[confusedstud]

When there is incomplete combusion, will it automatically mean that less of the hydrocarbon is being given off? For example, in a MCQ question a question stated: Which factor does not indicate that there is incomplete combusion a car and the only 2 that did not indicate was A) There is a small percentage of NO2 in the car's exhaust and B) There is some unburnt hydrocarbons in the car's exhaust

So the answer given was B but i don't understand why. Shouldn't the amount of unburnt hydrocarbon not be a problem?

[Dan]

When there is incomplete combusion, will it automatically mean that less of the hydrocarbon is being given off?

Hydocarbons are not given off by combustion of fuel. Fuel is made up of hydrocarbons, which are consumed in the burning reaction.

What is/are the equations for the combustion of a hydrocarbon fuel?

[confusedstud]

Hi dan, sorry for being vague. I meant that if I had less oxygen and cause incomplete combustion, would it mean that less of the hydrocarbons would react. If so why does this happen?

Eg CH4+O2 :rarrow:CO2+2H2O
CH4+1.5O2 :rarrow:CO+2H2O

[Dan]

I meant that if I had less oxygen and cause incomplete combustion, would it mean that less of the hydrocarbons would react. If so why does this happen?

Reaction 1: You have 100 mol of methane and 200 mol of oxygen
Reaction 2: You have 100 mol of methane and 100 mol of oxygen
Reaction 3: You have 100 mol of methane and 1 mol of oxygen

In which case is there more methane left at the end of the reaction?

[confusedstud]

Hi in those reactions will they be complete or incomplete combusion. Also, i realised i only know the defination of why incomplete combustion occurs. It is when there is insufficient oxygen. Does it mean in a reaction CH4+2O2-->CO2+2H2O if i have only 1 moles of O2, will only 1/2 mole of methane react with the oxygen or will there be incomplete combustion here. Will this reaction happen CH4+1.5O2-->CO+2H2O instead of that?

Thanks for the *delete me*

[Dan]

http://en.wikipedia.org/wiki/Limiting_reagent

If there is not enough oxygen (1 mol) to burn all the methane (100 mol), what do you think happens to the rest of the methane?

[confusedstud]

They don't get reacted? But does that small amount of methane react with the 1 mole of O2 with a complete or incomplete combustion? Because even though oxygen is the limiting reagents what's stopping the small amount of methane from completely combusting? Thanks Dan!

[Dan]

Incomplete combustion (forming CO) occurs when there is not much (not enough) oxygen relative to the fuel. If there is fuel left over, it indicates that the oxygen/fuel ratio was too low for complete combustion.

[confusedstud]

Oh so in reaction 2 and 3 that you gave, there would be incomplete combustion for the methane since there is not enough to totally reaction the 100 moles of methane? So all of the methane would incompletely combust? Thanks!

[confusedstud]

Reaction 2: You have 100 mol of methane and 100 mol of oxygen

When they react here will there be complete or incomplete combustion? If this reaction happens CH4+O2  CO2+2H2O, then there will be 0 moles of methane left behind. However, if this CH4+1.5O2 CO+2H2O there will be 33.3 moles of methane left behind.

There is sufficient oxygen so there would be more complete combustion happening than incomplete combustion? So only when there is insufficient oxygen then there definately will be some excess methane left behind?

[Dan]

Your equation for the complete combustion of methane is not balanced