i

 

 

 

 

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.

Blue Flame

Hydrocarbon (Methane)   +    Oxygen            Carbon dioxide    +    Water    +    Energy

                                                                  CH4   +   O2               CO2       +   H2O  +  ENERGY

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.

Yellow Flame

Hydrocarbon (Methane)   +    Oxygen            Carbon dioxide    +    Water    +    Carbon monoxide   +    Carbon    +    Energy

CH4   +   O2               CO2        +   H2O   +  CO  +    C    +    ENERGY

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:

Energy transferred to or from the substance 

= 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?

 

Energy transferred to or from the substance = mass x specific heat capacity x rise in temperature

=______ 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.

 

Alkane Energy of combustion (kj/ mole)
Methane (CH4) 882
Ethane (C2H6) 1542
Propane (C3H8) 2202
Butane (C4H12) 2877
Pentane (C5H12) 3487
Hexane (C6H16) 4141
   

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. 

 

Tags:Hydrocarbons, Methane, Ethane, Propane, Butane, Pentane, Hexane, Energy

 

 

 

© 2012 science-resources.co.uk. All rights reserved | Design by W3layouts