# Enthalpy Definition and 67 Threads

Enthalpy (listen) is a property of a thermodynamic system, and is defined as the sum of the system's internal energy and the product of its pressure and volume.
It is a state function used in many measurements in chemical, biological, and physical systems at a constant pressure, that is conveniently provided by the large ambient atmosphere. The pressure–volume term expresses the work required to establish the system's physical dimensions, i.e. to make room for it by displacing its surroundings. As a state function, enthalpy depends only on the final configuration of internal energy, pressure, and volume, not on the path taken to achieve it.
The unit of measurement for enthalpy in the International System of Units (SI) is the joule. Other historical conventional units still in use include the calorie and the British thermal unit (BTU).
The total enthalpy of a system cannot be measured directly because the internal energy contains components that are unknown, not easily accessible, or are not of interest in thermodynamics. In practice, a change in enthalpy is the preferred expression for measurements at constant pressure, because it simplifies the description of energy transfer. When matter transfer into or out of the system is also prevented, at constant pressure the enthalpy change equals the energy exchanged with the environment by heat.
In chemistry, the standard enthalpy of reaction is the enthalpy change when reactants in their standard states (p = 1 bar, T = 298 K) change to products in their standard states. This quantity is the standard heat of reaction at constant pressure and temperature, but it can be measured by calorimetric methods in which the temperature does vary, provided that the initial and final pressure and temperature correspond to the standard state. The value does not depend on the path from initial to final state since enthalpy is a state function.
Calibration of enthalpy changes requires a reference point. Enthalpies for chemical substances at constant pressure usually refer to standard state: most commonly 1 bar (100 kPa) pressure. Standard state does not strictly specify a temperature, but expressions for enthalpy generally reference the standard heat of formation at 25 °C (298 K). For endothermic (heat-absorbing) processes, the change ΔH is a positive value; for exothermic (heat-releasing) processes it is negative.
The enthalpy of an ideal gas is independent of its pressure, and depends only on its temperature, which correlates to its internal energy. Real gases at common temperatures and pressures often closely approximate this behavior, which simplifies practical thermodynamic design and analysis.

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1. ### Chemistry How to determine whether a single reaction is better than multi-stage reaction?

The above question is the final part of a problem in Silbey, Alberty, and Bawendi's Physical Chemistry. The main part of the problem had to do with various calculations of changes in enthalpy at 500K, which I completed successfully. The single reaction is endothermic with reaction enthalpy...
2. ### Chemistry How do we use enthalpy of formation in this calculation?

This is a problem from the book "Physical Chemistry" by Silbey, Alberty, and Bawendi. The end of the book says the answer is ##41.572\text{kJ/mol}##. Here is how to obtain this answer There is a table a the end of the book with the following entry The fourth column is...

5. ### I Calculating Entropy & Enthalpy: SRK, Wilson, Antoine & Clausius-Clapeyron

I would like to calculate the entropy or enthalpies (steam, specific and inner energy) using the SRK [suave-redlich-kwong] equation, the Wilson approximation and (if necessary) the Antoine equation. and the Clausius-Clapeyron equation for a mixture of 0.199 mol/l nitrogen and 0.811 mol/l carbon...
6. ### Thermodynamics Question Regarding Gibbs Free Energy, Enthalpy, Entropy

Hello, is someone able to explain why these two are wrong. I am not sure how to figure out the enthalpy direction as the reaction is not changing state of matter, nor is it changing temperature. (Please solve without calculating anything) Thank you
7. ### Calculating heat released from excess enthalpy

Hello, I am currently trying to determine the method to calculate the heat absorbed / released when a gas and liquid mix. I believe that this is known as the heat/enthalpy of mixing (or excess molar enthalpy). I also believe, that if this value is negative it means an exothermic reaction (heat...
8. ### I Is Enthalpy a Valid Concept Despite Dimensional Mismatch?

By definition (Anderson J) enthalpy, h = e + pV It looks to me like adding apples and oranges. Look at the dimensional analysis: e, energy is ML2T-2 P, pressure is ML-2 V, volume is L3 Thus PV is ML which is quite different from e:ML2T-2 Am I missing something?
9. ### High Temp γ-Fe Enthalpy > Low Temp α-Fe Enthalpy: Why?

