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# Enthalpy of Combustion of Alcohols Lab

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Introduction

MOLAR ENTHALPY OF COMBUSTION OF ALCOHOLS Lab Asad Ali Tayyab IB HL Chemistry 2 Grade 12 Lab Work Block 8 Mr. Graham MOLAR ENTHALPY OF COMBUSTION OF ALCOHOLS AIM: The purpose of this investigation is to use calorimeter to determine the molar enthalpy change in the combustion of each of a series of alcohols (Methanol, Ethanol and Butan-1-ol) RESEARCH QUESTION: How do the molar enthalpies of combustion change as the alcohol molecules become larger from methanol to ethanol to butan-1-ol? HYPOTHESIS: I think that as the molecules become larger, the enthalpy of combustion will increase. This hypothesis is based on my assumption that as the molecule becomes larger, more bonds are added to a substance. The more the bonds, the harder it is to separate the molecule and the more energy is required to combust the substance. INTRODUCTION: The molar enthalpy of combustion (?H) of a substance is the change of enthalpy, when 1 mole of a substance in its standard state (298 K and 1 atm pressure) is burnt to form products in their standard states. ?Hc may be calculated from standard enthalpies for formation of each of the substances involved in the combustion reaction, or can be estimated from bond enthalpies. Enthalpies of formation are usually obtained directly from combustion experiments. ?Hc is measured in calories. ...read more.

Middle

+ (5 x 1) + (16) + (1) M = 46.0 gmol-1 m = 1.06 g n = 1.06g/46.0 gmol-1 n = 0.0230 mol Step 2: Calculating heat energy given out Q = mC?T m = 100g C = 4.186 Jg-1k-1 ?T = 11.0k Q = (100g) (4.186 Jg-1k-1) (11.0k) Q = 4604.6 J Q = 4.60 kJ Step 3: Calculating the enthalpy of combustion ?HC = Q/mol ?HC = 4.60kJ 0.0230mol = 200 kJ/mol However it is exothermic, that is why it has to be negative = - 200 kJ/mol Step 4: Calculating the uncertainties Uncertainty for mass of alcohol combusted = (0.02g/1.06g) x 100 1.78% Uncertainty for volume of water: (0.05cm3/100cm3) * 100 = 0.05 % Therefore uncertainty for mass of water: 0.05% Uncertainty for change in temperature = (0.02/11.0) x 100 = 0.18% Total Uncertainty = 2.01 % ENTHALPY OF COMBUSTION OF ETHANOL = - 200 kJ/mol � 2.01 % BUTAN-1-OL: Step 1: Calculating the number of moles of Butanol combusted n = m/M C4H9OH M = (4 x 12) + (9 x 1) + (16) + (1) M = 74.0 gmol-1 m = 0.29 g n = 0.29g/74.0 gmol-1 n = 0.00390 mol Step 2: Calculating heat energy given out Q = mC?T m = 100g C = 4.186 Jg-1k-1 ?T = 11.0k Q = (100g) ...read more.

Conclusion

This was an error as methanol and ethanol are volatile liquids and the fact that the alcohol lamps were hot after the combustion indicates that some of the alcohols would have escaped which increases the uncertainty in the labs. In addition, one other reason that could affect the results of our lab is that the beaker might have absorbed some heat. The beaker was made of Pyrex glass and certainly absorbed some heat. This means that before the temperature of the water started to change, the beaker might have absorbed some heat and this affects the results as some energy is wasted in this process. Similar to this was the stirring rod and the thermometer which absorbed some heat as well further making the experiment less accurate. Furthermore, random errors certainly played a part in affecting the results as we had uncertainties for mass, temperature and volume. Even though random errors were comparatively small to affect the result, nevertheless, all these uncertainties add up to be large enough to affect the results of the lab. For random errors, we can take certain measures to reduce the uncertainty and increase the accuracy of our lab. Also, we can measure the mass of water to try and reduce the uncertainty due to our assumption made of the density of water. For lowering the uncertainties even more, we can use a digital thermometer so that if the thermometer has a lower uncertainty, the percentage uncertainty will decrease. ?? ?? ?? ?? ...read more.

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