# What is Thermochemistry: Definition and 81 Discussions

Thermochemistry is the study of the heat energy which is associated with chemical reactions and/or physical transformations. A reaction may release or absorb energy, and a phase change may do the same, such as in melting and boiling. Thermochemistry focuses on these energy changes, particularly on the system's energy exchange with its surroundings. Thermochemistry is useful in predicting reactant and product quantities throughout the course of a given reaction. In combination with entropy determinations, it is also used to predict whether a reaction is spontaneous or non-spontaneous, favorable or unfavorable.
Endothermic reactions absorb heat, while exothermic reactions release heat. Thermochemistry coalesces the concepts of thermodynamics with the concept of energy in the form of chemical bonds. The subject commonly includes calculations of such quantities as heat capacity, heat of combustion, heat of formation, enthalpy, entropy, free energy, and calories.

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1. ### Chemistry Confusion in relation of Gibbs free energy and equilibrium constant

SO2(g)+1/2O2(g)⇌SO3(g);ΔHo=-98.32KJ/mole,ΔSo=-95J/(mole-K). find Kp at 298 Kelvin? In given question at first Δ G will be calculated using formula ΔG = Δ H – T x ΔS, by putting the given values in formula we get ΔG = -70.01 kJ/mol. Then Keq will be calculated using equation = Δ G = -RT ln Keq...
2. ### I Problem regarding understanding entropy

I was reading about thermodynamics postulates when i came over the differnetial fundamental equation: I understand that the second element is just pressure and last element is chemical energy, but he problem is i don't understand what is the use of entropy and how does it contribute to a...
3. ### I Exploring Oscillating Electric Fields for Dipolar Molecules

Hi all! I was wondering, Is it possible, given a specific dipolar molecule, to create the perfect oscillating electric field so as to heat it and not, i.e. the water around it? What I'm basically asking is could there exist a specific microwave just for X and not all dipolar molecules without...
4. ### Thermodynamics of chemical reactions

In chemical reactions generally ΔG < 0 , but if we were to consider a reversible path between pure reactants and products at 1 bar pressure , shouldn't the ΔG = 0 for every reaction ? and if it is due to non-pv work , I don't see any non pv work being done in reactions happing in a closed...
5. ### Calculate the Mass of Ice cubes needed to cool a soft drink

Summary:: TLDR : Drink cooled from 17 C to 7 C with either ice cubes (method 1) or soapstone stones (method 2). Calculate the mass of each item that would be needed to cool the same drink. Given the information provided. In order to cool a drink (“cola” for example) from a storage-room...

7. ### Chemistry Thermochemistry enthelpy change

Can anyone explain how is this reaction ? I 've evaluated the moles of CO and N2 but now i can't see how is this reaction written CO + N2 ...?
8. ### Chemistry Thermochemistry and the heat of complete combustion

The balanced reaction wil be : ##2C_6H_{6(l)}+15O_{2(g)}->12CO_{2(g)}+6H_2O_(l)## in order to compute the the standard enthalpy of reaction : ##\Delta H°_{f} H_2O_(l)= -285,8 \frac {KJ}{mol}##; ##\Delta H°_{f} CO_{2(g)}= -393,5 \frac {KJ}{mol}##; ##\Delta H°_{f} C_6H_{6(l)}=49,04 \frac...
9. ### Enthelpy of combustion of acetylene and fluorine

Hi Folks, I have been trying to find data on the enthalpy of combustion of acetylene and fluorine. I have found the data for oxygen but am curious about the use of pure fluorine in stochiometric conditions and the resulting upper heating value when the oxidant is acetylene. I am happy to...
10. ### Can a Calorimeter Measure Fast Chemical Reactions?

My book introduces Thermochemistry with the concept of heat and how a calorimeter works. After that, it explains the story behind Hess Law and says that one of the reasons it was created is because a calorimeter can't calculate fast chemical reactions, but it doesn't really say the problem with...
11. ### Constant pressure heat of reaction -> constant volume q

Homework Statement For the reaction below, the constant pressure heat of reaction is qp = −3256 kJ mol−1 at 25 °C. What is the constant volume heat of reaction, qV , at 25 °C? 16 CO(g) + 33 H2(g) ⟶ C16H34(l) + 16 H2O(l) Enter your answer in kJ mol−1, rounded to the nearest kilojoule...
12. ### Thermodynamics: calculating work question

1. Homework Statement What are the values of q, w, ΔU, ΔH for the following constant pressure process for a system containing 0.596 moles of CH3OH ? CH3OH(g, 123.0 ºC, 1.00 atm) ⟶ CH3OH(l, 30.0 ºC, 1.00 atm)Molar heat capacity for CH3OH(g), Cp,m = 44.1 J K−1 mol−1 Molar heat capacity for...
13. ### How Do You Calculate Work When Compressing Gas?

