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Combustion.

posted Jan 28, 2015, 2:33 AM by ranmini@charliesresearch.com   [ updated Nov 30, 2015, 1:05 AM by Upali Salpadoru ]
                                                                 
                                                 

       


When a flammable substance comes in contact with a supporter of combustion, under certain
conditions burning takes place. The necessary conditions are:-
  1. 1.Close and continuous contact between the combustible substance and the su.pporter of combustion..
  2. 2.A suitable high temperature (Ignition temperature) and 
  3. 3. Removal of the products formed.

This is a chemical reaction. as the bonds between the molecules breakdown resulting in new molecular bonds. Observing a flame from an oil lamp or a candle is a very good  way to start the study of combustion. If you can light a candle you can compare what you see with the diagram given here.

A candle is made from a wax which can easily melt and evaporate. Wax is a fairly simple compound consisting of the two elements Carbon and Hydrogen. To start the process wax molecules must be split up into its elements. This requires energy; which is called ‘activation energy’. The supporter of combustion, which is Oxygen, comes from the air.

In lighting a candle, heat is applied to the end of the cotton wick. The heat that comes out of burning it melts the wax.  The molten liquid wax rises up the wick by the capillary force. As the liquid wax evaporates a colour less gas is produced. It is this gaseous wax that reacts with the Oxygen in the air resulting in a flame. The flame is made up of very hot molecules which are supposed to be in an excited state.


At first the melting wax may drip down along the candle but if there is no wind to waver the flame, a receptacle will form at the base to store the liquid wax.

Activity 1 :-  Fan the flame lightly from one side , observe and try to explain what happens.

 

Observations made by Nelly

Your observations

Probable reasons

1. The flame bends in the opposite direction.

 

 

2. More black smoke is given out.

 

 

3. The wax drips.

 

 

These observations make it clear that the shape of the flame is determined by the wind. So the upward directed flame should also be due to an upward wind.

In the absence of wind the flame will not waver and  the rising air currents will cool the edge and form a wall to store the liquid wax. If the wind blows this wall will melt and the wax will drip.


Activity  2 :-  Fold a piece of paper to make a fan and hold it above the flame.

  Fig.2 Lines to cut and holes to punch.

Take a square shaped paper and cut along the diagonals leaving off 1 cm from the centre.These lines are shown as dotted lines. Paste the edges at the centre and attach a piece of thread.

Fig.3  Folding the edges to make a fan. The thread is in red.

Hang the fan just above the flame and observe what happens.

Fig.4  The fan will rotate due to the rising current.

Activity 3:- Tilt the candle and observe what happens

Fig.5  The flame does not turn with the candle.

 

Observations made by Nelly

Your observations

Probable reasons

1. The molten wax drips down.

 

 

2.  The flame remains up right.

 

 

 

 

 

 

  The liquid wax will spill yet the flame will remain vertical. This shows that the shape of the flame is determined by the air currents around the flame. The arrows in the diagram indicate the rising air due to heat. These are called convection currents.

Activity 4:-

An Experiment :- To obtain a burning gas from a flame.

  Find a glass or a metal  tube with one narrow end. Insert the broad end into the flame and try to ignite any gas that may come out. You will be able to find out in which areas the unburnt gas is available.

 

Fig. 6.  A flame from a flame.

Activity 5:-  Touch the end of the glowing wick with an iron nail.
      What happens may surprise you. Iron is a good conductor of heat. So what ever that happens has to be due to loss of heat.

    

Fig.7 Why does the end of the wick glow?

Activity 6:-  Blow the flame out. 

A white smoke will be produced.  What's  this smoke?

 

Fig.8 The flame gets sucked in to the extinguished candle.

Chemistry of the candle flame.

Candles are made of wax. Wax is a compound belonging to a group of compounds called ‘ Hydro  carbons’.  The simplest member of the group is CH ( Methane)   As the number of Carbon atoms increase  the number of Hydrogen atoms double (x2). The general formula is given as  Cn H 2n +2.   One of the liquid members of the group is Octane in petrol having the formula C8H18.
The waxes range from C20H42 to about C40 H82.
As the supporter of combustion is oxygen, after complete combustion, the products have to be  the oxide of carbon and the oxide of hydrogen.  Yet in a candle flame the temperature is not sufficient to burn all the carbon atoms. Carbon comes out as soot. This becomes evident if you take some heat off the flame either by fanning or by inserting a metal rod.

Activity :- Reducing the flame temperature to get Carbon

 

 Fig.9. Hold a metal spoon or an egg shell inside the flame.

 Considering the change as a complete combustion, and taking the simplest hydrocarbon,  we may write the equation as follows:-
Methane + oxygen   =  carbon dioxide + water.
CH4 + 2 O2              =  CO2  + 2 H2O
What normally takes place is an incomplete combustion.
2CH4 + 3 O2              =  CO2 + C + 4 H2O
There is another kind of incomplete combustion which occurs at high temperatures, in ill ventilated places such as in the internal combustion engines.
3CH4 + 5 O2              =  CO2 + 2CO + 6 H2O

 

                                              

                                                              FORUM

Nelly  -  Burn a candle; matter changes to energy .

Ali-        No ! Never.

Nelly-     Why not ?  The mass of the used candle is less than the original.

Ali-         True, but the loss is not due to a conversion of matter to energy.

Nelly-    Then How’d you get heat and light.

Ali-        This is what you call an exothermic (heat producing) chemical reaction.  The energy that comes, is from a kind of potential (stored up) energy called chemical energy,

Nelly-    Then how does burning destroy matter.

Ali-         Up to the time of  Antoine Lavoisier, 1743 -1794, (Fr) everybody thought like you.  Lavoisier showed that burning does not destroy matter. What happens is changing wax, which is a solid to an invisible gases.  Lavoisier burnt magnesium and phosphorus and showed that they actually gained weight. That is due to the addition of oxygen . By that time Joseph Prriestly had obtained oxygen and named it de-pholgisticated air.

Nelly-  Oh yes I remember.  Matter can neither be destroyed nor created”

Ali-   Exactly !  This law, “The law of conservation of mass” was established by Lavoisier who was guillotined by the French people.

 This law does not hold for nuclear reactions.

 

The Law of conservation of mass

During a chemical reaction,the total mass of the products is equal to the Total Mass of the Reactants.

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