What is Heat of combustion: Definition and 31 Discussions

The heating value (or energy value or calorific value) of a substance, usually a fuel or food (see food energy), is the amount of heat released during the combustion of a specified amount of it.
The calorific value is the total energy released as heat when a substance undergoes complete combustion with oxygen under standard conditions. The chemical reaction is typically a hydrocarbon or other organic molecule reacting with oxygen to form carbon dioxide and water and release heat. It may be expressed with the quantities:

energy/mole of fuel
energy/mass of fuel
energy/volume of the fuelThere are two kinds of enthalpy of combustion, called higher and lower heating value, depending on how much the products are allowed to cool and whether compounds like H2O are allowed to condense.
The high heat values are conventionally measured with a bomb calorimeter. Low heat values are calculated from high heat value test data. They may also be calculated as the difference between the heat of formation ΔH⦵f of the products and reactants (though this approach is somewhat artificial since most heats of formation are typically calculated from measured heats of combustion). For a fuel of composition CcHhOoNn, the (higher) heat of combustion is 418 kJ/mol (c + 0.3 h – 0.5 o) usually to a good approximation (±3%), though it can be significantly off if o + n > c (for instance in the case of nitroglycerine (C3H5N3O9) this formula would predict a heat of combustion of 0). The value corresponds to an exothermic reaction (a negative change in enthalpy) because the double bond in molecular oxygen is much weaker than other double bonds or pairs of single bonds, particularly those in the combustion products carbon dioxide and water; conversion of the weak bonds in oxygen to the stronger bonds in carbon dioxide and water releases energy as heat.By convention, the (higher) heat of combustion is defined to be the heat released for the complete combustion of a compound in its standard state to form stable products in their standard states: hydrogen is converted to water (in its liquid state), carbon is converted to carbon dioxide gas, and nitrogen is converted to nitrogen gas. That is, the heat of combustion, ΔH°comb, is the heat of reaction of the following process:

CcHhNnOo (std.) + O2 (g, xs.) → cCO2 (g) + h⁄2H2O (l) + n⁄2N2 (g)Chlorine and sulfur are not quite standardized; they are usually assumed to convert to hydrogen chloride gas and SO2 or SO3 gas, respectively, or to dilute aqueous hydrochloric and sulfuric acids, respectively, when the combustion is conducted in a bomb containing some quantity of water.

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  1. A

    Chemistry Constant Volume Heat of Combustion from heat capacity of calorimeter

    Q=heat capacity calorimeter*(-)change in T*moles =0.009089mol*-6.8C*4.38kj/C =-0.2707kj/mol This answer is wrong but it was the only one I could come up with right now. I just noticed units in the answer would be wrong too. Any suggestions?
  2. J

    What is the heat of combustion of 1 kg carbon dioxide w/ magnesium?

    I am trying to find out the heat of combustion of one kilogram of carbon dioxide with magnesium. I am looking for results for stochiometric conditions of these two reactants. I have found the following equation online 2 Mg(s) + CO2 ---> 2 MgO(s) + C(s) yields a standard enthalpy of --810.1...
  3. S

    Calculate the molar heat of combustion of anthracene

    Homework Statement A 2.56g sample of anthracene, C14H10, was burned to heat an aluminum calorimeter (mass=945 g). The calorimeter contained 1.50 L of water with an initial temperature of 20.5 C and a final temperature of 34.3 C. a) Calculate the molar heat of combustion of anthracene...
  4. Saharka

    Heat of combustión of a mixture of two substances?

    Hello chemists, need some help here. I'm trying to calculate the power output of a certain otto cycle using different fuels, specifically gasoline, ethanol and different combinations of the two like 85% ethanol and 15% gasoline or 50% ethanol and 50% gasoline. However I have found it difficult...
  5. Hamal_Arietis

    Calculate Heat of Combustion of Graphite

    Homework Statement The heat of combustion of graphite and that of CO is 394kJ/mol and 283 kJ/mol, respectively. When 12g of graphite is combusted incompletely, the same volume of CO and CO2 are generated. Calculate the heat of generated by this combustion. Choose the closet value.(kJ) A.197...
  6. B

    Heat of Combustion of N2H4 - Calculating Final Temperature

    Homework Statement The combustion of hydrazine, N2H4, produces nitrogen gas and water vapor. The heat of combustion for this reaction is -618 kJ/mol. If 1.6 g of hydrazine are combusted in a bomb calorimeter at 298 K and with a heat capacity of 6.2 kJ/C, what will be the temperature of the...
  7. Stephanus

    Producing Methane from Composting: Benefits & Process

    Dear PF Forum, I'm interested in composting. I want to understand the benefit of composting. Besides Reducing organic waste Producing fertilizer This composting process also produce energy (methane) if properly managed. I wonder how, without oxygen addition/introduction, in a sealed composter...
  8. Stephanus

    How can I calculate heat of combustion and enthalpy using Hess's Law?

