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# Experiment to Compare the Enthalpy Changes of Combustion of Different Alcohols

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Introduction

Experiment to Compare the Enthalpy Changes of Combustion of Different Alcohols Introduction: This plan will try to outline how the experiment of comparing changes of combustion of different alcohols will be conducted and what results are expected. Background When chemical reactions take place they are often accompanied by energy changes. Chemical reactions most frequently occur in open vessels. That is, they take place at constant pressure. Enthalpy refers to energy at constant pressure (volume may vary). Enthalpy: An example is best to illustrate to show enthalpy works. Methane - how much energy does its molecules contain? The first thing needed is the amount of methane present = 1 mole (16 g). What ever its value, the total amount of energy in a given amount of a substance (sometimes called the Heat energy content) is known as the enthalpy, denoted H. Methane is a fuel to get energy from it, react it with oxygen. CH4(g) + 2O2(g) ? CO2(g) + 2H2O(l) The above chemical equation shows that 2 moles (64 g) of oxygen molecules are required to burn 1 mole of methane. Again, it is impossible to know the total enthalpy (heat energy content) of the oxygen. Likewise, we can't know the total heat energy content of 1 mole of CO2 and 2 moles of H2O (the products). Enthalpy Change ?H = (HCO2 + 2HH2O) - (HCH4 + 2HO2) In general, ?H = ?Hproducts - ?Hreactants But remember, this is theoretical; it is not possible to determine the absolute value of the enthalpy of a chemical element or compound. However, ?H values for chemical reactions can be obtained. They can be measured experimentally, or calculated using Hess's Law (see later), or worked out in other ways. Exothermic and Endothermic Reactions When chemical reactions take place they are often accompanied by heat changes. The system (the reactants which form products) may give out heat to the surroundings, causing them to warm up. ...read more.

Middle

* Splint * Metal stand + clamp * Safety goggles * Tin foil * Measuring cylinder * Bunsen burner. Aim: To determine the different enthalpy changes of combustion of different alcohols and to explain the trends and patterns. Prediction: As the amount of carbon atoms in the alcohol increases, the higher the enthalpy of combustion will be. I have made this prediction, using the values for the enthalpy change of combustion for each alcohol, calculated using bond enthalpies and Hess' law. Methanol's molecular formula is CH3OH. This is the basic structure for all the alcohols, then to make the larger ones an extra carbon is added to the existing carbon each time and the oxygen-hydrogen molecule gets added to the atoms added to the new carbon atom When methanol combusts in air, it reacts with oxygen molecules to from water and carbon dioxide. The balanced equation fort this is: CH3OH (l) + 1.5O2 (g) CO2 (g) + 2H2O (l) This means that the bonds broken are; 3 carbon- hydrogen, 1 carbon-oxygen, 1 oxygen-hydrogen and 1.5 oxygen- oxygen (double bond) and the bonds broken are; 2 carbon- oxygen (double bond) and 4 oxygen-hydrogen. Constructing a Hess' law cycle will show how these are linked together: CH3OH (l) + 1.5O2 (g) CO2 (g) + 2H2O (l) C (g) + 4H (g) + 4O (g) If a calculation for the amount of energy needed to break the bonds is made and then the amount of energy given out from bond formation, the resultant energy difference (negative because the reaction is exothermic) is the enthalpy change of combustion. Average bond enthalpies for elements in their gaseous states (kJmol-1): Carbon - Carbon (C-C) = +347 Carbon - Hydrogen (C-H) = +413 Oxygen - Hydrogen (O-H) = +464 Carbon - Oxygen (C-O) = +358 Carbon - Oxygen double bond (C=O) = +805 Oxygen - Oxygen double bond (O=O) = +498 Energy absorbed when bonds are broken (positive): (E=Energy) = E 3(C-H) + E (C-O) + E (O-H) + E 1.5(O=O) ...read more.

Conclusion

Record all results in a table. Results Methanol Test 1 Test 2 Test 3 Average Mass of the Spirit Burner, before the experiment (g to 2d.p.) 208.98 206.95 204.99 206.97 Temperature before experiment (oC) 21.5 19.00 20.50 20.3 Temperature After experiment (oC) 71.5 69.00 70.50 70.3 Temperature change (oC) 50.00 50.00 50.00 50.00 Time (Mins, secs, milli secs) 5.28.56 5.13.4 5.32.35 Mass of spirit Burner after experiment (g to 2d.p.) 206.95 204.99 203.11 205.02 Difference (g to 2d.p.) 1.95 Ethanol Mass of the Spirit Burner, before the experiment (g to 2d.p.) 224.40 233.49 232.00 229.96 Temperature before experiment (oC) 20.50 24.00 23.50 22.6 Temperature After experiment (oC) 70.50 74.00 73.50 72.6 Temperature change (oC) 50.00 50.00 50.00 50.00 Time (Mins, secs, milli secs) 4.10.39 5.03.00 4.53.25 Mass of spirit Burner after experiment (g to 2d.p.) 223.03 232.00 230.50 228.51 Difference (g to 2d.p.) 1.45 Propan-1-ol Mass of the Spirit Burner, before the experiment (g to 2d.p.) 252.84 251.40 250.05 251.43 Temperature before experiment (oC) 23.50 24.00 24.00 23.8 Temperature After experiment (oC) 73.50 74.00 74.00 73.8 Temperature change (oC) 50.00 50.00 50.00 50.00 Time (Mins, secs, milli secs) 3.49.54 3.29.37 3.25.68 Mass of spirit Burner after experiment (g to 2d.p.) 251.40 250.05 248.77 250.07 Difference (g to 2d.p.) 1.36 Butan-1-ol Mass of the Spirit Burner, before the experiment (g to 2d.p.) 144.13 142.80 141.71 142.88 Temperature before experiment (oC) 20.50 22.50 23.00 22.00 Temperature After experiment (oC) 70.50 72.50 73.00 72.00 Temperature change (oC) 50.00 50.00 50.00 50.00 Time (Mins, secs, milli secs) 3.60.79 2.54.39 2.54.63 Mass of spirit Burner after experiment (g to 2d.p.) 142.80 141.71 140.67 141.73 Difference (g to 2d.p.) 1.15 Hexanol Mass of the Spirit Burner, before the experiment (g to 2d.p.) 177.45 176.60 175.12 176.39 Temperature before experiment (oC) 20.50 23.50 21.00 21.6 Temperature After experiment (oC) 70.50 73.50 71.00 71.6 Temperature change (oC) 50.00 50.00 50.00 50.00 Time (Mins, secs, milli secs) 2.10.13 2.01.32 2.12.45 Mass of spirit Burner after experiment (g to 2d.p.) 176.67 175.12 173.87 175.22 Difference (g to 2d.p.) 1.17 ?? ?? ?? ?? Luke Smith 12MPC ...read more.

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