Unit 4: Energy and Combustion (Application Exercise)

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 when near the burning charcoal indicates the release of heat, thus an exothermic process.

b. Water evaporates from your skin.

The process leaves a cool sensation on one’s skin. This indicates that heat energy contributed by the human is lost to the evaporation process to break the inter-molecular forces of attraction between water molecules in liquid water and is hence an endothermic process.

c. Ice melts.

Likewise, the melting of ice is cold, indicating that heat is absorbed to overcome the inter-molecular forces of attraction between water molecules in solid ice. Hence, it is an endothermic 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.

There are 2 possible indicators of a ‘good’ explosion explained below:

a) It is easy to explode. This indicates low activation energy required to break the bonds in product molecules.

b) The explosion is huge. This indicates that a large amount of energy is released. This can only be achieved if the change in energy between the products and reactants is large, indicating that the bond strength in product molecules are very much weaker than the bond strength in reactant molecules.

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

Temperature Heat
Definition Measure of heat energy (average energy of molecular motion) A form of energy (thermal)
Relationship An increase in the amount of heat (thermal energy) will cause an increase in temperature.
Example 1 (different temperatures) A cup of cold water versus a cup of hot water

A cup of hot water would have a higher temperature than a cup of cold water. Intuitively, we know that a cup of hot water has more thermal energy (because it is hot). Hence, more heat energy (for the same volume) would indicate a higher temperature.

Example 2 (different volumes) A bathtub full of water versus a cup full of water

While both may have the same temperature (say 30 degrees Celsius), the bathtub full of water would have more thermal energy by virtue of the fact that it has a larger volume and hence more energy (thermal).

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 indicates a knocking characteristic of 98% isooctane and 2% heptane. Its ability to combust without causing knocking is similar to pure isooctane and  greatly higher than pure heptane. 

Practically, whether this causes knocking would also depend on the individual vehicle’s engine compression ratio.

b. whether the fuel contains oxygenates?

To our present knowledge, there appears to be no relationship between octane number and the presence of oxygenates. 

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