THERMODYNAMICS spontaneous reactions

Why do reactions occur? 14 KMnO C 3 H 5 (OH) 3 7 K 2 CO Mn 2 O CO H 2 O

Spontaneous Reactions Thermodynamics lets us predict whether a process will occur but gives no information about the amount of time (reaction rate) for the process. A spontaneous process is one that occurs without outside intervention. It may be fast, it may be slow, but it will eventually happen.

Thermodynamics

Spontaneity Whether a chemical reaction is spontaneous or non-spontaneous is dependent on two factors: Enthalpy (ΔH) and Entropy (ΔS)

Enthalpy Enthalpy is the total energy of a system. The total enthalpy, H, of a system cannot be measured directly. Thus, change in enthalpy, ΔH, is a more useful quantity than its absolute value.

Exothermic or Endothermic Exothermic reactions have a negative  H while endothermic reactions have a positive  H. What is the ΔH for the following? a) Propane undergoes combustion?  H is negative b) Water is boiled and converted to steam?  H is positive c) Iron rusts?  H is negative

Entropy Entropy, S, can be viewed as a measure of randomness. Nature spontaneously proceeds toward the states of more randomness. Things eventually obtain a state that have the highest probabilities of existing.

Entropy and States of Matter S solid < S liquid < S gas

Dissolving in water

The 2 nd Law of Thermodynamics The Second Law of Thermodynamics is commonly known as the Law of Increased Entropy. In any spontaneous process or reaction there is always an increase in the entropy of the universe.  S univ > 0

Entropy Question Which of the following has higher positional entropy? a) Solid CO 2 or gaseous CO 2 ? b) N 2 gas at 1 atm or N 2 gas at 1.0 x atm? c) H 2 O liquid or H 2 O solid ?

A New Look at Phase Changes Under what conditions would a substance have zero entropy? What happens to the entropy as temperature increases? What happens at a phase change?

Entropy Change What is the ΔS for the following? a) Solid sugar is added to water to form a solution?  S is positive b) I 2 vapor condenses on a cold surface to form crystals?  S is negative c) Water decomposes into its elements?  S is positive

Entropy Values Experimentally measured at Standard State (298K) Substance Absolute entropy at 298K, 1 atm (J K -1 mol -1 ) H2 (g) H2O (l) 69.9 H2O (g) Graphite Carbon 5.70 Diamond Carbon 2.40 Gaseous Carbon 198

Calculating Entropy Change Calculate the standard-state entropy change for the following reaction given the following information. What would be the standard-state entropy for the following reverse reaction? CompoundS o (J/mol-K) NO 2 (g) 240 N 2 O 4 (g) 304

Entropy and Phase Changes Why doesn’t water exist as a gas at room temperature? Under what conditions would H2O be a liquid, a solid, or a gas?

Driving Forces in Nature There are two forces that cause reactions to be spontaneous: tendency toward lower energy (-  H) tendency toward higher entropy (+  S) Any reaction in which entropy increases and is exothermic (releases energy) is spontaneous.

Gibb’s Free Energy Gibb’s Free Energy Equation shows the balance between the two driving forces of nature.  G =  H  T  S If the  G value of a reaction is negative, then the reaction is spontaneous.

Free Energy  G  G =  H  T  S  G = negative -- spontaneous  G = positive -- spontaneous in opposite direction  G = 0 -- at equilibrium Equilibrium represents the lowest free energy value available to a particular system (reaction). Would a galvanic cell have a +  G or -  G ?

Why does ice melt? What is the ΔH? Is it exothermic or endothermic?  H is positive – nature would not favor it What is the ΔS? Does entropy increase or decrease?  S is positive – nature favors it When does ice melt? At low or high temperature?

 H and  S on Spontaneity

Chemical Destiny ?!... when these reactants come in contact, this reaction must occur. (s) (l) 7 K 2 CO Mn 2 O CO H 2 O -H-H 14 KMnO C 3 H 5 (OH) 3 (s) (s) (g) (g) +S+S -  G

Practice AP Problem N 2 (g) + 2 H 2 (g)  N 2 H 4 (g) ΔH o 298 = kJ mol -1 and ΔS o 298 = -176 J K -1 mol -1 (a)On the basis of the thermodynamic data given above, compare the sum of the bond strengths of the reactants to the sum of the bond strengths of the products. Justify your answer. (b)Does the entropy change of the reaction favor the reactants or the products? Justify your answer. (c)For the reaction at 298 K, which is favored, the reactants or the products? Justify your answer.

More AP Problems When gram of propane gas, C 3 H 8, is burned at 25°C and 1.00 atmosphere, H 2 O(l) and CO 2 (g) are formed with the evolution of kilocalories. (a) Write a balanced equation for the combustion reaction. (b) Calculate the molar enthalpy of combustion, ΔH°, of propane. (c) Calculate the standard molar enthalpy of formation, ΔH f °, of propane gas. (d) Calculate the entropy change for the reaction and account for the sign ΔS°. Compound  H f  kcal/mol) S  cal/mol·K) H 2 O(l) CO 2 (g) O 2 (g) C 3 H 8 (g)?64.5

More AP Free Response (a)When liquid water is introduced into an evacuated vessel at 25°C, some of the water vaporizes. Predict how the enthalpy, entropy, free energy, and temperature change in the system during this process. Explain the basis for each of your predictions. (b)When a large amount of ammonium chloride is added to water at 25°C, some of it dissolves and the temperature of the system decreases. Predict how the enthalpy, entropy, and free energy change in the system during this process. Explain the basis for each of your predictions. (c)If the temperature of the aqueous ammonium chloride system in part (b) were to be increased to 30°C, predict how the solubility of the ammonium chloride would be affected. Explain the basis for each of your predictions.