Tag Archives: session 3

Answers for Application Exercise Session 3: Energy from Combustion

Qn 1. a) The burning of charcoal briquettes is an exothermic process. This because I went camping once and lit a fire surrounded by some rocks and sand. But some dry grass caught fire and it amplified quickly before it could be extinguished. Thus, it does not need additional energy to amplify and a lot of heat is released in the process.


         b) The evaporation of water from your skin in an endothermic process. This because once I forgot a bucket of water outside in the sun, and after a while the amount of water decreased. The water molecules gains energy from the environment and can therefore evaporate.


         c) The melting of ice is an endothermic reaction as well. This because if you place ice cubes in a glass of water they will melt. The melting speed depends on how warm the surrounded water is. This because the ice cubes gain energy from the surrounding which make them melt.

 

Qn2) Exothermic reactions are chemical reactions with releases energy. In order for this, the total energy of the reactants must be higher than that of the final products, in other words, the energy absorbed for breaking of bonds in the reactants must be less than the energy produced for the forming of bonds for the products. As such, the bond strength of the reactants needs to be lower than that of the products. For a good explosion to happen, a high enthalpy is needed for the release of large amount of energy.  Hence, the greater the difference between the bond strength of the reactants and products, the better the explosion.

 

Qn3. Temperature is the measurement of a heat source whereas heat is the energy found in that source. Heat can be transferred from a source to another source whereas temperature cannot be transferred. For example, when boiling water, the heat energy causes the water to boil; i.e. steaming and bubbling water under heat, whereas the temperature shows that the water is boiling at 100°C.

 

Qn 4. a) Gasoline with an octane rating of 98 has a high resistance to cause engine knock. Engine knock occurs when the fuel mixture ignites too early. High octane Gasoline burns slower and at a higher efficiency, thus it is more resistant to knock.

b) Oxygenates are added to motor vehicle fuels to make them burn more cleanly, thereby reducing toxic tailpipe pollution, particularly carbon monoxide. Oxygenates are favored not only for their vehicle emission benefits but also their blending properties in motor gasoline. Thus, with the gasoline at a high octane rating, it is probable that it contains oxygenates.