Questions tagged [enthalpy]

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

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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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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_\...
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Calorimetry, finding final temperature

I was looking for some help on understanding this practice question from my text book, I know how to figure out enthalpy, but this is my first temperature change question and I am stumped. Any ...
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Can the enthalpy change of a reaction be changed, and if so, will this affect the value of the equilibrium constant?

In the case of an arbitrary equilibrium reaction, is it possible to change the absolute enthalpy of the final products, thus the reaction enthalpy change? For example, say pH affects the charge ...
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Reaction enthalpy [closed]

In an experiment, we measured the temperature difference in a reaction of $2.5\ \mathrm g$ of $\ce{Na2SO3}$ with $25\ \mathrm{ml}$ of $10{-}13\ \%$ $\ce{NaClO}$ in a calorimeter with $300\ \mathrm{ml}$...
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Enthalpy of solution

If NaCl dissolves in water, it is said that it happens because the hydration energy is greater than the lattice energy. If so, the enthalpy of solution attained is about +3.88kJ/mol, implying that it ...
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Is this the true expression for isochoric enthalpy of non-ideal gases?

$\def\d{\mathrm{d}}$ $\def\D{\Delta}$ I recall seeing an equation in a thermodynamics lecture, $$\tag{non-ideal gas at constant $V$} \D H = n \int_{T_1}^{T_2}\! C_{P,\mathrm{m}}\ \d T$$ where $C_{...
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Reactivity order in hydrogenation reaction of but-2-yne and butene isomers

Question 1: The reactivity order towards hydrogenation of the following compounds is: but-2-yne ($\ce{CH3-C\bond{3}C-CH3}$) cis-but-2-ene ($\ce{CH3-CH=CH-CH3}$) trans-but-2-ene ($\ce{...
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Why are condensation reactions endergonic?

My lecture notes state that Condensation reactions, like all biosynthetic reactions, are endergonic. I am unsure as to why this would be the case. I thought bond formation was an exothermic ...
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Can a reaction of the form A + B → AB ever be endothermic? [duplicate]

I understand that forming a bond between $\ce{A}$ and $\ce{B}$ is an exothermic process, with negative enthalpy, but won't there be energy required to get $\ce{A}$ and $\ce{B}$ close enough together ...
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Enthalpy Change Of Formation

Enthalpy change of formation means the heat change when 1 mol of the compound is formed from its elements under standard condition. For example, the enthalpy change for the formation of carbon dioxide ...
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How to find the temperature change of a solution?

Right now I'm doing a calorimetry lab and for the pre-lab we have been asked to determine the temperature change (in Celsius) of ammonium chloride in water. The question asks for the expected ...
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Heat exchange between tin and ethanol

This is a pretty straightforward heat/energy exchange problem, but I can't figure the answer out. A constant-pressure calorimeter contains $250.0\ \mathrm{mL}$ of ethanol (density = $7.89\ \mathrm{...
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Total oxidation of toluene free enthalpy

I have trouble finding whether this reaction is spontaneous or not : $$\ce{C7H8 + 9O2 -> 7CO2 + 4 H2O}$$ I consider (from the CRC Handbook of Physics and Chemistry) : And so I find : $$\Delta_r ...
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How do you find the enthalpy change for a given reaction?

$0.500\,\mathrm g$ of magnesium chips are placed in a coffee-cup calorimeter and $100.0\,\mathrm{mL}$ of $1.00\,\mathrm M$ $\ce{HCl}$ is added to it. The reaction is: $$\ce{Mg(s) + 2HCl(aq) -> ...
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Validating Phase Diagram of a mixture

In lab I did several different runs to create a phase diagram for the mixture between biphenyl and naphthalene. My phase diagram is attached, just for visual purposes. My question is I'm supposed to ...
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Why is the reaction enthalpy the same as change in enthalpy for each of the reactants and products?

