E 0 exothermic E 0 endothermic u q w Chemistry Exam 4 Review Modules 8 and 9 First Law of Thermodynamics energy in the universe is constant Internal energy u the property of a system that can be changed by a flow of work heat or both Equation in action A system undergoes a process consisting of the following 2 steps 1 The system absorbs 72 J of heat while 35 J of work is done on it 2 The system absorbs 35 J of heat while performing 72 J of work Calculate u for the overall process 1 u 72J 35J 107J 2 u 35J 72J 37J 3 107J 37J 70J Expansion work done by a gas Compression work done to a gas Work force x distance KNOW 1 L atm 101 3 J Work P V Equation in action A piston performs work on 210 L atm on the surroundings while the cylinder in which it is placed expands from 10 0L to 25 0 L At the same time 45 J of heat is transferred from the surroundings of the system A Against what pressure was the piston working B Calculate u for the process 210 L atm P 25 0 10 0L a Work P V P 14 0 atm b 210 0L atm x 101 3 J 1 L atm 21273 J u 21273 J 45 J 21228 J Work if the system does work on the surroundings energy out expansion then w work is negative if the surroundings do work on the system energy in compression then w work is positive Add up moles on the reactants side and on the products side of an equation If you go from less to more moles the reaction demonstrates expansion If you go from more to less moles the reaction demonstrates compression Ex 5 3 4 expansion Reactants 6 moles products 7 moles Enthalpy heat content of a substance or the KE and PE of a substance at constant pressure enthalpy exothermic reaction enthalpy endothermic reaction Thermochemical equation a balanced chemical equation and its H enthalpy value If the reaction is reversed the sign of H is reversed if the reaction is doubled H is doubled etc Heat of fusion the energy required to convert one mole of a substance from a solid to a liquid at constant temperature Melting change in state when a solid becomes a liquid fusion endothermic Freezing change in state when a liquid becomes a solid exothermic Heat of vaporization the energy required to convert a mole of a substance from a liquid to a gas at constant temperature Vaporization change in state that occurs when a liquid evaporates to form a gas evaporation endothermic Condensation process by which vapor molecules reform a liquid exothermic Heat of sublimation energy required to convert one mole of a substance from a solid to a gas at constant temperature Sublimation process by which a substance goes directly from the solid to the gaseous state without passing through the liquid state endothermic Deposition process by which a substance goes directly from the gaseous state to the solid state without passing through the liquid state exothermic Sample problem How much energy is required to raise the temperature of 1 mole of water from 25 0 C to 125 C Important information about water is listed below Molar mass 18 016 g mol 0 C 100 C H vap 40 67 kJ mol H fus 6 01 kJ mol Solid molar heat capacity 30 57 J mol C Liquid specific heat capacity 4 18 J g C Steps to solve 1 Heat up solid from 25 C to 0 C q mc T q 1 mole 36 57 J mol C 25 C q 914 25 J 914 J 2 Melt ice into liquid q 1 mole 6 01 kJ mol 6 01 kJ 3 Heat up water from 0 C to 100 C 18 01g 4 18J g C 100 C 7528 18 J 7528 J 4 Vaporize liquid into gas q 40 67 kJ mol 1 mol 40 67 kJ 5 Heat up gas from 100 C to 125 C 36 8 J mol C 1 mol 25 C 920 J 6 Add kJ together 914 kJ 6 01kJ 7 528kJ 920kJ 40 67kJ 56 07 kJ Calorimetry science of measuring heat flow Sample Problem A 25 0g sample of water at 98 6 C was mixed with a 75 0g sample of water at 32 4 C What is the final temperature of the solution c 4 184 J g C q mc T m1 25 0g T1 98 6 C m2 75 0g T2 32 4 C q1 25 0g 4 184 J gC final temp 98 6 C q2 75 0g 4 184J g C final temp 32 4 C 25 0g 4 184 J gC final temp 98 6 C 75 0g 4 184J g C final temp 32 4 C final temp 48 95 C Hess s law in going from a particular set of reactants to a particular set of products the enthalpy change is the same whether the reaction takes place in one step or in a series of steps Standard heats of formation the enthalpy change that accompanies the formation of 1 mole of a substance at 25 C from its elements with all the substances in their standard states at that temperature Standard state a reference state for a specific substance defined according to a set of conventional definitions For a compound the standard state of a gaseous substance is a pressure of exactly 1 atm for a pure substance in a condensed state liquid or solid the standard state is the pure liquid or solid for a substance present in a solution the standard state is a concentration of exactly 1 M For an element the standard state of an eleent is the form in which the element exists under conditions of 1 atm at 25 C i e standard state for oxygen is O2 g Calculating Hf Bond dissociation energy D the amount of energy necessary to break a chemical bond in an isolated molecule in the gaseous state Pressure force per unit area applied in a direction perpendicular to the surface of an object atm Torr mm Hg Pa 1 atm 760 Torr 1 Torr 1 mm Hg Barometer measures atmospheric pressure Manometer measures pressure of gas in a container H D bonds broken D bonds formed Hrxn Hf products Hf reactants Combined gas law Kinetic Molecular Theory a model that assumes that an ideal gas is composed of tiny particles molecules in constant motion KE avg 3 2 RT 101 3 J 1L atm Same temperature Same average KE doesn t matter what type of gases Higher temperature higher KE Amonton s law P nr V T Root mean square velocity square root of the average of squares of the individual velocities of gas particles Effusion passage of a gas through a tiny ortifice into an evacuated chamber Thomas Graham Diffusion Mixing of gases An ideal gas is a hypothetical concept No gas exactly follows the ideal gas law although many gases …
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