Why is the enthalpy of γ-Fe at high temperatures higher than the enthalpy of α-Fe at low temperatures?
10. ### Question about the rules of thermodynamics

Hello , we learned in thermodynamics that to calculate ΔHr °(of the reaction ) using ΔHf °(standard heat of formation ), we have to respect that ΔHf °=ΔHr ° of a reaction forming 1 mol of compound from pure elements in their most stable form at standard state . the problem is when we want to...
11. ### Enthalpy Explained: All You Need to Know

Hello , I don’t think enthalpy makes sense, I am here because I want a clear explanation for what it really is. It has been said it is the internal energy and the volume it takes as in work (W) but why? Why do we even need enthalpy what is the problem with delta U! It is equal heat and...
12. ### Can Calorimeters Measure Enthalpy Variation of Fast Reactions?

First of all this is a question that I had while reading some concepts of my book, so this isn't a homework question. I have started reading the thermochemistry chapter of my book, and it shows the story of Hess' Law and says that it was created is because a calorimeter can't be used to measure...
13. ### Heat released during combustion at constant pressure vs volume

Greetings! I've been brushing up on some thermodynamics recently and came across a perplexing sentence in my notes and text from undergrad. It says that for a combustion reaction, such as the combustion of heptane: C7H16 (l) + 11O2 (g) ---> 7CO2 (g) + 8H2O (l) That this process carried out at...
14. ### Enthelpy & Internal Energy Change relation with Cp & Cv

Homework Statement Match the following Given : Processes do not include chemical reactions. Assume CP,m and CV,m are independent of temperature for given substance and consider only pressure-volume work in given all processes. Homework Equations ΔU = Q - W ΔH = ΔU +...
15. ### Chemistry: Heat of Sublimation and Hydrogem Bonds Problem

Homework Statement Using the heats of fusion and vaporization for water, calculate the change in enthalpy for the sublimation of water: H2O(s) --> H2O(g Using the delta H value given in Exercise 24 and the number of hydrogen bonds formed to each water molecule, estimate what portion of the...
16. ### Comparing delta(PV) vs P(deltaV)

Going from: E = q + w To: H = E + (PV) I'm confused as to why you add the product of the pressure and volume of the system to the internal energy to get enthalpy. Is it just because "that's what enthalpy is defined as"? I think I understand that when holding pressure constant, the (PV)...
17. ### Unsure Regarding Temperature Model for an Open System

Hi guys, I am currently working on my master thesis. I am supposed to make a dynamic model for a gas system and have some trouble setting up the energy balance. I am a noob when it comes to uploading pictures, so I don't have a figure for this, but let's consider a general control volume (CV)...
18. J

### Internal energy of an ideal gas as a function of pressure?

Assuming all gases in the combustion reaction of benzoic acid (C6H5COOH) behave ideally, what is the "exact" change in internal energy? The context in which this question is being asked is after a calorimetry experiment. For all the intents and purposes of calorimetry, the change in internal...
19. J

### Enthelpy of water / ethanol at different ratios -- trend?

Hello, I was doing some research on enthalpy of water-ethanol mixtures at different ratios, and when looking at all the information graphed, I don't understand why it produces the trend line it does. Why doesn't it produce a 'quadratic' trend line? Shouldn't having a 10:90 water-ethanol exert...
20. G

### B Could Copper's 29th Ionization Energy Be Used to Create Uranium-235?

So I was playing with periodic table and discovered Cu's 29th ionization energy(cu with no electron) is 1116105 KJ/mole . Then i searched on google to convert to MeV which is 6.9661798e+21 , then I thought that one uranium 235 produce 200mev so to make that you would need around 3.5e+19...
21. ### I Why is Joule-Thomson effect isenthelpic?

Let's have an experimental apparatus for realization of Joule-Thomson effect. It could be a thermally isolated pipe with a porous board separating two compartments inside. Different pressures can be set in the second compartment. The gas flows through the pipe and we measure temperature and...
22. ### What is enthelpy for combustion of glucose?

The enthalpies of formation with reference to (2) are: ∆(Glucose)= −1,268 /, ∆(Carbon Dioxide)= −393.5 ./, ∆(water)= −285.8 /. How do I calculate the enthalpy for the combustion of 1 mole of glucose? I know H = U + PV and ∆H = Q + Wother, but I am confused by all the chemistry involved!
23. ### A problem in finding enthelpy of metal oxides

Arrange the energy released upon hydration of Magnesium Oxide,calcium oxide,barium oxide,strontium oxide. I placed Magnesium oxide at the last,since magnesium hydroxide is a stable so lot of energy must be released in order to achieve that state,but the answer says calcium oxide.
24. ### Hess's Law Explained | Simple Explanation for Enthalpy Change