How come the $\Delta H$ of the reaction is the same $\Delta H$ for each of the reactants and products? For example, in an equation, the $\Delta H$ of the reaction is $244 \,\mathrm{kJ}$. When my ...
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Do the products in exothermic reactions have higher kinetic energy than the products? Do they also have lower chemical energy?

I understand that in an exothermic reaction energy is transferred to the surroundings. More energy is produced from making bonds than absorbed to break bonds. I would imagine that the products have ...
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Example of an endothermic reaction that can sustain itself for at least an hour?

Recall that in physical chemistry, the spontaneity of a reaction at constant pressure and a given temperature $T$ can be quantified by the Gibbs free energy $$\Delta G = \Delta H - T \Delta S$$ A ...
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Enthalpy of a reaction changes with temperature, does this mean bond strengths change with temperature?

During chemical reactions, the bonds between atoms break or form to either absorb or release energy. The result is a change to the potential energy of the system. The heat absorbed or released from a ...
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Enthalpy of formation of ammonium bicarbonate

I'm helping with correcting lab reports, and one of the experiments the students did was heating of ammonium bicarbonate, $\ce{NH_3HCO_3}$ in powder form, over Bunsen burners, which turns the compound ...
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How are standard bond enthalpy values experimentally produced?

The question's essentially it. I've always wondered about how these values are produced, since we refer to them heaps in the lab to assess our experimental results, but I don't actually know how they'...
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What is the correct definition of hydration enthalpy and why is it always negative?

The first definition of hydration enthalpy I found on the net is: The hydration enthalpy is the enthalpy change when $1$ mole of gaseous ions dissolve in sufficient water to give an infinitely ...
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Internal Energy Definition, Why is dU zero when dT is zero?

According to my Equation sheet, $$\delta U=\left(\frac{\delta U}{\delta T}\right)_V\,\delta T+\left(\frac{\delta U}{\delta V}\right)_T\,\delta V=C_V\,\delta T-C_V\left(\frac{\delta T}{\delta V}\right)...
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Discrepancies between specific heat of vaporization of water and enthalpy of formation of water and steam

According to Wikipedia, the enthalpy of formation of water is $-285.8~\mathrm{kJ/mol}$ while the enthalpy of formation of steam is $-241.818~\mathrm{kJ/mol}$, implying the following: $$\ce{H2O(l) -&...
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Internal Energy vs Enthalpy

I struggle to see the difference between internal energy and enthalpy, or at least the need for differentiation between the two. I understand that enthalpy, supposedly unlike internal energy, accounts ...
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Why does the energy required to break a bond decrease as the number of bonds increase?

I noticed that the energy to break a single C bond is $348\ \mathrm{kJ}$, to break a double C bond would be $612\ \mathrm{kJ}$, and to break a triple C bond would be $837\ \mathrm{kJ}$. How come the ...
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Are all bond-forming processes exothermic? [duplicate]

Are all bond-forming processes exothermic? If so, why don't helium form bonds with neon?
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C-C bond dissociation energy

Have the various $\ce{C-C}$ bonds in straight chain hydrocarbons, have the same dissociation energies? Or does it depend on the location of the carbon in the chain? In other words, are the terminal ...
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The dissolution of ammonium chloride

The dissolution of ammonium chloride is used to cool a container of water placed in the solution. It's an endothermic process. What absorbs the heat and what loses it?
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Differences in specific free radical reactions enthalpies

Iodination of alkanes using iodine $(\ce{I2})$ is usually an unfavorable reaction. Tetraiodomethane $(\ce{CI4})$ can be used as the iodine source for iodination, in the presence of a free-radical ...
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The natural equilibrium of water

Why can the equilibrium reaction which exists in water be simply represented as $$\ce{H2O <=> OH- + H+}$$ rather than $$\ce{2 H2O <=> OH- + H3O+}$$ Doesn't this change both the ...
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How is latent heat of vapourisation utilized in changing state?