Homework Statement What is w when a gas is compressed from 42.1 L to 25.1 L using a constant external pressure of 739 Torr? Remember to include a "+" or "−" sign as appropriate. Homework Equations W = -P(dV) The Attempt at a Solution Pressure = (739 Torr / 760 Torr)*(101.325 kPa) W =...
14. ### How much energy will be released?

Homework Statement The enthalpy of neutralization for the reaction of a strong acid with a strong base is -56 kJ/mol of water produced. How much energy will be released when 195.0 mL of 0.400 M HCl is mixed with 155.5 mL of 0.500 M NaOH? Homework Equations c=n/V The Attempt at a Solution to...
15. ### Calculate the final temperature of the mixture

Homework Statement Consider the following reaction. 2 HCl(aq) + Ba(OH)2(aq) BaCl2(aq) + 2 H2O(l) ΔH = -118 kJ Calculate the heat when 100.8 mL of 0.500 M HCl is mixed with 300.0 mL of 0.450 M Ba(OH)2. Assuming that the temperature of both solutions was initially 25.0°C and that the final...
16. ### Calculate the final temperature of the solution

Homework Statement Consider the dissolution of CaCl2. CaCl2(s) Ca2+(aq) + 2 Cl-(aq) ΔH = -81.5 kJ A 10.6-g sample of CaCl2 is dissolved in 109 g of water, with both substances at 25.0°C. Calculate the final temperature of the solution assuming no heat lost to the surroundings and assuming...
17. ### How Do I Solve This Isothermal Process Thermochemistry Problem?

Homework Statement area=2.25 p1=1bar v1=0.23m3 p2= v2=1m3 Homework Equations Isothermal process W=nrt ln(v2/v1) W=F*D The Attempt at a Solution I got the workdone then sub into the equation of isothermal process. i couldn't get the answer right
18. ### Calculating S-F Bond Energy in SF6 using Standard Heat of Formation Values

Homework Statement Estimate the average S-F bond energy in SF6 The standard heat of formation values of SF6(g) , S(g) and F)(g) are: -1100, 275 and 80 kj / mol respectively. Homework Equations ∆H=∆Hf (products) -∆Hf(Reactants) S(g) +6F)(g) ------>SF6(g) The Attempt at a Solution I...
19. ### Analogies between temperature and time in thermodynamics

Looking through the book of abstracts for «XXI International Conference on Chemical Thermodynamics in Russia (RCCT-2017)» I came across the abstract of talk given by Peter Atkins (University of Oxford) titled «Thoughts about thermodynamics» (you'll find the whole abstract at the end of the...
20. A

### Specific heat capacity, Q = mcθ

Homework Statement Here is the original question (just read the English version). Homework Equations Q = mcθ Specific heat capacity of water, c = 4200 J/kg °C The Attempt at a Solution I did Q_(absorbed) = Q_(released) mcθ = mcθ mθ = mθ And I solved for the final temperature, which is...
21. ### Equation of states for a gas that forms dimers

Homework Statement Show that to a first approximation the equation of state of a gas that dimerizes to a small extent is given by, ##\dfrac{PV}{RT} = 1 - \dfrac{K_c}{V}## Where ##K_c## is equilibrium constant for ##A + A \iff A_2## Homework EquationsThe Attempt at a Solution Using virial...
22. ### What is initial slopes of the plot Z versus P ?

Homework Statement Use the van der waals constant for ##H_2## and ##O_2## to calculate the initial slopes of the plots of compressibility factor Z versus P. Homework EquationsThe Attempt at a Solution Using virial expansion for van der waal gas in terms of ##P## I get ##Z = 1 +...
23. ### Will two isolated atoms form a bond by themselves?

For example: If I isolated (in a perfect vacuum), one Magnesium atom and one Oxygen atom, and they are close to each other, will they make a bond by themselves, with no outside energy? Does this work with any combinable pair of atoms, and what other pairs of atoms would make a bond by...
24. ### Challenge Thermochemistry Challenge Problem - Chet's Paradox

I have a reversible chemical reaction described by the balanced equation: ##aA+bB=cC+dD##. I devise a reversible process to take a closed system containing these species (and its surroundings) from thermodynamic equilibrium state 1 to thermodynamic equilibrium state 2: State 1: a moles of...
25. ### Thermochemistry & Gases in Electrical Engineering?

