Questions tagged [enthalpy]

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

Filter by
Sorted by
Tagged with
2
votes
2answers
3k 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 ...
0
votes
2answers
776 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, ...
1
vote
3answers
599 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 ...
0
votes
1answer
258 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 ...
0
votes
0answers
1k 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 ...
0
votes
2answers
7k 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 ...
0
votes
1answer
354 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} $ ...
4
votes
0answers
2k 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 ...
6
votes
5answers
1k 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 ...
1
vote
2answers
7k 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, ...
13
votes
1answer
783 views

How is enthalpy change equal to heat supplied at constant volume?

We know that: $$\begin{align} H &= U + pV \\ \Delta H &= \Delta U + \Delta (pV) \end{align}$$ Now, If $p$ is constant, $$\begin{align} \Delta H &= \Delta U + p\Delta V \\ &= q_p + ...
0
votes
0answers
57 views

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 ...
10
votes
3answers
1k 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 $\ce{O}$...
0
votes
1answer
2k 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 ...
1
vote
1answer
2k 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 ...
1
vote
2answers
699 views

(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?
1
vote
0answers
60 views

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. ...
2
votes
1answer
101 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 ...
1
vote
0answers
683 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 ...
2
votes
0answers
238 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 ...
1
vote
0answers
128 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 ...
3
votes
2answers
440 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 $...
1
vote
1answer
3k 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 ...
1
vote
0answers
1k 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? $$\ce{...
2
votes
2answers
2k 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 ...
1
vote
0answers
328 views

Standard Enthalpy of Formation for Branched Alkanes

I'm looking at the 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 ...
0
votes
1answer
3k 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
votes
2answers
10k 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 $\...
1
vote
1answer
568 views

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 ...
-1
votes
1answer
194 views

Difference between thermodynamic terms heat and enthalphy [closed]

What is the difference between heat and enthalphy?
1
vote
1answer
42k 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 3)...
2
votes
0answers
58 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 $\ce{...
1
vote
0answers
2k 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 ...
2
votes
1answer
6k 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 $...
1
vote
1answer
600 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 $0.00\ \mathrm{^\circ C}$. ...
4
votes
2answers
1k 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 $P_{\...
1
vote
1answer
6k 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 ...
2
votes
0answers
183 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 ...
1
vote
0answers
352 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 ...
4
votes
1answer
1k 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 ...
3
votes
0answers
732 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 $\pu{298 K}$, followed by an isothermal expansion of our ...
1
vote
0answers
1k views

Is the heat of reaction always negative?

For example, I have a question below that I know the correct answer to, but I don't know why it's positive rather than negative. When $\ce{NaOH}$ is neutralized by $\ce{HCl}$: $$\ce{NaOH(aq) +...
1
vote
0answers
2k 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\\ \ce{...
4
votes
2answers
216 views

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

Considering all gases to be ideal, the enthapy change for the reaction $$\ce{I2 (g) + H2 (g) -> 2 HI (g)}$$ is $\pu{−106.78 kJ}$ at $\pu{350 K}$. To find the change in internal energy the formula ...
1
vote
1answer
85 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 ...
-2
votes
1answer
11k 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 [duplicate]

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 ...
0
votes
1answer
652 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: $\ce{CH4}$ $M = 16\ \mathrm{g/mol}$...
2
votes
2answers
4k views

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 ...
0
votes
1answer
967 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 $7.793\ \mathrm K$. ...
1
vote
0answers
634 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 ...