- From personal experience, state whether these processes are endothermic or exothermic. Give a reason for each.
- A charcoal briquette burns
Exothermic, as when charcoal briquette burns, heat is released.
- Water evaporates from your skin
Endothermic. Water absorbs heat from my skin to break inter-molecular forces of attraction and evaporates.
- Ice melts
Endothermic, for example when ice melts on my hand, my hand feels cooler as the ice absorbs heat energy from my hand.
2) Chemical explosions are very exothermic reactions. Describe the relative bond strengths in the reactants and products that would make for a good explosion.
Reactants should have weaker bonds while products should have stronger bonds.
ΔH = Bonds broken – bonds formed, so to have a good explosion, it should have a largely negative enthalpy change. Hence bonds formed for products should have large values, while bonds broken of reactants should have a smaller value.
3) How might you explain the difference between temperature and heat to a friend? Use a practical everyday example.
Heat refers to when energy moves from a warmer body to a colder body. In comparison temperature refers to the measure of the average speed of the heat flow (it can be measured in Celsius, Kelvin etcetera).
For example, when a hot cup of coffee is placed on a table, the cup of coffee may have a temperature of 80°C, and the table a temperature of 25°C. Heat will travel from the coffee cup to the table. Simultaneously, heat is being transferred to the surrounding air, over time, both the coffee cup and the table will return to room temperature.
4) A premium gasoline available at most stations has an octane rating of 98. What does that tell you about:
- The knocking characteristics of this gasoline?
A gasoline’s octane rating is a measure of the gasoline’s resistance to causing knocking in a vehicle’s engine. It is able to burn with minimal knocking at a low octane rating of 98.
- Whether the fuel contains oxygenates?
They are not oxygenates as the octane rating is at 98 while compounds with oxygenates have a octane rating of above 100.
