Questions tagged [enthalpy]

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

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Change in Entropy and Enthalpy found over a temperature range and asked to compute the change in Gibb's Energy

I was given the following question by my tutor: "The variation of enthalpy ($\Delta H$) and entropy ($\Delta S$) in a reaction carried out at constant pressure in the temperature range between 300 ºC ...
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When salt dissolves in vinegar, is the process endothermic or exothermic? [closed]

We are asked to describe the process of dissolving sodium chloride in vinegar using the concept of enthalpy.
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Please explain why my textbook says this [closed]

The value of change in enthalpy becomes less and less negative during the course of the adsorption.
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Calculating change in enthalpy [closed]

I’ve got an assignment to solve this problem: Calculate the change in Enthalpy when 50cm3 if 0.2moldm-3 NaOH is added to 250cm3 of HCl. The initial temperature is 21.6C and it rose to 32.8C. I tried ...
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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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Change in temperature if salt is dissolved in water with different initial temperatures

I will mix a salt with room-temperature water and 8 degree Celsius water. Then I will compare the heat of solution with each other. Does the initial temperature of the water affect how much the heat ...
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Combining enthalpies of formation and combustion in the context of Hess's law

I know that you can use enthalpy of combustion to work out formation, and Formation to work out enthalpy change. But can you use enthalpy of formation and combustion data is the same Hess's cycle to ...
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What is the lattice enthalpy of zinc chloride?

I am trying to find a literature value for the enthalpy of solution of zinc chloride to compare with my experimental value but I can't seem to find it. So I'm trying to use the formula: enthalpy of ...
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Resolving ambiguity around Lattice Enthalpy in Born-Haber Cycle

According to my textbook, Lattice enthalpy is the enthalpy change that occurs when one mole of a solid ionic compound is separated into gaseous ions under standard conditions.$^1$ According to the ...
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Hyperconjugation, stability of molecules, and heat of formation

I recently asked a question about why the heat of formation of organic radicals and positive ions decreases with their size and degree of branching at the radical or ionic site. The user "Buttonwood" ...
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Why does the heat of formation of organic radicals and positive ions decrease with their size and degree of branching at the radical or ionic site?

I am currently studying the textbook Mass Spectrometry, third edition, by Jürgen H. Gross. Chapter 2.4.3 Bond Dissociation Energies and Heats of Formation says the following: The heat of formation ...
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Mechanistic explanation of salt lowering temperature of ice slush

As we all know, adding salt to ice water lowers its temperature. I've read plenty of system-level accounts of energy balances, enthalpies, vapor pressures, phase equilibria and freezing points—I ...
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Endothermic or Exothermic [closed]

Is an endothermic reaction always endothermic at all conditions of pressure, temperature etc.? Similarly for exothermic... I mean is sign of ∆H always the same as sign of ∆H°? This might seem to be a ...
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Are there any reactions that are endothermic and exothermic? [closed]

As it says: are there any reactions that require heat and produce heat simultaneously?
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Values for heat of reaction example

I am currently studying the textbook Mass Spectrometry, third edition, by Jürgen H. Gross. Chapter 2.4.3 Bond Dissociation Energies and Heats of Formation says the following: Energetics of $\ce{H}^\...
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Why are there so many different formulations for energy? When do I use each one? [closed]

I understand that energy is a fairly arbitrary concept, and that it completely depends on your definition of energy. So far in my class, we have discussed: Entropy, $S$ (not really energy, but it ...
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Which change occurs in all exothermic reactions? [closed]

Which change occurs in all exothermic reactions? A) Bubbles of gas are released from the mixture. B) Light energy is produced. C) The temperature of the mixture decreases. D) The ...
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Understanding the term 'heat of hydrogenation'

From Chemistry Libretexts: Heat of hydrogenation (symbol: $ΔH_\mathrm{hydro}$) of an alkene is the standard enthalpy of catalytic hydrogenation of an alkene. Does the heat of hydrogenation include ...
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Water becomes cold on mixing energy drink [closed]

I noticed that the temperature of water decreases noticeably when I mix an energy drink (Glucon-D) in it. The ingredients mentioned on the pack are majorly glucose (52%) and sucrose (45%). I believe ...
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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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Heats of formation of neutral molecules and homolytic vs heterolytic bond dissociation in mass spectrometry

I am currently studying the textbook Mass Spectrometry, third edition, by Jürgen H. Gross. Chapter 2.4.3 Bond Dissociation Energies and Heats of Formation says the following: Great efforts have ...
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Why for compounds of elements in a group with larger anions have nearly constant lattice enthalpy?

