Questions tagged [enthalpy]

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

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0answers
142 views

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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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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1answer
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Calculating the heat of formation of propane given its enthalpy of combustion

Given the equation $\ce{C3H8 + 5O2 -> 3CO2 + 4H2O}$ and that enthalpies of formation for $\ce{H2O (l)}$ is $\pu{-285.3 kJ/mol}$ and $\ce{CO2 (g)}$ is $\pu{-393.5 kJ/mol}$, and the enthalpy of ...
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714 views

Why can we use standard entropy when determining temperature at which a reaction becomes spontaneous?

In this chemistry problem, $\Delta H$ and $\Delta S$ are given at $298\text{ K}$, and it asks for the temperature at which the reaction becomes spontaneous: For the reaction $2\text{SO}_3 \...
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372 views

Can absolute enthalpy values be used to determine reaction enthalpy?

Our teacher added this question to an exam, and there was a huge disagreement with my classmates and teacher about the correct answer. One side favoured answer 2, the other side favoured answer 4 ...
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1answer
112 views

Is dH(S,P) = dH(T,P) true (always)?

I came across an interesting question with some physical chemistry students today. Based on the following steps, we're uncertain whether the statement in the title is/could be true. Assuming $dN = 0$, ...
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What is the difference between “spontaneous” and “energetically favourable”? [duplicate]

I read somewhere that the difference between energetically favorable reactions and spontaneous reactions is that energetically favorable reactions are ones where energy is released, i.e., $\Delta H<...
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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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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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1answer
95 views

How do scientist obtain the value of enthalpy of formation? [closed]

Do they do it experimentally or do/can they use bond enthalpy to figure it out? For example $\ce{C2H4 + H2 -> C2H6}$. The enthalpy of formation, $\Delta_fH$ of formation for the product is listed ...
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Which of the following species has an enthalpy change of formation = 0 kJ/mol?

Below was a question in the 2012 Australian Chemistry Olympiads: The enthalpy change of formation ($\Delta_\mathrm{f}H^\circ$) for a species at $\pu{298 K}$ is defined as the enthalpy change that ...
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1answer
178 views

How can I calculate the standard reaction enthalpy if I lack one formation enthalpy?

I have this reaction, for which I need the standard enthalpy of reaction: $$\ce{CuO (s) + H2S (g) \rightarrow CuS (s) + H2O (g)}$$ This is the data from the NIST web book (standard formation ...
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Why is the enthalpy of combustion higher for n-butane as compared to 2-methylpropane?

N-butane has an enthalpy of formation of $\pu{-125.6 kJ/mol}$, whereas it is $\pu{-134.2kJ/mol}$ for 2-methylpropane. When comparing the heat set free at the combustion process, why is it higher for n-...
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382 views

Enthalpy - unit confusion

I think I have trouble interpreting reaction enthalpy in these kind of situations: I am given that $\Delta _rH^0 = 25,9$kJ/mole in the reaction: $$\ce{1/2 H2 + 1/2 I2 \rightarrow HI}$$ which I ...
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How do I calculate the standard enthalpy of formation for liquid ethanethiol? Do I use Hess's law? [closed]

Calculate a value for the standard enthalpy of formation for liquid ethanethiol, $\ce{C2H5SH}$. Use the equation given below and enthalpy of combustion data from the following table. Just assume ...
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1answer
2k views

Why is $\Delta G = -T \Delta S_{\mathrm{total}}$ valid only at constant pressure?

According to my book(Elements of Physical Chemistry by Atkins and de Paula, 5th ed.), $\Delta G = - T \Delta S_{\mathrm{total}}$ is valid only for constant pressure and temperature. Enthalpy is ...
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1answer
286 views

Does temperature change pressure in isochoric system?

Consider the following equilibrium reaction: $$\ce{2 NOCl (g) <=> 2 NO (g) + Cl2 (g)} \qquad \Delta H > 0$$ My textbook has a statement that: If you decrease the temperature when ...
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1answer
350 views

What exactly does the “per mole” unit of heat of atomization mean?

I am facing a confusion in the definition of the standard enthalpy of atomization. The confusion arises because of the "per mole" in its units ($\pu{kJ mol^-1}$). My question: When I say "per mole", ...
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1answer
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Why must NaOH be in excess if we wish to measure the effect acid strength has on the enthalpy of neutralisation?

A few different acids are given i.e $\ce{HCl}$ , ethanoic acid, $\ce{H2SO4}$, ethanedioic acid. We want to conduct an experiment in which we measure the temperature rise for neutralisation as a ...
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1answer
202 views

Why does the standard enthalpy of formation diverge so far from the standard Gibbs free energy of formation for some substances?

If you look at a Table of Thermodynamic Values for chemical substances, most substances have very close values for their standard enthalpy of formation ($\Delta H_{\mathrm{f}}^\circ$) and their ...
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Why is the standard enthalpy of formation of black phosphorus not zero? [duplicate]

Black phosphorus is the most stable allotrope of phosphorus. Then why is its standard enthalpy of formation not taken as zero?
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Enthalpy Change

In calculating the change of enthalpy in converting Fulminic to Isofulminic acid, would one simply take the sum of the negative enthalpies of the bonds of Isofulminic acid minus the sum of the ...
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Why increase in enthalpy leads to increase in entropy? [closed]

Given statement: Enthalpy of adsorption is the strength at which molecule (say $\ce{H2}$) is bound to the adsorbent (say some metal center). For a free $\ce{H2}$ molecule, the entropy contribution is ...
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1answer
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Entropy and specific heat capacity

I have seen the equation $S(T_2)=S(T_1)+C_p\ln(T_2/T_1)$ where $C_p$ is the molar heat capacity at a constant pressure. I understand that this assumes that the temperature range is sufficiently small ...
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225 views

What does “heat is evolved at standard-state conditions” mean?