I'm a first year electrical engineering student and in my General Chemistry class, every other week, we're required to write a paragraph about how each weeks material relates to your future career. As I am in electrical engineering, I don't see much relation to any topic. This week is...
26. ### A problem from thermodynamics -- Freezing of water at 273 K and 1 atm

Homework Statement Freezing of water at 273 K and 1 atm which of the following is true for the above thermodynamics process p) q=0 q)w=0 r)ΔSsys<0 s)ΔU=0 t)ΔG=0 Homework Equations none[/B]The Attempt at a Solution [/B] i got r, s ,t since the reaction happens at constant...
27. ### I How endothermic reactions happen?

Ok, this is a large qiestion. Firstly, from second law of thermodynamics, thermal flow always happens between matters kept in different temparature. But while an endothermic reaction takes place, it may extract some heat from environment and consume it. Isnt it against second law? And for...
28. ### Classical Find the Best Thermodynamics Books for Clear Insight

I want a book on Thermo and statistical physics that will give me a clear insight on every basic thermodynamics systems. I am already done with Shroedar's introduction to thermal physics btw. I basically need a book that clears my concept more than solving problems. And the more on...
29. ### Is this correct Hess' law, thermodynamics

Homework Statement Calculate ∆rH° for the reaction c2h50h+3o2-->2co2+3h20 Given that ∆rU° = - 1373 kJ mol-1 at 298K. Homework Equations c2h50h+3o2-->2co2+3h20 The Attempt at a Solution delta H f CO2(g) = -393.5 kJ/mole delta H f H2O(l) = -241.8 kJ delta H f O2(g) = 0 these values are...
30. ### Entropy change in a reversible isothermal process

Why does ∆S = 0 for a reversible process, but for a reversible isothermal process, ∆S is given by nRln(Vf/Vi) (or other variations of that equation)?
31. ### Thermochem, thermodynamics, diatomic gas at constant volume

Homework Statement A sample consisting of one mole of a diatomic perfect gas is heated from 25 °C to 200 °C at constant volume. Calculate q, w, ΔU and ΔH for the process, given that Cv = 23.02 J K-1 mol-1 . Homework Equations Im studying for mid terms and i have no lecture notes on diatomic...
32. ### Ice in water question - what is final temperature?

Homework Statement An ice cube melts in a 10oC glass of water (mass of water is 225g). If the ice is allowed to melt completely, what will the final temperature of the water be? (we're told that the mass of ice can be ignored) Ti = 10oC Latent heat of fusion = Lf = 334000 J/kg specific heat...
33. ### Supercooled Steam Homework: Solving for Tf, Condensation Fraction & ΔS

Homework Statement You have 0.50 mol of steam which has been supercooled to 95˚C at 1 atm. Since the steam is below the boiling/condensation point, it proceeds to partially condense into liquid water. This happens in a thermally-insulated vessel at constant pressure. (a) When the system...
34. ### Lewis structure and thermochemistry problems

Hi I have a few questions that I have been working on but I don't get the right answer. After drawing the Lewis dot structure of HOClO2, pick the incorrect statement of the following. A. The oxygen bonded to the hydrogen has two lone pairs B. the oxygens not bonded to hydrogen have three lone...
35. ### Adsorption, coagulation, flocculation, and entropy

Homework Statement Adsorption, coagulation, and flocculation are all important processes to remove or separate target substances from mixtures. In these processes, we can observe the spontaneous decrease of entropy.; can we therefore conclude that the second law of thermodynamics doesn't always...
36. ### Thermochemistry relating delta H and q of surroundings

Homework Statement Why does ##\Delta H=\pm |q_{surroundings}|##? Homework EquationsThe Attempt at a Solution Exothermic reactions have a ##-\Delta H## but does that mean that ##q## is a ##+q##. Vice versa for a endothermic reaction. I get confused on which sign i should use for which when i do...
37. ### How Is the Enthalpy Balance Equation Derived for Chemical Reactions?

