10/1/ st Law of Thermodynamics (Law of Conservation of Energy) & Hess s Law. Learning Targets

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1 1 st Law of Thermodynamics (Law of Conservation of Energy) & Hess s Law 1 Learning Targets LT 5.02: I can relate temperature to the motions of particles and average kinetic energy. LT 5.03: I can generate explanations or make predictions about the transfer of thermal energy between systems based on this transfer being due to a kinetic energy transfer between systems arising from molecular collisions. LT 5.04: I can use conservation of energy to relate the magnitudes of the energy changes occurring in two or more interacting systems, including identification of the systems, the type (heat versus work), or the direction of energy flow. LT 5.06: I can use calculations or estimations to relate energy changes associated with a chemical reaction to the enthalpy of the reaction, and relate energy changes to PΔV work. 2 even though Chemistry is the study of matter, energy effects matter energy is anything that has the capacity to do work work is a force acting over a distance Energy = Work = Force x Distance energy can be exchanged between objects through contact collisions 3 1

2 Electrical kinetic energy associated with the flow of electrical charge Heat or Thermal Energy kinetic energy associated with molecular motion temperature = heat temperature = measure of average KE Light or Radiant Energy kinetic energy associated with energy transitions in an atom Nuclear potential energy in the nucleus of atoms Chemical potential energy in the attachment of atoms or because of their position 4 Kinetic energy is energy of motion or energy that is being transferred thermal energy is kinetic the amount of kinetic energy an object has is directly proportional to its mass and velocity KE = ½mv 2 when the mass is in kg and speed in m/s, the unit for kinetic energy is 2 kg m 2 s = 1 J 5 Potential energy is energy that is stored in an object, or energy associated with the composition and position of the object energy stored in the structure of a compound is potential 6 2

3 the internal energy is the total amount of kinetic and potential energy a system possesses (symbol = E or U) the change in the internal energy of a system only depends on the amount of energy in the system at the beginning and end a state function is a mathematical function whose result only depends on the initial and final conditions, not on the process used DE = E final E initial DE reaction = E products - E reactants Potential energy of hiker 1 and hiker 2 is the same even though they took different paths. 7 Conservation of Energy requires that the total energy change in the universe be zero DEnergy universe = 0 = DEnergy system + Denergy surroundings System - specific part of the universe that is of interest in the study. Surroundings everything else in the universe 8 when energy flows out of a system, it must all flow into the surroundings when energy flows out of a system, DE system is when energy flows into the surroundings, DE surroundings is + therefore: DE system = DE surroundings Surroundings DE + System DE 9 3

4 when energy flows into a system, it must all come from the surroundings when energy flows into a system, DE system is + when energy flows out of the surroundings, DE surroundings is therefore: DE system = DE surroundings Surroundings DE System DE + 10 System vs Surroundings Exchange between mass & energy in a system open closed closed Exchange: mass & energy energy nothing 11 Endothermic vs Exothermic Exothermic process is any process that gives off heat (H), i.e. transfers thermal energy from the system to the surroundings. 2H 2 (g) + O 2 (g) H 2 O (g) 2H 2 O (l) + energy H 2 O (l) + energy DH is Endothermic process is any process in which heat has to be supplied to the system from the surroundings. energy + 2HgO (s) energy + H 2 O (s) 12 2Hg (l) + O 2 (g) H 2 O (l) DH is + 4

5 Energy Diagram 13 Heat (Enthalpy) Enthalpy (H) heat content at constant pressure Enthalpy of formation (ΔH f ) heat absorbed or released when ONE mole of compound is formed from elements in their standard states Enthalpy of reaction (ΔH rxn ) heat absorbed or released by a chemical reaction Enthalpy of combustion (ΔH comb ) -- heat absorbed or released by burning (usually with O 2 ) Enthalpy of fusion (ΔH fus ) -- heat absorbed to melt 1 mole of solid to Enthalpy of vaporization (ΔH vap ) -- heat absorbed to change 1 mole liquid to 14 Turn to appendix page A-8 the formation reaction C + O CO(g) the elements must be in their standard state C(s, graphite) + O 2 (g) CO(g) the equation must be balanced, but the coefficient of the product compound must be 1 use whatever coefficient in front of the reactants is necessary to make the atoms on both sides equal without changing the product coefficient C(s, graphite) + ½ O 2 (g) CO(g) 15 5

6 The standard enthalpy of reaction (DH 0 rxn ) is the enthalpy of a reaction carried out at 1 atm. a A + b B c C + d D DH 0 rxn = S ndh 0 f (products) - S ndh 0 f (reactants) DH 0 ddh rxn = [ cdh 0 (C) + 0 (D)] - [ adh 0 (A) + bdh 0 (B)] f f f f S means sum n is the coefficient of the reaction 16 if a reaction can be expressed as a series of steps, then the DH rxn for the overall reaction is the sum of the heats of reaction for each step 17 the enthalpy change in a chemical reaction is an extensive property the more reactants you use, the larger the enthalpy change we calculate the enthalpy change for the number of moles of reactants in the reaction as written C 3 H 8 (g) + 5 O 2 (g) 3 CO 2 (g) + 4 H 2 O(g) DH = kj DH reaction for 1 mol C 3 H 8 = kj DH reaction for 5 mol O 2 = kj 2044 kj 1 mol C3H8 or 1 mol C3H kj 2044 kj 5 mol O2 or 5 mol O kj 18 6

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