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

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How to calculate a reaction enthalpy using the Hess law [on hold]

Given the reactions depicted, what is the enthalpy for the last reaction shown?
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17 views

Ideal gasses reaction under const temp and pressure

Conceptual problem: let $A + 2B \rightarrow C + 2D$ be a reaction under constant pressure (P) and temp (T), that goes all the way towards the products. A,B,C,D are ideal gases. let $\Delta H$ be the ...
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55 views

Why is enthalpy of atomization of Mn exceptionally low?

Transition elements usually have high values, especially so for greater number of unpaired electrons. Why the exception here?
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29 views

Can a reaction with positive Gibbs free energy change ever occur?

If you have an endothermic reaction with a negative entropy change, is it still possible to induce the reaction in some way despite the fact that the Gibbs free energy change is positive or all ...
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1answer
30 views

How to measure the enthalpy change of hydration?

I am learning about lattice enthalpies, and my text book talks about determining the lattice enthalpy from the standard enthalpy change of solution and the enthalpy change of hydration of the ions, ...
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1answer
35 views

Helium gas expand isobaric isothermal

If 4 mol helium gas expand against constant pressure in isothermal process. How can calculate w? Is it w=-nrtLn(v2:v1) as define by isothermal or is it w=-p(v2-v1)as define from isobaric? Also, is ...
2
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1answer
37 views

How to calculate bond enthalpies from first principles?

Is it possible to calculate bond enthalpies using the periodic table and the molecule's geometric structure? I am interested in a way of calculating this without getting the answer from measurements ...
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2answers
50 views

Gibbs free energy or reaction enthalphy

If I conduct a reaction say, $$\ce{CaO(s) + H2O(l) \rightarrow Ca(OH)2 (aq)}$$ let us say , I want to measure the energy generated from the reaction.Will this energy I calculate equal the change in ...
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1answer
34 views

What is an example of a very low temperature endothermic reaction?

Shakashiri describes the reaction of solid Ammonium Thiocyanate and solid Barium Hydroxide Octahydrate resulting in an endothermic reaction leading to subzero temperatures. I've performed this ...
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74 views

Heat given off from an electrochemical cell compared to mixing reactants

My confusion regarding the difference between the heat given off in a reaction that takes place in an electrochemical cell compared to in a beaker/appropriate container when the reactants are simple ...
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3answers
58 views

What value of temperature does T represent in Gibbs Free Energy Equation?

Temperature before, or after reaction? Temperature change? Or what is it?
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1answer
52 views

How to calculate a reaction's DeltaH using a MM forcefield calculator software?

I want to calculate $\Delta H$ for hydrogenation of Benzene and producing Cyclohexane using MM+ Force Field Energy. I am using Hyperchem for energy calculation and optimization. I calculated the ...
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1answer
45 views

If the standard state symbol means that the substance is pure (and at 1 bar) how is it possible to have a standard REACTION enthalpy?

For the reaction one mole of substance $A$ in equilibrium with one mole of substance $B$ the standard reaction enthalpy is defined: $$\triangle_rH^{\theta}(T)=H_m^{\theta}(B;T)-H_m^{\theta}(A;T)$$ ...
3
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1answer
173 views

How to determine the molar enthalpy of hydration of sodium sulfate in the laboratory?

There are two separate experiments done in this problem. First experiment 106.6 gram of water at 23.28 C and 8.56 gram of sodium sulfate places into a foam cup (a calorimeter) the temperature to ...
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1answer
42 views

Why is activation enthalpy found and not just rate of reaction?

What is the benefit of finding activation enthalpy and not rate of reaction?
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1answer
22 views

Which enthalpy changes map to the alternative reaction routes under Hess' law?

I am searching for a method that would always allow me to determine which enthalpy changes map to which reaction routes using Hess' law. For example this exam question: And then I end up doing ...
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2answers
152 views

Ethane and ethene: which is easier to burn? Which one burns hotter? Why?

Anything that burns "easy", has a low activation energy for the burning process. Anything that burns hotter, will have a lesser enthalpy and thus, will have a more aggressive exothermic reaction. ...
3
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1answer
420 views

Is a C-F bond stronger than a C-Cl bond?