Why do compounds of elements in a group with larger anions have nearly constant lattice enthalpy? In the reasons for the decrease of the solubility of sulphates of alkaline earth metals, it is given ...
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How to determine the formula for calculating the enthalpy change of a reaction? [closed]

$$\ce{4 HCl(g) + O2(g) -> 2 Cl2(g) + 2H2O(g)}$$ In the reaction above, knowing the enthalpy of formation of each individual compound, in order to calculate the standard enthalpy change of the ...
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Which of the following reaction will have greater enthalpy and why? [closed]

Which of the following reaction will have greater enthalpy and why? $\ce{C (g) + 4 H (g) -> CH4(g)}$ (reaction enthalpy = $x\ \mathrm{kJ/mol}$) $\ce{C (graphite) + 2H2 (g) -> CH4 (g)}$ ...
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Need help understanding Enthalpy

My chemistry textbook says that: Enthalpy (Internal Energy) = chemical potential energy + thermal energy Now, I know that Enthalpy and Internal Energy are different so I really don't understand ...
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how to compute enthalpy of combustion for methanoic acid [closed]

My data source gives the following values for various enthalpies of formation and combustion: formation of CO2 = -393.5 formation of HCOOH = -425 combustion of HCOOH = -254.3 Wikipedia gives ...
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Enthalpy of reaction between ammonia and oxygen

I have to ask this here because I asked literally everybody I know and I still disagree with them. Using the information in the table below, calculate the heat of released from the following ...
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Enthalpy of formation for Lithium ion in ethylene carbonate

The following link has the enthalpy of formation for aqueous lithium http://nshs-science.net/chemistry/common/pdf/R-standard_enthalpy_of_formation.pdf I am wondering how to get the enthalpy of ...
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How do you find the bond enthalpy from an equation?

Which equation represents the $\ce{N-H}$ bond enthalpy in $\ce{NH3}?$ $$ \begin{align} &\textbf{A.} &\ce{NH3(g) &-> N(g) + 3 H(g)}\\ &\textbf{B.} &\ce{1/3 NH3(g) &-> ...
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Ground state elevation for phase transition

I have a question about the illustration from chem.libretexts.org: the ground states of the liquid and gas are offset from that of the previous state by the heats of fusion and vaporization, ...
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Is the heat for constant pressure the same as for constant volume in the formation of nitric oxide?

In the reaction $\ce{N2 (g) + O2(g) -> 2NO (g)}$ my textbook[1] says $q_v$ and $q_p$ are not the same, without any explanation. I know that \begin{align} \Delta U &= q_p + w\\ \Delta U &=...
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How to calculate the temperature change when CaCl2, NaCl, and water added together at an initial temperature of 298 K?

My question: How to calculate the temperature change when adding $\pu{0.1 kg}$ of $\ce{CaCl2}$ and $\pu{0.1 kg}$ of $\ce{NaCl}$ to $\pu{1 kg}$ water with an initial temperature of $\pu{298 K}$? I ...
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Calculating ΔH with non-integer coefficients in the balanced equation

I am working a practice problem out Ebbing's General Chemistry and for part of the problem I needed to calculate $ΔH.$ The problem considers the burning of ethane in oxygen to yield carbon dioxide and ...
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Enthalpy Changes [closed]

0.50 g of Mg is added to a 50 mL HCl solution. The temperature increased by 5.2 degree Celsius. Calculate the enthalpy change for the reaction per mole of Mg that reacted.
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What does higher electron gain enthalpy mean here?

While talking about ionic bonds my book states : Higher the value of electron gain enthalpy of the atom, greater the ease of formation of the anion from it, i.e., other atom should have high value ...
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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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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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Is it possible to use the laws of termodynamics for non-ideal systems?

For the reaction $$\ce{2CO(g) + O2(g) -> 2CO2(g)}\quad\Delta H = \pu{−560 kJ}$$ it is given that the reaction occurs at constant volume of $\pu{1L}$ and the initial and final pressures are $\pu{...
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Heat involved in isenthalpic process

Question 28 from [1, p. 4.4]: An ideal gas is expanded irreversibly from $\pu{5 L}$ to $\pu{10 L}$ against a constant external pressure of $\pu{1 bar}.$ The value of heat involved $(q)$ in this ...
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Is enthalpy of combustion the 'heat released' or 'enthalpy change'?

In a constant volume calorimeter, $\pu{3.5 g}$ of a gas with molar weight $\pu{28 g}$ was burnt in excess $\ce{O2}$ at $\pu{298 K}.$ Temperature of calorimeter increased from $298$ to $\pu{298.45 K}$ ...
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Change in enthalpy equation [closed]

If the formula for change in enthalpy is ΔH=ΔU+pΔV, and ΔU = q - pΔV then shouldn't the formula be simplified to ΔH = q regardless of whether the volume or pressure is constant? Is this only true when ...
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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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Enthalpy and heat relationship

They say that enthalpy of reaction is the heat when at constant temperature and pressure. So if enthalpy is $$\mathrm dH = \mathrm dU + p\,\mathrm dV = T\,\mathrm dS - p\,\mathrm dV + p\,\mathrm dV + ...
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Enthalpy change of water [closed]

A given amount of (liquid) water in an open vessel is heated from 20 degree celsius to 50 degree celsius. Suppose it absorbs x joule in the process. What is the enthalpy change in the process? Is it = ...
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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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Chemistry 12: Calculating changes in enthalpy [closed]

I was wondering if I could please get some help with this: In a coffee-cup calorimeter $\pu{100.0 mL}$ of $\pu{1.0 M}\ \ce{NaOH}$ and $\pu{100.0 mL}$ of $\pu{1.0 M}\ \ce{HCl}$ are mixed. Both ...
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How to find the Cp, difference of Cp and heat of fusion from the dependence of vapor pressure on temperature?

The vapor pressure of zinc varies with temperature as $$\log P(\pu{mm Hg}) = -6850/T - 0.755\log T + 11.24 \label{eqn:1}\tag{1}$$ and that of liquid zinc as $$\log P(\pu{mm Hg}) = -6620/...
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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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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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