enthalpy, dh entropy, ds gibbs free energy,...

Chemical

Thermodynamics

UNIT 15: THERMODYNAMICS

• ENTHALPY, DH

• ENTROPY, DS

• GIBBS FREE ENERGY, DG

Chemical

Thermodynamics

ENTHALPY, DH

Chemical

Thermodynamics

Heat is the transfer of thermal energy between two bodies that

are at different temperatures.

Energy Changes in Chemical Reactions

Temperature is a measure of the thermal energy.

Temperature = Thermal Energy

900C400C

greater thermal energy

Chemical

Thermodynamics

Exothermic process is any process that gives off heat –

transfers thermal energy from the system to the surroundings.

Endothermic process is any process in which heat has to be

supplied to the system from the surroundings.

2H2 (g) + O2 (g) 2H2O (l) + energy

H2O (g) H2O (l) + energy

energy + 2HgO (s) 2Hg (l) + O2 (g)

energy + H2O (s) H2O (l)

Chemical

Thermodynamics

Enthalpy (H) is used to quantify the heat flow into or out of a

system in a process that occurs at constant pressure.

DH

DH = heat given off or absorbed during a reaction at constant pressure

Hproducts < Hreactants

DH < 0

Hproducts > Hreactants

DH > 0

Chemical

Thermodynamics

Thermochemical Equations

H2O (s) H2O (l) DH = 6.01 kJ

Is DH negative or positive?

System absorbs heat

Endothermic

DH > 0

6.01 kJ are absorbed for every 1 mole of ice that

melts at 00C and 1 atm.

Chemical

Thermodynamics


CH4 (g) + 2O2 (g) CO2 (g) + 2H2O (l) DH = -890.4 kJ

Is DH negative or positive?

System gives off heat

Exothermic

DH < 0

890.4 kJ are released for every 1 mole of methane

that is combusted at 250C and 1 atm.

Chemical

Thermodynamics

H2O (s) H2O (l) DH = 6.01 kJ

• The stoichiometric coefficients always refer to the number

of moles of a substance


• If you reverse a reaction, the sign of DH changes

H2O (l) H2O (s) DH = -6.01 kJ

Chemical

Thermodynamics

ENTROPY, S

Chemical

Thermodynamics

First Law of Thermodynamics

• You will recall that energy cannot be created nor destroyed.

• Therefore, the total energy of the universe is a constant.

• Energy can, however, be converted from one form to another or transferred from a system to the surroundings or vice versa.

Chemical

Thermodynamics

Spontaneous Processes

Processes that are

spontaneous in one

direction are

nonspontaneous in the

reverse direction.

Chemical

Thermodynamics

Spontaneous Processes

• Processes that are spontaneous at one

temperature may be nonspontaneous at other

temperatures.

• Above 0C it is spontaneous for ice to melt.

• Below 0C the reverse process is spontaneous.

Chemical

Thermodynamics

Reversible ProcessesIn a reversible process

the system changes in such a way that the system and surroundings can be put back in their original states by exactly reversing the process.

Changes are infinitesimally small in a reversible process.

Chemical

Thermodynamics

Entropy

• Entropy (DS) is a term coined by

Rudolph Clausius in the 19th century.

• Clausius was convinced of the

significance of the ratio of heat

delivered and the temperature at which

it is delivered, q

T

Chemical

Thermodynamics

Entropy

• Entropy can be thought of as a measure

of the randomness of a system.

• It is related to the various modes of

motion in molecules.

Chemical

Thermodynamics

Entropy

• For a process occurring at constant

temperature (an isothermal process):

qrev = the heat that is transferred when the

process is carried out reversibly at a constant

temperature.

T = temperature in Kelvin.

Chemical

Thermodynamics

Entropy on the Molecular ScaleImplications:

• more particles

-> more states -> more entropy

• higher T

-> more energy states -> more entropy

• less structure (gas vs solid)

-> more states -> more entropy

Chemical

Thermodynamics

Entropy on the Molecular Scale

• The entropy tends to increase with

increases in

Temperature.

Volume (gases).

The number of independently moving

molecules.

Chemical

Thermodynamics

Entropy Changes

• In general, entropy

increases when

Gases are formed from

liquids and solids.

Liquids or solutions are

formed from solids.

The number of gas

molecules increases.

The number of moles

increases.

Chemical

Thermodynamics

The Concept of Entropy (S)

Entropy refers to the state of order.

A change in order is a change in the number of ways of arranging the

particles.

solid liquid gas

more order less order

crystal + liquid ions in solution



crystal + crystal gases + ions in solution

Chemical

Thermodynamics

GIBBS FREE ENERGY

Chemical

Thermodynamics

Gibbs Free Energy

Very key equation:

This equation shows how DG changes with

temperature.

(We assume S° & DH° are independent of T.)

Chemical

Thermodynamics

Gibbs Free Energy

1. If DG is negative, the

forward reaction is

spontaneous.

2. If DG is 0, the system

is at equilibrium.

3. If DG is positive, the

reaction is spontaneous

in the reverse direction.

Chemical

Thermodynamics

Free Energy and Temperature

• There are two parts to the free energy

equation:

DH— the enthalpy term

TDS — the entropy term

• The temperature dependence of free

energy comes from the entropy term.

Chemical

Thermodynamics

Free Energy and Temperature

By knowing the sign (+ or -) of DS and DH,

we can get the sign of DG and determine if a

reaction is spontaneous.

enthalpy, dh entropy, ds gibbs free energy,...

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