Ch 15 Entropy and Gibbs Energy

Define spontaneous process + give examples (know these)

• one that occurs without any external input to the system

• examples

  • ice melting at room temp

  • a ball rolling downhill

  • The rusting of iron at room temp

  • water freezing at -10 degrees Celsius

  • Sodium metal reacting violently with water to produce sodium hydroxide and hydrogen gas

Define Nonspontaneous process + give examples (know these)

• one that requires external input to the system

• examples

  • water freezing at room temp

  • a ball rolling uphill

  • the conversion of rust back to iron metal at room temp

  • ice melting at -10 degrees Celsius

  • Sodium hydroxide reacting with hydrogen gas to produce sodium metal and water

A process that results in a decrease in energy of a system means that the process is…?

1. Spontaneous

2. Nonspontaneous

2. Nonspontaneous

What is the difference between ΔH and ΔE?

• ΔH is the change in enthalpy which accounts for energy exchange

• ΔE is the change in energy which focuses only on the internal energy of a system

If we are given the ΔH of a process, can we predict its spontaneity? Why or why not?

1. No, because spontaneity depends on both ΔH and ΔS, as described by Gibbs Free Energy.

2. Yes, because ΔH alone determines whether a process is spontaneous.

3. No, because spontaneity depends only on temperature.

1. No, because spontaneity depends on both ΔH and ΔS, as described by Gibbs Free Energy.

What is the difference between enthalpy and entropy?

• Enthalpy (ΔH) is the total heat content of a system, indicating whether a process absorbs or releases heat = Heat flow

• Entropy (ΔS) measures the disorder or randomness of a system, reflecting how energy is spread out = energy dispersal

How do we calculate Entropy?

• S (entropy) = k(lnW)

  • k = boltzmann constant (1.38×10^(-23) J/K)

  • W = number of different arrangements (given on tests)

What does possible arrangements mean in regards to entropy?

refers to the different ways particles in a system can be distributed among energy levels or positions.

True or False: The most probable state for a system has the largest number of arrangements. 

It is true because >arrangements = >entropy = >statistically favored 

What does “probable state” mean in regards to entropy?

it refers to the distinct particle arrangements that a system could be in and result in the same observable conditions

What are the three ways to calculate the change in entropy of a system?

• ΔS_sys = S_final - S_initial

• Use these two when gas laws are being referred to 

  • ΔS_sys = n(R)(ln(V_final/V_initial) 

  • ΔS_sys = -R (n₁lnx₁) + (n₂lnx₂)

    • x = mole fraction

What is standard entropy?

• S°

• it is the absolute entropy of a substance at 1 atm

• *Temp is not part of this and must be specified.

What are the trends in S°?

• S° liquid > S° Solid

• S° gas > S° liquid

• S° increases with molar mass

• S° increases with molecular complexity

• S° increases with mobility of a phase 

How can we calculate the change in standard entropy of a reaction?

• ΔS°_rxn = ∑nS°(products) - ∑mS°(reactants)

  • n and m = coefficients in a balanced equation

What are processes that lead to an increase in entropy?

• melting

• vaporization or sublimation

• temperature increase

• reaction resulting in a greater number of gas molecules

TRUE or FALSE? You can correctly predict the spontaneity of a process knowing the entropy of system.

False, you must know the entropy of the system and its surroundings to correctly know the spontaneity of the process

What is the formula for the change in entropy of surroundings?

• ΔS_surr = -ΔH_sys/T

What is the formula for the change in entropy of the universe?

• ΔS_universe = ΔS_sys + ΔS_surr

• If the change in entropy of the universe is positive, the process is …

• If it is negative, then the process is…

• Process is spontaneous

• Process is not spontaneous

What is the 2nd law of thermodynamics?

• for a process to be spontaneous, ΔS_universe must be positive.

What is the 3rd law of thermodynamics?

• The entropy of a perfect crystalline substance is 0 at absolute 0

• Entropy inc. in a substance as temp inc. from absolute zero.

• The most order is at absolute 0

What is Gibbs energy?

(G) A value used to express spontaneity

What is the equation to calculate Gibbs energy?

G = H - TS

What is the equation for the change in Gibbs energy?

• ΔG = ΔH - TΔS

• What can we infer from the value of the change in Gibbs energy?

• ΔG < 0, reaction is spontaneous in the forward direction

• ΔG > 0, reaction is nonspontaneous in the forward direction

• ΔG = 0, the system is at equilibirum

What is the standard Gibbs energy of reaction?

• (ΔG°_rxn) it is the Gibbs energy change for a reaction when it occurs under standard-state conditions

What are the standard states for different pure substances for Gibbs energy?

• Gases = 1 atm pressure

• Liquids = pure liquid

• Solids = pure solid

• Elements = the most stable allotropic form at 1 atm and 25 °C

• Solutions = 1 molar concentration

What is the formula for Standard Gibbs Energy Change of a reaction?

• ΔG°_{rxn =}∑nΔG°_f (products) - ∑mΔG°_f (reactants)

What is ΔG°_f for an element in its most table allotropic form at 1 atm?

It is 0

If a reaction has a positive ΔG°, can it still move forward?

Yes, because it can be coupled with a negative ΔG° reaction.