Questions tagged [enthalpy]

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

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Explanation of the strength of phosphorus-oxygen bond

When discussing the Wittig reaction, Clayden's Organic Chemistry cites the strength of the $\ce{P=O}$ formed in triphenylphosphine oxide as a driver of the reaction through enthalpy: The $\ce{P=O}$ ...
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639 views

Entropy of Activation and Temperature Dependence

I am trying to understand the extrapolation of enthalpy $\Delta H^{\ddagger}$ and entropy of activation $\Delta S^{\ddagger}$ from the Eyring equation. It's typically cast as: $$\ln\left(\frac{k}{T}\...
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Why are IUPAC's definitions of exo- and endothermic disconnected from the direction of heat flow?

I'm wondering if anyone can provide a rationale for IUPAC's definitions of exothermic and endothermic, since they disconnect these terms from the direction of heat flow. Specifically, here are IUPAC'...
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What is wrong with my enthalpy calculations?

I need to calculate the enthalpy of combustion for butane using bond formation energies. Thus I first balance the equation: $$\ce{C4H10 + 6.5 O2 -> 4 CO2 + 5 H2O}$$ Then, I calculate the total ...
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Why are Born-Haber enthalpy calculations inaccurate for transition metal compounds?

A senior high/secondary school level exam question is about discrepancies between calculated and empirical enthalpies of formation. It asks why a discrepancy “usually” exists for d-block compounds ...
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Bond Energy in P-O , S-O , Cl-O

My text says that Bond energy between P-O , S-O , Cl-O follows the trend P-O < S-O < Cl-O. But I think it should be a reverse of what's given because more the electronegativity difference ...
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Theoretical temperature change in lime hydration

Calcium oxide (quicklime) and water have an exothermic reaction $\ce{CaO + H2O \rightarrow Ca(OH)_2} \quad \quad \Delta H^{\circ}_f=-65.3 \; \mathrm{kJ \cdot mol^{-1}}$ Assume that I have 1 kg of ...
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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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259 views

Precise explanation of macrocyclic effect

In most of the books, the reason of macrocyclic effect is given that It occurs due to preorganised structure of macro-cyclic ligand.  But a proper explanation of these two particular examples isn't ...
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443 views

Example of an endothermic reaction that can sustain itself for at least an hour?

Recall that in physical chemistry, the spontaneity of a reaction at constant pressure and a given temperature $T$ can be quantified by the Gibbs free energy $$\Delta G = \Delta H - T \Delta S$$ A ...
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Determining pressure of sublimation of iodine

The pressure of sublimation of iodine can theoretically be determined by considering the two hypothetical steps: the sublimation of iodine at $\pu{298 K}$, followed by an isothermal expansion of our ...
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Calcium chloride + starch desiccant packs inside PPE for ebola workers?

(First question on stackexchange, apologies for any faux pas) Ebola workers get drenched in sweat while working inside their personal protective equipment. There's a discussion happening over on ...
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Why when deriving the Clapeyron equation can you assume that dH = dq, as surely the pressure is not constant?

Part of the Clapeyron equation involves replacing $\Delta S$ with $\Delta H/T$. I can see the step from $\Delta S$ to $\Delta q/T$ but I'm not sure why you can replace $\Delta q$ with $\Delta H$. From ...
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Why do alkyl substituents stabilize alkenes?

My textbook states that because $sp^2-sp^3$ bonds are stronger than $sp^3-sp^3$ bonds, alkenes are more stable than alkanes. The rule given is that bonds with more $s$ character are stronger. However,...
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Why can't calorimetry be used to measure fast reactions?

As my previous post was a bit unclear and unspecific, I am reformulating it. First of all this is a question that I had while reading some concepts my book introduces, so this isn't a homework ...
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What is total enthalpy?

I've seen that it's defined as $h_0 = h + \frac{v^2}{2}$, where $h_0$ is the total enthalpy per unit mass. I can see that the $\frac{v^2}{2}$ term probably has something to do with kinetic energy, but ...
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1answer
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Enthalpy of neutralization of strong acid and strong base differing from enthalpy of formation of water from ions

If the neutralization between strong acid and bases has enthalpy of neutralization of around $\pu{-57.1 kJ mol^-1}$, why is it that when I try to calculate the enthalpy of formation of water from $\ce{...
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202 views

What factors contribute in making a solution more endothermic?

For example, a solution between potassium nitrate and water is more endothermic than sodium nitrate in water. Can someone explain?
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211 views

Why are negative enthalpies becoming more positive described as decreasing?

This website and my lecturer have describe negative enthalpies that are becoming less exothermic as decreasing. A relevant diagram from the linked to website to illustrate what I mean: I see the ...
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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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Change in enthalpy and entropy for the formation of nickel tetracarbonyl

I was working on an enthalpy/entropy problem and I had a question on predicting the signs of some reactions. For the reaction $$\ce{Ni (s) + 4 CO (g) -> Ni(CO)4 (g)}$$ my textbooks says that ...
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Why don't we measure mass of oxygen used in the calorimeter

The apparatus shown in the diagram can be used to measure the enthalpy change of formation of magnesium oxide (MgO). List the measurements you would need to make using this apparatus in order to ...
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Differences in specific free radical reactions enthalpies

Iodination of alkanes using iodine $(\ce{I2})$ is usually an unfavorable reaction. Tetraiodomethane $(\ce{CI4})$ can be used as the iodine source for iodination, in the presence of a free-radical ...
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Enthalpy change with temperature

Recently I stumbled across a book that describes enthalpy change with temperature, specifically for a product of combustion process (e.g. CO as a result of solid carbon combustion). The book says that ...
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Why do some chemical reactions lead to less stable state?

If the universe is always trying to find a more stable energy state, then how do reactions take place that lead to a less stable state e.g. the formation of ammonia from extremely stable diatomic $\ce{...
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Calculating enthalpy change

My friends and I were talking casually about electrolysis, fuel cells, and whatnots when the following question arose: Is $\ce{H2 (g) -> 2H+ (aq) + 2e-}$ endothermic or exothermic? I have the ...
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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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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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Reaction enthalpy using bond energy in resonating molecules

To find out and enthalpy of a reaction using bond energy we use the formula $$\Delta H = \sum\Delta_\mathrm{bond}H_\mathrm{reactants} - \sum\Delta_\mathrm{bond}H_\mathrm{products}$$ since we use all ...
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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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1answer
50 views

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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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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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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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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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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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) +...
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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 $...