    Dear PF Forum, I have read this link in Wiki. But I don't understand what it means. Can someone help me? https://en.wikipedia.org/wiki/Heat_of_combustion A: Heat of combustion of CH4 is 50.09 MJ/kg https://en.wikipedia.org/wiki/Standard_enthalpy_change_of_formation_(data_table) B: Enthalpy of...
  9. kester

    Effect of temperature on the heat of combustion

    Like in a topic. I just got this question and I am looking for some help. I found this one http://wikieducator.org/The1stLawofThermodynamicsLesson5 I don't know, is the heat of combustion is the same as entalphy?
  10. gfd43tg

    Can the heat of combustion only be found experimentally?

    Hello, Can the heat of combustion only be found experimentally? I am looking on this page at the heat of combustion of ethanol http://www.ausetute.com.au/heatcomb.html and I see that ##\Delta H^{o}_c = 1368 \hspace{0.05 in} kJ/mol##. But if you calculate from its constituents ##C_{2}H_{6}O...
  11. Q

    Calculate Heat of Combustion of C2H2

    Homework Statement Given the following enthalpies calculate the molar heat of combustion of C2H2. Heat of formation of liquid water: -285.8 kJ/mol Heat of formation of C2H2 (gas): -26.7 kJ/mol Heat of formation of CO2 (gas): -393.5 kJ/mol Homework Equations Heat of combustion is...
  12. D

    Determining Heat of Combustion for a Fuel and Oxygen Mixture

    Homework Statement n A student performs a combustion experiment by burn- ing a mixture of fuel and oxygen in a constant-volume metal can surrounded by a water bath. During the experiment, the temperature of the water is observed to rise. Regard the mixture of fuel and oxygen as the system...
  13. T

    Heat of Combustion for D2 Gas

    Hey Guys, do you know where I can find the heat of combustion for D2 gas? Kind Regards, -TP
  14. P

    Thermochemical question about the heat of combustion of a hydride

    Homework Statement What is the heat released by the complete combustion of 22.4x10^3 litres of FeTiH2 - a fuel used in hydrogen cars -, producing liquid water? Homework Equations ΔH°(reaction) = ΔH°f(products) - ΔH°f(reactants) The Attempt at a Solution 1 mol FeTiH2 - 22,4 L (STP)...
  15. B

    What is the molar heat of combustion for ethanol?

    The question i am stuck on is "If 2.9g of ethanol burn with the evolution of 87 kj of heat, what is the molar heat of combustion of ethanol?" thank you for your help if it possible you can tell me how to do this question.
  16. G

    Why heat of combustion is not the reverse of the heat of formation

    Using Methane gas as an example (CH4 gas), explain why one can not assume the heat of combustion is the reverse of the heat of formation. I can't find a decent answer to this anywhere.
  17. B

    What exactly is heat of combustion? How would I calculate it?

    1. Calculate heat of combustion for butane. Given: mass water- 129.91g mass butane 0.15g old temp- 16.4 degrees C new temp.- 26.7 degrees C 2. Homework Equations 2C4H10 + 3O2 yields 8CO2 + 10H2O 3. The Attempt at a Solution I've tried, but I really have no idea. I only know...
  18. L

    Heat of Combustion Candle Wax Calorimetry

    Homework Statement I'm supposed to find the heat of combustion for candle wax (which was used to boil water in a simple calorimeter), yet I have no clue on how to do it, the lab books at school do not tell anything about the calorimeter's specific heat or give a formula for heat of combustion...
  19. H

    Calculate Heat of Combustion: Grams, Heat Capacity, Temp Change

    What's the formula for calculating this, given: grams, heat capacity of substance, and temperature change? How about heat combustion per kJ/Mol? Thanks
  20. S

    Specific heat of combustion? help

    specific heat of combustion?? help Homework Statement Chemical Properties of gasoline components Using C5 to C10 hydrocarbons, conduct an experiment to determine the effect of one property of the hydrocarbon on the specific heat of combustion. Homework Equations The Attempt at a...
  21. M

    Finding the heat of combustion of magnesium

    Homework Statement Finding the heat of combustion of magnesium.Homework Equations 1.) MgO(s) + 2HCl(aq) → MgCl2(aq) + H2O(l) ∆Hr = 1047.61KJ/mol *This value was found experimentally 2.) Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g) ∆Hr = 374.36KJ/mol*This value was found experimentally 3.)...
  22. 3

    How to calculate heat of combustion?

