Combustion of Hydrocarbons
When a hydrocarbon is completely
burnt in excess oxygen to produce ENERGY, it will only produce carbon
dioxide and water as waste
products. These two are very clean and harmless. During COMPLETE
COMBUSTION a clean blue flame results. The complete combustion of hydrocarbons is therefore safe. Hydrocarbon
(Methane) + Oxygen CH4 + O2 To keep warm we use gas fires to heat our homes.
All these gas fired heaters release these waste
gases (CO2 and H2O) into the room, which is not
a problem. The difficulty only arises when the room is not properly ventilated and
there is a lack of oxygen. When
there is lack of oxygen the combustion of hydrocarbons is INCOMPLETE.
This results in the production of
Carbon monoxide and Carbon. It also produces
a smoky yellow flame. Hydrocarbon
(Methane) + Oxygen CH4 + O2 The
problem with incomplete combustion is that it produces carbon monoxide, a colourless and poisonous gas and it is very
dangerous. It kills people in their
sleep when it is released in rooms with faulty heater and boilers. The production of carbon (soot or smoke) is an indication that
the fuel is not burning
properly. Measuring energy produced
from burning fuels It is important to know how much
energy a fuel transfers before you choose it. The amount of energy transferred
can be measured by using a calorimetric technique. The energy released by
the fuel is absorbed by the water and can be calculated by using the following
equation: Energy transferred = mass
x specific heat capacity x rise in temperature to or from the substance The specific heat capacity of water is
4 200 J/ kg/°C or 4.2 J/g/°C. The mass of the water (in grams) is the same
as the volume of the water (in cm3), because the density of water
is equal to 1g/cm3. Example The temperature of 500cm3 of water rose from 15°C to 90°C when heated in a metal can. Calculate the
energy produced per gram of paraffin if 5 grams of paraffin were burned in
the spirit burner. Calculation method: = mass x specific heat capacity x
rise in temperature = 500 x 4.2 x 75 = 157 500 J per 5g of paraffin = 31 500 J per gram of paraffin. Try the following problem. The temperature of 2kg of water increases
from 05°C to 95°C after heating it over a methalated spirits burner that used
500g of spirits. How much heat has been transferred to the water per gram
of methalated spirits burning? =______ x __________ x ___________ = _________________J per 500g of
spirits. = ____________________J per gram
of spirits. During any chemical reaction the old
covalent bonds brake and new ones form. Energy is needed to break bonds (endothermic)
and given out when new bonds are formed (exothermic). During combustion reactions
of fuels, the energy released is more than that absorbed. That is why these
reactions are exothermic. Larger the alkane molecule the more energy
is released. See table below. Coal, oil and natural gas are non-renewable
resources of fuel. Once we have used them up, they are gone for ever.
That is why we must use them sensibly, efficiently and find other sources
of energy to replace them.
Carbon dioxide + Water + Energy CO2 + H2O + ENERGY
Carbon dioxide + Water + Carbon
monoxide + Carbon + Energy CO2 + H2O + CO
+ C + ENERGY
Energy transferred to
or from the substance
Energy transferred to
or from the substance
= mass x specific heat
capacity x rise in temperature
Alkane
Energy
of combustion (kj/ mole)
Methane
(CH4)
882
Ethane
(C2H6)
1542
Propane
(C3H8)
2202
Butane
(C4H12)
2877
Pentane
(C5H12)
3487
Hexane
(C6H16)
4141
Tags:Hydrocarbons, Methane, Ethane, Propane, Butane, Pentane, Hexane, Energy