In hydrolysis/nucleophilic substitution of haloalkanes, the bond enthalpy indicates the rate of hydrolysis (e.g. the $\ce {C-I}$ bond is weaker than the $\ce {C-Cl}$ bond so in hydrolysis the alkane ...
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0answers
21 views

Why do enthalpy of formation values differ for liquid water and gaseous water? [closed]

Why do the enthalpy of formation values differ for liquid water and gaseous water?
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1answer
413 views

Difference between internal energy of combustion and enthalpy of combustion?

I am having real trouble understanding the first law of thermodynamics and I think the question, below, highlights most of my issues: 1) (a) A bomb calorimeter was calibrated by igniting a 0.825 g ...
3
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0answers
45 views

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 ...
3
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1answer
508 views

Validity of Trouton's Rule

I would like to know under what circumstances is Trouton's rule obeyed by liquids and why certain systems i.e. substances may deviate from Trouton's rule. By the way I know what Trouton's rule is ...
2
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1answer
163 views

Why is the Standard enthalpy of formation of O2 and other reference forms of element zero?

Well, from what I know bonding releases energy. And does anybody know a source for finding the reference form of element at standard conditions?
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1answer
476 views

Metal vs. Styrofoam cup calorimeter?

For a metal cup as opposed to a styrofoam cup calorimeter, do you expect the enthalpy of the reaction to be higher, lower, or the same? I think the enthalpy of the reaction would be the same in the ...
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I'm disagreeing with my chemistry teacher over this enthalpy of formation problem, so can someone tell me if I am right or why I am wrong?

The enthalpy of formation of gaseous sulfur trioxide is $-396~\mathrm{kJ/mol}$. What is the enthalpy of the reaction represented by the following balanced equation? $$\ce {2 SO_3 (g) -> 2 ...
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1answer
875 views

What kind of chemicals would exist at the heat death of the universe?

As the universe reaches the maximum entropy, presumably only the most stable chemicals would exist. Assume that all the elements haven't turn into iron yet, what kind of chemicals would dominate at ...
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Why doesn't diamond have $\Delta H_f^\theta=0$?

Why doesn't diamond have $\Delta H_f^\theta=0$, when graphite does? Is it something to do with the definition - diamonds can't really form at STP, even though it is naturally occuring?
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2answers
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Can lattice enthalpy only be determined by the Born-Haber cycle?

Since we are able to determine the enthapy of formation experimentally, why it is impossible to determine the lattice enthalpy directly by experiment?
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1answer
114 views

Is sign convention for delta H and enthapy change of a solution different?

I have got some confusion . In the Ncert Chemistry Tb , it is given in bold that $\Delta H$ is negative for exothermic reactions which evolve heat during the reaction and $\Delta H$ is positive for ...
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3answers
676 views

Why is the enthalpy of vaporization greater than the enthalpy of fusion?

Why is the molar enthalpy of vaporization of a substance larger than its molar enthalpy of fusion (at constant pressure); for example, in the case of ice and water.
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1answer
69 views

Endothermicity of Reaction

Regarding this question, how can one solve this without access to thermodynamic data? I suppose we should keep the following information in mind: 1) Double bonds are not double the strength of ...
4
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3answers
139 views

Is the bond enthalpy of S-O stronger in SO₃ or SO₃²⁻?

Can you please explain the difference between the bond enthalpies in sulfurtrioxide $\ce{SO3}$ and the sulfite anion $\ce{SO3^{2-}}$?
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3answers
41 views

Hess Cycle, determining enthalpy change of formation

I am asked to find the enthalpy change of formation of the following: $$\ce{N2 + 1/2O2 -> N2O}$$ I am given the following enthalpies of reaction: $$1:\:\ce{C + N2O -> CO + N2} \:\:\:\Delta H_f ...
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Will heating diatomic oxygen enough break the O=O bonds?

