- From personal experience, state whether these processes are endothermic or exothermic. Give a reason for each.
a) A charcoal briquette burnns
b) Water evaporates from your skin
c) Ice melts
a) Exothermic. Burning of charcoal releases heat to surroundings.
b) Endothermic. Water needs to absorb the heat from the skin for evaporation to occur.
c) Endothermic. Ice requires energy to be absorbed to break the lattice structure to melt.
- Chemical explosions are very exothermic reactions. Describe the relative bond strengths in the reactants and products that would make for good explanation.
C2H6 + 3.5 O2 ——-> 2 CO2 + 3 H20
The above equation states a complete ethane explosion.
Given that the bond energies for reactants and products are :
C-H bond : 411 kJ/mole
O=O bond : 498 kJ/mole
H-O bond : 459 kJ/mole
C=O bond : 803 kJ/mole
C-C bond : 346 kJ/mole
The reactants will have energy level of 4555kJ/mole, while the products will have a higher energy level of
This give an overall net energy of -493 kJ/mole, which indicates the explosion of ethane is an exothermic reaction.
3. How might you explain the difference between temperature and heat to a friend? Use some practical everyday examples?
Heat refers to the TOTAL energy of molecular motion inside a substance and exists as an energy that can be transferred from one to the other. While temperature is a measure of AVERAGE kinetic energy present in molecular motion of the substance and cannot be transfer.
For example , a hot water bath of 37 degrees celsius is prepared by a person. When the person is inside the bath, we say that the heat energy of the hot water is transferred to the person’s body. While we can say that the temperature ,which is a measure of average kinetic energy of molecules in the water, is 37 degrees celsius.
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
b) whether the fuel contains oxygenates
a) This states that the gasoline contains 98% iso-octane and 2% heptane. It will be unlikely for the gasoline to cause much knocking to the engines.
b) The octane rating does not tells us whether the fuel contains oxygenates.