If latent heat of vapourisation is utilized in overcoming intermolecular forces of attraction between the participle of a material then how is it still hidden there? If latent heat is used in ...
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How is heat physically released in an exothermic process?

Many non-covalent interactions (e.g. electrostatic interactions) result in decreased entropy and are driven by negative enthalpy. If the heat released in the reaction has its physical basis in an ...
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Is enthalpy of activation the same as activation energy? Is there a purely mathematical way to find that value?

I asked about the effect of pressure on a reaction, and since then I learned a lot more. One of the things I learned was the Eyring equation: $$\kappa = \frac{\kappa_B T}{h} e^\frac{\Delta^\ddagger S°...
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Looking at the equation of a reaction, how do we know it is endotermic or exotermic

For example, $MnCO_3 → MnO + CO_2$ is endotermic, and occurs at 473 K the polymerization of ethylene $H_2C=CH_2$ to $n(H_2C=CH_2)$ is exotermic I suspect it has something to do with the standard ...
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The road to solvation

To understand the behavior of solubility, it is helpful to look at the chemicals that misbehave, and apply the rules, and look for underlying explanations and common threads. Generally speaking, the ...
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Why don't we use enthalpy of fusion and enthalpy of vaporisation as two separate terms in Born-Haber's cycle?

I am not able to understand why we use enthalpy of sublimation rather than enthalpy of fusion and enthalpy of vapourisation of solid sodium metal in a Born-Haber cycle for the formation of NaCl from $...
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Does ∆Gº exist?

To the best of my understanding, the quantity of Gibbs free energy of an entity depends on its concentration in the system. Equilibrium is reached when ∆G = 0, or when the Gibbs free energies of all ...
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For what is reaction enthalpy more relevant than reaction free energy?

I know enthalpy is a component of the Gibb's Free Energy (along with entropy). For practical purposes, though, isn't Gibb's Free Energy what a chemist is actually interested in, as that determines the ...
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Is ΔS of a system related to temperature and change in enthalpy?

I often see the equation $\Delta S_\text{universe}$ = $\Delta S_\text{surroundings}$ + $\Delta S_\text{system}$, where $\Delta S_\text{surroundings}$ can be re-expressed in terms of the enthalpy ...
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Why is the specific heat of ethanol not representing experiments

I am in my last year of high school completing an assignment hot and cold packs, and particular the enthalpy behind them. My major lack of understanding is why my calculations are working for heat of ...
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Is the reaction of silver nitrate and copper endothermic or exothermic?

Is the following reaction endothermic or exothermic and why? $$\ce{2 AgNO3 + Cu -> Cu(NO3)2 + 2 Ag}$$ EDIT: I have a feeling that it is exothermic due to the fact that it is spontaneous and will ...
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Understanding Gibbs free energy and enthalpy

Let's say that a reaction releases -50kJ/mol of Gibbs free energy (delta G), and -75kJ/mol of enthalpy (we know that -25kJ/mol went to make sure that the total entropy of universe increased). Now let'...
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How does the relative amount of oxidation of different alcohols affect the amount of energy released?

I am investigating the enthalpy of combustion of different primary alcohols and a website I found briefly mentioned how the relative amount of oxidation of different alcohols affects the amount of ...
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Is the standard enthalpy of formation always non-positive?

If the standard enthalpy of formation is defined as the energy change when 1 mole of a substance is formed from its elements in their standard states and forming chemical bonds is an exothermic ...
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Why are there different values for enthalpy of combustion, depending on the calculation method?

Why are there different values for enthalpy of combustion, depending on the calculation method? Take for example the combustion of ethanol: $$\ce{C2H5OH(l) + 3O2(g) -> 2CO2(g) + 3H2O (l)}$$ If I ...
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Predicting Extent of Exothermic Reactions

Sodium Acetate is widely used in hot packs due to the fact that it produces an exothermic reaction when it reacts with water. I decided to see if other sodium salts, such as sodium bromide, sodium ...