Experiment Hypothesis: The energy released by an alcohol increases as the number of carbon atoms increases.

by rhiannon548courtenay (student)

Applied Science coursework: Energy change in a reaction

Aim: To carry out a series of combustion reaction in which energy is released and the various energies are compared for each reactant used.

Introduction

The presence of the –OH means that hydrogen bonding occurs between the molecules of alcohol and also between the water.

The hydrogen bonding has two consequences:

The first it leads to boiling temperature (and melting temperature) being much higher than those of alkanes of comparable relative molecular mass. For example, butan-1-ol (Mr = 72) which boils at 309 K. The shorter chain alcohols are all liquids at room temperature. Decanol is the first solid.

Secondly, alcohols with relatively short carbon chain tend to have a lower boiling and melting points than those that tend to have a larger carbon chain. The shorter carbon chain alcohols will tend to have weaker van der Waals forces, intermolecular force, between the alcohol molecules meaning that it will have a lower activation energy compared to an alcohol with a comparatively larger carbon chain. Intermolecular forces and bonds are of a number of types; dipole-dipole interactions and van der Waals forces and the hydrogen bond. Van der Waals forces between shorter chain molecules are weak. Between molecules with a longer chain of atoms, giving many points of contact multiple van der Waals forces operate. Branched-chain hydrocarbons are more volatile (have lower boiling temperatures) than unbranched-chain hydrocarbons. Hydrocarbon polymers have molecules which are continuous chains, containing thousands of repeating units. The long, strand-like molecules of poly (ethane) can align themselves to give thousands of contacts between atoms and set up very strong attractions through the operation of multiple van der Waals forces.

When a fuel burns the energy released may be used to do work, as in driving a vehicle, or to heat the surroundings. Heat energy leaves the reaction mixture if the reaction mixture is at a higher temperature than its surroundings. (Energy flows out of us when we touch a cold object; energy flows into us when we step into a hot bath.) The surroundings are the containers and everything else outside the reaction mixture.

Heat is a total measure of energy in a given amount of substance. It does depend on how much of the substance is present. Heat always flows high to low temperature, so heat will flow from a red hot nail into a bucket full of lukewarm water, even though the total heat content of the water is greater than that of the nail. To measure heat we must take into account the temperature, the mass of the substance and the type of substance itself, because some substances take more heat to raise their temperature than others.

Hypothesis: The energy released by an alcohol increases as the number of carbon atoms increases.

Variables:

Mass of water used: The mass of water used throughout the reaction was kept constant so that there would be consistency during the process of calculating the energy released.
Time of combustion of alcohol: The time of combustion was kept fairly constant throughout the reaction for about 30s. This is because alcohols burn very fast and it was more convenient for consistency throughout the reaction.
Temperature of water: The temperature of the water used was not constant throughout the reaction. This is ...