CM8001

Intermolecular forces are broken.

a. Exothermic
It is exothermic as it releases energy when it burns. It causes us to feel warmer when we are around it.

b. Endothermic
It is endothermic as it absorbs energy to evaporate. When water evaporates from our skin, we tend to feel cold. This is consistent with the fact that endothermic processes causes its surroundings to become colder.

c. Endothermic
It is endothermic as it absorbs energy to melt – it undergoes a change of state from solid to liquid state. This means that bonds were broken between the water molecules in solid state to give rise to a more disordered liquid state. Bond breaking requires energy (it absorbs energy). When we place ice at room temperature, or in room temperature drinks, they absorb the energy of the surroundings and melt. This in turn makes the drinks colder.

For chemical explosions to be exothermic, they have to release energy to its surroundings. This means that the energy used for bond breaking (of the reactants) must be very small and the energy used for bond forming (of the products) is very large; this means the energy released would be a lot. The products of the reaction should be of a lower energy than the reactants. Hence, the products of such exothermic reactions are typically gases.

Heat is the consequence of motion at the molecular level, and temperature is the measure of average speed of the motion. Temperature determines the direction of heat, which flows from a warmer body to a colder body. Heat is a form of energy.

Temperature can be measured directly using a thermometer. However, to measure heat, we need to know the mass, specific heat capacity and temperature of the compound.
When we apply heat to water and the water boils, we can observe bubbles (water vapour). Temperature rises at the same time.

Knocking is the likelihood of the compound undergoing preignition, where the fuel-air mixture ignites before sparking, when compression occurs. It is affected by the different chain lengths.

a. Unlikely to knock as it has a knocking characteristic of 98% isooctane (branched) and 2% heptane (straight-chained). This gasoline behaves like a mixture of these two compounds.

b. Adding oxygenates to gasoline increases the gasoline’s octane rating. Hence, it can be deduced that the fuel contains a good amount of oxygenates.