WU 5/3

PCl5 PCl3 + Cl2,

1. At equilibrium [PCl3]= 6.4x10-3, [Cl2]=2.5x10-2, [PCl5]=4.0x10-3

a. Write the equilibrium expression for this reaction

b. Calculate K. c. If the concentration of chlorine is

doubled, how will the equilibrium shift?

WU 5/4 **Take Out HW**1. What is entropy?

2. Which has more entropy:

a. Br2(g) or Br2(l) b. High or Low Temp?

3. a. What two factors determine whether a reaction is spontaneous?

b. List the types of changes from 3a that would be considered spontaneous? (ΔH, ΔS)

Learning Goals

• Classify reactions as increasing or decreasing in entropy.

• Analyze reactions to determine whether they are spontaneous or non-spontaneous.

18-4 Driving Forces of Reactions

• Two driving forces:

1. Enthalpy

2. Entropy

• Combine & Evaluate these factors to determine whether a reaction is spontaneous.

Enthalpy (H):• a measure of the energy of a

substance• ∆H is the change in enthalpy due to a

reaction• A positive value is:

– endothermic • A negative value is:

–exothermic

Spontaneous Processes• Occur without any outside intervention• Exothermic processes are

spontaneous.• Endothermic processes are non-

spontaneous

Entropy (S)• a measure of the disorder of the system. • How random something is.• Things tend to become more disordered. • Disorder is easier than becoming more

ordered• Entropy increases are spontaneous.

Evaluating Entropy

• Egg in shell splattered egg.• Which process (forward or reverse) is

spontaneous?• Place the three states of matter in

order from least to most entropy• Draw an arrow that indicates the

direction of spontaneous changes.

Evaluating Entropy (ΔS)

• Which demonstrates an increase in entropy?

EX: Messy Room Clean Room

EX: H2O(l) H2O(g)

EX: H2O(g) H2O(s)

EX: NaHCO3(s) + HCH3COO(l) CO2(g) + H2O(l) + NaCH3COO(aq)

Entropy Increases• Increase the number of molecules

present.–Think: only child vs. “John & Kate + 8”

• Increase temperature–Think: bumper cars

18-4 Spontaneous Reactions• Happen without outside intervention.• Favor formation of products• Predicting spontaneity: two factors

–enthalpy change, ΔHand –the entropy change, ΔS.

18-4 Spontaneous Reactions• A Negative ΔH = exothermic. • Positive ΔH = endothermic.

–Which process is spontaneous?• A positive ΔS = entropy increases.• Negative ΔS = entropy decreases.

–Which process is spontaneous?

Spontaneous?

• -ΔH & +ΔS • +ΔH & -ΔS • -ΔH & -ΔS OR +ΔH & +ΔS

–Depends on temperature

18-4 Free Energy

• Energy that can be used to do work.

• Reactions that produce free energy are considered spontaneous.–Is this a positive or negative value?

• Thus: objects that require free energy are not spontaneous.

18-4 Gibb’s Free Energy

• The maximum amount of free energy that can be gained from a reaction.

• A mathematical way of comparing evaluating enthalpy and entropy

• ΔG = ΔH – TΔS–Temperature is in Kelvin–ΔG = spontaneous

+ΔG = non-spontaneous

Sample Problem

H2O(l) H2(g)  +  ½O2(g)

ΔH=  +285 kJ, ΔS =  0.822 kJ/K   

• At 25°C, What is the value of ΔG?• Is this reaction spontaneous?

Practice:CH3CO2H(l) + 2O2(g) →2CO2(g) + 2 H2O(g)

ΔH = -786.1 kJ, ΔS = 0.235 kJ/K

• Will this reaction be spontaneous at 25° C?

• At what temperature will this reaction not be spontaneous?