THERMODYNAMICS

spontaneous reactions

Why do reactions occur?

14 KMnO4 + 4 C3H5(OH)3

7 K2CO3 + 7 Mn2O3 + 5 CO2 + 16 H2O

Spontaneous ReactionsThermodynamics 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.

Thermodynamics16_343

En

erg

y

Reaction progress

Reactants

Products

Domain of kinetics(the reaction pathway)

Domain ofthermodynamics(the initial andfinal states)

SpontaneityWhether a chemical reaction is

spontaneous or non-spontaneous is dependent on two factors:

Enthalpy (ΔH) and Entropy (ΔS)

EnthalpyEnthalpy 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 EndothermicExothermic 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 negativeb) Water is boiled and converted to steam?

H is positivec) Iron rusts?

H is negative

EntropyEntropy, 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.

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Entropy and States of Matter

Ssolid < Sliquid < Sgas

Dissolving in water

The 2nd Law of ThermodynamicsThe 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.

Suniv > 0

Entropy QuestionWhich of the following has higher positional

entropy?

a) Solid CO2 or gaseous CO2?

b) N2 gas at 1 atm or N2 gas at 1.0 x 10-2 atm?

c) H2O liquid or H2O solid ?

A New Look at Phase ChangesUnder what conditions would a substance have zero entropy?

What happens to the entropy as temperature increases?

What happens at a phase change?

Entropy ChangeWhat is the ΔS for the following?

a) Solid sugar is added to water to form a solution?

S is positiveb) I2 vapor condenses on a cold surface to form crystals?

S is negativec) Water decomposes into its elements?

S is positive

Entropy ValuesExperimentally measured at Standard State

(298K)Substance      Absolute entropy at 298K, 1 atm     (J K-1mol-1)

     H2 (g)      130.6

    H2O (l)       69.9

    H2O (g)       188.8

    Graphite Carbon       5.70

    Diamond Carbon       2.40

    Gaseous Carbon       198

Calculating Entropy ChangeCalculate 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? 

Compound So   (J/mol-K)

NO2 (g) 240

N2O4 (g) 304

Entropy and Phase ChangesWhy 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 EnergyGibb’s Free Energy Equation shows the balance between the two driving forces of nature.

G = H TS

If the G value of a reaction is negative, then the reaction is spontaneous.

Free Energy GG = H TS

G = negative -- spontaneousG = 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 SpontaneityH S G Result

+ spontaneous at all temps

+ + ?? spontaneous only at high temps

spontaneous only at low temps

+ not spontaneous at any temp

Chemical Destiny ?!... when these reactants come in contact, this reaction must occur.

(s) (l)

7 K2CO3 + 7 Mn2O3 + 5 CO2 + 16 H2O

-H14 KMnO4 + 4 C3H5(OH)3

(s) (s) (g) (g)

+S

- G

Practice AP ProblemN2(g) + 2 H2 (g) N2H4 (g)

ΔHo298 = +95.4 kJ mol-1 and ΔSo

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 ProblemsWhen 1.000 gram of propane gas, C3H8, is burned at 25°C

and 1.00 atmosphere, H2O(l) and CO2(g) are formed with the evolution of 12.03 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, ΔHf°, of propane gas. (d) Calculate the entropy change for the reaction and account for the sign ΔS°.

Compound Hfkcal/mol) Scal/mol·K)H2O(l) - 68.3 16.7CO2(g) -94.1 51.1O2(g) 0.0 49.0

C3H8 (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.