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16 Cards in this Set
- Front
- Back
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First law of Thermodynamics |
Conservation of energy.
For an exothermic reaction, “lost” heat from the systemgoes into the surroundings. There are two ways energy is “lost” from a system: Converted to heat, q Used to do work, w Energy conservation requires that the energy change in the system is equal to the heat released plus work done. ΔE = q + w ΔE = ΔH - P ΔV ΔE is a state function. Internal energy change independent of how done |
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The Energy Tax
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To recharge a battery with 100 kJ of useful energy will require more than 100 kJ because of the second law of thermodynamics.
Every energy transition results in a “loss” of energy. |
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Thermodynamics and Spontaneity
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Thermodynamics predicts whether a process will occur under the given conditions.
Processes that will occur without ongoing outside intervention are called spontaneous. Nonspontaneous processes require energy input to go. Spontaneity is determined by comparing the chemical potential energy of the system before the reaction and after the reaction. If the system after reaction has less potential energy than before the reaction, the reaction is thermodynamically favorable. |
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Reversibility of Process |
Any spontaneous process is irreversible because there is a net release of energy when it proceeds in that direction. It will proceed in only one direction. A reversible process will proceed back and forth between the two end conditions. Any reversible process is at equilibrium. This results in no change in free energy. If a process is spontaneous in one direction, it must be nonspontaneous in the opposite direction. |
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Enthalpy Change
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A reaction is generally exothermic if the bonds in the products are stronger than the bonds in the reactants. Exothermic = energy released; ΔH is negative. Most spontaneous processes proceed in the direction of lowest enthalpy. Exothermic A reaction is generally endothermic if the bonds in the products are weaker than the bonds in the reactants. Endothermic = energy absorbed; ΔH is positive. The enthalpy change is favorable for exothermic reactions and unfavorable for endothermic reactions. But there are some spontaneous processes that proceed in the opposite direction. Endothermic Example: ice spontaneously melts above 0°C. <--- Endothermic |
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Can the sign of ΔH predict spontaneous change? |
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Enthalpy
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The enthalpy, H, of a system is the sum of theinternal energy of the system and the product ofpressure and volume. H = E + PV H is a state function. The enthalpy change, ΔH, of a reaction is the heatevolved in a reaction at constant pressure: ΔHreaction = qreaction at constant pressureUsually ΔH and ΔE are similar in value; thedifference is largest for reactions that produce oruse large quantities of gas. ***kJ/K |
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Entropy |
More freedom of motion increases the randomness of the system. When systems become more random, energy is released. We call this energy entropy.
Entropy, S, is a thermodynamic function that increases as the number of energetically equivalent ways of arranging the components increases. The units of entropy are J/K It is a key factor in determining the direc7on of a spontaneous process. |
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Changes in Entropy, ΔS |
For a process where the final condition is more random than the initial condition, ΔSsystem is positive and the entropy change is favorable for the process to be spontaneous. For a process where the final condition is more orderly than the initial condition, ΔSsystem is negative and the entropy change is unfavorable for the process to be spontaneous. ΔSsystem = ΔSreaction = Σnp(S°products) − Σnr(S°reactants) ΔS = Sfinal − Sinitial Entropy change is favorable when the result is a more random system: ΔS is positive. Some changes that increase the entropy are as follows: Reactions whose products are in a more random state Liquid is less ordered than solid Gas is less ordered than liquid Reactions that have larger numbers of product molecules than reactant molecules Solids dissociating into ions upon dissolving Increase in temperature More atoms per molecule and higher molecular mass give rise to higher entropy. 1. CH3OH(g) - 6 atoms 2. O2 (g) - 2 atoms amu 32 3. N2 (g) - 2 atoms amu 28 ***J/K |
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Entropy Change in State Change |
The more degrees of freedom the molecules have the higher the entropy. solid --> liquid --> gas low to high Predict the sign of ΔS for each process: a. H2O(l) → H2O(g) b. Solid carbon dioxide sublimes. c. 2 N2(g) + O2(g) → 2 N2O(g) a. S>0 b. S>0 c. S<0 |
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Enthalpy Change in State Change |
The phase changes solid to liquid, liquid to gas, and solid to gas all have positive changes in entropy (ΔS is positive).
Liquid to solid, gas to liquid, and gas to solid all have negative changes in entropy (ΔS is negative). The heat of fusion, heat of vaporization and heat of sublimation are all positive numbers, meaning that as a substance goes from solid to liquid, liquid to gas, and solid to gas, the enthalpies increase. H gas < H liquid < H gas Solid to liquid, liquid to gas, solid to gas H+ and S+ Liquid to solid, gas to liquid, gas to solid H- and S- |
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Homework Problem: Find Ssys Ssurr Suniv 2H2(g) + O2(g) ---> 2H2O(g) deltaH: -483.6kJ |
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The Second Law of Thermodynamics
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The second law of thermodynamics states that the total entropy change of the universe must be positive for a process to be spontaneous.
For an irreversible spontaneous process: ΔSuniv > 0 ΔSuniverse = ΔSsystem + ΔSsurroundings > 0 If the entropy of the system decreases, then the entropy of the surroundings must increase by a larger amount for the process to be spontaneous. Example: water vapor condensing is spontaneous even though water vapor is more random than liquid water. The entropy increase of the surroundings must come from heat released by the system; the process must be exothermic! |
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Classify the possible combinations of signs for a reaction's ΔH and ΔS values by the resulting spontaneity.
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question wrong |
An exothermic reaction is always spontaneous if ΔS is positive, and only spontaneous at low temperatures if ΔS is negative. An endothermic reaction is spontaneous only at high temperatures if ΔS is positive and never spontaneous if ΔS is negative. In other words, the only time the spontaneity is temperature dependent is when ΔΗ and ΔS have the same sign. You can calculate ΔH°rxn and ΔS°rxn using the formulas to the left with these values.
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Homework Problem: What is the value of K for this aqueous reaction at 298 K? A + B (--><--) C + D Delta G =14.16kJ/mol |
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