To investigate the relationship between the structure and heat provided by combustion of a range of alcohols.

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COURSE WORK PIECE 1:

INVESTIGATION OF THE COMBUSTION OF ALCOHOLS

HARRY HUDSON 10 U                                                                                                                     25/10/02

AIM: To investigate the relationship between the structure and heat provided by combustion of a range of alcohols.

INTRODUCTION:

In this investigation I will be burning a range of alcohol in a method known as calorimetry. This will allow me to see the amount of energy produced by each alcohol, and then look at the structure of the alcohol and investigate why an amount of energy is produced for each alcohol. Before I go on with the experiment there are several factors that must be first understood.

What is an alcohol?

The definition of an alcohol as taken from Richard Harwood’s Chemistry textbook is “a series of organic compounds containing the functional group – OH.” The – OH group, called a hydroxyl group is what defines the compound as an alcohol. The alcohol compounds are very similar to the alkanes, however alcohols contain one oxygen atom, creating the hydroxyl group, and making the alkane an alkanol (alcohol). As the hydroxyl group makes the compound different to an alkane, the hydroxyl group is seen to be “functional”. The formula for alcohol is:

In this investigation I will look at the first five alcohols. These are methanol, ethanol, propan-1-ol, butan-1-ol and pentan-1-ol. The classification of alcohols is similar to the classification of alkanes, where the name refers to the number of carbon atoms i.e. ‘meth’- one carbon atom, ‘eth-’=two carbon atoms, ‘prop-’=3 carbon atoms, ‘but-’=four carbon atoms, ‘pent-’=five carbon atoms and so on. The carbon atom that the hydroxyl group is bonded to may classify the alcohol further. If it is joined to the end carbon atom it is classified as ‘-1-ol’, if it is bonded to the second carbon atom it is classified as ‘-2-ol’ and so on. The five alcohols I am testing are shown with formulas, and atomic structure diagrams below:

What is combustion?

The definition of combustion as taken from Richard Harwood’s Chemistry textbook is “a chemical reaction in which a substance reacts with oxygen – the reaction is exothermic. Burning is a combustion reaction that produces a flame.” The reactions that will take place in our investigation will be combustion reactions, as we burn the alcohol in calorimetry.

The combustion of an alcohol produces carbon dioxide and water. This is represented by the following equation:

ALCOHOL + OXYGEN → CARBON DIOXIDE + WATER

e.g. The combustion of methanol produces carbon dioxide and water:

METHANOL + OXYGEN → CARBON DIOXIDE + WATER

2CH3OH         +      3O2        →              2CO2                  +    4H2O

What is calorimetry?

Calorimetry is defined as the science of heat. It may be used in chemistry to measure the heat energy exchanged for a substance during a reaction, by allowing the substance reacting to heat a measured mass of liquid. The temperature change of this liquid is measured and recorded, and the following equation may be used to calculate the heat energy transferred to the liquid: Heat change = MCΔT (where M = mass of liquid, C = critical temperature, and T = temperature change). The critical temperature is the proportionality between heat energy applied and the subsequent temperature rise. E.g. for 1g of water a temperature rise of 1 °C requires 4.8 joules of heat energy.

Using calorimetry may be used to measure both reactions in solution, and reactions not in solution. “Wet reactions” or those in solution are more accurate than those not in solution, as the actual energy release of the reaction is being measured directly as a thermometer is placed in the solution. For “dry reactions”, (a reaction not in solution such as the burning of an alcohol) the reaction must be used to heat a mass of water and the temperature increase measured from the water. As the combustion reaction must transfer its heat energy from the reaction to the water, calorimetry for “dry reactions” can be inaccurate due to heat loss.

HYPOTHESIS

I believe that the increased complexity of a molecule and the energy released by it are proportional.

I found this idea upon the theory of breaking and making bonds. In a molecule, bonds hold atoms together. When these bonds are formed energy is given out to the surroundings (exothermic), and when these bonds are broken energy is absorbed from the surroundings (endothermic). This may be seen by the alcohols boiling points:

Using this theory of breaking/making bonds, we can say that there will be a greater amount of energy released from the combustion of a more complex alcohol, as more bonds will be formed. When more bonds are formed, more heat energy is released.

The hypothesis may be seen on the flowing graph:

TESTING

To investigate the relationship between the structure and heat provided by combustion of a range of alcohols, we will use the method of calorimetry. We will do this by burning an alcohol, and allowing it to heat a mass of water. Measuring the temperature rise of this mass of water we can use the formula to find the heat energy released during the reaction.

Apparatus

  • 1 can
  • 1 measuring cylinder
  • Water
  • 1 thermometer
  • 1 stand
  • 1 clamp
  • 1 boss
  • 1 heat mat
  • 1 alcohol burner/candle
  • 1 balance
  • Draught shields
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 Fig1: Experiment setup.

Method

Safety must be ensured at all times, as highly flammable substances are being burned here. All students must wear eye protection during the experiment, stay standing during the experiment, and be aware of the flames around them.

  1. The apparatus is set up as shown in fig 1.
  2. The alcohol candle is weighed on the balance, and the mass recorded.
  3. The mass of water in the can measured and recorded
  4. The height between the candlewick, and the base of the can must be set and recorded, measuring the distance with a ruler.
  5. The ...

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