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

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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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14 views

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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20 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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21 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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25 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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16 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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12 views

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 ...
3
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2answers
72 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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19 views

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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31 views

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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13 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. ...
4
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41 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
33 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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0answers
18 views

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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0answers
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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23 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
174 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 ...
4
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2answers
141 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
30 views

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 ...
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1answer
43 views

Difference between thermodynamic terms heat and enthalphy [closed]

What is the difference between heat and enthalphy?
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38 views

Joules to kJ/mol conversion

In my AP Chem class we are working on testing Hess's Law and conducted three reactions. Then enthalpy changes of the 1) $\ce{NaOH + HCl}$ and 2) $\ce{NaOH + NH4Cl}$ to predict the enthalpy change of ...
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32 views

Why do these factors affect Enthalpy?

I have done some research into factors that affect energy change and discovered that the physical state of reactants, allotropic modification, pressure of gases and concentration of reactants affects ...
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37 views

Why do some chemical reactions lead to less stable state?

If the universe is always trying to find a more stable energy state, then how do reactions take place that lead to a less stable state e.g. the formation of ammonia from extremely stable diatomic ...
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48 views

Why are absolute enthalpies of reactants not available?

The standard enthalpy of reaction is given as $$\Delta_r H = \sum\nu\; H_m(\text{products})- \sum\nu\;H_m(\text{reactants}).$$ Peter Atkins in his book Elements of Physical Chemistry then writes ...
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1answer
88 views

What is the exact definition of state functions? How to determine state functions?

How do you know if something is a state function? I know Internal Energy (U) is a state function since it is the sum of energies, which is constant at a given time. However I don't know for sure if ...
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25 views

heat of fusion in an equation

You add $100.0\ \mathrm g$ of water at $55.0\ \mathrm{^\circ C}$ to $100.0\ \mathrm g$ of ice at $0.00\ \mathrm{^\circ C}$. Some of the ice melts and cools the water to $0.00\ \mathrm{^\circ C}$. ...
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2answers
69 views

Does enthalpy equal heat when PV work is done?

The derivation that enthalpy equals heat at constant pressure goes like: $$\begin{align} H &= U + P_{\mathrm{int}}V \\ \Delta H &= \Delta U + \Delta (P_{\mathrm{int}}V) \end{align}$$ If ...
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1answer
59 views

Calculating enthalpy of dissolution

In a polystyrene cup calorimeter, $4.3\ \mathrm g$ of ammonium nitrate, $\ce{NH4NO}$, was added to $60.0\ \mathrm g$ of water and stirred to dissolve the solid completely. The initial temperature ...
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39 views

Calculating enthalpy change

My friends and I were talking casually about electrolysis, fuel cells, and whatnots when the following question arose: Is $\ce{H2 (g) -> 2H+ (aq) + 2e-}$ endothermic or exothermic? I have the ...
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24 views

Enthalpy of mixing in a regular solution

In a mixture of $A$ and $B$ where $A$ and $B$ are randomly mixed and the average coordination number is $z$. If there are $n_B$ molecules of $B$ in the mixture these will be in contact with a total of ...
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21 views

What is the standard enthalpy of combustion of maltodextrin?

Writing a lab about yeast metabolism and one metabolite we used was maltodextrin. I cannot find the standard enthalpy of combustion anywhere for this sugar but I can find it for the other sugars we ...
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99 views

What assumptions underlie Hess' law and and its analogues?

Let me first briefly describe the laws as I understand them. Hess' law states that the enthalpy of reaction can be obtained by taking the sum of the enthalpies of formation for the compounds involved ...
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30 views

Entropy and enthalpy change

From an exothermic rxn where ∆H=negative, we know that heat is released to the surroundings so ∆Ssurr=positive. But if it's given ∆Ssys=positive, can we assume that heat is absorbed by the system and ...
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68 views

Determining Pressure of Sublimation of Iodine

The pressure of sublimation of iodine can theoretically be determined by considering the two hypothetical steps: the sublimation of iodine at 298 K, followed by an isothermal expansion of our ...
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2answers
35 views

Finding enthalpy change for an element [closed]

I'm having troubles solving this question since my understanding for enthalpy is not solid. I don't want the answer I just need an explanation and the way to solve it and I will do the rest on my own ...
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66 views

Why does it not matter what mass of NaOH is used to solve for q?

