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

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How to determine exactly which bonds are in a compound? [on hold]

I am writing a program that calculates enthalpy change using formation, combustion and bond enthalpies. My question is, how can I determine all the bonds in a given compound? More specifically, how ...
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9 views

Enthalpy stoichiometry [on hold]

How much heat will be released when 12 grams of H2 reacts with 76 grams of O2 according to the equation 2H2 + O2 --> 2H2O with an enthalpy of reaction of -571.6 KJ ?
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7 views

Exothermicity of reactions

Which of the following is the most exothermic: a) Sodium Bromide and Water b) Sodium Nitrate and Water c) Sodium Fluoride and Water d) Sodium Carbonate and Water e) Sodium Sulfate and Water I ...
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155 views

Why are there different values for enthalpy of combustion, depending on the calculation method?

For example: Combustion of Ethanol $\ce{C2H5OH(l) + 3O2(g) -> 2CO2(g) + 3H2O (l)}$ If i calculate this using enthalpy of formation: $\begin{array}{c|c} \textrm{Substance} & \Delta \ce{Hº ...
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26 views

What is the meaning of the double tilde in the enthalpy diagram?

My highschool book presents this diagram. I have searched the web and did not found the meaning of this ~~ tildes on y-axis.
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9 views

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 ...
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1answer
45 views

Enthalpy change vs. change in internal energy in a system

I've been trying to get my head around the differences between enthalpy change and internal energy change for a system. Here's my understanding of it so far. I'd appreciate any feedback on my ...
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1answer
37 views

Standard Enthalpy of Formation of H2(g) vs H(g)

My question is somewhat related to this quesion. When I look at a table of standard values there are two entries for Hydrogen: $\ce {H2(g)} $ - 0 kJ/mol $\ce {H(g)} $ - 218 kJ/mol I understand ...
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23 views

Enthalpy change defined at constant pressure only?

Is change in enthalpy defined only at constant pressure? I know Q-W=del(U) And Q at constant pressure equals del(H) (enthalpy change). Can Q (heat given to the system) be used interchangeably ...
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Help with partial pressure question

For the reaction 3 O2(g) -> 2 O3(g), At 30 km above the surface of the earth, the temperature is about 230. K, and the partial pressure of oxygen is about 1.0 ✕ 10-3 atm. Estimate the partial ...
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14 views

Temperature when greater work leaving than heat entering

A reaction takes place in an open system where q is entering the system, work is leaving the system and the magnitude of q is less than w. The question (among others) is would the temperature of the ...
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2answers
22 views

Enthalpy and Internal Energy in Constant Volume Calorimeter

It is my understanding that enthalpy differs from internal energy only in that the change in enthalpy takes into account any pressure-volume work done on the system (or by the system) in a reaction, ...
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3answers
66 views

Intuition on Gibbs Free Energy

In many places, I read that Gibbs Free Energy is called "free energy" because some of the enthalpy that comes from the chemical reaction becomes "waste heat" due to a change in the entropy of the ...
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1answer
14 views

Finding the energy given out when 2 moles of reactants are mixed and ignited from a balanced equation

The question I am trying to answer is this: I only know about Hess' law, bond enthalpies and enthalpies of combustion and have tried to use all concepts in order to somehow get an answer but none of ...
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21 views

Entropy and enthalpy of hydrogen ion?

The absolute entropy of $\ce{H^+ (g)}$ is $\ce{108 kJ/mol.K}$. However, the absolute entropy of $\ce{H^+ (aq)}$ is exactly 0. Why is that? Also, the standard enthalpy of $\ce{H^+ (aq)}$ is 0, so ...
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2answers
57 views

derivation of enthalpy internal energy relationship [closed]

A question asks us to derive a relationship between the enthalpy change and the change in internal energy for a reaction. Now, I know the relationship is: $\Delta H = \Delta U + V \Delta p + p \Delta ...
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37 views

Difference between enthalpy and internal energy for a reaction

Zinc metal reacts with dilute nitric acid according to the equation: Zn + 2(HNO3) = H2 + Zn(NO3)2 calculate the difference between the delta H and delta U for this reaction at a temperature of 298 ...
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1answer
26 views

