The Driving Forces The Driving Forces of Reactionsof Reactions

AP Chemistry

In chemistry we are concerned with In chemistry we are concerned with whether a reaction will occur whether a reaction will occur spontaneously, and under what spontaneously, and under what conditions will it occur spontaneously.conditions will it occur spontaneously.

Spontaneous reactionsSpontaneous reactions are reactions are reactions that naturally favor the that naturally favor the productsproducts at at specific conditions. They also release specific conditions. They also release free energyfree energy (the energy available to (the energy available to do work)do work)

Non-Spontaneous reactionsNon-Spontaneous reactions are are ones that do not favor the products and ones that do not favor the products and do not release free energy, they use it!do not release free energy, they use it!

Free EnergyFree Energy Free EnergyFree Energy is given the symbol “ is given the symbol “GG” and we ” and we

look at the look at the change in free energy (ΔG)change in free energy (ΔG) in a in a reaction to determine if it is spontaneous or not.reaction to determine if it is spontaneous or not.

A positive ΔG means the reaction is not A positive ΔG means the reaction is not spontaneous .spontaneous .

A A negative ΔG means the reaction is negative ΔG means the reaction is spontaneousspontaneous..

ΔG = 0 if the system is at equilibrium. (such ΔG = 0 if the system is at equilibrium. (such as at melting or boiling temperature)as at melting or boiling temperature)

The change in free energy, ΔG, is determine by The change in free energy, ΔG, is determine by the the Gibbs Free Energy EquationGibbs Free Energy Equation

ΔG = ΔH - TΔS ΔG = ΔH - TΔS

T = Temperature in KelvinT = Temperature in Kelvin

ENTHALPYENTHALPY

ΔH ΔH represents the change in represents the change in enthalpy enthalpy for for the reaction. the reaction.

(remember: (remember: +ΔH is endothermic+ΔH is endothermic and and

-ΔH is exothermic-ΔH is exothermic.).) Most reactions are exothermic because Most reactions are exothermic because

there is a natural tendency in nature to there is a natural tendency in nature to move to a lower energy state, and that is move to a lower energy state, and that is what happens in an exothermic reaction what happens in an exothermic reaction (think about the potential energy (think about the potential energy diagrams!)diagrams!)

ENTROPYENTROPY ΔS ΔS represents the represents the change in change in

entropy (S)entropy (S)

EntropyEntropy is the measure of the degree is the measure of the degree of randomness in a system. Gases of randomness in a system. Gases have higher entropy than a liquid or have higher entropy than a liquid or solid, because the particles have less solid, because the particles have less order.order.

There is a tendency in nature to There is a tendency in nature to proceed in the direction that increases proceed in the direction that increases entropy (to get more random). entropy (to get more random).

EntropyEntropy

+ΔS+ΔS means there is an means there is an increase in increase in entropyentropy for a reaction for a reaction

( liquid( liquid gas) gets more random. gas) gets more random.

––ΔSΔS means there is a means there is a decrease in decrease in entropyentropy for a reaction. for a reaction.

( a liquid ( a liquid solid) gets more solid) gets more orderedordered

Gibbs free energy equationGibbs free energy equation

We use the Gibbs free energy equation to We use the Gibbs free energy equation to determine that ΔG for a reaction if we determine that ΔG for a reaction if we know the enthalpy change, the entropy know the enthalpy change, the entropy change and the temperature (in Kelvin). change and the temperature (in Kelvin).

This will tell us if a reaction will be This will tell us if a reaction will be spontaneous at that temperature.(you spontaneous at that temperature.(you look at the sign (+ or -) on the answer for look at the sign (+ or -) on the answer for ΔG)ΔG)

- ΔG = spontaneous- ΔG = spontaneous + ΔG = non-spontaneous+ ΔG = non-spontaneous

You can calculate ∆GYou can calculate ∆Goo from ∆H from ∆Hoo and and ∆S∆Soo and Temperature or you can look and Temperature or you can look up the standard free energies of up the standard free energies of formation ∆G of and use the formation ∆G of and use the products minus reactants procedure products minus reactants procedure we used for ∆H and ∆S.we used for ∆H and ∆S.

(see appendix handout for values or (see appendix handout for values or look at appendix in book!)look at appendix in book!)

ΔG = ΔH - TΔSΔG = ΔH - TΔS

ΔHΔH ΔSΔS ΔGΔG

-- ++ -- Always Always SpontaneousSpontaneous

++ -- ++ Never Never SpontaneousSpontaneous

-- -- Depends Depends on on

TemperatuTemperaturere

Spontaneous atSpontaneous at

Low TempsLow Temps

++ ++ Depends Depends on on

TemperatuTemperaturere

Spontaneous atSpontaneous at

High TempsHigh Temps

Example:Example: Determine the values of Determine the values of H, H, S and S and

G at standard conditions (25oC G at standard conditions (25oC and 1 atm) for the following and 1 atm) for the following reaction using the data given.reaction using the data given.

2SO2SO22(g) + O(g) + O22(g) (g) 2SO 2SO33(g)(g)

Substance S (J/K mol) H kJ/ mol

SO2 (g) 248 -297

O2 (g) 205 0

SO3(g) 257 -396

∆∆H = 2∆HH = 2∆HSO3 SO3 - [2∆H∆HSO2SO2 + ∆H + ∆HO2O2]] = 2(-396) – [ 2(-297) + 0]= 2(-396) – [ 2(-297) + 0] = -198 kJ= -198 kJ

∆∆S = 2 ∆SS = 2 ∆SSO3SO3 - [2(∆S - [2(∆SSO2SO2 + ∆S + ∆SO2O2]] = 2(257) – [2(248) + 205]= 2(257) – [2(248) + 205] = -187 J/K= -187 J/KNow plug into the Gibbs Equation using Now plug into the Gibbs Equation using

T=298K (convert ∆S to kJ from J first!)T=298K (convert ∆S to kJ from J first!)∆∆G = ∆H - T ∆SG = ∆H - T ∆S = (-198kJ - (298K)(-0.187kJ/K)= (-198kJ - (298K)(-0.187kJ/K) = -142 kJ= -142 kJ

HH22O(s) = HO(s) = H22O(l)O(l) Is Enthalpy (+) or (-)?Is Enthalpy (+) or (-)?

(+) because it is an endothermic change(+) because it is an endothermic change Is Entropy Increasing or Decreasing?Is Entropy Increasing or Decreasing?

IncreasingIncreasing ∆∆S = (+)S = (+)

Is the reaction Spontaneous?Is the reaction Spontaneous? Depends on Temperature.Depends on Temperature. Spontaneous, ∆G = (-), at High Temps.Spontaneous, ∆G = (-), at High Temps. Heat is required to melt Ice. Heat is required to melt Ice. If you are at the right temperature, it will If you are at the right temperature, it will

spontaneously melt.spontaneously melt.

HH22SOSO44 = 2H = 2H++ + SO + SO442- 2- + +

HeatHeat Is Is ∆H (+) or (-)?∆H (+) or (-)?

(-) because it is exothermic(-) because it is exothermic

Is ∆S (+) or (-)?Is ∆S (+) or (-)? (+) because entropy is increasing, more (+) because entropy is increasing, more

particles are more random!particles are more random!

Is the reaction spontaneous?Is the reaction spontaneous? Yes, it is Yes, it is alwaysalways spontaneous. spontaneous.

The universe favors lower energy and The universe favors lower energy and higher entropy!higher entropy!