When $\pu{0.539 g}$ of $\ce{Na(s)}$ reacts with excess $\ce{F2(g)}$ to form $\ce{NaF(s)}$, $\pu{13.3 kJ}$ of heat is evolved at standard-state conditions. What is the standard enthalpy of formation ($\...
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1answer
127 views

Can the enthalpy change of allotrope transformation can be done with Hess cycle?

My A- level textbook has some points about the uses of Hess cycle. For example: indirectly determining the lattice enthalpy, vaporisation enthalpy etc. But, my book also says it is possible to ...
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1answer
62 views

Do electric charges increase activation barriers?

The activation energy governs the rate of reaction, and is thought to be equal to the energy needed to break the bonds needed to initiate the reaction. But if we have a reaction like: $$\ce{A +B -&...
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1answer
473 views

How to determine the mass of ammonium nitrate needed and enthalpy of solution?

I am trying to figure out how to determine the mass of ammonium nitrate needed to reduce the temperature of solution from $\pu{23 ^\circ C}$ (measured room temperature at start) to $\pu{3 ^\circ C}$, ...
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481 views

Enthalpy definitions. What are their main differences?

The definition of enthalpy is properly described in here. The principal objective of this question is mainly academic. In order to any students with internet connection can find a definition and an ...
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256 views

Equation of enthalpy

This question arises because: by giving classes in thermodynamics, I have observed that students are often confused between the different definitions (or applications) of the enthalpy concept. The ...
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4answers
1k views

Why change in enthalpy is negative?

I noticed that the enthalpy of the products of an exothermic reaction (bonds formed are stronger than bonds broken) is lower than the enthalpy of the reactants. Enthalpy equals internal energy plus $...
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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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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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1answer
659 views

Why do enthalpy changes of combustion and formation not take into account oxygen?

I've seen many enthalpy changes in my studies and I don't understand why the oxygens on either side of reactions are not calculated (especially if the unreacted oxygens don't balance). For example: ...
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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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1answer
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Do both H (enthalpy) and ∆H, or ∆H alone have a physical significance?

My chemistry teacher said that H (enthalpy) alone has no physical significance it is ∆H or change in enthalpy that has a physical significance. But my book says that both have a significance and H ...
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1answer
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Finding change in enthalpy for a reaction with an atom in elemental state using bond energy

For example, consider $\ce{C(s) + O2(g) -> CO2(g)}$. If we know the bond energies for $\ce{O=O}$ and $\ce{C=O}$, can we determine $\Delta H$, even though there is carbon in the reaction? Can we ...
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1answer
349 views

Relationship between heat and standard enthalpy for a reversible reaction

The enthalpy change for a reaction is equal to the heat absorbed at a constant pressure. My notes say that the standard enthalpy change is the heat absorbed when the reactants (in their standard ...
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1answer
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Enthalpy of reaction [closed]

Please someone help me with this question How is d possible?
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1answer
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How do I evaluate the PV term when calculating a change in enthalpy?

As we know enthalpy is defined by the equation $H = E+PV$. Now to calculate this $PV$ factor for $\Delta H$, Sometime we take $P \Delta V$ and sometime $P_2V_2-P_1V_1$. Why are there so many ...
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Can somebody help me understand this sentence?

"The molar enthalpy of vaporisation of a liquid is the enthalpy change when one mole of the liquid changes into one mole of its gas at the boiling temperature." The part which i am not sure of is the ...
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How to determine the specific heat of aluminum?

I have a question from my AP preparation book that I got wrong and do not understand the explanation: A student designs an experiment to determine the specific heat of aluminum. The student heats a ...
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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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2answers
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Difference between lattice energy and thermal stability

For an ionic compound, is it always true that the greater the thermal stability, the greater the lattice energy. E.g. for 2 ionic compounds MX and MY, if MX has a higher thermal stability than MY, ...
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3answers
448 views

Calculating standard combustion enthalpy of dodecane from that of heptane & octane

These are the problem from UKChO-2015. Given the standard enthalpy changes of combustion ($\Delta_\mathrm{c}H$) for heptane ($n = 7$) and octane ($n = 8$) are $-4816$ and $-5470~\mathrm{kJ~mol^{-1}}...
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Thermodynamics: Relation between different types of energy: My Analogy

I have always been confused between the different kind of energies in thermodynamics so today I thought of finally getting it right. Here's my analogy(which is wrong and I need help in improving it, ...
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1answer
468 views

how do i find how much energy needs to be removed?

How much energy must be removed to convert $8.49\ \mathrm{g}$ of water at $24.5\ ^{\circ}\mathrm{C}$ to an ice cube at $-12.0\ ^{\circ}\mathrm{C}$? Give your answer as a positive value in Joules ...
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Why is N2 enthalpy zero, entropy 191.5, yet G is listed as zero and not [-T*0.1915]?

In standard tables, $\ce{N2}$ at STP has an enthalpy formation ($\Delta H_\mathrm f$) listed as $0\ \mathrm{kJ/mol}$, entropy ($S$) as $0.1915\ \mathrm{kJ/K}$, and free energy of formation ($\Delta G_\...