Homework Statement I need to research Hess' law, but I can't find a simple explanation. I know that the enthalpy change between the reactants and products is the same as the enthalpy change between the reactants and something else, and between that something else and the products, but I don't...
25. ### Calculating Power of a Water Pump: Enthalpy vs Pressure Difference

Trying to calculate the power of a pump, it's pumping water. It has a flowrate in and out in kg/s. Surely I should look up the enthalpy values and just mutiply with the flow rate to get power in kW. However I know that if I convert flowrate to m3/s by dividing 1000 and multiplying by the...
26. ### Calculating Steam Flow in a Nuclear Power Plant Turbine System

I have a thermodynamics question I'm having difficulty with... I have a steam generator heated by a 39MW nuclear reactor that powers two 4.5MW turbine generators. It produces dry saturated steam at 330psig, and enters the turbine at those conditions. It exits the turbine to a condenser...
27. ### Does the amount of energy affect the dissociation of methane

I hear that methane is known dissociate into free radicals and sometimes into ions (carbocation and hydrogen proton). Is it different amounts energy responsible for the same molecule (methane) to undergoing different kinds of dissociation?
28. ### Why is Qir < than Qr? Carnot cycle and change in G

I am learning physics on khan academy and they do a proof to show that delta G for a reversible reaction is negative and how for a irreversible reaction it is positive. However in the proof, they assume that the heat put in by the isotherm is less for an irreversible reaction compared with a...
29. ### Exploring Enthalpy: Magnesium Reactivity in Hydrochloric Acid Lab Results

I just conducted a lab in which I am testing varying amounts of magnesium to react with a consistent amount of hydrochloric acid, determining which amount of magnesium produces a reaction whose enthalpy is the closest to the theoretical/ideal enthalpy of this equation. I created 3 conditions...
30. ### Change in Enthelpy for Nitrogen at constant temp

Homework Statement Calculate ΔH for 2 mols of Nitrogen for the following transformation: 1 bar, 21°C → 200 bar, 21°C Given that molar Cp = 4R 2. The attempt at a solution How am I supposed to find the ΔH? From what I learned, there can only be a change in enthalpy when there's a change in...
31. ### Heating upon sudden elastic compression of material

Hello, I would expect the heating of an elastic material upon sudden elastic compression to be given simply by the first law of thermodynamics, i.e. Delta Q=Delta U + P Delta V where P is constant since the compression is applied suddenly as in a square-wave pressure pulse (this is equivalent...
32. ### What is Enthelpy Conceptually?

So I understand that enthalpy is energy. But what energy specifically? Is enthalpy heat energy? Is it the energy that bonds atoms together and that's why it can either be released or absorbed? Or is that what Gibbs free energy is? I'm trying to make sense of the equation: ΔG=ΔH-TΔS...
33. ### How is enthelpy different than heat?

Background I'm taking grade 12 chemistry. During a lesson, my teacher briefly mentioned that enthalpy is equal to the internal energy of a system plus the product of the pressure and volume of that system. However, she remarked that the concept wasn't an important part of the curriculum and...
34. ### Constant Volume Calorimetry - Why does (ΔnRT)=(Δn)RT

When doing constant volume calorimetry, the enthalpy can be calculated as follows: ΔH = ΔU + Δ(PV) ΔH = w + q + Δ(PV) ΔH = PΔV + q + Δ(PV) and at constant volume: ΔH = q + VΔP which I've then see people rewrite using the ideal gas law as follows: ΔH = q + (Δn)RT where Δn is the change in...
35. ### What is enthelpy of a system, Macroscopic thermodynamics.

What is enthalpy of a system based on macroscopic POV of thermodynamics and not chemistry? And how do we use it to calculate the total heat transfer in isentropic processes?
36. ### Calculating Enthalpy ΔH of H2O at 101 kPa: Detailed Process & Equations

Homework Statement What is the enthalpy ΔH, when 1 mole of water is heated at 101 kPa from 353 K to 393 K? Outline the process in an H (T) diagram. The following values are available: Cp (H2O, l) = 75.0 J K-1 mol-1; ΔHvap = 47.3 kJ mol-1 at 373 K; Cp (H2O, g) = 35.4 J K-1 mol-1 Homework...
37. ### Need Help understanding energy and enthelpy please

I was studying about Ionization enthalpy and I found that in many books it is described in terms of enthalpy change while in others it is described in terms of energy, although in all of them the basic definition is almost same. It is the enthalpy change when ... or it is the energy required to...
38. ### Calc ∆Ho & ∆Eo for CH3OH Reaction @ 298K

