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16-4-2011

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Thermodynamics

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Thermodynamics examines the heat and work as well as spontaneity of a reaction.

Spontaneity is the notion of whether or not a process can take place unassisted.

Entropy is a mathematical concept describing disorder and randomness in a system.

Free energy is a thermodynamic function that relates enthalpy and entropy to spontaneity, and can also be related to equilibrium constants.

Introduction

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• With a knowledge of thermodynamics and by making a few calculations before embarking on a new venture, scientists and engineers can save themselves a great deal of time, money, and frustration.

• Thermodynamics tells us what processes are possible.– (Kinetics tells us whether the process is practical.)

Why Study Thermodynamics?

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Law of Conservation of Energy

• energy cannot be created or destroyed

– First Law of Thermodynamics

• energy can be transferred between objects

• energy can be transformed from one form to another

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Spontaneous processes and entropy

As far as we are concerned, we would like to be able to predict whether a chemical reaction proceeds spontaneously under a specified set of conditions or not.

A chemical reaction is said to be spontaneous if it occurs under a given set of conditions. If it does not occur under this set of conditions it is said to be non-spontaneous.

If a reaction is spontaneous in one direction, it is non-spontaneous in the other, and vice versa.

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Examples of spontaneous processes

• A waterfall runs downhill, but never uphill.• A lump of sugar spontaneously dissolves in

a cup of tea, but never dissolved sugar lumps back spontaneously.

• Heat transfers from a hot object to colder surroundings, but never heat from a cold surrounding transfers to heat a body of higher temperature

• Iron exposed to water and oxygen rusts, but never the opposite.

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Spontaneous Processes

• Spontaneous processes are those that can proceed without any outside intervention.

• The gas in vessel B will spontaneously effuse into vessel A, but once the gas is in both vessels, it will not accumulate in one only.

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Many exothermic processes are Spontaneous

In general, product-favored reactions are exothermic and

spontaneous

E.g. thermite reaction

Fe2O3(s) + 2 Al(s)

2 Fe(s) + Al2O3(s)

H = - 848 kJ

H+ + OH- H2OH = - 56.2 kJ

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Many endothermic processes are also spontaneous

NH4NO3(s) + heat NH4+ (aq) + NO3

- (aq)

Hsol = +25.7 kJ/mol

H2O (s) H2O (l)

Ho = +6.01 kJ/mol

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Water falling (higher to lower potential energy) is

a spontaneous process.

H2 and O2 combine spontaneously to form water

(exothermic)… liquid water vaporizes

spontaneously at room temperature; an

endothermic process.

Conclusion: enthalpy alone is not a sufficient criterion for prediction of spontaneity.

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In some cases, endothermic reactions that are non-spontaneous at room temperature may become spontaneous when the temperature is raised. Example:

2HgO(s) 2 Hg(l) + O2(g)Ho = +90.7 kJ/mol

Conclusion: Exothermicity favors spontaneity of a reaction but does not guarantee it. In addition, endothermic reactions can be spontaneous. Therefore, we can not judge spontaneity considering H only.

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Entropy

• Entropy, S, is a measure of the disorder of a system.

• Spontaneous reactions proceed to lower energy or increase entropy.

• In ice, the molecules are very well ordered because of the H-bonds, therefore, ice has a low entropy.

• Melting ice will increase entropy, a spontaneous process although endothermic.

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Entropy is a state function. For a system, S = Sfinal - Sinitial

If S > 0 the randomness increases if S < 0 the order increases

Ssolid < Sliquid << Sgas

Entropy

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There is a balance between energy and entropy considerations.

When an ionic solid is placed in water two things happen: the water organizes into hydrates

about the ions (so the entropy decreases), and

the ions in the crystal dissociate (the hydrated ions are less ordered than the crystal, so the entropy increases).

Entropy

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A gas is less ordered than a liquid which is less ordered than a solid

Any process that increases the number of gas molecules leads to an increase in entropy

When NO(g) reacts with O2(g) to form

NO2(g), the total number of gas

molecules decreases, and the entropy decreases

2NO(g) + O2(g) 2NO2(g)

The Molecular Interpretation of Entropy

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Boiling corresponds to a much greater change in entropy than melting.

Entropy will increase when: Liquids or solutions are formed from

solids Gases are formed from solids or liquids The number of gas molecules increases The temperature is increased The number of atoms in a molecule

increases without change in number of moles of gas molecules

The molar mass increases

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Which of the following processes produces a decrease in the entropy of the system?

A. Boiling water to form steamB. Dissolution of solid KCl in waterC. Mixing of two gases into one

containerD. Freezing water to form iceE. Melting ice to form water

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Consider a pure crystalline solid that is heated from absolute zero to a temperature above the boiling point of the liquid. Which of the following processes produces the greatest increase in the entropy of the substance?

A. melting the solidB. heating the liquidC. heating the gasD. heating the solidE. vaporizing the liquid

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Predict whether each of the following leads to an increase or decrease in the entropy of a system.

(a) The synthesis of ammonia:N2(g) + 3 H2(g) 2 NH3(g)

(b) Preparation of a sucrose solution:

C12H22O11(s) C12H22O11(aq)

(c) Evaporation to dryness of a solution of urea, CO(NH2)2, in water:

CO(NH2)2(aq) CO(NH2)2(s)