Title: Lesson 8 Spontaneity and Entropy

Learning Objectives:– Understand the term ‘spontaneous’ in relation to a chemical

reaction

– Understand the term entropy, and predict the sign of changes in entropy

– Understand the relationship between enthalpy, entropy and absolute temperature

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Refresh Which equation represents the electron affinity of calcium?

A. Ca(g) →Ca+(g) + e–

B. Ca(g) →Ca–(g) + e–

C. Ca(g) + e– → Ca–(g)

D. Ca+(g) + e– → Ca(g)

Which reaction has the most negative ∆Hο value?

A. LiF(s) → Li+(g) + F–(g)

B. Li+(g) + F–(g) → LiF(s)

C. NaCl(s) → Na+(g) + Cl–(g)

D. Na+(g) + Cl–(g) → NaCl(s)

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Spontaneity

A spontaneous reaction is one that keeps itself going i.e. It doesn’t require an external energy input

to keep going

E.g. A carbonated drink will go flat if left open, the carbon dioxide will diffuse into the surroundings, and will not come back.

Some reactions are spontaneous at all temperatures: Although if you lower the temperature enough,

the rate might be too low to matter

Some reactions only become spontaneous above or below a certain temperature.

These reactions can be reversed but only at the expense of using energy.

Burning wood is a spontaneous reaction

The thermal decomposition of copper carbonate is a non-spontaneous reaction

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Entropy Matter and energy tend to disperse and become more disordered.

The degree of disorder of a system is quantified by its entropy (s). The more ways the energy can be distributed the higher the entropy. Units: J K-1 mol-1 (Joules per Kelvin per mole)

Ordered states small energy distribution low entropy Disordered states high energy distribution high entropy

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2nd Law of Thermodynamics

“for a spontaneous process, the total entropy change in a system and in the system’s surroundings will increase”

System: this is the reactants and productsSurroundings: container and anything in

contactwith the container

Reactions are only feasible if the total entropy change is +ve

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Entropy Changes, ∆S The entropy of a truly closed system* can only remain

fixed or increase… (relates to 2nd law of thermodynamics)* A closed system is one where the total energy content is fixed, nothing can enter or leave. The only truly closed system is (probably) the universe, but we can approximate them in the lab

TASK: Think of three chemical/physical changes that result in an increase in entropy, and explain why they do so.

Lower Entropy

Pure substancesSlow moving

particlesParticles close

togetherFewer particles

Solid

Higher Entropy

Mixtures of substancesFast moving

particlesParticles spread

outMore particles

Gas

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Chemical/Physical reactions that results in an increase in entropy… 1. A solid melts 

2. A liquid vaporizes 3. A solute dissociates into ions 4. The decomposition of a compound 5. Two substances are mixed together. 6. The decay of a radionuclide 

... to name a few.

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Predicting Entropy Changes Doubling amount of substance (e.g. particles)

increase (+) S

Physical changes tabulated below:

12 molecules

24 atoms

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Solutions

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What is enthalpy? Measure of the heat content of a chemical

system. What is entropy? Measure of disorder in a chemical system. Always some degree of disorder as particles

always have some sort of motion – value always positive.

Motion is related to enthalpy (energy) so entropy and enthalpy are interlinked.

A better description for entropy is therefore the dispersal of energy.

Entropy

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Calculating Entropy Change, ∆So

The absolute entropy of different substances can be calculated. Entropy depends on temperature and pressure so given values will be at standard conditions.

Standard Molar Entropy values, So, for many organic compounds can be found in Table 12 of the data booklet.

Entropy values increase as you move from solid liquid gas. All entropy values are positive. A perfectly ordered solid at absolute zero has an entropy of zero.

We can use these to calculate the standard entropy change of a reaction, ∆So

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Calculating Entropy Change, ∆So

Entropy change of the system during a reaction can be calculated from the differences between total entropy of the products and total entropy of the reactants.

The strategy and pitfalls of calculating entropy changes are similar to those discussed when calculating enthalpy changes. E.g.

Not all the heat produced/absorbed by the reaction is kept in the reactants of the system.

The reaction may not be complete.

The experiment was not performed under standard conditions.

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Calculating Entropy Change, ∆So

For example: Calculate the standard entropy change when pentane (So=261 JK-1mol-1) is ‘cracked’ to form propane (So=270 JK-1mol-1) and ethene (So=220 JK-1mol-1) :

C5H12(l) C2H4(g) + C3H8(g) ∆So = (270 + 220) – 261 = 229 J K-1 mol-1

The entropy of the reactants is subtracted from the entropy of the products

The positive nature of this result should be expected, as you have increased the number of molecules, and changed state from liquid to gas

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Calculate ∆So for:For each one, explain why ∆So has the sign it does∆So are written underneath the formula if not in data booklet.

C6H14(l) C2H4(g) + C4H10(g)

CH4(g) + 2O2(g) CO2(g) + 2H2O(l)

205 214 189

6CO2(g) + 6H2O(l) C6H12O6(s) + 6O2(g)

214 189 209 205

Now write some problems for a class-mate to answer. You can easily find ∆So for most compounds on the web – for example the side box of most Wikipedia entries

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Entropy changes of the surroundings So far, we have looked at total entropy of the system.

To calculate total enthalpy change of a reaction we must consider the total entropy change in the surroundings.

Consider the reaction between Zinc and Copper Sulphate:

How does this reaction increase the total entropy of the universe?

E.g. Exothermic reaction and a cooling cup of coffee will cause an increase in entropy of the universe as heat is dispersed…

Enthalpy and Entropy of the system

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Change in entropy of the surroundings is proportional to - H(system)

Exothermic reactions in the system increase entropy in the surroundings:

This can explain why exothermic reactions are generally more common than endothermic reactions. It is not about decreasing energy in the system but increasing entropy of the surroundings.

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Change in entropy is inversely proportional to the absolute temperature

The impact of a transfer of heat to the surroundings depends on the current state of disorder in the surroundings.

E.g. if the surroundings are hot, the addition of a little heat will make little difference to the disorder.

If the surroundings are cold, the addition of the same amount of heat will cause a more dramatic change in entropy.

Will a sneeze cause more disruption in a busy street or a quiet library?

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S(surroundings) and the units of entropy

Combined expression showing relationship between change in enthalpy and absolute temperature:

E.g. Using the displacement reaction:

At T = 25oC = 298K

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Calculating total entropy changes and understanding endothermic reactions

The second Law of Thermodynamics tells us for a spontaneous change:

Substitute S(surroundings) for:

This equation allows us to understand how endothermic reactions occur.

If the change of entropy of the system can compensate for the negative entropy change of the surroundings (heat from the surroundings moving to the system) endothermic reactions are feasible.

=S(surroundings)

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Key Points Entropy is a measure of ‘disorder’

Entropy tends to increase

Entropy increases as: Substances mix Particles move faster Particles spread out The number of particles increases State changes from solid liquid gas