Questions for CM8001-AE 3
1.From personal experience, state whether these processes are endothermic or exothermic. Give a reason for each.
a. A charcoal briquette burns
Hot sensation felt by the skin suggest that the burning of charcoal produces heat, thus an exothermic process.
b. Water evaporates from your skin.
Skin feels cold. Heat energy from the human is used by the water to break the inter-molecular forces of attraction between water molecules to form water vapor. Liquid phase to gaseous phase. Hence, an exothermic process.
c. Ice melts.
Similar to part B, the feeling is the same. However, the change of physical states are different. Ice melts, takes in energy to break the inter-molecular forces of attraction. Solid phase to liquid phase. Hence, an exothermic process.
2. Chemical explosions are very exothermic reactions. Describe the relative bond strengths in the reactants and products that would make for a good explosion.
An exothermic reaction produces heat. The larger the explosion the more heat is given out. This can be explained by calculating the Enthalpy of a reaction, ΔH = – [ (sum of product bond energies) – (sum of reactant bond energies)]
For a reaction to be exothermic, reactants must have higher energy than products in terms of bond energies. The higher the reactants energy and the lower the products energy, the bigger the difference and this difference will be the result of the amount of energy(heat) given off. As a result, in order to produce a BIG explosion, the amount of energy given off must be huge.
3. How might you explain the difference between temperature and heat to a friend? Use some practical, everyday examples?
By taking two objects of the same temperature and place them into contact of each other, there will not be overall transfer of energy as the average energies of the particles in each object are the same. However, if one of the object has a different temperature, the object that has a higher temperature will transfer its energy to the colder object until both reach the same temperature. This shows that heat is energy and will increase or decrease the temperature. Heat is the total energy of molecular motion in a substance and is dependent on the speed and the number of particles. On the other hand, temperature is not energy but a measure of it. Temperature is a measure of average energy of molecular motion (rotation or vibration) in a substance. Temperature does not depend on size or type of object.
4. 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?
An octane rating of 98 suggests that this gasoline has a knocking characteristic of 98% isooctane and 2% heptane. Its ability to burn smoothly without knocking is lower but close to isooctane and greatly higher than heptane.
b. whether the fuel contains oxygenates?
Oxygenated fuels tend to have octane rating above 100 as these gasoline are able to burn more smoothly without knocking compared to isooctane. An octane rating of 98 suggests that this gasoline is not oxygenated as its ability to burn smoothly without knocking is below that of isooctane which has an octane rating of 100. The oxygenation of fuels generally increases the octane number above 100 as oxygenated fuels requires a higher activation energy to initiate combustion. As such, the occurrence of uncontrolled ignition becomes less unlikely. Therefore, since isooctane is a non-oxygenated fuel, a fuel with even lower octane rating would not be oxygenated as well.