    Homework Statement Calculate the heat of combustion per mole of glucose(s), C6H12O6(s). This is the only information given. There is no Hess' law equations or any of the sort. Homework Equations The Attempt at a Solution I already know the answer is roughly -2813kJ/mole but I...
  23. C

    Calculating Heat of Combustion for C2H6: Homework Statement Help

    Homework Statement Calculate the heat of combustion for C2H6 from the following information: C2H4+ 3O2----> 2 CO2 + 2 H2O (-1409.5kJ) C2H4 + H2 ----> C2H2 (-136.7 KJ) H2 + 1/2 O2 -----> H2O ( -285.5 kJ) The Attempt at a Solution C2H6 --> C2H4 + H2 +136.7kJ C2H4 + 302...
  24. T

    The molar heat of combustion for phenol

    You burn 103.4 mg of phenol in a calorimeter of heat capacity 1,483 J K-1 and the temperature rises by 1.83 oC. Determine the molar heat of combustion for phenol in kJ mol-1. The molar mass of phenol is 94.12 g mol-1 Ccal=deltaH/ deltaT This is what I did. n=.1034/94.12 =1.09859E-3mol...
  25. A

    Heat of Combustion for C6H4O2: q (kJ/g) & q (kJ/mol)

    I am needing help on some homework questions: A 2.200 g sample of quinone, C6H4O2, is burned in a bomb calorimeter whose total heat capacity is 7.854 kJ/ degrees C. The temperature of the calorimeter increases from 23.50 degrees C to 30.63 degrees C. 1. What is the HEAT OF COMBUSTION PER...
  26. C

    Discover the Heat of Combustion of Methane (CH4) Using Bond Energies

    Determine the heat of combustion of methane (CH4(g)) by writing a balanced equation and using the bond energies indicated below. C--H = 414 KJ/mol O==O = 502 KJ/mol C==O = 799 KJ/mol O--H = 464 KJ/mol ________________ I've googled it and apparently the answer is -890 KJ/mol but when...
  27. J

    Is this right? chem heat of combustion

    Hi guys can you please check if this is right...thanks in advance The heat released by the reaction 2C + 2H2 = CH4 is 74.8kJ/mol. Using this information and a table of standard enthalpy of formation values, determine the heat of combustion for the burning of methane to form CO2 and H2O(g)...
  28. 1

    Calculating the Heat of Combustion for C2H6 Using Hess's Law

    Calculate the heat of combustion for C2H6 from the following information: C2H4 + 3 CO2 --> 2 CO2 + 2 H2O /\H f -1409.5 kJ C2H4 + H2 --> C2H2 /\H f - 136.7 kJ H2 + 1/2 O2 --> H2O /\H f - 285.5 kJ i saw someone else ask this question and they were referred to look at...
  29. R

    Calculating heat of combustion

    Hello I am trying to solve this question and have been stumped for quite a while. Can some tell me the steps I have to take in order to solve this. Question Calculate the heat of combustion for C2H6 from the following information: C2H4+ 3O2----> 2 CO2 + 2 H2O -1409.5kJ C2H4...
  30. B

    Calculating Heat of Combustion

    A 0.196 g sample of quinone (C6H4O2) is burned in a bomb calorimeter that has a heat capacity of 1.56 kJ/C. What is the heat of combustion (kJ/mol) if the temperature increases by 3.2 C? -I have no idea how to do this problem...
  31. D

    Calculating Heat Of Combustion

    Hey, I need some help with the following question: A sample of 0.1964g of Quinone (C6 H4 02, Relative Molar Mass = 108.1) was burned in a bomb calorimeter that heas a heat capacity of 1.56 kJ/M. The temperature in the caolorimeter rose from 19.3 to 22.5 degrees celcius. (a) write a...