CM8001

1a) Exothermic: Charcoal briquette releases heat when it burns

1b) Endothermic: Water absorbs heat needed for evaporation by breaking bonds (H-bonding)

1c) Endothermic: (solid) –> (liquid) Ice absorbs heat so as to allow it to melt. When ice absorbs heat as energy, it gains kinetic energy until it reaches its melting point where it will exist as a liquid.

An example of a good explosion: methane combustion.

CH4 + 2 O2 –> CO2 + 2 H2O

For the reaction to be exothermic, the net change of energy must be negative i.e. the energy absorbed to break the bond must be less than the energy released during bond formation. The bond energies of the products should be larger than those of reactants.

In methane combustion, the bond energies involved are: C-H: 416KJ/mole, O=O: 498KJ/mole, H-O: 467KJ/mole and C=O: 803KJ/mole

Bond energies of reactants = 4 C-H + 2 O=O = 4 X 416 + 2 X 498 = +2660KJ/mole
Bond energies of product = 2 C=O + 4 O-H = 2 X 803 + 4 X 467 = -3474KJ/mole

Net change = 2660 – 3474 = -814KJ/mole

The bond energies of the product > bond energies of the reactant. Hence, net energy change is negative (as shown above). This indicates a large exothermic reaction.

Heat = energy transfer.
e.g. touching a hot coffee.

Saying that a hot coffee has “heat” = wrong.
A better statement = “This coffee can transfer energy as heat towards the lower energy (finger).”

Heat depends on the speed of particles; the number of particles and the type of particles in an object whereas temperature does not depend on size or type of object. Temperature is an intensive property.

e.g. when you boil eggs
You need X amount of water at 100-degree Celsius for you to cook one hard boiled egg. However, you will need either/both more than X amount of water or/and more time to cook two or more hard boiled eggs.

Octane rating = The measure of the ability of the gasoline to burn smoothly without knocking.

a) 98% isooctane + 2% heptane

b) It does not indicate if the fuel contains oxygenates