Hello PF! I have a very short question. We're currently doing enthalpy balances for chemical reactions in our Thermo class. We are using the simple enthalpy balance equation: \Delta H_{\textrm{Reaction}}^0 = \sum \Delta H_{\textrm{Products}}^0 - \sum \Delta H_{\textrm{Reactants}}^0 My question...
38. ### Calculating Heat Transfer in Melting Ice: A Thermochemistry Problem

Homework Statement If 13.4 kJ of heat is added 1.00 kg of ice at 0ºC, how much ice will be melted Homework Equations Change of Heat = mass*change of temperature*constant change of heat fusion (water) = 333 J/g change of heat water vapor = 2260 J/g change of heat total = change of heat...
39. ### Thermochemistry Books: Comprehensive & Fundamental Coverage

Which books can anyone recommend with the most comprehensive or fundamental coverage of chemical thermodynamics and thermochemistry, linking up to equilibrium (including electrochemical equilibrium and potentials preferably), starting from the basics? I find the explanations of the topics of...
40. ### Find the relative mass of the blocks (thermochemistry)

Homework Statement A block of copper at 100.0° C comes into contact with a block of aluminum at 20.0°C. The final temperature of the blocks is 60.0°C. What are the relative masses of the blocks? Homework Equations mcΔT copper specific heat = .386 J/g°C aluminum specific heat = .900...
41. ### When do you use q=mc(Tf-Ti) versus q=c(Tf-Ti) in thermochemistry calculations?

Hello, I am currently studying intro to thermochemistry. I noticed in some problem solutions the equation q=mc(Tf-Ti) but in other the equation is q=c(Tf-Ti). How come sometimes the mass is not used in the equation? When do you know which one to use? Thanks!
42. ### Thermochemistry significance of infinitely dilute solution

Thermochemistry significance of "infinitely dilute solution" Homework Statement (see attachment) Homework Equations The Attempt at a Solution I don't know where to start from and it seems to me that data given is insufficient to solve this problem. Also, what's the significance...
43. ### Thermochemistry, heat calculation.

Homework Statement When 10.00 mL of a solution of strong acid is mixed with 100.00 mL of a solution of weak base in a coffee-cup calorimeter, the temperature rises from 22.8 oC to 26.8 oC. Determine q for the acid-base reaction, assuming that the liquids have densities of 1.00 g/mL and the...
44. ### Thermochemistry- Finding Delta G

Homework Statement Calculate Delta G Degree for 2Mg(s) + O2(g) -> 2MgO(s) Homework Equations Delta G = (Delta G of products - Delta G of reactants) The Attempt at a Solution So according to my book's appendix, the dG of Mg is 0, the dG of O2 is 0, and the dG of MgO is -596.6...
45. ### Chem 1 thermochemistry problem? Am I doing this right?

Homework Statement When aluminum reacts with chlorine how much heat in KJ is released on reaction of 5.00 g of Al for the following reaction: 2AL (s) + 3 CL2(g) ----->2AlCl3 (s) delta h = -1408.4 Homework Equations The Attempt at a Solution Is it right if you start...
46. ### Thermochemistry and the formation of pure oxygen

Homework Statement Pure oxygen was first obtained in the laboratory by the French scientist Antoine Lavoisier in the 18th century by decomposing red mercury(II) oxide, HgO, to give silver mercury liquid and gaseous oxygen: 2HgO(s) \Leftrightarrow 2Hg(l) + O2 (a) From values of \Delta f...
47. ### Thermochemistry - work, heat, total energy

Homework Statement (a) A certain battery runs a toy truck and becomes partially discharged. In the process, it performs a total of 117J of work on its immediate surroundings. It also gives off 3.0J of heat, which the surroundings absorb No other work or heat is exchanged with the surroundings...
48. ### How much heat is evolved Thermochemistry Question

"How much heat is evolved" Thermochemistry Question This is a question from a general chemistry I quiz i had a few days ago. I got this question, and all other questions like it correct, but here's the thing. Calcium oxide and water react in an exothermic reaction: CaO(s) + H2O(l) →...
49. ### Calculating Heat of Reaction in Thermochemistry Problems

I am a high school senior having a problem with a problem in my thermochem class. Could someone please help me understand the following: if 16.8 kJ are released when 1.20 g of F sub 2 (F2) reacts with SiHsub4 (SiH4), what is the heat of reaction when 1.90 moles of Fsub2 react?
50. ### Difference between 'q' and 'ΔH' in thermochemistry?

What is the basic difference between 'q' and 'ΔH' in thermochemistry? I get confused between them! Is there any criteria for ΔH to become equal to q?