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

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Find the amount of moles [closed]

How can I calculate the amount of moles of water that is formed given a theoretical value of -55.90 kJ/mol $\ce{H2O}$ that is formed and from using the amount of joules of heat that is liberated.
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38 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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25 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 ...
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1answer
120 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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52 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
103 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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2k views

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
835 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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57 views

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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58 views

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
49 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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183 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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54 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 ...
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99 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
26 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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7answers
839 views

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
40 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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2answers
101 views

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
325 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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993 views

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
269 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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48 views

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
41 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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55 views

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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37 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
42 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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265 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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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
88 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
95 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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133 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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359 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 ...
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58 views

Intuition for electrode potential

If I connect a zinc half cell (Left hand side) and a copper half cell (right hand side), I get an E cell value of 1.1V taken from the voltmeter. If I replace the voltmeter with a light bulb, I will ...
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330 views

Bond enthalpy vs. enthalpy of atomization confusion

H-H bond enthalpy is 436 kJ/mol. To break apart a mole of H2 into H atoms, I apparently need to heat it by 436 kJ. Does this only apply at the standard temperature of 298 kelvin? What about H2 gas ...
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143 views

Mathematical basis of why enthalpy of mixing is 0 for ideal gas

For this case, let's say there are two identical chambers connected by a small tube, constant pressure and temperature. There's a closed valve that separates the two chambers. Within the chambers ...
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38 views

Enthalpy: when and how does it vary?

I have been looking for a physical explanation for entropy and enthalpy variations. An example would be the air system of a vehicle, where people say we have a strong enthalpy and entropy variations ...
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3k views

Calculating Standard Enthalpy of reaction $\Delta_rH^{\Theta}$, which reactant to choose for the number of mole

For example, this reaction: $$\ce{2NaOH + H2SO4 -> Na2SO4 + H2O}$$ Suppose both concentration are 1M (standard condition). There is 0.02 mol (20$cm^3$) of $\ce{NaOH}$, and 0.01 mol (10$cm^3$) of ...
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310 views

What non-toxic non-water substances have a freezing point very close to water's?

I was linked to a Kickstarter for a cooling ball to be placed in drinks which claims with constant marketing hyperbole how much it relies on "Phase Change" ! which is a neat concept and seems not ...
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1answer
54 views

Why do molecules have to first transform to gas before doing a reaction?

I have to calculate the enthalpy of the reaction $$\ce{Cs(l) + 1/2I2(s) -> CsI(s)}$$ and the answer is $$\Delta H_f = \Delta H_{vap}^{\ce{Cs}} + \frac{1}{2}\Delta H_{subl}^{\ce{I2}} + ...
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Difference between exothermic and exergonic

In High School I learned that an exothermic reactions releases energy, while an endothermic reaction needs energy to occur. Now I learned that there is a separate, somewhat similar classification ...
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1answer
137 views

A theoretical molecule that could match uranium mole-for-mole

XKCD #1162 piqued my curiosity. Obviously, there's nothing we generally use as a chemical fuel that can match the energy density of uranium fission. But I wondered what it would take, hypothetically, ...
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1answer
630 views

Is the activation barrier of a reaction related to the free energy or heat of formation of its reactants?

The energy barrier for activation appears in the derivation of a bimolecular reaction through collision or transition state theory, but it seems that from looking at diagram such as this one it seems ...
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1answer
128 views

Thermodynamic Cycle

What is the value for $\Delta_f G^0(\ce{Cl^-}, \ce{aq})$ in this thermodynamic cylce: Do you agree it should be (with the formation free energy of $\ce{H^+} = 0$) $$-\Delta_f G^0(\ce{Cl^-}, ...
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1answer
91 views

Calculating $\Delta_fH$ of $\ce{AgNO3}$

Given the standard enthalpy of formation of $\ce{AgNO3(aq)} = -99 \text{ kJ/mol}$ and the standard enthalpy of formation of $\ce{Ag+(aq)} = +105 \text{ kJ/mol}$, what's the standard heat of formation ...
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When is it okay to use $Q=mc\Delta T$, is this equation only for calorimetry questions?

When is it okay to use $Q=mc\Delta T$, is the equation only for calorimetry questions? When a question talks about heat how do I know if i'm measuring $\Delta H$ or Q?
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106 views

What defines the specific enthalpy of atomization?

1 mol of F2 is turned into 2F (atoms). Is this the specific enthalpy of atomization, or two times that?
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Increasing the solubility of Sodium Hydroxide

$\ce{NaOH -> Na^+ + OH^-}\ \ \ \ \Delta H ^o = -44.51 \ kJ/mol$ The dissolution of sodium hydroxide in water is an exothermic process, and so, according to Le Chatelier's principle, cooling the ...
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3k views

Enthalpy Calculations, Heat of Formation/Combustion

I just don't understand how to do these and would really appreciate your help. 1) Ammonium dichromate decomposes in a reaction when heated. Calculate the heat transferred for the decomposition of ...
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1answer
90 views

How can oxygen atoms suddenly appear on the product side of an equation?

The question states the following equation: $$ \ce{C(s) + 2H2(g) -> CO2(g) + 2H2O}$$ And then asks you to calculate the enthalpy change for it, given enthalpies of combustion. The stated values ...
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1k views

Methane seems to dissolve more readily in cold water. Why?

When you are diluting air in the system and/or properly evacuating it, you must consider water temperature. Methane is completely dissolved in water at 42 degrees Fahrenheit (5.5 C) but can be ...