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Energy Chemical Reactions Part 2 Enthalpy continued Many reactions involve little if any PV work so most or all of the energy change occurs as a transfer of heat. Reactions that do not involve gases. Gases do not appear in many reactions. Liquids and solids undergo very small volume changes so V 0 and PV 0. H E Reactions in which the amount (mol) of gas does not change. When the total amount of gaseous reactants equals the total amount of gaseous products, V = 0, PV = 0, and H = E. Reactions in which the amount (mol) of gas does change. In these cases PV 0. But since q is usually so much larger than PV, H is very close to E. Most of E occurs as heat transfer so H E. Exo- and Endothermic Processes Enthalpy is a state function. The enthalpy change of a reaction is called heat of reaction, Hrxn, which is Hfinal - Hinitial or Hproducts - Hreactants. Exothermic process releases heat, H < 0. Endothermic process absorbs heat, H > 0. Problem: Draw an enthalpy diagram for the decomposition of 1 mol of nitroglycerine which releases 5.72 x 103 kJ of heat.
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Types of Enthalpy Change When 1 mol of a substance combines with O2 in a combustion reaction, the heat of reaction is the heat of combustion (Hcomb). When 1 mol of a compound is produced from its elements, the heat of reaction is the heat of formation (Hf). When 1 mol of a substance melts, the enthalpy change is the heat of fusion (Hfus). When 1 mol of a substance vaporizes, the enthalpy change is the heat of vaporization (Hvap). Where does the heat of reaction come from? H2(g) + F2(g) 2HF(g) + 546 kJ Focus on the HF molecule: Kinetic Energy Contributions Translation, rotation, vibration, electron motion Potential Energy Contributions Force between vibrating atoms, forces between nucleus and electrons and between electrons in each atom, forces between protons neutrons in each nucleus, forces between nuclei and shared electron pair in each bond.
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Breaking and forming bonds to contribute energy change! Bonds of the reactants absorb energy when they break and bond of the products release energy when they form. Weaker bonds are easier to break than stronger bonds because they are higher in energy. Energy released or absorbed during a chemical change is due to differences between the strengths of reactant bonds and product bonds. Bonds in Fuels and Food Fuels for machines are hydrocarbons or coal (fossil fuels). Fuels for organisms are fats and carbohydrates. Both contain large organic molecules with many C C and C H bonds. When a fuel reacts with O2, C O and O H bonds form in the products CO2 and H2O. The C O and O H bonds are more stable and of lower energy than the C H bond. Compare the energy released when fats and carbohydrates are burned. Hcomb (kJ / g) Vegetable Oil -37.0 Table Sugar -16.2 Calorimetry Measuring Heats of Reaction Specific Heat Capacity (c) the quantity of heat required to change the temperature of 1 gram of a substance by 1 K. Units: J / g K q = c x mass x T
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Molar Heat Capacity (C) the quantity of heat required to change the temperature of 1 mole of a substance 1 K. Problem: Calculate the heat transferred when a 5.5 g iron nail is cooled from 37oC to 25oC. For iron, c = 0.450 J / g K.
Calorimetry Constant Pressure Calorimetry Coffee cup Calorimeter Constant Volume Calorimetry Bomb Calorimeter Problem: As a purity check for industrial diamonds, a 10.25 carat (1 carat = 0.2000 g) diamond is heated to 74.21oC and immersed in 26.05 g of water in a constant pressure calorimeter. The initial temperature of the water is 27.20oC. Calculate T of the water and of the diamond (cdiamond = 0.519 J / g K). -qsolid = qwater
Problem: A chemist burns 0.8650 g of graphite (a form of carbon) in a new bomb calorimeter and CO2 forms. If 393.5 kJ of heat is released per mole of graphite and T increases 2.613 K, what is the heat capacity of the bomb calorimeter? -qsample = qcalorimeter