In this final part (Part 4) of Berry Berry Easy’s SPM Chemistry Form 5 notes on Thermochemistry, the topic is finally on a kind of heat change which students are most familiar with! Combustion! Boom! The phrase ‘heat of combustion‘ might immediately be linked to internal combustion engines, especially for our male readers who could also be engine enthusiasts. However, heat of combustion in its simplest form simply means heat change (release) when a substance is oxidised under standard conditions. It can be the combustion of anything. (Please don’t also link it only to explosion, that’s another topic in far far away future when you are in the university).
[Tips: Berry Readers should at least be familiar with all the previous kind of 'heats' discussed previously such as heat of precipitation, heat of displacement and heat of neutralisation before moving on to the heat of combustion.]
SPM Chemistry Form 5 – Thermochemistry (Part 4 – Final)
Heat of Combustion (Form 5, Chapter 2 Carbon Compounds)
- Heat of combustion – the heat change when one mole of a substance is completely burnt in oxygen under standard conditions.
- Combustion – redox reaction between substance (fuel) reacts rapidly with oxygen with the production of heat energy.
- Combustion reaction gives out heat and always an exothermic reaction.
- Heat evolved in combustion of fuel = Heat absorbed by water.
- Bomb calorimeter is used to determine the heat of combustion.
- The more carbon and hydrogen atoms per molecules in a fuel, the more heat that is released when 1 mol of fuel combusts.
There are differences in heats of combustion:
|Chemical equation||ΔH (kJ mol-1)|
|H2(g) + ½ O2(g) –> H2O(l)||-286|
|C(s) + O2(g) –> CO2(g)||-392|
|CH4(g) + 2O2(g) –> CO2(g) + 2H2O(l)||-890|
|CH3OH(l) + 3/2 O2(g) –> CO2(g) + 2H2O(l)||-728|
|C2H5OH(l) + 3O2(g) –> 2CO2(g) + 3H2O(l)||-1376|
|C3H7OH(l) + 9/2 O2(g) –> 3CO2(g) + 4H2O(l)||-2016|
The selection of suitable fuel:
- Fuel value (the amount of heat energy given out when one gram of the fuel is completely burnt in excess of oxygen): The higher the fuel value, the more energy is released.
- Effect on the environment: Production of soot which caused air pollution. Hydrogen fuel is known as clean fuels (no soot or poisonous gases).
- Cost per gram of fuel.
Fuel values of common fuels
|Substances||Fuel value (kJ g-1)|
|Dry cow dung||15.5|
|Biodiesel (Vegetable oil)||42.2|
Qualities of a fuel are based on the following:
- Easily available
- Cheap in cost
- High fuel value
- Do not pollute the environment
- Less storage space
The Existence of Various Energy Sources
- The Sun: solar cell (still expensive and inefficient)
- Fossil Fuels: relatively high fuel value and convenient to use (non-renewable and cause greenhouse effect and acid rain)
- Water: hydroelectric power. It is clean, renewable, convenient and economical to use (high cost of construction and destruction of the surrounding environment)
- Biomass: plants and droppings of animals – biodiesel (large areas of land to grow plants)
- Radioactive substances: uranium and plutonium (non-renewable and very destructive if an accident occurs)
With this, we conclude the simple but important chapter of Form 5 Thermochemistry for SPM students.