Finding value of equilibrium constant

I tried to find the value of equilibrium constant several times. I failed.. the value of equilibrium constant at 500K has enthalpy 250kJ/mol and entropy 48J/mol*K the answer is $ 2.45*10^{-24} $ ...
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33 views

Solubility Enthalpy of NaHCO3 [closed]

What is the solubility enthalpy of $\ce{NaHCO3}$ at 25 degree Celsius ($\mathrm{298K}$)?
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41 views

Theoretical temperature change in lime hydration

Calcium oxide (quicklime) and water have an exothermic reaction $\ce{CaO + H2O \rightarrow Ca(OH)_2} \quad \quad \Delta H^{\circ}_f=-65.3 \; \mathrm{kJ \cdot mol^{-1}}$ Assume that I have 1 kg of ...
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2answers
90 views

How much energy is stored when H+ is converted to H2?

I'm doing a write-up for this for my students: Source: Amirav Research Group (Facebook) The energy of formation of isopropyl alcohol is $-318.2~\mathrm{kJ/mol}$ (kilojoules per mole), that of ...
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2answers
84 views

Relationship between enthalpy change and endo/exothermic reactions

Something that confused me, is there a relationship between enthalpy change and endo/exothermic? Does region "x" represent the enthalpy change? What represents enthalpy change, kinectic energy, ...
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80 views

How is ΔH = q_v?

We know that: $$H = U + PV$$ $$\Delta H = \Delta U + \Delta(PV)$$ Now, If $P$ is constant, $$\rm \Delta H = \Delta U + P\Delta V$$ $$\rm \Delta H = q_p+w+P\Delta V$$ $$\rm \boxed{\Delta H = ...
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How to use the enthalpy of formation to assess the energy stored in a molecule?

The standard enthalpy of formation for glucose is -2801 kJ/mol, while the standard enthalpy of formation for carbon dioxide is −393.5 kJ/mol. If one simply examine the numbers attached to the ...
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299 views

How to calculate an enthalpy change for this reaction

I've got an assignment to solve problem number 4 in this worksheet, that is to calculate change in enthalpy for reaction: $$\ce{CHCl3 + O -> COCl2 + HCl}$$ How do I solve it? Why is oxygen ...
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35 views

Determining Enthalpy Change of the Combustion of Ethanol

A worked example went something like this: Use the following experimental data to determine the enthalpy change when 1 mole of ethanol (C2H5OH) is burnt: Mass of water = 150.00 g Initial ...
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70 views

Why are reactions of acids with metals exothermic?

I realised the other day that I'm not sure why the reactions of strong acids with metals are exothermic. Any reaction is exothermic if there's more energy released from bonds being made than is used ...
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(P1,T1) -> to (P2,T2) then what will be the change in enthalpy for ideal gas?

I need to prove that it is equal to the integral of $C_pdT$ from $T_1$ to $T_2$. But should not be the case only when the pressure is constant?
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Where can I find the standard enthalpy of formation for Sodium Cyanamide (Na2NCN)?

I am trying to find the standard enthalpy of formation for sodium cyanamide. Ive looked on various databases that list data about specific compounds but I can't find anything on this particular one. ...
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20 views

Find dE dissolution for salt

Part of my lab is to find the dEdissolution for a series of salts. I am told that dE ~ dH and a table which looks like this: Unfortunately in class we have never been given a lattice energy or ...
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1answer
41 views

When is the Assumption that Enthalpy is Constant over Temperature Change Invalid?

In a question, I was asked to integrate the Gibbs-Helmholtz equation to derive some formula. Then the next question asked: Identify one assumption that you have made in deriving the above equation ...
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How to calculate enthalpy change of neutralisation - what mass?

How to calculate enthalpy change of neutralisation - what mass? I made an experiment where I reacted 100ml HCL 2M with 100ml NaOH 2M in 1300ml Water, and got a temperature change dT. My question is ...
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35 views

What steps would you take to solve this enthalpy change problem?

