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Comparing the enthalpy of combustion of various alcohols.

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Introduction

Comparing the enthalpy of combustion of various alcohols Aim My aim is to find out the energy changes of combusting alcohols. (Methanol, Ethanol, Propanol and Butanol) Background Info Key science Chemistry by Eileen Ramsden says that "an alcohol is a series of organic, homologous compounds, with the general formula CnH2n+1OH." Each member of the series has the O-H group in its molecule. As you go up the homologous series of the alcohols, their physical properties start to vary but their chemical properties remain the same. Alcohols react with the oxygen in the air to form the products water and carbon dioxide: Methanol = CH3OH +1.5O2 CO2 + 2H2O Ethanol = C2H5OH +3O2 2CO2 +3H2O Propanol = C3H7OH +4.5O2 3CO2 +4H2O Butanol = C4H9OH +6O2 4CO2 + 5H2O This is an example of combustion. "Reactions of this kind, which give out energy are exothermic reactions (ex = out; therm = heat)." (Key science Chemistry by Eileen Ramsden). Another example of an exothermic reaction is neutralisation. The amount of energy produced by such exothermic reactions can be calculated by using the formula Mass of the substance x rise in temp x specific heat capacity. The specific heat capacity is the number of joules required to heat one gram of water by 1�C. ...read more.

Middle

10 3 90s 18 86 Methanol 50 50 30s 18 31 Methanol 50 50 60s 18 41 Methanol 50 50 90s 16 57 Methanol 100 100 30s 18 21 Methanol 100 100 60s 18 27 Methanol 100 100 90s 18 34 Method The apparatus is set up as shown above. The apparatus will be placed in a card box over a heat proof mat. This is to minimise the amount of heat lost to the environment. Firstly, I will fill a beaker of water containing 50cm, measuring it out using a measuring cylinder. Then, I will measure the temperature of the water. This will give me the start temperature. After that I will weigh a spirit burner containing an alcohol with its lid. This will give me the starting mass of the spirit burner. Having done this, I will clamp the beaker so the bottom is about 5 cm above the wick of the spirit burner. Then I will light the spirit burner and the stir the water with the thermometer. I will allow the spirit burner to burn for 1 minute. After that, I will reweigh the spirit burner with the lid, which will allow me to work out the change in mass. ...read more.

Conclusion

I can see that it takes more energy to break up the bonds of butanol (+7507 mol) in comparison to methanol (+2803 mol). This is also evident from the data book value. This is also evident from the data book values which I stated in my Bond energy calculation section. There is a pattern and a numerical trend from these data suggesting that as you go up the homologous series the more exothermic the reaction will become. This is because there are more carbons present in alcohols high in the homologous series. Therefore I predict that Butanol will be the most exothermic because it has the greatest mass per mole of the four alcohols provided; therefore it will produce the most heat in a given time. As a result I predict the alcohol with the greatest relative molecular mass will be the most exothermic and Methanol with the least relative molecular mass will be the least exothermic. I think that the order of the alcohols (highest heat produced to lowest heat produced) will be: butanol, propanol, ethanol and methanol. Safety I will also need to ensure that I conduct the experiments safely. As alcohols are very dangerous and highly flammable I will wear my safety goggles at all times. I will need to keep all lose items off. The lids on the alcohol must be kept on at all times to prevent evaporation or any spillage. ...read more.

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