Homework Statement For the reaction CH3OH(l) → CH4 (g)+ 1/2 O2 (g)[/B] (a) Calculate ∆Ho298 (b) Calculate ∆Eo298 (c) Write an equation that would allow you to determine ∆H at 500°C and 1atm Homework Equations ∆H=∆E+∆(PV) ∆H=Σ∆Hproducts-Σ∆Hreactants The Attempt at a Solution a)...
39. ### Designating States of Water [Thermo]

Homework Statement Case, P[bar], T[C], v[m3/kg], u[kJ/kg], x[quality], State a) 1.5, ?, ?, 3000, ?, ? b) ?, 120, .6, ?, ?, ? c) 3, ?, ?, 2543.6, ?, ? d) ?, 190, ?, ?, 20%, ? e) 25, 100, ?, ?, ?, ? f) ?, 240, ?, ?, 0, ? Homework Equations We are supposed to use the steam tables in the back of...
40. ### Why is electron affinity positive?

I was reading thermodynamics and about Born Haber cycle. There, I found that the values given for electron affinity are positive. When an electron is added to flourine, it attains noble gas configuration. So i t becomes more stable. So shouldn't the reaction be exothermic?
41. ### What is the Correct Way to Determine the Enthalpy of Subcooled Water?

Hi! I was looking at some problems in Geankoplis' book on Transport Processes and Unit Ops and I came across this problem involving water at 85°C, atmospheric pressure. The problem required the enthalpy of the water and so the book used the enthalpy of saturated water at 85°C from a steam...
42. ### Transient state energy balance -- Variable volume system

Hello PF! I have some questions regarding the accumulation term for the energy balance on a variable volume system. Suppose we have a tank storing a liquid substance. The tank has a moving boundary at the top, which can expand unlimitedly. The system has a mass input \dot{m}_1 and mass output...
43. ### Ice added to water at 273K, calculate new ice mass

Hi! I'm new here and I can't find anything helpful through google so I thought I'd give PF a try. Sorry for not using the template fully but I have no ideas on how to solve this problem. 1. 0.1kg ice at 263K is added to 1kg water at 273K. Calculate new mass of the ice. 2. I know how to do this...
44. ### Is the difference of two state functions a state function?

Hello everybody, For my thermodynamics test I have to tell whether or not a quantity is a state function, which is obviously not all too difficult when regarding entropy, enthalpy etc. on their own. However there are a lot of questions where it is about "H-S" or "G-H". Are these not always...
45. ### Enthelpy of reaction calculation?

Limestone stalactites and stalagmites are formed in caves by the following reaction: Ca2+(aq) + 2HCO3-(aq) --> CaCO3(s) + CO2(g) + H2O(l) . If one mol of CaCO3 forms at 298 K under 1 atm pressure, the reaction performs 2.48 kJ of P - V work, pushing back the atmosphere as the gaseous CO2 forms...
46. ### Internal Energy Change: Homework Solutions

Homework Statement Internal Energy Change Homework Equations N/A The Attempt at a Solution If internal energy change equals enthalpy change, then I am looking for the reaction which has the enthalpy change closest to zero. I know it cannot be A, as the formation of water is highly exothermic...
47. ### Entropy change in irreversible processes

The equation for entropy S=delta(Q)/T is derived from reversible processes such as Carnot cycle. The delta(Q) in the equation is the reversible heat added or taken out from the system. So, why is this equation valid in the case of processes like cooling of a body which is irreversible?
48. ### Thermodynamics: Enthelpy vs. Gibbs Free Energy usage

I'm currently taking a Biophysics lecture. There's a vast usage of the terms Enthalpy and Gibbs Free Energy. I understood that most of the time, we're dealing with the Gibbs Free Energy, because our experiment is at constant Temperature (e.g. room temperature) and constant Pressure (e.g. 1 atm)...
49. ### Exothermic reaction in adiabatic reactor

Homework Statement One mole of gas A, two moles of gas B, and one mole of inert gas I are fed into an adiabatic reactor of variable volume and constant pressure at 25 °C. At this temperature, the reaction yielding liquid R proceeds normally as: \textrm{A} (g) + \textrm{B} (g) \rightarrow...
50. ### Internal energy, enthelpy and heat capacity

Hello PF! I have some questions regarding these concepts. First of all, are the following expressions valid for any case? i.e. any kind of process, like isochoric, isobaric. \Delta U = \int C_v \ dT \Delta H = \int C_p \ dT Or is the ΔU expression only valid when dV = 0, and ΔH when dP = 0...