Chapter 17

# Chapter 17 - THERMODYNAMICS - The study of energy in matter...

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THERMODYNAMICS - The study of energy in matter - Thermodynamics allows us to predict whether a chemical reaction occurs or not. - Thermodynamics tells us nothing about how fast a reaction occurs. - i. e., thermodynamics can’t explain kinetics and vice versa DEFINITIONS OF PROCESSES Spontaneous Processes - A process that occurs without outside help. Reversible Processes - A reversible process is a process that is always in equilibrium. Examples 1.) Ice melting at O ° C H 2 O (s) D H 2 O (l) - Equilibrium is adjusted by adding or subtracting heat. - Water can go from solid to liquid to solid to liquid etc… 2.) Haber process in a closed container N 2 (g) + 3 H 2 (g) D 2 NH 3 - Production of ammonia can increase or decrease by adjusting the external pressure. (also temperature) - Aside: Haber process is extremely important for support of human population. Natural fertilizer is insufficient. Thank goodness for synthetic fertilizer. Irreversible Processes - A process not in equilibrium Examples: Ice melting at 25 ° - Process only happens in one direction - i. e., melting occurs, never freezing Spontaneous Pants fall to floor Raw egg becomes hard boiled in water Dry ice sublimates at room temp. CH 4 + 2 O 2 CO 2 + 2 H 2 O Nonspontaneous Pants hang themselves in closet Hard-boiled egg becomes raw egg 2 gas deposits as solid at room temp. 2 + 2 H 2 4 + 2 O 2

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2.) Precipitation of AgCl Ag + (aq) + Cl - (aq) AgCl (s) - As a solution of Ag + and a solution of Cl - are mixed, the system is out of equilibrium. - Thus, precipitation occurs - The solid AgCl will never redissociate into ions. - However, once precipitate forms, system is in equilibrium. Now the formation of solid can be considered reversible (e. g. addition or subtraction ammonia causes amount of solid to change AgCl(s) + 2 NH 3 (aq) D [Ag(NH 3 ) 2 ] + (aq) + Cl - (aq) REVIEW OF FIRST LAW OF THERMODYNAMICS - The change in the energy of a system is due to heat and/or work. E = q + w - The first law is a conservation of energy statement. - We will see that although energy is conserved, not all of it will be useful. - i. e., some energy in a process will always be wasted. - Be sure to review first law, especially sign convention of heat and work from Chapter 4. ENTROPY - Review notes from Chapter 13 about entropy. - Entropy is disorder - Increasing the number of places a particle in a system can be, increases the entropy of the system. Example: Which has more entropy: 1 mole of gas in 10 L or 1 mole of gas in 20 L? - Larger volume means molecules can be in more places, the gas in the 20 L has more entropy. - When heat exchange is not involved, spontaneous processes always increase entropy. Consider diffusion of gas in two connected gas bulbs. - Gas spontaneous expands, increasing entropy. open stopcock 1 atm 0 atm ½ atm
Thermodynamic definition of entropy - Entropy is defined via a change in heat (i. e., a change in choatic motion) S q T rev = - q is often difficult to measure for irreversible processes, but is usually easy to measure for reversible processes. Example: Calculate the change of entropy when 36.02 g of ice melts at 0.00 ° C. The molar enthalpy of melting for ice is 6.008 kJ/mol.

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## This note was uploaded on 09/27/2010 for the course CHEM 220 taught by Professor Bates during the Spring '10 term at Skyline College.

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Chapter 17 - THERMODYNAMICS - The study of energy in matter...

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