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Enthalpy change for different alcohols.

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Investigation -Enthalpy change for different alcohols: An alcohol is a series of organic, homologous compounds, Alcohols may be classified according to which carbon of the alkyl group is bonded to the hydroxyl group with the general formula CnH2n+1OH. The alcohol reacts with the oxygen in the air to form water and carbon dioxide: The structure of the molecules in this reaction is: This is an exothermic reaction, as heat is given out from the formation of water and carbon dioxide. This is because there is more energy given out when bonds are broken, furthermore less energy is absorbed when bonds are formed therefore the energy left over after the formation (water and carbon dioxide) is given out to the surrounding as heat. This can be shown in an energy level diagram: The values of enthalpy change for any exothermic reaction is always negative as the diagram above shows that energy is 'lost' as heat. Enthalpy is defined as the energy of reaction, or the heat energy associated with a chemical change. The bonds that are formed in an exothermic reaction can be of two types. ...read more.


H Reaction = - 746 kJ mol-1 Combustion of Ethanol To calculate the H we need know the bond enthalpy of the bonds present. C=O = + 805 kJ mol-1 C-H = + 413 kJ mol-1 C-O = + 360 kJ mol-1 O=O = + 498 kJ mol-1 O-H = + 464 kJ mol-1 C-C = + 347 kJ mol-1 Bonds broken( H Reactant ) 5 x C-H = 5 x (+ 413 ) = + 2065 kJ mol-1 1 x C-O = 1 x (+ 360) = + 360 kJ mol-1 1 x O-H = 1 x (+ 464) = + 464 kJ mol-1 3 x O=O = 3 x (+ 498) = + 1494 kJ mol-1 1 x C-C = 1 x (+ 347) = +347 kJ mol-1 Total H Reactant = +2065 + 360 + 464 + 1494 + 347 = + 4730 kJ mol-1 Bond formation ( H Products) 6 x O-H = 6 x (+ 464) = + 2784 kJ mol-1 4 x C=O = 4 x (+ 805) = + 3220 kJ mol-1 Total H Products = +2784 + 3220 = +6004 kJ mol-1 The total enthalpy change of formation : H Reaction = H Products - H Reactant H Reaction = -(+ 6004 - 4730) ...read more.


So when I come to predicting results I can safely say that Propan-2-ol will need more input of energy than methanol simply because it has more bonds to break. How to calculate energy transfer of alcohol after combustion. Heat transfer = mc T x M y 1. m = mass of water 2. c = specific heat capacity 3. T= rise in temperature 4. M = relative molecular mass of alcohol 5. y = (initial mass of alcohol - mass of alcohol after combustion) y = Difference > mass of water = 200cm3 > Specific heat capacity of water is 4.2 J > Rise in temperature is 20 oc Example Ar (H) = 1, (c) = 12 (O) = 16 RMM of (CH3OH) = 12 + (4X 1) + 16 = 32g Heat transfer = 200cm3 x 4.2 J x 20 oc x (32g) 0.21(rough estimate) Heat transfer = 16800 J x (32g) 0.21 Heat transfer = 16800 J x 15.23 Heat transfer = 25586400 J Heat transfer = -(2558) kJ mol-1 Heat transfer = - 2558 kJ mol-1 Through this method I will calculate my energy transfer of different alcohol. Predicted table Alcohol Structural formula Mass of water cm3 Rise in temperature oc Methanol CH3OH 200 20 Ethanol CH3CH2OH Propan-1-ol CH2 (OH)CH2CH3 Propan-2-ol CH3 CH2 (OH)CH2 Butan-1-ol CH2 (OH)CH2CH2 CH3 ...read more.

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