Unless otherwise stated, all images in this file have been reproduced from:

Blackman, Bottle, Schmid, Mocerino and Wille,     Chemistry, 2007 (John Wiley)

     ISBN: 9 78047081 0866

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e CHEM1002 [Part 2]

A/Prof Adam Bridgeman (Series 1)Dr Feike Dijkstra (Series 2)

Weeks 8 – 13

Office Hours: Monday 2-3, Friday 1-2Room: 543ae-mail: adam.bridgeman@sydney.edu.aue-mail: feike.dijkstra@sydney.edu.au

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Lecture 6:• Physical States and Phase Changes• Blackman Section 7.3

Entropy

Lecture 7:• Entropy• Blackman Section 8.4

Lecture 8:• Crystal Structures• Blackman Chapter 7, Section 7.4 (pages 259-265)

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H2O(s) H2O(l) DH = +6.02 kJ mol-1

• The temperature of the surroundings is also important!

Spontaneous Reactions?

• In which direction is this reaction spontaneous?

At -10 °C: liquid solid

At 10 °C: solid liquid

(exothermic)

(endothermic)

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e

Twarm Twarm

Thermal Equilibration

Thot Tcold

• Clearly, energy (heat) will move from the hotter body to the cooler body. Overall energy is constant.

ENTROPY may be thought of as this tendency for energy to spread out as far as possible.

The appropriate definition of thechange in entropy (S) is:

Derivation of this equation in CHEM2402

?

Δ = qST

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e Energy Spreading Out…

Energy can be spread out in two main ways:• the molecules and their energy can be spread out further, or• the energy can be spread across more molecules

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NO

NO2

N2O4

Entropy and Molecular Complexity

• Entropy increases as molecule grows more complex.

animations

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e Amount of Entropy (S)• In many cases, the relative entropy of two different

systems can be understood intuitively.

S(gas) >> S(solid)

S(solid+liquid) < S(solution)

S(gas+liquid) > S(solution)

S(3 mol) > S(2 mol)

S(C2H6) > S (CH4)

Energy spreads further in gas

Energy localised in solid

Energy spreads further in gas

Entropy amount of substance

More bonds to spread energy around

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x Which Has Higher Entropy?

• Which has the higher entropy in each of the following pairs?

(a) 1 mol of SO2(g) or

(b) 1 mol of CO2(s) or 1 mol of CO2(g)

(c) 3 mol of O2 or 2 mol of O3(g)

(d) 1 mol of KBr(s) or 1 mol of KBr(aq)

(e) Seawater in midwinter (2 0C) or in midsummer (23 0C)

more atoms

gas > solid

more molecules

solution > solid

higher T

1 mol of SO3(g)

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DfusS

Entropy: Qualitative

• More disordered = higher S• For given substance: • Same substance:

meltingpoint

T

Ent

ropy

(S)

boilingpoint

Solid

Liquid

Gas

DvapS

(increasing vibrations)

(increasing translations)

(increasing speeds)

Sliquid > SsolidSgas > > higher T → higher S

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x Practice Examples1.When one mole of ice melts to liquid at 0 °C,

A. The entropy of the system decreases.B. The entropy of the system remains the same.C. The entropy of the system increases.D. The order of the system increases. E. None of the above

2. The entropy of a chemical system will usually increase whenA. A molecule is broken down into two or more smaller

fragments.B. A reaction occurs that results in an increase in the moles

of gas.C. A solid changes to a liquid.D. A liquid changes into a gas.E. All of the above

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e Summary: Entropy,Learning Outcomes - you should now be able to:

• Complete the worksheet• Explain what entropy is to a non-science friend• Predict whether entropy will increase or

decrease in a chemical or physical process• Apply the entropy concept qualitatively to

explain direction of phase change

Next lecture:

• Crystal structures