Chapter 5 Thermochemistry Every chemical and physical process involves an exchange of energy between the reaction and the rest of the universe. Thermochemistry involves the study of this exchange of energy. Energy energy is broadly defined as the capacity to do work. (includes force electrical work) potential energy: energy due to position or composition kinetic energy: energy of motion System, Surroundings, and Universe; eat and Work distance as well as internal energy, E : sum of potential and kinetic energy of all particles that make up system E = E FINAL E INITIAL = E PRDUCTS E REACTANTS heat, q: energy transfer as a result of a difference in temperature (NLY) work, w: energy transfer when an object is moved by a force (also includes electrical work) E SYS = q + w pressure-volume work: work in which volume changes against an external pressure. w = PV First Law of Thermodynamics aka Law of Conservation of Energy Energy is conserved ; you can convert energy from one form to another but you can not create or destroy energy. E SYS + E SURR = E UNIV = 0 Units of Energy SI: joule (J) 1 J = 1 kg m 2 s 2 (= 1 N m, units of force distance) In chemistry and many other sciences, the calorie (cal) still common 1 cal = 4.184 J NTE: the nutritional Calorie = 1000 chemical calories (= 1 kcal) 5 1
State Functions a state function depends only on the present state of the system, not on the path taken to get there o e.g., T, P, V, E o state functions usually written with CAPITAL LETTER SYMBLS Enthalpy, Enthalpy is the heat of a reaction at constant pressure. = q P Enthalpy always written as, not just. No convenient uniform reference point to set as zero point. In many cases, E at constant P if little or no work.* *This is not so if w is very large, i.e., if PV is a large quantity (a large V either way). Names for Signs of Enthalpy Change exothermic: heat out negative endothermic: heat in positive Types of Enthalpy Change heat of reaction( RXN ): enthalpy change for a chemical reaction 2Al(s) + Fe 2 3 (s) Al 2 3 (s) + 2Fe(s) = RXN = 851 kj heat of combustion ( CMB ): enthalpy change for the chemical reaction when 1 mole of a substance reacts with 2 (combustion) C 3 8 (g) + 5 2 (g) 3C 2 (g) + 4 2 (l) = CMB = 2202 kj heat of formation ( F ): enthalpy change for the chemical reaction when 1 mole of a compound is produced from its component elements Na(s) + 1 Cl 2 2(g) NaCl(s) = F = 411.1 kj heat of fusion ( FUS ): enthalpy change for the melting of 1 mole of a substance 2 (s) 2 (l) = FUS = +6.0 kj heat of vaporization ( VAP ): enthalpy change for the vaporization of 1 mole of a substance 2 (l) 2 (g) = VAP = +44.0 kj eat of fusion and heat of vaporization are both ALWAYS endothermic. For a substance, the heat of vaporization is always larger than the heat of fusion. 5 2
Bond Energies and Enthalpy RXN results primarily from differences in the strengths of bonds in the reactants and products. Breaking bonds is ENDthermic. Forming bonds is EXthermic. Average Bond Enthalpy: the amount of energy required to break 1 mole of bonds in the gas phase. Table of average bond enthalpies in Table 9.2. Example: Estimate RXN : C + C + C 4 (g) + 2 2 (g) C 2 (g) + 2 2 (g) = nd(bonds broken) nd(bonds formed) [ D for bond D issociation energy, old-fashioned name for bond enthalpy] 5 3
eat Capacity specific heat capacity, c: quantity of heat required to raise temperature of 1 gram of material by 1 K (or 1 C); an intensive property. (molar heat capacity C is similar for 1 mole of substance) Constant-Pressure Calorimeter SYSTEM = reaction SURRUNDINGS = water, thermometer, cups, stirrer, everything else! ess s Law of eat Summation ess s law: if a reaction is carried out in a number of steps, for the overall reaction is the sum of for each individual step. Example: What is for this reaction C 4 (g) + 2 2 (g) C 2 (g) + 2 2 (l) given the following information? C 4 (g) + 2 2 (g) C 2 (g) + 2 2 (g) = 802 kj 2 (l) 2 (g) = +44 kj o reverse equation: change sign of o multiply all coefficients by a constant: multiply by same constant o add equations: add values 5 4
Standard States (e.g., ) 1. Compounds gas: P = 1 bar (1 bar = 100 kpa 1 atm) liquids/solids: most stable form at T of interest and P = 1 bar aqueous solution: 1 M concentration 2. Elements: most stable form at T of interest and P = 1 bar usually tabulated at 25 C (298 K) Standard eat of Formation, F always tabulated per one mole of substance Note: extensive list of F values in Appendix B of text 1. F for element in standard state = 0 (KNW) 2. Most compounds have negative F (some don t) RXN = n F (products) n F (reactants) Example: Calculate RXN: C 3 8 (g) + 5 2 (g) 3C 2 (g) + 4 2 (g) F 5 5