OB: Combustion Reactions

Combustion reactions require a hydrocarbon compound to combine rapidly with oxygen, forming oxygen and carbon dioxide, and lots of energy.

There is little challenge recognizing these reactions, and the products NEVER vary.

Hydrocarbon: a compound made of carbon and hydrogen.

Oxygenated hydrocarbon: a compound made of carbon, hydrogen and oxygen. These also combust, but the balancing is one step more work for you.

Examples of hydrocarbons: methane, propane, butane, octane, wax.

Examples of oxygenated hydrocarbons: ethanol (alcohol), wood, ethers, acetone.

Every single combustion reaction looks like this:

Hydrocarbon + oxygen carbon dioxide + water

First practice example: the simplest of all hydrocarbons, methane combusts.

We set up the skeleton reaction, then YOU balance this while I take attendance.

CH4 + O2 CO2 + H2O

CH4(G) + 2O2(G) CO2(G) + 2H2O(G)

John Dalton, simple whole number ratio check: 1:2:1:2 all okay.

Due to the heat released, the water is a gas too.

Balance these, we’ll see how you did on the next slide.

C2H6(G) + O2(G)

C3H8(G) + O2(G)

2C2H6(G) + 7O2(G) 4CO2(G) + 6H2O(G)

C3H8(G) + 5O2(G) 4H2O(G) + 3CO2(G)

Combustion reactions always have a hydrocarbon* and oxygen reacting to form water and carbon dioxide.

*you could switch in an oxygenated hydrocarbon too

Demo Diagram #5 Combustion Reactions

Combustion reactions require a hydrocarbon (or oxygenated hydrocarbon) to combine with oxygen, forming carbon dioxide, water, and HEAT (always exothermic!)

Abstract: Hydrocarbon + O2 CO2 + H2O

The “abstract” is not too abstract because of the limitations of the reaction.

Word Equation:

Methanol + oxygen yields carbon dioxide & water

(Methanol is NOT the “alcohol” in wine and beer)

BALANCE THIS REACTION TO SEE IT, IT’S CERTAINLY WORTH IT!

CH3OH(G)+ O2(G) CO2(G)+ H2O(G) + energy!

BALANCE THIS REACTION TO SEE IT, IT’S CERTAINLY WORTH IT!

2CH3OH(G)+ 3O2(G) 2CO2(G) + 4H2O(G)

plus energy!

In an exothermic reaction, energy is a product.

Later in the year, we’ll be able to calculate how much energy is released

if we know how many moles of hydrocarbon we start with.

That’s called STOICHIOMETRY, and it’s fun.

CH3OH

Methanol liquid, shaken to a gas

Oxygen in the air, is also inside the bottle.

candle

Diagram of Methanol Cannon

Balance these last few combustion reactions…

Butane (C4H10) combusts.

Octane (C8H18) combusts.

2C4H10 + 13O2 8CO2 + 10H2O +E

2C8H18 + 25O2 16CO2 + 18H2O +E

Back and forth, back and forth,

pull hair as needed. Count, they always work out.

Homework due Tuesday:Combustion HW #4.

DUE Wednesday: Reactions Lab

Butane

Octane