So we recently did a lab in Chem to prove Hess's law. The reactions we performed were $$\begin{align} \ce{NaOH + HCl &-> NaCl + H2O}\tag1\\ \ce{NaOH(s) + HCl &-> NaCl + H2O}\tag2\\ ...
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2answers
106 views

Does the temperature during this reaction remains constant? [duplicate]

Consider all gases to be ideal The enthapy change for the reaction $$ \ce{I2(g) + H2(g) -> 2HI(g)} $$ is −106.78 kJ at 350 K. To find the change in internal energy the formula we use is $$ \Delta ...
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22 views

Enthalpy Change

If I mixed $25.0\ \mathrm{cm^3}$ of $0.350\ \mathrm{mol\space dm^{-3}}$ sodium hydroxide solution with $25.0\ \mathrm{cm^3}$ of $0.350\ \mathrm{mol \space dm^{-3}}$ hydrochloric acid. The temperature ...
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126 views

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 the combustion of butane is: $$\ce{2C4H10(g) + 13O2(g) -> 8CO2(g) + 10H2O(l)} , \quad\Delta H = -5748 \ \mathrm{kJ\cdot mol^{−1}}$$ So far, I've calculated the heat ...
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1answer
57 views

Calculate the amount of energy released when 500 cm³ of methane gas at STP reacts with excess air according to the equation:

$$\ce{CH4(g) + 2O2(g) -> CO2(g) + 2H2O(l)} \qquad \Delta H = -890\ \mathrm{kJ\ mol^{−1}}$$ I am not totally sure how to work this out. What I did, though, was: $\ce{CH4}$ $M = 16\ ...
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78 views

Thermodynamics Enthalpy and Internal Energy for Isothermal Expansion

Considering a system with constant atmospheric pressure , i.e a massless piston sitting in a cylinder containing water. At constant temperature say $\rm 100\,^{\circ} C$, the water turns into vapour ...
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1answer
34 views

Concept of Bomb caloriemeters

Considering the example where $0.3212\ \mathrm g$ of glucose (molar mass = 180) being burnt in bomb calorimeter of heat capacity $641\ \mathrm{J\ K^{-1}}$: the temperature rises by $7.793\ \mathrm K$. ...
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69 views

Molar heat of reaction for water in a neutralization reaction

I have a simple lab problem: $30\ \mathrm{mL}$ of $1\ \mathrm{mol/L}$ $\ce{H2SO4}$ reacts with $50\ \mathrm{mL}$ of $1\ \mathrm{mol/L}$ $\ce{NaOH}$ in a neutralization reaction as per the following ...
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1answer
35 views

Do entropy changes have an equivalent of bond enthalpy?

Is it possible to calculate entropy changes in a way similar to enthalpies from bond enthalpies? For example, to work out the enthalpy change of a reaction you can get the sum of all bonds in the ...
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1answer
33 views

What is the significance of the “Standard Temperature” in Standard Enthalpies of Formation tables?

In my data book, there's a list of common compounds and their molar enthalpies of formation -- at 298.15 K. What's the meaning of this given temperature value (is it the final temperature of the ...
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108 views

Bond Energy Calculation

Dichlorine monoxide (which has the skeleton $\ce{Cl‐O‐Cl}$) is an unstable molecule and explodes readily to form chlorine gas and oxygen gas. The heat of the reaction $\Delta ...
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1answer
74 views

Is there a way to make a liquid cold by a chemical reaction?

I'm looking for a way that 2 substances make a chemical reaction that uses a lot of energy, therefor use heat, and get a substance cold. I'm thinking about a 2 chamber thermos that in the middle ...
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41 views

Find ΔHr for HCl + Mg Reaction

I am having trouble calculating the value of $\Delta H_\mathrm r$ for the reaction: $$\ce{Mg + HCl}$$ The total heat capacity ($C_\text{cal} + C_\mathrm r$) is $6.88\ \mathrm{kJ/{^\circ C}}$, and the ...
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50 views

Bond Dissociation vs Hess' Law Enthalpy

I am trying to determine two of the theoretical values for the enthalpy of the combustion of methanol. I have calculated (using Hess' law) that the $ \Delta H_{\mathrm {reaction}}\; = ...