CM8001

A. Exothermic. Heat is released when charcoal briquette burns.
B. Endothermic. Heat is absorbed for water to evaporate and hydrogen bonding between water molecules are broken.
C. Endothermic. Heat is absorbed and required to reach the melting point of ice.

Heat is released from forming bonds and heat is absorbed to break bonds. For an exothermic reaction, heat is released. Therefore, heat released during bond formation (products) is greater that the heat absorbed for bond breaking (reactants). Hence the product would have a greater bond strength than the reactants and the products are more stable.

Heat is the energy that flows from the hotter object to the colder object, it is the consequence of motion at the molecular level whereas temperature is a measure of the average speed of that motion.

A practical example would be a hot cup of coffee cooling down. As it cools gradually, heat from the coffee is transferred to the surrounding air which is at a lower temperature. The rate at which heat transfers is the temperature. Temperature difference causes heat to flow from the hotter region to cooler region.

A. Octane rating of 98 means a knocking characteristic of 98% isooctane and 2% heptane.

B. Octane rating measures the ability of gasoline to burn smoothly without knocking. Fuels that contain oxygenates generally have a higher octane rating. The higher the octane rating, the higher possibility that it contains oxygenates. Octane rating does not tell us whether oxygenates are present or not but only the likelihood.