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Thermochemistry. Second law of thermodynamics

PlanPlan1.1. The subject of the The subject of the

thermochemistrythermochemistry. . Thermal effect of Thermal effect of the chemical reaction.the chemical reaction.

2.2. The Hess law and its conclusions.The Hess law and its conclusions.3.3. Second low of thermodynamicsSecond low of thermodynamics..4.4. Concepts about the entropy.Concepts about the entropy.

Assistant Kozachok S.S. prepared

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Bomb calorimeter for the determination of change in Bomb calorimeter for the determination of change in internal energyinternal energy

The process is carried out at constant volume, i.e., ΔV=0, then the product PΔV is also zero.Thus, ΔU=QvThe subscript v in Qv denotes that volume is kept constant.Thus, the change in internal energy is equal to heat absorbed or evolved at constant temperature and constant volume

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Bomb calorimeter is made up of heavy steel. The steel vessel coated inside with gold or platinum. It is fitted with a pressure tight screw cap. The two electrodes R1 and R2 are connected to each other through a platinum wire R dipping in the platinum cup C.A small known amount of the substance under investigation is taken in the platinum cup which is supported on the rod R2.The bomb is filled with excess of oxygen under the pressure of 20-25 atmospheres and is sealed. The water bath is also provided with a mechanical stirrer and a Beckmann thermometer which can read correctly upto 0,01 degree.The initial temperature of water is noted and reaction , i.e. combustion is started by passing an electric current through the platinum wire. The heat energy evolved during the chemical reaction raises the temperature of water which is carefully recorded from the thermometer.By knowing the rise in temperature and the heat capacity of the calorimeter, the amount of heat evolved in the reaction can be calculated

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Specific heat capacity (often shortened to specific heat) is the measure of the heat

energy required to increase the temperature of a unit quantity of a substance by a unit of

temperature. For example, at a temperature of 15 Celsius, the heat energy required to raise water’s temperature one kelvin (equal to one

degree Celsius) is 4186 joules per kilogram. So this measure would be expressed as

4186 J/(kg·K) or 1000 cal/(kg·K) 1 calories = 4,184 joules

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Kirchhoff equation

dT

dHC p

dT

dUCV

Mayer equation ( for mol of ideal gas)

RdT

pdVCC Vp

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ThermochemistryThe study of the energy transferred as heat

during the course of chemical reactions.

Thermochemical reactions:

H2(g) + Cl2(g) = 2HCl; ▲ H = -184,6 kJ

1/2 H2(g) + 1/2 Cl2(g) = HCl; ▲ H = -92,3 kJ/mol

▲ H is calculated for 1 mole of product▲H = ▲U + p▲V▲H = ▲U + ▲nRTEnergy change at constant P = Energy change at constant V + Change in the number of geseous moles * RT

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Correlation U і Н:

RTUVpUH

If υ0, то НU: СаО + СО2 → СаСО3

If υ0, то НU: Na + H2O → NaOH + H2

If υ=0, то Н=U: H2 + Cl2 → 2HCl

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Calculation of standard enthalpies of reactions

▲ H = ( Sum of the standard enthalpies of formation of products including their stoichiometric coefficients ) – ( Sum of the standard enthalpies of formation of reactants including their stoichiometric coefficients )

For elementary substances Н0298 = 0

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If the volume or pressure are constant the total amount of evolved or absorbed heat depends only on the nature of the initial

reactants and the final products and doesn’t depend on the passing way of reaction.

The Hess’s law

Н1

Н2 Н3

Н4

Init

ial r

eact

ans T

he p

rod

ucts

of reactio

nНН11 = = НН22 + + НН3 3 + + НН44

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Conclusions from the Hess lawConclusions from the Hess law1. Нc

298(the standard enthalpy of combustion) =-Нf

298(the standard enthalpy of formation)

2. Н = ΣnНf298(prod.) - ΣnНf

298(reactants)

3. Н = ΣnНс298(reactants) - ΣnНс

298(prod.)

4.Н3=Н1-Н21

2

3

5.Н1=Н3-Н21

2 3

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The standard enthalpy of formation ▲ H2980

is defined as the enthalpy change that takes place when one mole of the product is formed under standart condition.

C + O2 = CO2 ▲ Hf1 - ?

C + ½ O2 = CO ▲ Hf2

CO + ½ O2 = CO2 ▲ Hf3

▲ Hf1 = ▲ Hf2 + ▲ Hf3

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The extent of disorder or randomness in a system may be expressed by a property known as entropy.

Entropy can be defined asthe property of a system which measures the degree

of disorder or randomness in the system

Units of entropy joules per mol or calories per degree

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Conformities to the law of heat formation of mattersConformities to the law of heat formation of matters

fH 298 (simple materials) = 0

2

)(3

1)(

)(3

2

AlClHNaClHMgClH

ff

f

fH 298- (organic compounds):c

NOHCcH

cC

f HHHNOHCH2562

5,26)( 252

-Berkenheym’s rule of “stars”: heat formation of substances is subjected to the rule of “Stars” by the periodic table (Mg situates between Na and Al that’s why we can use this rule

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Second law of thermodynamics.1) Heat cannot be transferred from one body to a second

body at a higher temperature without producing some other effect.

2) The entropy of a closed system increases with time3) The entropy of the universe always increases in the

course of every spontaneous (natural) change

The second law of thermodynamics introduces the concept of entropy and its relation with spontaneous processesIn an isolated system such as mixing of gases, there is no exchange of energy or matter between the systemAnd the surroundings. But due to increase in randomness, there is increase entropy ΔS›0

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However, if the system is not isolated, we have to take into account the entropy changes of the system and the surroundings.

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Calculation of entropy for the phase passing from the ice to the

vapour state at heating.

Where Where ССрр’’ і і ССрр’’’’ – – molar heat (capacity)molar heat (capacity) of water of water

at the isobaric processat the isobaric process

...'''fusion

1

boiling

boiling

fusionТ

H

T

dTС

T

H

T

dTCS boiling

Т

Т

р

fusion

fusionT

T

p

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Free energy and free energy change

The maximum amount of energy available to a system during a process that can be converted into useful work

It’s denoted by symbol G and is given by

▲G = ▲H - T ▲S where ▲G is the change of Gibbs energy (free energy)

This equation is called Gibbs equation and is very useful in predicting the spontaneity of a process.

N.B. Gibbs equation exists at constant temperature and pressure

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▲ F = ▲ U – T▲S where ▲F is the change of Helmholtz energy

N.B. Helmholtz equation exists at constant temperature and volume

ΔG would be negative under the following condition:

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Effect of Temperature on Feasibility of a Processa)Exothermic reactionsFor exothermic reactions ΔH is always negative, and

therefore, it is favourable.b) Endothermic reactionsFor exothermic reactions ΔH is positive and always

opposes the process.

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1) Spontaneous (irreversible) process :

▲ G < 0, ▲S > 0, ▲H < 0

2) Unspontaneous (reversible) process :

▲ G > 0, ▲S < 0, ▲H > 0

3) Equilibrium state

▲ G = 0

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Third law of Thermodynamics

The third law of thermodynamics is a statistical law of nature regarding entropy and the impossibility of reaching absolute zero of temperature. The most common enunciation of third law of thermodynamics is:

“As a system approaches absolute zero, all processes cease and the entropy of the system approaches a minimum value.”Note that the minimum value is not necessarily zero, although it is almost always zero in a perfect, pure crystal.

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The third law helps to calculate the absolute entropies of pure substances at different temperatures. The entropy (S) of the substance at different temperatures T may be calculated by the measurement of heat capacity changes

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