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

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Enthalpy and Bond Dissociation Energies

I'm not sure if I understand the reasoning behind this particular question. The sigma and pi bonds of $\ce{C=C}$ have a combined bond dissociation energy of 632 kJ/mol. Using this information, ...
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33 views

Hess' Law contact process question

In the contact process, the commercial process of producing $\ce{H2SO4}$, several reactions occur before the final product is produced: $\ce{S8_{(s)} +8O2_{(g)} -> 8SO2_{(g)}}~~~~~~~~~~\Delta ...
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20 views

What is heat of vaporization? How can it be used at temperature as low as 25 °C?

When I first learned this term. I was introduced to this graph: The heat in this case is at 100 °C, which I had ever since believed to be the only temperature of vaporization, assuming vaporization ...
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1answer
27 views

Is there more than one equation for Delta T(reaction)?

So, I have to find the $\Delta T_{\mathrm{reaction}}$ of a solution for my lab report. Earlier in the lab, it gave the equation as $\Delta ...
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50 views

If the temperature of a solution decreases, what is the sign of the enthalpy change?

Here is what seems to be a straightforward question: you dissolve some ammonium chloride in water at $25\ \mathrm{^\circ C}$, lowering the temperature of the solution. Is the change in enthalpy for ...
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45 views

Plotting ΔG vs Temp for Electrochemical Reaction

I’m doing an electrochemistry lab that requires me to construct various galvanic cells in an attempt to solve for $\Delta H$, $\Delta G$, and $\Delta S$ values of an unknown. During one part of the ...
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How can I work out the entropy change of benzene when it changes from vapour into liquid?

I’m stuck on a particular problem which I can’t work my head around: Information has been given that: The boiling point of benzene at atmospheric pressure is $353~\mathrm{K}$ and the enthalpy of ...
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50 views

How do I convert J to kJ/mol?

I'd like to convert from J to kJ/mol. I understand how to convert between J to kJ, however, I don't know which amount of moles I'm supposed to use from the reaction. Products? What if I have two ...
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30 views

Hess's Law - enthalpy of combustion

The enthalpy of combustion of a solid carbon to form carbon dioxide is $-393.7\ \mathrm{kJ/mol}$ carbon, and the enthalpy of combustion of carbon monoxide to form carbon dioxide is $-283.3\ ...
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29 views

Using enthalpies of formation and combustion to find formation enthalpy of Acetylene (Hess' Law)

I understand this is a simple question (I’m in high school), but for some reason the numbers just are not working for me. Given: $$\Delta H^\circ_\text{c} = -1301\ \mathrm{kJ}\ \text{for}\ ...
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92 views

Is time a measurement of Entropy?

Is time a measurement of entropy? Is it because time is increasing forever? Well after reading about the Second Law of Thermodynamics and working with Entropy, I began to realize that if time can be ...
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76 views

What is the enthalpy of combustion for ethene given the values of enthalpy of formation?

I have answered this question and got a value of $\mathrm {1409 \space kJ\ mol^{-1}}$ however, the answer sheet says $\mathrm{-1009}$. Using the equation: $$\ce{C2H4 + 3O2 -> 2CO2 + 2H2O}$$ I ...
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42 views

What is the significance of enthalpy definition not under constant pressure?

Under constant pressure (and only PV work), change in enthalpy can be shown to equal change in heat. If enthalpy is not meaningful outside of constant pressure, then why not use heat instead? So I ...
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1answer
47 views

Heat Capacity temperature dependence and Gibbs energy

I got this problem in school as a tool to get ready for International Chemistry Olympiad, but I have some problems with this challenge. So, the problem is to calculate as precise as possible the Gibbs ...
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39 views

What is the enthalpy change for the following equation? [closed]

The equation we need it to change to \[\ce{FeO + CO ->Fe + CO2}\] The three equations and delta heat: \[ \begin{align*} \ce{3Fe2O3 + CO &-> 2Fe3O4 + CO2} &\Delta H=-47.0\ \mathrm{kJ}\\ ...
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66 views

Are all naturally occuring reactions exothermic?

If a reaction is endothermic, then the reaction must take energy from the environment to occur (as far as I know), and if this is the case, then to me the reaction seems energetically unfavorable, as ...
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15 views

Why is it that reactions that cause an increase of temperature are exothermic? [duplicate]

I've been taught that if you have two aqueous reactants that react together and produce heat it is exothermic. Shouldn't that be endothermic? Did my professor misspeak? If it's gaining heat, that ...
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122 views

Where to find data for gibbs energy, enthalpy, and entropy digitally

Where can you find the enthalpy and gibbs free energy of formation and entropy of compounds either as a data file or web api (I technically only need two since I can figure out the other one from two ...
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97 views

Determining the Average Bond Enthalpy for the C-F bond

Given the following data, how would you work out the average bond enthalpy for C-F bond. I've tried setting up the chemical equations and applying Hess's Law, but that's not getting me anywhere. ...
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23 views

How do you figure out the dissociation energy for ions?

Specifically $\ce{KF}$ into the $\ce{K+}$ and $\ce{F-}$ ions. I can find tables on the DE into atoms, but where would I go to find the DE between ions? I am also given that the bond length is ...
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68 views

How to calculate the enthalpy change for the formation of ethylene epoxide from ethylene?

Question 4, iii.) in here: (iii) The equation for an important reaction of ethene is given below. I get everything up until the very end. We need to identify the bond enthalpies first: ...
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39 views

How to determine the sign of the enthalpy of formation?

During the formation of $\ce{Al2O3}$ from 5.4 grams of $\ce{Al}$ and enough oxygen, the temperature of 2 kg of water climbs by 20 degrees. What’s the enthalpy of formation of $\ce{Al2O3}$ (per mole) ? ...
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1answer
41 views

Trying to figure out heat of reaction

A question on a textbook is to find the "standard heat of reaction" of this reaction: $\ce{C8H16N2O3(aq) + H2O(l) = C6H13NO2(aq) + C2H5NO2(aq)}$ As far as I understand it, it should be a rather ...
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Relationship between thermodynamic reversibility and reactions

Often we're concerned with changes in thermodynamic properties during reactions, like $\Delta H_p = Q_{reaction}$ (change in enthalpy for an isobaric process). What seems odd to me is that in the ...
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19 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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91 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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64 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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71 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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45 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 ...
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58 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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69 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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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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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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102 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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61 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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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: $$\triangle_rH^{\theta}(T)=H_m^{\theta}(B;T)-H_m^{\theta}(A;T)$$ ...
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1answer
362 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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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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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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390 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. ...
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733 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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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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969 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 ...
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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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1answer
752 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 ...
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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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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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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?