Spontaneous ReactionsSpontaneous Reactions

Proceed forward on their own without Proceed forward on their own without outside or external cause.outside or external cause.

Certain conditions can help us predict Certain conditions can help us predict whether a reaction will occur whether a reaction will occur spontaneously or not.spontaneously or not.

Factors that Determine SpontaneityFactors that Determine Spontaneity

Change in EnthalpyChange in Enthalpy ( (∆H)∆H)

ExothermicExothermic ( (- ∆H)- ∆H)reactions are more likely to reactions are more likely to be spontaneous.be spontaneous.

A + B A + B → C + D + energy→ C + D + energy

Why?Why? Release energy, producing lower energy more Release energy, producing lower energy more

stable products.stable products. Require less activation energy, so are easier to Require less activation energy, so are easier to

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Factors that Determine SpontaneityFactors that Determine Spontaneity

Change in EntropyChange in Entropy ( (∆S)∆S) Reactions with an Reactions with an INCREASE in EntropyINCREASE in Entropy are are

more likely to be spontaneous.more likely to be spontaneous.

Entropy is a measure of the disorder, Entropy is a measure of the disorder, randomness or lack of organization in a system.randomness or lack of organization in a system.

+ ∆S+ ∆S = = increase entropy increase entropy

or disorderor disorder

Entropy deals with the Entropy deals with the Second Second Law of ThermodynamicsLaw of Thermodynamics, which , which basically states that all systems basically states that all systems in the universe progress to a in the universe progress to a state of higher disorder.state of higher disorder.

+∆S

-∆S

Entropy =Entropy =

Chaos andChaos and

DisorderDisorder

http://www.youtube.com/watch?v=CgppGozbFd4&safe=active

http://www.youtube.com/watch?v=uQSoaiubuA0&safe=active

How to Determine the Change in EntropyHow to Determine the Change in Entropy+∆S entropy increases+∆S entropy increases - - ∆S entropy decreases∆S entropy decreases

Changes to Phases of MatterChanges to Phases of Matter

Solids Solids Liquids Liquids GasesGases

Most orderedMost ordered Least orderedLeast ordered

Ex: H2O(s) → H2O (l)

+∆S

Ex: H2O(l) → H2O (s)-∆S

Gas particles are highly disbursed with random Gas particles are highly disbursed with random motion and high entropy. motion and high entropy.

Liquids have much lower entropies Liquids have much lower entropies Solids are most ordered and lowest entropy.Solids are most ordered and lowest entropy.

If Phases Stay the SameIf Phases Stay the Same

Look at:Look at:

Is there an increase in number of moles of gas? Is there an increase in number of moles of gas? +∆S

More highly disorganized gas = more chaos

Is the substance breaking apart or dissolving?Is the substance breaking apart or dissolving? +∆S

More pieces, more disbursed = more chaos

What is the Sign of What is the Sign of +∆S?

ReactionReaction Sign Sign of of ∆S

AgAg++(aq) + Cl(aq) + Cl--(aq) (aq) →→ AgCl(s) AgCl(s) --∆S

HH22O(l) O(l) →→ 2 H 2 H22(g) + O(g) + O22(g) (g) ++∆S

CO(g) + 3 HCO(g) + 3 H22(g) (g) →→ CH CH44(g) + H(g) + H22O(g) O(g) --∆S

2 NaHCO2 NaHCO33(s) ---> Na(s) ---> Na22COCO33(s) + H(s) + H22O(g) + COO(g) + CO22(g) (g) ++∆S

Given the balanced equation representing a Given the balanced equation representing a phase change:phase change:

CC66HH44ClCl22(s) + energy (s) + energy → → CC66HH44ClCl22(g)(g)

Which statement describes this change?Which statement describes this change?

(1) It is endothermic, and entropy decreases.(1) It is endothermic, and entropy decreases.

(2) It is endothermic, and entropy increases.(2) It is endothermic, and entropy increases.

(3) It is exothermic, and entropy decreases.(3) It is exothermic, and entropy decreases.

(4) It is exothermic, and entropy increases. (4) It is exothermic, and entropy increases.

