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. M

    Can the Nature of Reactants Predict Reaction Rates?

    So if a reaction involves Fe2+ and the same reaction involves Fe3+, which one would occur at a faster rate? I know that the nature of the reactants has to due something with this difference in the rate of the reaction but I'm not sure how to explain it. what about C2O42- (oxalate) and 2CO2...
  2. M

    Thermochemistry : What happens when too much hea tis added to a reaction?

    I need to find out whether there is a maximum amount of heat that one can apply to a certain reaction in order to increase its rate, as described by the collision theory. In other words, is there such thing as too much heat when referring to speeding up the reaction in question? Can enough heat...
  3. R

    Thermochemistry Help: Calculating Enthalpy Change of NaOH Dissolving in Water

    Homework Statement When 2.0 g of NaOH were dissolved in 53.0 g water in a calorimeter at 24.0 degrees C, the temperature of the solution went up to 33.7 degrees C. Homework Equations Enthalpy change is equal to the opposite sign of the heat of water. The Attempt at a Solution For...
  4. V

    Thermochemistry: Heat of a Solution

    Homework Statement The heat of solution of potassium acetate (KC2H3O2) in water is -15.3 kJ/mol. If 12.7 g of KC2H3O2(s) is dissolved in 525 mL of water that is initially at 22.2 °C, what will be the final temperature (in degrees Celcius) of the resulting aqueous solution? Homework...
  5. S

    Thermochemistry Homework Problem

    Homework Statement 1. What is the kinetic energy in joules of a 950-lb motorcycle moving at 50 mph ? 2. If the speed of the motorcycle changes from 50 mph (with kintic energy K1) to 25 mph (with kinetic energy K2), what is the ratio of K1 to K2? Homework Equations 1. KE=1/2mv2...
  6. K

    Thermochemistry Help Again How to find mass?

    Homework Statement Imagine that your water heater has broken, but you want to take a bath. You fill your bathtub with 25 kg of room-temperature water (about 25 degrees Celsius). You figure that you can boil water on the stove and pour it into the bath to raise the temperature. How much...
  7. K

    Thermochemistry Help: Finding the final temperature of a mixture.

    Homework Statement A 1.22 kg piece of iron at 126.5 degrees Celsius is dropped into 981 g of water at 22.1 degrees Celsius. The temperature rises to 34.4 degrees Celsius. What will be the final temperature if this same piece of iron at 99.8 degrees celsius is dropped into 350 mL of glycerol...
  8. H

    {Thermochemistry} Hess' law problem

    Homework Statement Benzene (C6H6) is a liquid which burns in air to form liquid water and carbon dioxide. Calculate the heat released by burning one gram of benzene. \DeltaHof, kJ/mol carbon dioxide (g) = -393.5 water (l) = -285.8 benzene (l) = 49.04 Homework Equations 2C6H6 (l) +...
  9. Y

    Thermochemistry (Enthelpy) queery

    I know that: Change in Internal Energy = Heat change - (pressure X change in volume) and at constant volume PV is negligable. But i heard that ENTHALPY is like internal energy but at constant PRESSURE, so PV becomes negligable and enthalpy = heat change - (PV) PLUS (PV) = heat change...
  10. I

    Thermochemistry Problems: How Do I Solve These Two Heat Release Calculations?

    I have more than one problem I need help with. Any help is extremely appreciated. Homework Statement Problem 1. The heat of neutralization of HCl by NaOH is delta H of reaction = -56.2 kj/mol. How much heat is released when 125mL of 1.750 M HCl is mixed with 195mL of 0.667 M NaOH...
  11. B

    How Much Heat is Needed to Convert Ice at -10C to Steam at 110C?