The bond enthalpy associated with a O=O double bond is equal to 495kJ/mol. Does that mean that adding enough kinetic energy in the form of heat will eventually cause the bonds to break and create ...
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1answer
41 views

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 changes (comparing methods)

I learnt that given a reaction $A \to B$, the enthalpy change is given by, $$\Delta H = \left( \begin{array}{c} \text{total enthalpy of}\\ \text{bonds broken}\end{array}\right)-\left( ...
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1answer
903 views

What is the difference between enthalpy of formation and Gibbs free energy of formation?

I'm having trouble (not alone, apparently) understanding the concept of Gibbs free energy, but I'm beginning to. What I still don't understand, however, is how Gibbs free energy of formation differs ...
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Temperature changes when salts dissolve in water

Please help me! I got this as an assessed investigation question and I really do not know where to start, or how I going to conduct this investigation. I understand that I should include information ...
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1answer
762 views

at constant volume why the change of enthalpy doesn't equal the change of heat

At constant pressure $p=\text{const.}$, change of internal energy equals heat subtract work(PV). So, $q=\Delta E - pV$. So is the the change of enthalpy. So we say at constant pressure, the change of ...
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1answer
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Calculation of the enthalpy change of reactions

First, I'm a high school student and I wonder why every textbook just tells you how to set up those equations and cancel them to get the entropy change but never tells you why you can cancel the same ...
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1answer
54 views

Finding the enthalpy of formation of MagnesiumOxide

I have the following reactions: $\ce Mg_{(v)}+2HCl_{(aq)} \rightarrow MgCl_{2(aq)}+H_{2(g)}$ $\ce MgO_{(v)}+2HCl_{(aq)} \rightarrow MgCl_{2(aq)}+H_2O_{(aq)}$ $\ce H_{2(g)}+\frac{1}{2}O_{2(g)} ...
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1answer
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How much heat (in J) is required to produce 2.0L of $N_2(g)$ at 77.36 K and 1.0 atm?

The only thing that is given is: The enthalpy of vaporization for $N_2(l)$ is 5.56 kJ/mol. There aren't any examples similar to this problem in the textbook. I only know how to get energy using: ...
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0answers
42 views

Enthalpy change of a constant pressure process

In the question, we were expected to find "enthalpy change of a constant pressure process where internal energy of gas is increased $x\ \text{kJ}$ of energy and $y\ \text{kJ}$ of work is done by the ...
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1answer
56 views

Quotable Literature for the Enthalpy of a Proton

A proton obviously has no electronic energy, no vibrational and no rotational degrees of freedom. Therefore I think it is fair to assume, that $$H(\ce{H+})=\frac32\cdot \mathcal{R}\cdot T$$ holds for ...
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2answers
393 views

Does covalent character increase or decrease bond strength?

On one hand, covalent character increases the bond dissociation enthalpy (i.e., more energy is needed to be put in) and on the other it lowers the melting point of $\ce{Al2O3}$ to a value lower then ...
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0answers
43 views

Covalent character and ionic dissociation enthalpy? [duplicate]

I am a bit confused does covalent characteristic within a bond increase or decrease its strength as i have found two counter examples: firstly covelent character increases the enthalpy of dissociation ...
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1answer
119 views

How much activation energy is required to combust propane?

According to WolframAlpha, the change in enthalphy in the combustion of propane is -2220 kJ/mol. How do I calculate the activation energy required start the reaction in the first place? Do you use the ...
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1answer
107 views

Clarifications on the calculation of the $\Delta\mathrm{G}$ of a reaction

I'm trying to calculate the $\Delta\mathrm{G}$ of the following reaction. Are my calculations correct? I'm also a bit puzzled, 'cause this is a endoergonic reaction right? But the $\Delta\mathrm{G}$ ...
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1answer
212 views

How do I calculate the enthalpy change when a gas is being used to heat water?

The question I'm trying to answer is: Calculate the mass of butane gas that would be needed to heat 724 cm³ of water from an initial temperature of 7.44°C to 50.7°C. The thermochemical equation for ...
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1answer
652 views

How to determine Delta H from the heat of combustion?

The heat of combustion of 2-propanol at 298.15 K, determined in a bomb calorimeter, is -33.41 kJ/g. For the combustion of one mole of 2-propanol, determine (a) $\Delta U$ and (b) $\Delta H$ So I ...