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

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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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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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29 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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37 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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17 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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28 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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36 views

Does covalent character increase or decrease bond strength?

I am a bit confused 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 Al2O3 to a value ...
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34 views

Covalent character and ionic dissociation enthalpy?

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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42 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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48 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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54 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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168 views

chem question enthalpy changes and determining Delta H

The heat of combustion of 2-propanol at 298.15 K, determined in a bomb calorimeter, is For -33.41 kJ/g. the combustion of one mole of 2-propanol, determine (a) delta U and (b) Detla H So I managed to ...
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45 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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116 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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83 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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35 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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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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208 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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42 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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80 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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489 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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105 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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82 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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74 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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2k 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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68 views

Is this enthalpy change question possible?

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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902 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 ...
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94 views

What is the energy package released to the outside world?

Consider a chemical reaction where two different particles form another one $$O+O_2\rightarrow O_3.$$ I find it confusing how this can be an exothermic process. How to picture the release of ...
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170 views

Algebra of conserved quantities for chemical reactions

So there is the standard formation enthalpy $\Delta H^0_f$ and there is also formation entropy. Are there more (independent) quantities? What about the heat capacity? Is there a general theoretical ...
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Are there endothermic dissolution reactions? [closed]

Are there reactions like $X(s) + n Y(l) \to X \centerdot Y_n(l)$ that are endothermic? What are X and Y then? The material X should also be soluble to material Y.