    How much heat (in joules) must be added to change 20.0g of H2O(s) at -10C to steam at 110C? The specific heat of H2O(s) is 2.09 J/gC; of H2O(L) is 4.184 J/gC; and of H2O(g) is 1.84 J/gC. The heat fusion of water is 334.7 J/g and the heat vaporization of water is 2259.4J/g.I feel like we have to...
  12. W

    Calculate \DeltarH0 and \DeltarU0 for Hydrogenation of Ethyne to Ethene at 348 K

    Homework Statement Calculate \DeltarH0 and \DeltarU0 at 348 K for the hydrogenation of ethyne (acetylene) to ethene (ethylene) from the enthalpy of combustion and heat capacity data in Tables 2.5 and 2.7. Assume the heat capacaties to be constant over the temperature range involved. \DeltafH0...
  13. H

    Thermochemistry and Equilibrium of Reaction

    Homework Statement An ice cube of mass 36 g at temperature -10 Celsius is placed in 360 g of water at 20 Celsius. Find thermal equilibrium temperature. \DeltaH(fusion) = 6.007 kJ mol\wedge-1. Molar heat capacity of ice = 38 water = 75 (Joules)/(K mol)...
  14. P

    Thermochemistry 2: Zn w/ HCl & Sulfuric Acid -150 kJ

    hi zinc reacts with HCl according to the eq Zn + 2HCl ------> ZnCl2 + H2 , delta H= -150 kj * IF SULPHURIC ACID IS USED INSTEAD HCl ..jow many moles of sulphuric acid react with excess amount of zinc so the change in heat content is -150 kj
  15. L

    Urgent need Help on Chem (average velocity, Thermochemistry, ionization energy)

    Urgent need Help on Chem... (average velocity, Thermochemistry, ionization energy) 1.) The average velocity of Cl(2) molecules is 2.50 x 10^3 m/sec at a temp of 25 C therefore the average velocity of helium atoms is ?? [ i got the answer 1.05 x 10^4 m/Sec] but I am not sure 2.) A gas is...
  16. E

    Thermochemistry : % Mass Question

    Homework Statement When a 10.0g sample of a mixture of CH4 and C2H6 is burned in excess oxygen, exactly 525 KJ of heat is produced. What is the percentage by mass of CH4 in the original mixture? Given: CH4(g) +2 O2(g) -> CO2(g) + 2 H2O(l) \DeltaH= -890.4 KJ C2H6(g) + \frac{7}{2} O2(g)...
  17. H

    Calculating Molar Heat of Reaction for Ca + HCl

    Homework Statement When 0.612 g of Ca metal is added to 200.0 mL of 0.500 M HCl(aq), a temperature increase of 100C is observed. Assume the solution's final volume is 200.0 mL, the density is 1.00 g/mL, and the heat capacity is 4.184 J/gC. (Note: Pay attention to significant figures. Do...
  18. A

    Calculating the heat of vaporization of chloroform [thermochemistry]

    I am doing a lab writeup and am completely lost. Summary of lab itself: Calculated the vapour pressure of chloroform (TCM) across a range of temperatures (using flask apparatus). Objective is to determine the heat of vaporization of TCM using the variation of vapour pressure with temperature...
  19. H

    How can I calculate the heat lost by the metal in a styrofoam cup calorimeter?

    I'm given: mass metal temp metal mass cool water temp water final temp (after insertion of metal) (note: this is in a styrofoam cup calorimeter) I need to find the amount of heat lost by the metal, qmetal. The only hint is: qmetal = - (qwater + qcalorimeter). Desperately tried...
  20. T

    Thermochemistry Problem: Heat Transfer in the Reaction of Sulfur and Oxygen

    Homework Statement How much heat will be transferred when 0.54g of sulfur reacts with 0.54g of oxygen to produce sulfur trioxide according to the following reaction: 2 S (s) + 3 O2 (g) --> 2 SO3 (g) change in Enthalpy = -790kJ Homework Equations The Attempt at a Solution...
  21. U

