrm{atm}$. What is the $\Delta E$ for the decomposition process? $$\ce{...
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Standard Enthalpy of Formation for Branched Alkanes

I'm looking at the heat of formation for branched and straight alkanes and I've noticed a few odd trends I'm having trouble explaining. For straight-chained alkanes, the addition of carbon atoms to ...
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Why are absolute enthalpies of reactants not available?

The standard enthalpy of reaction is given as $$\Delta_r H = \sum\nu\; H_m(\text{products})- \sum\nu\;H_m(\text{reactants}).$$ Peter Atkins in his book Elements of Physical Chemistry then writes ...
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Calculating enthalpy of dissolution

In a polystyrene cup calorimeter, $4.3\ \mathrm g$ of ammonium nitrate, $\ce{NH4NO}$, was added to $60.0\ \mathrm g$ of water and stirred to dissolve the solid completely. The initial temperature ...
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Enthalpy of mixing in a regular solution

In a mixture of $A$ and $B$ where $A$ and $B$ are randomly mixed and the average coordination number is $z$. If there are $n_B$ molecules of $B$ in the mixture these will be in contact with a total of ...
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Is the heat of reaction always negative?

For example, I have a question below that I know the correct answer to, but I don't know why it's positive rather than negative. When $\ce{NaOH}$ is neutralized by $\ce{HCl}$: $$\ce{NaOH(aq) + HCl(aq)...
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Why does it not matter what mass of NaOH is used to solve for q?

So we recently did a lab in Chem to prove Hess's law. The reactions we performed were $$\begin{align} \ce{NaOH + HCl &-> NaCl + H2O}\tag1\\ \ce{NaOH(s) + HCl &-> NaCl + H2O}\tag2\\ \ce{...
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Molar heat of reaction for water in a neutralization reaction

I have a simple lab problem: $30\ \mathrm{mL}$ of $1\ \mathrm{mol/L}$ $\ce{H2SO4}$ reacts with $50\ \mathrm{mL}$ of $1\ \mathrm{mol/L}$ $\ce{NaOH}$ in a neutralization reaction as per the following ...
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Find ΔHr for HCl + Mg Reaction

I am having trouble calculating the value of $\Delta H_\mathrm r$ for the reaction: $$\ce{Mg + HCl}$$ The total heat capacity ($C_\text{cal} + C_\mathrm r$) is $6.88\ \mathrm{kJ/{^\circ C}}$, and the $...
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Enthalpy change of reaction

Suppose you have the following equation: $$\ce{2NH4SCN(s) + Ba(OH)2(s) ->Ba(SCN)2(s) + 2H2O(l) + 2NH3(g)}$$ You initiate this reaction using 15.22g of the $\ce{NH4SCN}$ (and excess of the barium ...
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Enthalpy: when and how does it vary? [closed]

I have been looking for a physical explanation for entropy and enthalpy variations. An example would be the air system of a vehicle, where people say we have a strong enthalpy and entropy variations ...
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Comparing the enthalpy change between the formation of water in two states

The enthalpy of the reaction $\ce{H2(g) + 1/2 O2 (g) -> H2O (g)}$ is $\Delta H_1$ and that of $\ce{H2(g) + 1/2 O2 (g) -> H2O (l)}$ is $\Delta H_2$. Then, a) $\Delta H_1 < \Delta H_2$ b) $\...
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Where can I find the total enthalpy and heat of dissolution in water of a given components?

I don't really have a background in chemistry but I'm actually studying in aerospace engineering, sorry if this question may seem trivial. I am interested in the decomposition process of the hydrogen ...
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Calorimetry, finding final temperature

I was looking for some help on understanding this practice question from my text book, I know how to figure out enthalpy, but this is my first temperature change question and I am stumped. Any ...
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