Methane Combustion

CH4 + 2 O2 CO2 + 2H2O + heat

Exothermic reaction

(releases heat – how much?)

Heat of Combustion

Energy given off upon combustion of

a specific amount of a substance

kJ/mol Cal/g cal/mol etc.

1 mol of CH4 802.3 kJ

Heat is given off; products have less PE

than reactants

The system (CH4) LOSES energy

Energy change = - 802.3 kJ/mol

Other molecules?

Bonds are broken, others are formed

Energy absorbed Energy released

If E released > E absorbed : Exothermic reaction

2

One more time:

• It COSTS energy to BREAK a bond

• Energy is PRODUCED to MAKE a bond

Bond Energy

Depends on the atoms sharing electrons

Methane:

Break: Make:

4 C-H bonds 2 C-O double bonds

2 O-O double bonds 4 O-H bonds

Products: stronger bonds than reactants

Reactions proceed toward more stable species

What about endothermic?

Endothermic reactions occur,

but need “outside” energy

O3 + photon O2 + O

(p160)

4.4 Calculating Energy Change

Combustion of hydrogen:

2 H2 + O2 2 H2O + energy

Bonds break/form @ same time

Easier to think of as:

1. Bonds break

2. Bonds form

NET energy change

Bond energy

Energy needed for breaking a specific bond

Ex.: O-H is 467 kJ/mol

How many bonds (moles?)

Chem. equations can be read as moles!

Combustion of H2

2 H2 + O2 2 H2O + energy

Broken: Energy (kJ/mol) Change (kJ)

2 mol H-H 436 + 872

1 mol O=O 498 +498

Formed:

4 mol O-H 467 -1868

Total energy change: - 498 kJ

Exothermic

Combustion of Methane CH4 + 2O2 CO2 + 2H2O

Broken: Energy (kJ/mol) Change (kJ)

4 mol C-H 416 +1664

2 mol O=O 498 +996

Formed:

2 mol C=O 803 -1606

4 mol O-H 467 -1868

Total energy change: - 814 kJ

Exothermic

Heats of Combustion

Related to energy change EXCEPT

values are positive

CH4 + 2O2 CO2 + 2H2O DE = -814 kJ

Heat of Combustion: +814 kJ

Good fuels

H2O and CO2: poor fuels (combusted)

Bonds are strong (O-H and C=O)

Can’t be converted to stronger bonds

Large energy change (exothermic)

Reactants: Weak bonds

Products: Strong bonds

Energy required to start a reaction

Activation energy

Strike a match:

Heat from friction

Hindenberg explosion:

Caused by a static spark

More energy is given off than it takes to activate

Useful reactions

Neither too fast nor too slow

Too slow: energy release takes too long

(rotting wood)

Too fast: explosion – difficult to control,

(H-bomb)

Optimizing reactions: Increasing rate

1. Maximize surface area (stirring a solution)

2. Raise temp. (add energy to the system)

3. Catalysts (lower activation energy w/o

being consumed)

Energy Consumption (4.5)

Fig. 4.8

No Fuel Like an Old Fuel

Sunlight in plants: stored energy

2800 kJ + 6 CO2 + 6 H2O 6 O2 + C6H12O6

(glucose) (Animals eat plants:

6 O2 + C6H12O6 6 CO2 + 6 H2O +2800 kJ )

C6H12O6 High temp, pressure

LONG time Fossil fuels (CxHy)

Fuels Fossil fuels

Coal (4.6)

Petroleum (4.7)

Gasoline (4.8)

“Newer” fuels (4.9)

Coal (4.6)

• Yields more energy per gram than wood

– 2-3X as much

•Why?

–More C, less O2 and H2O

–Coal: C135H96O9NS (85% C)

–Wood: C6H12O6 (40% C)

Why is O2 & H2O “Bad”? • Low heats of combustion

•Bonds are stable

–Lots of energy to break

–Less energy out

•Less oxygen = more energy

Advantages of Coal

• Large supply

• Widely used

– Lesser dev. Nations

• More efficient than wood

• Doesn’t need processing

Drawbacks

• Difficult to get

– Mining (Accidents, health risks)

• Safe = Expensive

– Strip mining

• Erosion

• Difficult transport

– No “pipelines”

• Slurry

• Capsules

Drawbacks (cont.)

• Pollution (esp. low qual. coal)

– Carcinogens

– Acid rain

– Greenhouse gases

– Mercury

• Anthracite: running low!

Future of Coal

• Petroleum will run out 1st

• Demand for coal will rise

• Will need refinement

– (make it cleaner)

• Cost will rise