Chemical Thermodynamics the study of Reaction Feasibility.

Chemical Thermodynamics

the study of Reaction Feasibility

Reaction Feasibility• Thermodynamics is concerned with questions such as:

why do some reactions take place while others don’t?

can we predict whether or not a reaction will occur?

under what conditions will a reaction occur?

• Increasingly we consider all reactions to be reversiblereversible, but, under certain conditions the reaction will be more likelymore likely to go in one direction than the other

• In one direction the reaction will be spontaneousspontaneous while the other direction will be non-spontaneousnon-spontaneous

Most spontaneous reactions are exothermic

But not ALL !

Spontaneous Exothermic

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Spontaneous Endothermic• solid ammonium carbonate reacts with conc. ethanoic acid(NH4)2CO3 + 2CH3COOH 2NH4CH3COO + CO2 +

H2O

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Spontaneous Processes• H2O(s) ⇋ H2O(l)

Ice turning to water is spontaneous at T > 0°C,

Water turning to ice is spontaneous at T < 0°C.

• Both Exothermic & Endothermic processes can be spontaneous• The direction of a spontaneous process will depend on temperature

Energy Rules!The key to understanding Thermodynamics is the appreciation of the ways in which energy interacts with matter.

Energy doesn’t just determine the speed at which particles move (TemperatureTemperature) it is part of everything that affects particles.

Energy Rules!Some Processes involve no change in temperature no change in temperature but a major change in the energy of particles is still occurring.

EntropyThese changes in the energy of particles have an overall affect on the level of disorderlevel of disorder shown by a substance.

The disorder in a substance is known as its ENTROPY, S ENTROPY, S .

The Third Law of Thermodynamics provides a reference against which Entropies can be measured.

“the Entropy of a perfect crystal at 0 K is zero”

Entropy - State

Molecular MotionTranslation Rotation Vibration

no freedom to move

restricted freedom to move

total freedom to move

no freedom to rotate

some freedom to rotate

total freedom to rotate

free to vibrate

free to vibrate

free to vibrate

Entropy - Temperature

Entropy(S)

Temperature

Entropy of Fusion

Entropy of Vaporisatio

n

Entropy - Dissolving

Less Randomness More Randomness

Less EntropyLess Entropy More EntropyMore Entropy

Entropy - Molecules

NO NO2

N2O

4Fewer

Vibrations

Less EntropyLess Entropy

More Vibrations

More EntropyMore Entropy

More Vibrations

More EntropyMore Entropy

Entropy - Numbers

Less Randomness More Randomness

Less EntropyLess Entropy More EntropyMore Entropy

Entropy - Mixtures

Less Randomness More Randomness

Less EntropyLess Entropy More EntropyMore Entropy

Entropy Values

Entropy Calculations

Similar to a previous formula:

∆∆SSoo = ∑ S = ∑ Sooproductsproducts - ∑ - ∑

SSooreactantsreactants

Entropy Changes, ∆S• spontaneous endothermic reactions tend to have certain characteristics in common

• the number of moles of product are greater than the number of moles of reactant

• a large proportion of the products are either liquids or gases• reactants are often solids or liquids

(NH4)2CO3 + 2CH3COOH 2NH4CH3COO + CO2 +

H2O

The trend solids liquids gases is associated with an increase in disorder .

Entropy - The Answer?Is an Increase In EntropyIncrease In Entropy the driving force behind a spontaneous chemical reaction ?

Both ∆S = +ve & ∆S = -ve processes can be spontaneousThe direction of a spontaneous process will depend on temperatureA spontaneous process will depend on both ∆S and ∆H

Entropy - The Answer?The ‘problem’ can be resolved if we take into account changes taking place in the SurroundingsSurroundings.

The driving force behind a spontaneous process turns out to be an Overall Increase In EntropyOverall Increase In Entropy

Entropy - The Answer?

Water freezing is a spontaneous process whenever there is an Overall Increase In EntropyOverall Increase In Entropy

Water freezing leads to a decrease in entropy within the system.

Being Exothermic, however, leads to an increase in entropy in the surroundings

Entropy - The Answer?

Water melting is a spontaneous process whenever there is an Overall Increase In EntropyOverall Increase In Entropy

Being Endothermic leads to a decrease in entropy in the surroundings

There will have to be an increase in entropy within the system.

Measuring ∆Sosurr

Trying to Calculate the effect on the surroundings would appear, at first, an impossible task.

Where do the surroundings start & finish?

What is the entropy of air? Glass? etc.

How many moles of ‘surroundings’ are there?

Fortunately it is much, much simpler than that.

Measuring ∆Sosurr

Firstly the change in Entropy of the Surroundings is caused by the Enthalpy change of the Surroundings, and…….

∆∆HHoosurrsurr = -∆H = -∆Hoo

systsyst

soso ∆S∆Soosurrsurr ∝∝ -∆H -∆Hoo

systsyst

Temperature has an inverse effect. For example, energy released into the surroundings has less effect on the entropy of the surroundings, the hotter the surroundings are.

Measuring ∆Sosurr

In fact, it turns out that ..

It is the Overall Entropy ChangeOverall Entropy Change that must be considered.

