Group 4 – Chemistry for Engineers
Canosa, Mark Nathaniel
Camarillo, Shane Collen
Cepe, Francis John A.
HEAT COMBUSTION
Objectives:
This report aims to let the learners:
- know what is heat of combustion.
-perform calculations involving heat of combustion.
-And know the importance of it.
Introduction
Heat of combustion, heat released during combustion. In particular, it is the amount of heat released when
a given amount (usually 1 mole) of a combustible pure substance is burned to form incombustible
products (e.g., water and carbon dioxide); this amount of heat is a characteristic of the substance. Heats of
combustion are used as a basis for comparing the heating value of fuels, since the fuel that produces the
greater amount of heat for a given cost is the more economic.
Heat of combustion is an analytical technique that is used to measure the amount of energy in a molecule.
It is basically a machine that blows up molecule to see how energetic they are. The theory behind it is that
the higher the heat of combustion or the more heat that is released by the explosion, the higher the energy
of the molecule and if the energy is very high, you would expect it to not be very stable and vice versa.
Motivational/ Prompting Questions
What is Heat of combustion? What is Calorimetry? What is a Calorimeter? How do we solve for the Heat
of combustion? How do we solve for the Molar Heat of combustion?
Discussion
The thing about heat combustion that the reporters will explain are:
Calorimetry − is the process of measuring the amount of heat released or absorbed during a chemical
reaction. By knowing the change in heat, it can be determined whether or not a reaction is exothermic or
endothermic.
Calorimeter − device for measuring the heat developed during a mechanical, electrical, or chemical
reaction, and for calculating the heat capacity of materials.
Exothermic reaction − it is chemical energy (enthalpy) is being turned into heat energy which is
transferred to the surroundings, so the temperature we measure increases.
Endothermic reaction − heat energy is taken from the surroundings and converted into chemical energy
(enthalpy), so the temperature decreases, or we have to heat the reaction constantly to make it work.
Heat of combustion − is the amount of energy released when a substance is burned in the presence of
oxygen. The heat of combustion, also known as the heating value, is reported in two forms. The higher, or
gross, heating value assumes that all of the water produced by combustion is in the liquid phase. The
lower, or net, heating value assumes the water is vapor.
Example Problem:
Calculation:
2.50 of ethanol (C2H5OH) is burned completely in excess oxygen and the heat Released can raise the
temperature of 750 g of water by 20ºC. Calculate the heat of Combustion of ethanol. Specific Heat
Capacity of water 4.186 J/g ºC.
Step 1: Calculate the heat change
Heat Change: Q (kJ) Heat change formula: Q = mC∆T
Mass of water: m (g)
Specific Heat: c (J/g ºC)
Change in temperature: ∆T (ºC)
Q = (750 g) x (4.186 J/g ºC) x (20ºC) Convert J to kJ: 63000 J ÷ 1000= 63 kJ = 63 kJ
Step 2: Calculate the number of moles of the fuel
Number of moles: n (mol) Number of moles formula: n = m / MM
Mass of fuel: m (g)
Molar mass: MM (g/mol)
Molar Mass of ethanol = (2x12.01g/mol) + (6x1.01g/mol) + (1x15.999g/mol)
Molar Mass of ethanol = 46.079 g/mol
N = 2.50 g. ÷46.079 g/mol
N = 0.054mol
Step 3: Calculate the heat of combustion
Heat of Combustion: -∆H Heat of Combustion formula: -∆H = Q/n
Heat Change: Q (kJ)
Number of moles: n (mol)
Heat of combustion = 63 kJ÷0.054mol
= 1166. 67kJ/mol
= - 1166.67kJ/mol
Molar Heat of Combustion − the molar heat of combustion (He) is the heat released when one mole of a
substance is completely burned.
Example Problem:
A 1.55 g sample of methanol (CH3OH) is burned and produced a temperature increase of 55ºC in 200g
of water. Calculate molar heat of combustion.
Step 1: Calculate the heat change Heat Change: Q (kJ)
Heat change formula: Q = mC∆T
Mass of water: m (g) Specific Heat: c (J/g ºC)
Change in temperature: ∆T (ºC) Q = (200 g) x (4.186 J/g ºC) x (55ºC)
Convert J to kJ: 46046 J ÷1000=46046 J = 46.046 kJ
Step 2: Calculate the number of moles of the fuel Number of moles: n (mol)
Number of moles formula: n = m / MM Mass of fuel: m (g) Molar mass: MM (g/mol)
Molar Mass of ethanol = (1x12.01g/mol) + (4x1.01g/mol) + (1x15.999g/mol) = 32.049 g/mol n = 1.55 g
32.049 g/mol = 0.0484 mol
Step 3: Calculate the heat of combustion Heat of Combustion:
-∆H Heat of Combustion formula: -∆H = Q/n
Heat Change: Q (kJ) Number of moles: n (mol)
Molar heat of combustion = 46.046 𝑘𝐽 0.0484 𝑚𝑜𝑙 = 951.36 kJ/mol
Learning Activities/ Exercises
The reporters will choose from the learners and to answer the following questions:
1.) Find the heat of combustion of butane (C4H10) undergoes combustion at 25ºC with 250 g of water.
2.) A 1.72 grams sample of methane (CH4) is burned and produced a temperature increase of 30ºC in
300 g of water. Calculate the molar heat of combustion and heat of combustion.
Practice Task/ Assessment
The learners should answer the following question on a one piece of paper and submit in the GC.
Instruction:
0.379g of propane (C3H8) undergoes complete combustion in a spirit burner. The heat energy release is
used to heat 100 g of water. The temperature of the water rises from 20.24ºC to 37.65ºC.
a.) molar heat of combustion of propane
b.) calculate the heat of combustion
References
The Engineering Toolbox. (2020). Retrieved from
https://www.google.com/amp/s/www.engineeringtoolbox.com/amp/standard-heat-ofcombustionenergy-content-d_1987.html Libretexts. (2020).
Heat of Combustion. (2019). Retrieved from https://www.ck12.org/chemistry/heat-ofcombustion/lesson/Heat-of-Combustion-CHEM
Heat of Combustion. (2020). Retrieved from https://www.sciencedirect.com/topics/earth-andplanetary-sciences/heat-of-combustion
Molar Heat of Combustion: Definition & Calculations. (2020). Retrieved from
https://study.com/academy/lesson/molar-heat-of-combustion-definition-calculations.html