Application Exercise 3: Energy from Combustion

plantbynight

  1. From personal experience, state whether these processes are endothermic or exothermic. Give a reason for each.

a. A charcoal briquette burns

charcoal

Exothermic. A charcoal briquette releases heat as it burns.

b. Water evaporates from your skin

Endothermic. Water absorbs the heat necessary for evaporation from your skin, and your skin feels cooler.

c. Ice melts

ice-melt

Endothermic. Ice absorbs the necessary heat to melt from the environment.

  1. Chemical explosions are very exothermic reactions. Describe the relative bond strengths in the reactants and products that would make a good explosion.

chemical-explosion

The bond energies involved are: C–H single bonds, 416 kJ/mole; O=O double bonds, 498 kJ/mole; H–O single bonds, 467 kJ/mole; C=O double bonds, 803 kJ/mole. The bond energies of the products are larger than those of the reactants. This will lead to a large negative net energy change indicating an exothermic reaction.

  1. How might you explain the difference between temperature and heat to a friend? Use some practical, everyday examples?

Wouldn’t you rather spill a drop of hot coffee on you than the whole cupful at the same temperature? Although the drop and the cup full of coffee may initially have the same temperature, you will receive a bigger burn from the bigger volume of coffee because it has the higher heat content. Heat is a form of energy. In contrast, temperature is a measurement that indicates the direction heat will flow. Heat always flows from an object at high temperature to an object at lower temperature. This means that if hot coffee is added to cold coffee, heat will flow from the hot liquid to the cold liquid, and the final temperature of the mixture will be between the original temperatures of the two individual solutions. Heat depends on the temperature and on how much material is present.

  1. A premium gasoline available at the most stations gas an octane rating of 98. What does that tell you about:

gasoline

a. The knocking characteristics of this gasoline

Gasoline with an octane rating of 98 has the same knocking characteristics as a mixture composed of 98% isooctane and 2% n-heptane. As a “premium gasoline,” it has a higher octane rating than other blends sold at gasoline stations and hence is more resistant to knocking than these blends.

b. Whether the fuel contains oxygenates

The octane rating provides no information about whether or not the fuel contains oxygenates. Other labels around the pump should reveal this information.