∆∆SSoosurrsurr = - = -

∆H∆Hoosystsyst

TT

∆∆SSoototaltotal = ∆S = ∆Soo

systsyst + +

∆S ∆Soosurrsurr

∆∆SSoototaltotal

==∆∆SSoo

surrsurr -∆H -∆Hoosystsyst

TT

Measuring ∆Sototal

We are interested in the point at which the Total EntropyTotal Entropy becomes a positive value (ceases being a negative

value). We can ‘solve’ for ∆S∆Stotaltotal= 0= 0

0 = T∆S0 = T∆Soosystsyst -∆H -∆Hoo

systsyst

0 =0 =∆∆SSoosurrsurr -∆H -∆Hoo

systsyst TT

Multiplying throughout by T T gives us

Measuring ∆Sototal

Remember that this is really the formula for ∆S∆Stotaltotal

∆∆SSoototal total = T∆S= T∆Soo

systsyst -∆H -∆Hoosystsyst

Armed with ∆S∆S , , ∆H∆H and values for T T we can calculate the overall change in Entropy and a positive value would be necessary for a spontaneous reaction.

However, for reasons that are beyond this Topic, a term called the Gibbs Free Energy, GGibbs Free Energy, G, is preferred. A negative value for ∆G∆G is equivalent to a positive value for ∆S∆S. This requires a slight adjustment in our final formula.

Gibbs Free Energy ∆Go

∆∆GGoo = ∆H= ∆Hoo

systsyst - T∆S - T∆Soosystsyst

The convenient thing about this expression is that it allows us to do calculations using only values that can be directly measured or easily calculated.

Strictly speaking, the Second Law of ThermodynamicsSecond Law of Thermodynamics states that Entropy must increaseEntropy must increase for a Spontaneous Process.

In practice, the Second Law of ThermodynamicsSecond Law of Thermodynamics means that Gibbs Free Energy must decreaseGibbs Free Energy must decrease for a Spontaneous Process.

Gibbs Free Energy ∆Go

Calculating ∆Go

∆∆HHoo = ∑ ∆H = ∑ ∆Hffoo productsproducts - ∑ ∆H - ∑ ∆Hff

oo reactantsreactants

∆∆SSoo = ∑ S = ∑ Sooproductsproducts - ∑ S - ∑ Soo

reactantsreactants

∆∆GGoo = ∆H= ∆Hoo - -

T∆ST∆Soo

FeFe22OO3 3 + 3 CO + 3 CO ➝➝ 2 Fe + 3 2 Fe + 3

COCO22

T T in in KelvinKelvin

∆Go of Formation

∆∆GGoo = ∑ ∆G = ∑ ∆Gffoo productsproducts - ∑ ∆G - ∑ ∆Gff

oo reactantsreactants

The ∆G∆G of a reaction can be calculated from ∆G∆Gff values.

By themselves, they give useful information about relative relative stabilitiesstabilities.

Ellingham Diagrams∆∆GGoo

= ∆H= ∆Hoo - -

T∆ST∆Soo

yy = c + = c +

mxmx

Reversible Reactions

∆∆GGoo = ∆H= ∆Hoo - -

T∆ST∆Soo

For a Chemical Reaction

If ∆G is negative∆G is negative for one direction, it must be positivepositive for the reverse.

This implies that only one reaction can proceed (spontaneously) under a given set of conditions.

However, ∆S ∆S calculations are based on 100% Reactant & 100% Product.

In reality, mixtures exist, so larger ∆S ∆S values will be obtained than those calculated.

Reversible Reactions

Equilibrium position∆∆G G for 100% Reac ➝ 100% Prod is positivepositive but 100% Reac ➝ mixture is negativenegative so forward reaction can take place.

∆∆G G for 100% Prod ➝ 100% Reac and and 100% Prod ➝ mixture is more more negativenegative so backward reaction is favouredfavoured

Equilibrium lies over to the left but only slightly since value of ∆G∆G is positiveis positive but relatively small

Equilibrium position∆∆G G for 100% Reac ➝ 100% Prod is very positivevery positive but 100% Reac ➝ mixture is still slightlyslightly negativenegative so forward reaction can take place.

∆∆G G for 100% Prod ➝ 100% Reac and and 100% Prod ➝ mixture is more more negativenegative so backward reaction is favouredfavoured

Equilibrium lies well over to the left since value of ∆G∆G is positiveis positive and relatively large

Equilibrium position∆∆G G for 100% Reac ➝ 100% Prod is very negativevery negative but 100% Prod ➝ mixture is still slightlyslightly negativenegative so backward reaction can take place.

∆∆G G for 100% Reac ➝ 100% Prod and and 100% Reac ➝ mixture is more more negativenegative so forward reaction is favouredfavoured

Equilibrium lies well over to the right since value of ∆G∆G is negativeis negative and relatively large

Equilibrium position

Equilibrium Position

∆∆GGoo = - RT ln K= - RT ln K

There is a mathematical relationship:

More simply:

Thermodynamic Limits

Thermodynamics can predict whether a reaction is feasible or not.

Thermodynamics can predict the conditions necessary for a reaction to be feasible.

Thermodynamics can predict the position of equilibrium

Thermodynamics cannotcannot predict how fasthow fast a reaction might be.

Kinetics

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