Unit #5- Chapter #6. Types of chemical reactions. Energy: its forms 10/15/2013. Thermodynamics

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1 Unit #5- Chapter #6 Thermodynamics Types of chemical reactions PRODUCT-FAVORED: when the reaction converts reactants to products completely-it may take a small amount of activation energy but releases more energy. H is negative (exothermic) REACTANT-FAVORED: when the reaction will be sustained only as long as energy is supplied. It will not occur on its own. H is positive (endothermic) Energy: its forms Kinetic Energy the energy of motion Thermal energy of atoms, molecules or ions in motion at the submicroscopic level Mechanical energy of a macroscopic object moving Electrical energy the motion of electrons moving through a conductor Sound the compression and expansion of space between molecules Potential Energy the energy of position Chemical potential energy results from attractions among electrons and nuclei in molecules Gravitational energy like a ball held well above the floor Electrostatic energy positive and negative ions a small distance apart 1

2 Energy: some facts Law of Conservation of Energy energy cannot be created nor destroyed, the total energy of the universe is constant. Energy always transfers spontaneously from the hotter to the cooler object. The total thermal energy depends on temperature, the types of atoms or molecules, and the number of them in the sample. K.E. = ½ mv 2 where m = mass in kg and v=velocity in m/s and K.E. = energy in joules Energy: its units A calorie is defined as the amount of heat needed to raise one gram of water one degree celcius. (cal) A food calorie is really a kilocalorie (Cal) or (kcal). A joule (J) is a kg m 2 /s 2 The conversion for joules and calories is 1 cal = J 1 kwh = 3.61 x 10 6 J Thermodynamics Thermodynamics is the science of heat (energy) transfer. Heat energy is associated with molecular motions. Heat transfers until thermal equilibrium is established. 2

3 Directionality of Heat Transfer Heat always transfer from hotter object to cooler one. EXOthermic: heat transfers from SYSTEM to SURROUNDINGS. T(system) goes down T(surroundings) goes up Chemical Reactivity What drives chemical reactions? How do they occur? The first is answered by THERMODYNAMICS and the second by KINETICS. Have already seen a number of driving forces for reactions that are PRODUCT- FAVORED. formation of a precipitate gas formation H 2 O formation (acid-base reaction) electron transfer in a battery Chemical Reactivity But energy transfer also allows us to predict reactivity. In general, reactions that transfer energy to their surroundings are product-favored. So, let us consider heat transfer in chemical processes. 3

4 Heat Energy Transfer in a Physical Process Heat transfers from surroundings to system in endothermic process. CO 2 (s, -78 o C) ---> CO 2 (g, -78 o C) A regular array of molecules in a solid -----> gas phase molecules. Gas molecules have higher kinetic energy. Energy Level Diagram for Heat Energy Transfer CO 2 gas E = E(final) - E(initial) = E(gas) - E(solid) CO 2 solid Heat Energy Transfer in Physical Change CO 2 (s, -78 o C) ---> CO 2 (g, -78 o C) Two things have happened! Gas molecules have higher kinetic energy. Also, WORK is done by the system in pushing aside the atmosphere. 4

5 FIRST LAW OF THERMODYNAMICS heat energy transferred E = q + w energy change work done by the system Energy is conserved! heat transfer in (endothermic), +q heat transfer out (exothermic), -q SYSTEM E = q + w w transfer in (+w) w transfer out (-w) ENTHALPY Most chemical reactions occur at constant P, so Heat transferred at constant P = q p q p = H where H = enthalpy and so E = H + w (and w is usually small) H = heat transferred at constant P E H = change in heat content of the system H = H final - H initial 5

6 ENTHALPY H = H final - H initial If H final > H initial then H is positive Process is Process is ENDOTHERMIC If H final < H initial then H is negative Process is Process is EXOTHERMIC Calculations involving Heat Formula is q = m T C Specific heat capacity = C= the heat required to produce a given temperature change per gram of material. C is measured in J/g K Heat = q= is measured in J Mass = m = is measured in g Change in temperature = T = is measured in K (remember one degree K is also one degree Celsius) Specific Heat Capacity Use page Appendix 4 for C (heat capacity) values If 25.0 g of Al cool from 310. o C to 37.0 o C, how many joules of heat energy are lost by the Al? heat gain/lose = q = (C) (mass)( T) where T = T final - T initial q = (0.902 J/g K)(25.0 g)(37-310)k q = J Notice that the negative sign on q signals heat lost by or transferred OUT of Al. 6

7 Thermochemical Equations If a reaction is EXOTHERMIC, heat is a product and should be written on the product side If a reaction is ENDOTHERMIC, heat is a reactant and should be written on the reactant side 7