Q1: From personal experience, state whether these processes are endothermic or exothermic. Give a reason for each.
a. A charcoal briquette burns.
This is an exothermic reaction. Heat is released when a charcoal briquette burns.
b. Water evaporates from your skin.
This is an endothermic reaction. When water evaporates from our skin, heat is absorbed, thus our skin feels cooler.
c. Ice melts.
This is an endothermic reaction. Heat is absorbed by the ice from the surrounding, allowing it to melt.
Q2: Chemical explosions are very exothermic reactions. Describe the relative bond strengths in the reactants and products that would make for a good explosion.
Methane is a type of chemical explosive.
During a methane explosion,CH4 + 2 O2 -> CO2 + 2 H2O.
The bond energies involved are: 4 C-H single bonds (99kcal/mol*4), 2 O=O double bonds (119kcal/mol*2), 2 C=O double bonds (192kcal/mol*2) and 4 O-H single bonds (111kcal/mol*4). The bond energies of the reactants should be lower than the bond energies of the products. The products has a higher bond energy as it releases more energy during bond formation, than give out energy during bond breaking of the reactants. Thus, leading to an exothermic reaction; net negative energy change.
Q3: How might you explain the difference between temperature and heat to a friend? Use some practical, everyday examples.
Heat is the total energy of the molecular movement, while temperature is a measurement of the average thermal energy of the molecule.
A larger beaker of water (beaker A) will have more heat than a smaller beaker of water (beaker B), even though they are of the same temperature. Beaker B has lower energy, so it will take a shorter time to cool down.
It is similar to how a person with more mass has more kinetic energy than a person with less mass, even though they are travelling at the same velocity.
Q4: A premium gasoline available at most stations has an octane rating of 98. What does that tell you about:
a) the knocking characteristics of this gasoline?
The octane rating of gasoline is 98. This means that it has the knocking characteristics similar to a mixture composed of 98% isooctane and 2% n-heptane.
It is categorized as a premium gasoline as it has a higher octane rating than other blends sold at gasoline stations. This means that gasoline is more resistant to knocking than the other blends.
b) whether the fuel contains oxygenates?
The octane rating does not tell us whether the fuel contains oxygenates (such as ethanol). Information on whether the fuel contains oxygenates may be available on other labels found around the pump.