    Thermochemistry Lab Help: Solving for CO2's Second Virial Coefficient

    Could someone please provide some assistance as to how to approach this problem. I am doing a thermochemistry lab: The second Virial coefficient of CO2 and I am not sure how to approach this problem. Just a few hints would really be appreciated. I have tried differentiation, integrals no luck...
  22. D

    Thermochemistry combustion problem

    Homework Statement How many grams of methane (CH4 (g)) must be combusted to heat 1,200 grams of water from 26.0 degrees Celsius to 87.0 degrees Celsius, assuming water is in liquid state as a product and 100% efficiency in heat transfer? Given data: mass of H2O = 1.20 kg specific heat...
  23. A

    Verify Thermochemistry answer (no calculations)

    I just want to make sure my logic is correct. Hot coffee in a vacuum flask (thermos) is an example of what type of sytem? Choices: Closed Open Isolated I thought it was closed system because the coffee is not interacting with the surroundings. But one of my classmates said he...
  24. A

    Thermochemistry: Understanding Delta H and Work in Constant Pressure Reactions

    1. For a certain reaction at constant pressure, delta U= -72 KJ, and 39KJ of expansion work is done by the system. What is delta H for this process. 2. I basically have no idea what to do because it's not in my textbook and my professor has never taught this. Maybe If I knew what the delta U...
  25. A

    What am I doing wrong? Thermochemistry

    [SOLVED] What am I doing wrong? Thermochemistry 1. Calculate the heat per mole of CO2 produced for octane. 2. I just balanced the equation and calculated the total heat produced. Subtracted the heat of water produced from it. Then took that and divided it by the number of moles. 2(C8H18)...
  26. A

    Calculating Delta H using Hess's Law for Thermochemistry Homework

    1. The problem is to calculate the delta H. (1/2)A====> B 150 KJ/mol 3B====> 2C + D -125 KJ/mol E + A====> D 350 KJ/mol We have to find the delta H for: B + D====> E + 2C 2. The equation to use is just Hess's Law II 3. Now I have tried to combine these three reactions...
  27. C

    Calculate Grams of Propane Needed for Hot Water Tank | Thermochemistry Help

    Thermochemistry please help! Homework Statement Not sure how to go about this problem. Calculate the number of grams of propane (C3H8) required to heat all of the water in a 50-gallon hot-water tank from 23.4 degrees C to 65.0 degrees C if the water tank itself has a heat capacity of...
  28. S

    How Does Heat Release Affect Water Temperature in Thermochemical Reactions?

    I know this is a physics forum, but this problem is mainly physics, and exceedingly simple: Homework Statement A certain reaction releases about 90 kJ of heat when stoichiometric amounts of reactants react. If the reaction goes to completion in 100 g of water whose initial temperature is...
  29. tandoorichicken

    Help with Thermochemistry Problems: Cu & Water

    I don't remember at all how to do these thermochemistry problems. Can someone please guide me through? You add 30.0 g of Cu (Specific Heat Capacity = 0.3846 J/K·g) at 92.0°C to 150.0 mL of water at 25.0°C in a very insulating calorimeter. Assuming the calorimeter constant (Cc) is zero, and...
  30. P

    What is the Enthalpy and Internal Energy Change for a Thermochemical Reaction?

    N_2_{(g)} + \frac{5}{2}O_2 \longrightarrow N_2O_5 + \mathfrm 533 kJ energy 533kJ energy is released as heat We are given that 101.7g N2 , 102.97g O2 reacted. And this happens at T= 25C and P = 1atm what is the delta H and delta U for this process per mol N2(g). And how much work is done...
  31. denian

    Thermochemistry - Reaction between phosphorous and oxygen

    i come across this fact. red and white phosphorus are allotropes, but the reaction between white phosphorus with oxygen gives out slightly more heat than the reaction btw red phosphorus and oxygen. P4 (s) [red] + 5O2 (g) --> 2P2O5 (s) delta H = -30.2kJmol-1 P4 (s) [white]+ 5O2 (g)...
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