Chemistry Stack Exchange
Chemistry Stack Exchange

Questions tagged [enthalpy]

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

A thermodynamic state function describing the total energy content of a system. The sum of the system's internal energy, $U$, and the product of its volume and pressure, $pV$.

$H = U + pV$

592 questions
16
votes
3 answers

Enthalpy of formation and reaction equations

I'm disagreeing with my chemistry teacher over this enthalpy of formation problem, so can someone tell me if I am right or why I am wrong? The enthalpy of formation of gaseous sulfur trioxide is $-396~\mathrm{kJ/mol}$. What is the enthalpy of the…
2mia
  • 169
  • 1
  • 1
  • 5
9
votes
2 answers

Which of the following species has an enthalpy change of formation = 0 kJ/mol?

Below was a question in the 2012 Australian Chemistry Olympiads: The enthalpy change of formation ($\Delta_\mathrm{f}H^\circ$) for a species at $\pu{298 K}$ is defined as the enthalpy change that accompanies the formation of one of the…
George Tian
  • 1,361
  • 2
  • 15
  • 30
4
votes
0 answers

Why are Born-Haber enthalpy calculations inaccurate for transition metal compounds?

A senior high/secondary school level exam question is about discrepancies between calculated and empirical enthalpies of formation. It asks why a discrepancy “usually” exists for d-block compounds such as copper (II) chloride. For something like…
Tony B
  • 41
  • 1
3
votes
1 answer

What is the standard heat of formation of the nitrate ion, based on the standard enthalpy of formation of silver nitrate?

Given the standard enthalpy of formation of $\ce{AgNO3(aq)} = \pu{-99 kJ/mol}$ and the standard enthalpy of formation of $\ce{Ag+(aq)} = \pu{+105 kJ/mol}$, what's the standard heat of formation of $\ce{NO3-(aq)}$? How can I know this if the…
TMOTTM
  • 795
  • 6
  • 21
3
votes
1 answer

Why is the zero of standard enthalpy of formation a convention?

The standard enthalpy of formation $\Delta H_f^°$ of pure elements is zero by definition. Why is that a convention? It is true that enthalpy is defined unless a constant (like energy and entropy), but enthalpy of formation is actually a variation of…
Mr. Feynman
  • 171
  • 6
3
votes
1 answer

What is meant by electrons moving to lower potential energy?

I am currently learning about chemical bonds in chemistry, and came across somebody who wrote this (source): "Chemical bonds certainly "contain" potential energy, and the atoms want to move to a lower potential energy (become more stable). When…
Christopher U
  • 339
  • 1
  • 2
  • 8
2
votes
0 answers

Why are negative enthalpies becoming more positive described as decreasing?

This website and my lecturer have describe negative enthalpies that are becoming less exothermic as decreasing. A relevant diagram from the linked to website to illustrate what I mean: I see the enthalpy of hydration is decreasing in magnitude but…
whois
  • 29
  • 1
2
votes
0 answers

Why is ΔH (enthalpy)=Σreactants-Σproducts and not Σproducts-Σreactants?

I was told that ΔH=Σreactants-Σproducts in regards to finding the enthalpy, given average bond enthalpies. Why is the case? It seems counterintuitive, because of the delta, which would imply ΔH=Σproducts-Σreactants. Is this the case, because of the…
Ron Lauterbach
  • 29
  • 1
  • 3
2
votes
1 answer

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 enthalpy of formation of the different terms of the…
FinnTheHuman
  • 580
  • 5
  • 15
2
votes
0 answers

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 enthalpy of formation at $298$ K is…
Marin
  • 21
  • 2
2
votes
1 answer

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) $\ce{HCl + NH3}$. I used the equation $q =…
Ungeheuer
  • 121
  • 1
  • 1
  • 5
2
votes
0 answers

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 following questions (by the way I am a physicist): Is…
Taiki
  • 121
  • 2
2
votes
1 answer

Determining the Average Bond Enthalpy for the C-F bond

Given the following data, how would you work out the average bond enthalpy for $\ce{C-F}$ bond. I've tried setting up the chemical equations and applying Hess's Law, but that's not getting me anywhere. $\Delta H_\mathrm…
deusy
  • 497
  • 2
  • 5
  • 14
2
votes
1 answer

Does enthalpy of dissolution change with temperature?

Dissolution can be either endothermic or exothermic. At higher temperature dissolution is much faster. However, supplying heat also leads to an increase in the internal energy of the system. Ultimately, this affects the enthalphy of product and…
ushan
  • 21
  • 2
1
vote
2 answers

What is V times dP?

Work is defined when there is a change of volume. Mathematically, $W=-\int P\, dV$. When pressure is constant $W= P\, \Delta V$. For an ideal gas expansion we have $W= nRT\ln\frac{V_2}{V_1}$. If its constant volume then $W=0$ in both cases.…
Zillai
  • 211
  • 2
  • 9
1
2 3 4