The entropy of a sample of HThe entropy of a sample of H22O increases O increases as the sample changes from aas the sample changes from a

(1) gas to a liquid    (1) gas to a liquid   

(2) gas to a solid (2) gas to a solid 

(3) liquid to a gas(3) liquid to a gas

(4) liquid to a solid (4) liquid to a solid

Which 1-mole sample has the least Which 1-mole sample has the least entropy?entropy?

(1) Br(1) Br22(s) at 266 K     (s) at 266 K    

(2) Br(2) Br22(l) at 266 K (l) at 266 K 

(3) Br(3) Br22(l) at 332 K(l) at 332 K

(4) Br(4) Br22(g) at 332 K (g) at 332 K

In terms of energy and entropy, systems in In terms of energy and entropy, systems in nature tend to undergo changes towardnature tend to undergo changes toward

(1) higher energy and higher entropy(1) higher energy and higher entropy

(2) higher energy and lower entropy(2) higher energy and lower entropy

(3) lower energy and higher entropy(3) lower energy and higher entropy

(4) lower energy and lower entropy (4) lower energy and lower entropy

Which list of the phases of HWhich list of the phases of H22O is O is

arranged in order of increasing entropy?arranged in order of increasing entropy?

(1) ice, steam, and liquid water(1) ice, steam, and liquid water

(2) ice, liquid water, and steam(2) ice, liquid water, and steam

(3) steam, liquid water, and ice(3) steam, liquid water, and ice

(4) steam, ice, and liquid water (4) steam, ice, and liquid water

Given the balanced equation:Given the balanced equation:

II22(s) + energy (s) + energy →→ I I22(g)(g)

As a sample of IAs a sample of I22(s) sublimes to I(s) sublimes to I22(g), the entropy(g), the entropy

(1) increases because the particles are less (1) increases because the particles are less randomly arrangedrandomly arranged

(2) increases because the particles are more (2) increases because the particles are more randomly arrangedrandomly arranged

(3) decreases because the particles are less (3) decreases because the particles are less randomly arrangedrandomly arranged

(4) decreases because the particles are more (4) decreases because the particles are more randomly arranged randomly arranged

At STP, a sample of which element has At STP, a sample of which element has the highest entropy?the highest entropy?

(1) Na(s) (1) Na(s)           (2) Hg(l)(2) Hg(l)

(3) Br(3) Br22(l)(l)

(4) F(4) F22(g) (g)

Which of these changes produces the Which of these changes produces the greatest increase in entropy?greatest increase in entropy?

(1) CaCO(1) CaCO33(s)  (s)  →→ CaO(s) + CO CaO(s) + CO22(g)(g)

(2) 2 Mg(s) + O(2) 2 Mg(s) + O22(g)  (g)  →→ 2 MgO(s) 2 MgO(s)

(3) H(3) H22O(g)  O(g)  →→ H H22O(l)O(l)

(4) CO(4) CO22(g)  (g)  →→ CO CO22(s) (s)

Even though the process is endothermic, Even though the process is endothermic, snow can sublime. Which tendency in snow can sublime. Which tendency in nature accounts for this phase change?nature accounts for this phase change?

(1) a tendency toward greater entropy(1) a tendency toward greater entropy

(2) a tendency toward greater energy (2) a tendency toward greater energy

(3) a tendency toward less entropy(3) a tendency toward less entropy (4) a tendency toward less energy (4) a tendency toward less energy

Which process is accompanied by a Which process is accompanied by a decrease in entropy?decrease in entropy?

(1) boiling of water(1) boiling of water

(2) condensing of water vapor(2) condensing of water vapor

(3) subliming of iodine(3) subliming of iodine

(4) melting of ice(4) melting of ice

Answer: AAnswer: A

Answer: 2Answer: 2

Answer: 1Answer: 1

Is Reaction Spontaneous?Is Reaction Spontaneous?

A reaction will A reaction will always be spontaneousalways be spontaneous if: if: - - ∆H ∆H + ∆S+ ∆S

A reaction will A reaction will never be spontaneousnever be spontaneous if: if: + + ∆H∆H - ∆S- ∆S

Sometimes SpontaneousSometimes Spontaneous + + ∆H∆H + ∆S+ ∆S

More likely at higher temperaturesMore likely at higher temperatures

- - ∆H∆H - ∆S- ∆S

More likely at lower temperaturesMore likely at lower temperatures

These situations depend on the These situations depend on the temperaturetemperature conditions.conditions.

