Tag Archives: Energy from Combustion

Application Exercise 3 – Energy from Combustion

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

a) A charcoal briquette burns.

This reaction is exothermic because hear is released as the charcoal burns. The heat released is used to cook your food!

b) Water evaporates from your skin.

This reaction is endothermic because water (liquid state) absorbs heat from your skin necessary for it to change into water vapour (gaseous state). Therefore, your skin will feel cooler.

c) Ice melts:

The reaction is endothermic as ice (solid state) absorbs heat for it to melt into water (liquid state).

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

In this case, we would consider a natural gas (methane) explosion:

CH4 + 2 O2 –> CO2 + 2 H2O

In this example, the bond energies involve:

-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

From this, we can see that the energies of the product (CO2 + 2 H20) are larger than those of the reactants. This will lead to a large negative net energy change, thus highly exothermic explosions.

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

Heat is a form of energy while temperature is a measurement indicating the direction of the heat flow. Heat always flows from a hotter object to a colder object. Take for example choosing between spilling a drop of tea on yourself than the whole cup of tea at the same temperature. Even at the same temperature, it will definitely be more painful from the bigger volume of tea because of the higher heat content. Heat depends on the temperature and volume of material. In the case of temperature, when cold water is added to the hot tea, the heat will flow from the hot tea to the cold water and the final temperature of the new mixture will be between that of the original temperatures of the cold water and tea.

4. A premium gasoline available at most stations has an octane rating of 98. What does this tell you about:

a) the knocking characteristics of this gasoline?

It has the same knocking characteristics as a mixture made up of 98% isooctane and 2% n-heptane. The word ‘premium’ gasoline would indicate lower knocking characteristics and that this company’s gasoline has a higher octane rating than other gasoline blends sold at other stations.

b) whether the fuel contains oxygenates?

The octane rating does not provide additional information about whether the fuel contains oxygenates. This can be found on other labels around the pump instead.

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