Q1) From personal experience, state whether these processes are endothermic or exothermic. Give a reason for each: A) a charcoal briquette burns, B) water evaporates from your skin, C) ice melts.
Answers:
A) It is exothermic. Burning is combustion which is an exothermic reaction.
B) It is endothermic. In order to change from liquid to gaseous state, water takes in heat from the surroundings in order to evaporate.
C) It is endothermic. In order to change from the solid state to the liquid state, ice needs to take in heat from the surroundings.
Q2) Chemical explosions are very exothermic reactions. Describe the relative bond strengths in the reactants and products that would make for a good explosion.
Answers:
The bond energies in the reactants should be higher than the bond energies in the products. This results in an exothermic reaction as heat is released during the reaction.
For example: CH4 + 2O2 -> CO2 + 2H2O
Bond breaking:
4 (C-H) + 2 (O=O)
4(416) + 2(498) = 2660
Bond forming:
2 (C=O*) + 2 (2 H-O bonds)
2(803) + 4(467 )= 3474
3474 – 2660 = -814
Q3) How might you explain the difference between temperature and heat to a friend? Use some practical, everyday examples.
Answers:
Heat is form of energy which flows from a hotter body to a colder body. This can be seen when you hold a hot cup of coffee and your hand becomes warm. This is due to heat flowing from the hotter body (the cup) to a colder body (your hand). Temperature determines the direction of heat flow. In this case, since the cup has a higher temperature than your hand, the heat will flow from the hot cup to your hand.
Q4) A premium gasoline available at most stations has an octane rating of 98. What does this tell us about A) the knocking characteristics of this gasoline and B) whether the fuel contains oxygenates?
Answers:
A) The fuel has a knocking characteristic of 98% isooctane and 2% heptane
B) The fuel does not contain oxygenates as the octane rating of the fuel (98) is too low for the fuel to contain oxygenates.