Unit 3, chap 10

Unit 3, chap 10 - Chapter 10 Spontaneous Processes  ...

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Unformatted text preview: Chapter 10 Spontaneous Processes   A process that occurs without outside intervention is spontaneous   This may occur quickly or slowly   Thermodynamics predicts whether or not a process will occur, but gives no information of the amount of time required Example: diamond and graphite Spontaneous Processes   Examples of spontaneous processes:   A ball rolls down a hill   Iron rusts   A gas fills a container uniformly   Heat flows from a hot object to a cooler one   Wood burns to form CO2 and H2O   At temperatures below 0°C, water freezes   At temperatures above 0°C, ice melts Spontaneous Processes   Initially, ΔH was used as an indicator of spontaneity   Many exothermic reactions are spontaneous   However, there are some exceptions, such as ice melting   ΔH is a factor, but not the only factor   The other factor affecting spontaneity is entropy (ΔS)   A state function   A measure of change in disorder   Naturally, the universe moves from order to disorder   Your sock drawer spontaneously becomes disordered, but it will never become more ordered on its own Entropy   Entropy is closely related to probability   If you put a deck of cards in order and then throw them up in the air, they will probably land in a way other than how you arranged them   There is only one way they can land in the order you had them, but there are billions of other possible ways they can land   Nature favors the state that has the highest probability of existing, the most disordered state Entropy   Entropy increases when:   A substance is melted or vaporized (Ssolid < Sliquid < Sgas)   A substance is heated   A solid is dissolved   A substance decomposes   The volume of a gas increases   Two substances are mixed The Second Law of Thermodynamics   The second law of Thermodynamics states that, in any spontaneous process, there is always an increase in the entropy of the universe   ΔSuniv = ΔSsys + ΔSsurr   ΔSuniv is positive when the process is spontaneous The Effect of Temperature   When water evaporates, ΔSsys is positive   Ssolid < Sliquid < Sgas   To evaporate water, heat must flow into the system from the surroundings   The surroundings therefore lose heat and molecules slow down slightly, ΔSsurr is negative   ΔSuniv = ΔSsys + Δssurr   In this example, the components of ΔSuniv are in opposition, so which one determines the sign of ΔSuniv?   It depends on the temperature The Effect of Temperature   ΔSsurr is directly proportional to the quantity of heat (J) and is indirectly proportional to temperature (K)   An exothermic process ( ­ΔH) increases the entropy of the surroundings (+ΔSsurr)   Mathematically stated, ΔSsurr =  ­ΔH/T   ΔSuniv = ΔSsys + ΔSsurr = ΔSsys  ­ ΔH/T   ΔSuniv (the indicator of spontaneity) is a function of ΔSsys, ΔH, and T Free Energy   The more common indicator of spontaneity is Gibb’s free energy, G   A state function   A process is spontaneous when the free energy decreases (ΔG is negative)   ΔG = ΔH – TΔS Free Energy   Extensive properties are physical properties that depend on the amount of material in a sample   Example ­ mass, volume, number of particles   Intensive properties do not depend on amount   Example ­ temperature, viscosity, velocity, specific heat capacity, boiling point, pressure   ΔG, ΔH, and ΔS can be either extensive or intensive   When doing problems dealing with these properties, pay special attention to units Free Energy   ΔG = ΔH – TΔS ( ­ΔG means a spontaneous process)   The temperature at which ΔH and –TΔS are equal and cancel each other out is equilibrium   ΔG = 0 ...
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