Catalyst

Come in quietly and begin working on the Quiz that is on your desk

End

Learning Log Assessment

• Rate yourself 1 – 4 on LTs 7.1-7.7

Checklist Manifesto!• How do I know which equation to use?Does it have specific heat or enthalpy??

Then use calorimetry!Does it give you ΔH or ΔT

If ΔH then use q = mΔHIf ΔT then use q = m x Cp x ΔT

Box the one it’s asking you to find!Plug in the right numbers and solve!

Connect 4

Lecture 7.5 – Gibbs Free Energy

Today’s Learning Targets• 7.8 – I can discuss what entropy is, the conditions under

which it is favored, and how entropy changes when we change the phase of a solution.

• 7.9 – I can describe what enthalpy is, how it relates to the idea of endothermic and exothermic, and how it changes the favorability of a reaction.

• 7.10 – I can use the idea of Gibbs Free Energy to predict whether a reaction will be spontaneous.

Today’s Focus Question

• What chemistry allows for the breakdown of Flamin’ Hot Cheetos to occur?

What are enthalpy and entropy?

EndothermicExothermic

I. Entropy and Enthalpy• Enthalpy is a measure of energy a system has.

– Endothermic = +ΔH– Exothermic = -ΔH

• Entropy is the measurement of the disorder of a system.– Lots of Disorder = +ΔS– Minimal Disorder = -ΔS

How do we predict spontaneous reactions?

I. Favorability of Reactions• Only certain values of ΔH and ΔS are favorable• Reactions favor disorder, so +ΔS is favored.• Reactions favor reactions that release energy, so

–ΔH is favored.

+ΔH -ΔH

+ΔS Sometime Spontaneous (High Temp.

Required)

Always Spontaneous

-ΔS Never Spontaneous

Sometimes Spontaneous (Low Temp. Required)

Class Example

• Without calculating, predict if the following reaction will be spontaneous. If it is spontaneous, specify under what conditions.

You run the following reaction:N2 (g) + O2

(g) 2 NO (g)

You have ΔH = 180.7 kJ and ΔS = 24.7 J/K

Table Talk• Without calculating, predict if the following reaction will

be spontaneous. If it is spontaneous, specify under what conditions.

• You run the following reaction:• P4 (g) + 6 Cl2 (g) 4 PCl3 (g)

• You have ΔH = -190.2 kJ and ΔS = 423.4 J/K

Stop and Jot• Without calculating, predict if the following reaction will

be spontaneous. If it is spontaneous, specify under what conditions.

• You run the following reaction:• N2 (g) + 3 H2 (g) 2 NH3 (g) + 567 kJ

White Board Races

White Board Questions• Using only the signs of Ho and So, predict the signs and

temperature dependence.1. CH3OH (l) + 3/2 O2 (g) CO2 (g) + 2 H2O (g)

2. Exothermic reaction 3. 2. The vaporization of water 4. 3. CO (g) + H2O (g) CO2 (g) + H2 (g)

5. Ho = -41.2 kJ and So = -135 J/K6. 4. The condensation of water7. 5. P4 (g) + 6 Cl2 (g) 4 PCl3 (g)

8. The reaction is endothermic9. 6. N2 (g) + O2

(g) 2 NO (g)

10.The reaction is endothermic11.7. The freezing of carbon dioxide

How do we calculate spontaneity?

I. Gibbs Free Energy (ΔG)

• We can calculate ΔG by:ΔG = ΔH – TΔS

• ΔG is the Gibbs Free Energy• T is Temperature (in KELVIN)• A positive ΔG means the

reaction is NOT spontaneous.

• A negative ΔG means that the reaction IS spontaneous

Class Example• For the vaporization reaction:

Br2 (l) Br2 (g)

ΔH = 31 kJ/mol and ΔS = 93 kJ/(mol x K). Is this spontaneous at 298 K?

Table Talk• A reaction has ΔH = -385 kJ and ΔS = -36 kJ/(mol x

K). Calculate ΔG at 25 oC and state whether the reaction is spontaneous.

Stop and Jot• You have a reaction with a ΔH = 125 kJ/mol, ΔS =

0.0350 kJ/(mol x K), and T=293 K. Is this spontaneous and how could you make this reaction spontaneous?

BINGO!

Below 333 K Spontaneous Endothermic Decrease Entropy

195.7 kJ Not Spontaneous 13.5 kJ 13.4 kJ

Increase Entropy 135.2 kJ/mol Above 333 K Less Disorder

More Disorder Exothermic 11,000 kJ 11 kJ

BINGO Questions• I have a reaction with a positive enthalpy and a

negative entropy. Is this a spontaneous or non-spontaneous reaction?

• I have a reaction with a negative enthalpy and a positive entropy. Is this a spontaneous or non-spontaneous reaction?

• I turn solid ice into liquid water. Does this increase or decrease the entropy of the system?

• Determine the ΔG of the hydrolysis of urea when it is 25 oC, ΔH=119.2 kJ/mol, and ΔS=0.3548 kJ/(mol x K).

BINGO Questions• Calculate ΔG for the following reaction: ΔH = -85.2

kJ, T= 127 oC, and ΔS = 0.125 kJ/(mol x K). • Calculate ΔG for the following reaction: ΔH = -275

kJ, T= 773 oC, and ΔS = 0.450 kJ/(mol x K).• A reaction has ΔH = 98 kJ/mol and ΔS = 292 J/(mol x

K). If the temperature is 25 oC, what is the ΔG?• For the vaporization reaction

Br2 (l) Br2 (g)

ΔH = 31 kJ/mol and ΔS = 93 kJ/(mol x K). At what temperature will it become spontaneous?

BINGO Questions

• Which is more favorable: more disorder or less disorder?• Which is more favorable: an exothermic reaction or an

endothermic reaction?

Learning Log Assessment

• Rate yourself 1 – 4 on LTs 7.8, 7.9, and 7.10.

Online Simulator• Use the online simulator to explore the

relationship between entropy, enthalpy and the favorability of a reaction.

• You may work with a partner

• Begin working on your “Homework 7.5”• There will be an exit slip after this activity

Work Time

Exit Slip1. You have a reaction that has a +ΔS and a +ΔH.

Under what conditions would this reaction be spontaneous?

2. For the reaction:NH4Cl (s) NH3 (g) + HCl (g)

ΔH = 176 kJ/mol and ΔS=0.285 kJ/(mol x K) and T = 400 K. Determine ΔG and state whether this is a spontaneous reaction.

Learning Log Assessment

• Using your exit slip score, re-rate yourself on LTs 7.8, 7.9, and 7.10

Review Stations• Rotate between the 7 stations (10 for honors)

ROTATE!

• Honors Station 8,9,10 – LT 7.8, 7.9, 7.10

• Homework 7.5 – Gibbs Free Energy• Test Tuesday!

Closing Time