The answer is shown below: But is there a defined process for solving this problem?
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42 views

Estimate bond strength from bond energies

The standard enthalpy of formation for NO(g) is 90. kJ/mol. Use this and the values for the O=O and N≡N bond energies to estimate the bond strength in NO. Attempted work: 90= 2(NO) - (941+495) NO ...
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29 views

Enthalpy change with temperature

Recently I stumbled across a book that describes enthalpy change with temperature, specifically for a product of combustion process (e.g. CO as a result of solid carbon combustion). The book says that ...
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24 views

Energy required to break salt bridges in peptides and proteins

Is there a way to calculate the energy required to break a salt bridge in a peptide or on the exposed surface of a protein in aqueous solution? Is there any useful literature on this topic? I'm ...
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Chemical energy + energy of electrons

The amount of energy released in a chemical reaction is equal to variation of the enthalpy of this system (In general). The energy released is usually is in form of light and heat. When light is ...
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93 views

What is the qualitative definition of enthalpy in beginner terms?

So in my coursebook it says enthalpy is a property that accounts for the increase in internal energy and the accompanying work done by the system and that its equal to the sum of the internal energy ...
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Why is the enthalpy of solution for NaCl 3.88 and not 4.1 kJ/mol?

Based on Hess' law, the molar enthalpy of solution is equal to the sum of the enthalpies of formation of products minus reactants. In the case of dissolving table salt ($\ce{NaCl}$) in water, those ...
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Calculating energy change for calcium carbonate decomposition

The $\Delta H^0$ for the decomposition of $\ce{CaCO3(s)}$ is $178.3~\mathrm{kJ/mol}$ at $25~\mathrm{^\circ C}$ and $1.000~\mathrm{atm}$. What is the $\Delta E$ for the decomposition process? ...
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14 views

Why does n-butane have a higher enthalpy of combustion than i-butane? [duplicate]

I noticed that n-butane has a higher enthalpy of combustion (-2878 kJ/mol) than i-butane (also known as isobutane or 2-methylpropane) (-2869 kJ/mol). However, I don't see why there is a difference. ...
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49 views

Enthalpy of Reversible Adiabatic Expansion

Suppose an ideal gas undergoes a reversible adiabatic expansion. Explain why the the enthalpy change is given by $\mathrm{\Delta H = c_v\Delta T}$ My Attempt I know that $\mathrm{q = 0}$ and ...
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8 views

Enthalpy of Formation of Alkali Metal Halides [duplicate]

A book that I am currently reading contains the following lines :- The Enthalpy of formation for fluorides become less negative as we go down the group, whilst the reverse is true for the Enthalpy ...
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2answers
42 views

Enthalpy change and equilibrium

I don't understand why enthalpy change won't change at an equilibrium? The reason I ask this is because of this question: At first I thought the answer is C, but I realised that the amount depends ...
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ΔHf for Branched Alkanes

So I'm looking at heat of formation for branched and straight Alkanes and I've noticed a few odd trends I'm having trouble explaining. For straight-chained Alkanes the addition of Carbon atoms to the ...
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14 views

Factors Affecting Single Displacement Exothermic Reaction [duplicate]

For the reaction of a solution of copper (II) sulphate with zinc. $$\ce{CuSO4_{~aq} + Zn_{s} -> Cu_{s} + ZnSO4_{aq}}$$ Would increasing the concentration of the copper (II) sulfate solution ...
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27 views

Use bond dissociation energies to calculate reaction enthalpy

Find $\Delta_\mathrm{r}H$ for the reaction $\ce{2CH4 -> C2H6 + H2}$, given the following bond dissociation energies: $\ce{C-H}$: $410~\mathrm{kJ~mol^{-1}}$ $\ce{C-C}$: ...
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1answer
387 views

Finding/explaining equilibrium constant, chemical significance, and standard enthalpy

I tried doing these questions and just wanted to know if there are any that I should look back at and fix. Please don't edit this. This looks exactly how it looks on my paper and in the book I am ...
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442 views

Does Gibbs free energy of formation always equal zero for elements in standard state?

I was looking through the appendix in my chemistry textbook when I noticed that every compound that had a $\Delta H_f$ of zero also had a $\Delta G_f$ of zero. So of course the compounds that have a ...
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1answer
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Doubt related to Relation between enthalpy and Cv

MY ATTEMPT We already know that $\Delta U=nC_v\Delta T$ which is for constant volume Now we know that $\Delta H = \Delta U + P\Delta V$ so $\Delta H = nC_v\Delta T + 0$ (as at constant volume ...