Thermodynamics The study of energy changes that accompany chemical and physical changes.

First Law of Thermodynamics the total amount of energy in the universe is constant. Also known as the Law of Conservation of Energy which states that energy cannot be created or destroyed, but can only change form.

Enthalpy Enthalpy change (∆H) is the quantity of heat (q) transferred in or out of a system. ∆H = Hfinal – Hinitial ∆H = Hproducts - Hreactants

Exothermic Reactions ∆H = energy of products – energy of reactants If the reaction is exothermic, ∆H < 0.

Endothermic Reactions ∆H = energy of products – energy of reactants If the reaction is endothermic, ∆H > 0.

Spontaneous Processes A process is spontaneous if it occurs without outside intervention. Note: For many spontaneous processes, some energy (Ea) from the surroundings must be supplied to get the process started. (Example: using a match to light a bunsen burner) Spontaneous processes may be either fast or slow. Thermodynamics lets us predict whether a process will occur but gives no information about the amount of time required for the process.

Entropy Entropy (S) is a measure of the molecular randomness or disorder. This is related to the freedom of the system’s particles to move and the number of ways they can be arranged. The driving force of spontaneous processes is an increase in entropy of the universe. (2nd Law of Thermodynamics) The natural progression of things is from order to disorder, from lower entropy to higher entropy.

Predicting Entropy Changes ∆S = Sfinal - Sinitial If the entropy increases, ∆S is > 0 (becoming more disordered) If the entropy decreases, ∆S is < 0 (becoming less disordered)

Predicting Entropy Changes (Continued) Entropy changes associated with changes in state can be predicted. In solids, particles have limited movement while in gases, molecules can move freely. In order of increasing entropy: Solids< liquids< gases

Predicting Entropy (continued) The dissolving of a gas in a solvent results in a decrease in entropy. The entropy of a system increases when the number of gaseous product particles is greater than the gaseous reactant particles.

Predicting Entropy (continued) Entropy increases when a solid or a liquid dissolves in a solvent. Entropy increases as temperature increases.

Which of the following has the greatest entropy? 10 1 mole of solid CO2 1 mole of gaseous CO2 Both are equal 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Which has the greatest entropy? 10 1 mole of N2 gas at 1 atm 1 mole of N2 gas at 0.01 atm Both have the same entropy 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

What is the sign for the entropy change when solid sugar is added to water to form a solution? 10 Positive Negative 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

What is the sign for the entropy change when iodine vapor condenses on a cold surface to form crystals? 10 Positive Negative 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Predict the sign of ∆S for each of the following: ClF (g) + F2(g)  ClF3(g) NH3(g)  NH3(aq) CH3OH(l)  CH3OH(aq) C10H8(l) C10H8(s)

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. In other words, the entropy of the universe is increasing and is not conserved. ∆Suniv = ∆Ssys + ∆Ssurr If ∆Suniv > 0, the process is spontaneous as written. If ∆Suniv < 0, the process is spontaneous in the opposite direction.

The Effect of Temperature on Spontaneity An exothermic process in the system causes heat to flow to the surroundings, increasing the random motions and entropy of the surroundings. ∆Ssurr >0 The opposite is true for endothermic processes. As a result, nature tends to seek the lowest possible energy.

Free Energy It is possible to determine if a reaction is spontaneous by evaluating the enthalpy and entropy involved in the reaction. Free energy (G) is the energy that is available to do work. ∆G = ∆ H – T ∆ S where H is enthalpy, T is Kelvin temp, and S is entropy. A process is spontaneous (at constant T and P) if the sign of the free energy change is negative (∆G <0) If the sign of the free energy change is positive, the reaction is not spontaneous.

Reaction Spontaneity Example Burning Wood Freezing water ∆H ∆S ∆G Reaction Spontaneity Example Negative (exothermic) Positive (Entropy Increasing) Negative Always Spontaneous Burning Wood Negative (Entropy Decreasing) Negative of Positive Spontaneous at lower temperatures Freezing water Positive (endothermic) Negative or Positive Spontaneous at higher temperatures Boiling water Negative (Entropy decreasing) Positive Never Spontaneous Oxygen changing into ozone Note: A reaction that is not spontaneous can be made to occur. Example given above occurs when lightning passes through the air. Reaction stops, however, when the electrical discharge stops.

Which of these reactions would you expect to be spontaneous? 92kJ + NH3(g)  N2(g) +3H2(g) 2NO2(g)  N2O4 (g) + 58 kJ 178 kJ +CaCO3(g)  CaO(s) + CO2(g)