You would have to do a calculation to determine.You would have to do a calculation to determine.

Gibbs Free EnergyGibbs Free EnergyGibbs Free Energy (Gibbs Free Energy (∆G)∆G)

A portion of energy from a spontaneous reaction that can A portion of energy from a spontaneous reaction that can perform useful work.perform useful work. Ex: battery, glowstickEx: battery, glowstick

Spontaneous Reactions are - ∆GSpontaneous Reactions are - ∆GThey give off free energyThey give off free energy

Nonspontaneous Reactions are + ∆GNonspontaneous Reactions are + ∆GThey require the input of free energy to occurThey require the input of free energy to occur

When a rxn is no longer spontaneous and producing free When a rxn is no longer spontaneous and producing free energy the system is at equilibrium and ∆G = 0energy the system is at equilibrium and ∆G = 0

(- (- ∆H) (+ ∆S) Always Spontaneous so ∆H) (+ ∆S) Always Spontaneous so - ∆G- ∆G

(+ (+ ∆H) (- ∆S) Never Spontaneous so ∆H) (- ∆S) Never Spontaneous so + ∆G+ ∆G

What about sometimes spontaneous? What about sometimes spontaneous?

How would you know if it is or not?How would you know if it is or not?

Use the Gibbs Free Energy EquationUse the Gibbs Free Energy Equation

∆∆G = ∆H - T ∆SG = ∆H - T ∆S

Kelvin Temperature

Gibbs Free Energy of Formation Gibbs Free Energy of Formation ((∆G∆Gff) (Honors Only)) (Honors Only)

Quantity of free energy involved in the Quantity of free energy involved in the formation of one mole of a compound from formation of one mole of a compound from it’s elements.it’s elements.

Way to determine overall Way to determine overall ∆G ∆G of a reactionof a reaction

∆∆GGrxnrxn = = ΣΣ (∆G (∆Gff Products)Products) - - ΣΣ (∆G (∆Gff Reactants)Reactants)

This should look familiar!!! Same as ∆H∆Hff problems from last test.

An Endothermic reaction may proceedAn Endothermic reaction may proceedspontaneously if there is an increase inspontaneously if there is an increase in

A. Potential EnergyA. Potential Energy

B. OrderB. Order

C. ConcentrationC. Concentration

D. EntropyD. Entropy

A reaction must be spontaneous if itsA reaction must be spontaneous if its

occurrence isoccurrence is

A. exothermic with an decrease in entropyA. exothermic with an decrease in entropy

B. exothermic with an increase in entropyB. exothermic with an increase in entropy

C. endothermic with an decrease in entropyC. endothermic with an decrease in entropy

D. endothermic with an increase in entropyD. endothermic with an increase in entropy

Above 0Above 0°°C, ice changes spontaneously to waterC, ice changes spontaneously to water

according to the following equation:according to the following equation:

HH22O(s) + heat --> HO(s) + heat --> H22O(l)O(l)

The change in HThe change in H22O(s) involveO(s) involve

A. an absorption of heat and a decrease in entropyA. an absorption of heat and a decrease in entropy

B. a release of heat and a decrease in entropyB. a release of heat and a decrease in entropy

C. an absorption of heat and a increase in entropyC. an absorption of heat and a increase in entropy

D. a release of heat and a increase in entropyD. a release of heat and a increase in entropy

The ΔG of a chemical reaction refers to theThe ΔG of a chemical reaction refers to the

change in change in 

A. entropyA. entropy

B. free energyB. free energy

C. stateC. state

D. activation energyD. activation energy

The change of the reactants into productsThe change of the reactants into products

will will alwaysalways be spontaneous if the products, be spontaneous if the products,

compared to the reactants, havecompared to the reactants, have

A. lower enthalpy and higher entropyA. lower enthalpy and higher entropy

B. higher enthalpy and higher entropyB. higher enthalpy and higher entropy

C. higher enthalpy and lower entropyC. higher enthalpy and lower entropy

D. lower enthalpy and lower entropyD. lower enthalpy and lower entropy