Chemical Thermodynamics

Chemical Thermodynamics

Therme = HeatDynamikos = work

Thermodynamics = flow of heat

THERMODYNAMICSThermodynamics is a branch of science that deals with the study of inter conversion of heat with other forms of energy during physical and chemical process

HEAT LIGHTExample?

Electric Energy HeatExample?

Thermodynamic terms

System

System

SystemIt is a specified portion of the universe which is under thermodynamic study and which is separated from the rest of the universe with a definite boundary.

Eg.?

SurroundingIt is the portion of the universe excluding the system and capable of exchanging matter and energy with the system

Eg.?

Surrounding

BoundaryThe real or imaginary surface that separates the system from the surrounding is called boundary

Types of system

1.Open system2.Closed system3.Isolated system

Open system

A system which can exchange both matter and energy with the surroundings.

Open system

Closed system

Closed system

A system which can exchange energy but not matter with the surroundings.

Isolated system

A system which cannot exchange both energy and matter with the surroundings.

Isolated system

State of a systemIt is the condition of the system

expressed by giving definite values for its properties such as temperature, pressure, volume etc.

Hydrogen gas

P1 V1 T1

STATE -1

Hydrogen gas

P2 V2 T2

STATE -2

State functionsThe thermodynamic properties whose values depend only on the initial and final state of the system and are independent of the manner as to how the changes is brought about .

Eg. Pressure, temperature , volume, internal energy, enthalpy, entropy

Analogy

Height = h

Height h of a mountain is independent of the path followed in reaching the top of the mountain. h is similar to a state function

Path functions

What ?Example?

Path functionsCommon path functions

1. Work

2. Heat

Work as Path functionsWork = force x displacement

The definition of work indicates that work depends on its path it takes, because the movement of an object is dependent upon the path taken to execute that movement.

Eg. Work done by a person for climbing stairs is different from using a lift.

Heat as Path functionsFor instance, if a gas expands isothermally, then heat has to be supplied to the system so that the gas maintains its temperature as it expands. But if you do this adiabatically, then the system does work. Same final state (pressure and volume) but different work and heat.

Thermodynamic processThe operation which brings about the change in the state of the system .

Thermodynamic process1. Isothermal process: A process which is carried out at constant temperature. ∆T = 0

2. Isobaric: A process which is carried out at constant pressure. ∆P = 0

3.Isochoric : A process which is carried out at constant volume. ∆V = 0

4. Adiabatic: A process in which there is no heat exchange occurs between system and surrounding ∆q = 0

Pressure

Volume

Isoc

horic

Adiabatic

Isobaric

Isothermal

For a given amount of ideal gas P – V relation

Reversible processIt is a process which is carried out infinitely slowly through a series of steps so that system and surroundings always remain almost in equilibrium state. The process is conducted in such a manner that any moment it could be reversed by a infinitesimal change.

Reversible process

Gas V1

Gas V2

Remove one particle of sand each time

Reversible expansion process involves infinite

number of steps.

Sand

It is a process which is carried out infinitely slowly through a series of steps so that system and surroundings always remain almost in equilibrium state. The process is conducted in such a manner that any moment it could be reversed by a infinitesimal change.

Irreversible processA process which is carried out rapidly so that the system does not get a chance to attain equilibrium.

Cyclic processA process during which the system undergoes a series of changes and return to its initial state.

A (P 1, V1, T1 )

D (P 4, V4, T4 ) B (P 2, V2, T2 )

C (P 3, V3, T3 )

Properties of the systema. Intensive propertyProperty of a system which does not

depend upon the quantity of substance present in the system.

Eg. Density, temperature, refractive index, viscosity, pressure ,surface tension, specific heat, freezing point, boiling point, melting point, emf, pH, mole fraction, molarity etc.

Intensive is independent of quantity

Properties of the systemb. Extensive property.Property of a system which depends upon

the quantity of substance present in the system.

Eg. Mass , volume, energy, enthalpy, internal energy etc.

HEAT Form of energy How can we feel it?

From the change in temperature

Heat is the amount of energy transferred between the system and the surrounding when they are at different temperatures.

International conventions

Symbol of heat = qHeat absorbed by the system = +q Example?Heat liberated by the system = -q

Example?

Other method of exchange of energy between system and surrounding

WORK

1. Mechanical work2. Electrical work3. Pressure volume work

Pressure volume work

It is also called expansion work. It is significant in system which consists of gases and involve change in volume against external pressure

Gas V1

Gas V2

Gas V1

Gas V2

International conventions

work done on the system = + w

Com

pres

sion

work done by the system = - w

Expa

nsio

n

Fire

Internal energyIt is the energy possessed by the system

due to its nature, chemical composition and thermodynamic state.

Internal energyCharacteristics:

1. It is the sum of translational E + rotational E + vibration E + Bond E

2. It depends on mass of system3. It depends on state of system4. It is indicated by U5. The absolute value of internal energy

cannot be measured.6. Change in internal energy of a system

can be measured7. ∆U = U2 –U1

Internal energy of a system may change when:

1. Heat passes into or out of the system

2. Work is done on or by the system

3. Matter enters or leaves the system

Change in internal energy in an adiabatic system

How?1. By rotating a small paddle inside2. By heating with a immersion heater

STATE 1 (Before the work)Temperature = T1

Internal energy = U1

STATE 2 (After the work)Temperature = T2

Internal energy = U2

Change in internal energy ∆U = U2 –U1

Change in temperature ∆T = T2 – T1

Change in internal energy in terms of work

∆U = U2 –U1 = Wad

Change in internal energy due to heat transfer

Change in internal energy ∆U = U2 –U1

Change in temperature ∆T = T2 – T1

Change in internal energy in terms of heat

∆U = U2 –U1 = q

Change in internal energy in terms of both adiabatic work and heat transfer

∆U = U2 –U1 = q + w

Mathematical expression for 1st law of thermodynamics

When q = 0 and w = 0 ( a state possible in an isolated system)

∆U = 0

Statement of 1st law of thermodynamicsThe energy of an isolated system in constant

First law of thermodynamicsEnergy can neither be created nor be destroyed but can be transformed from one form to another

Example?

1.Work done in an isothermal reversible compression of an ideal gas

w = - 2.303 n RT log (Vf / Vi)

w = - 2.303 n RT log (P1 / P2)

n = number of moles of the gasR = universal gas constant = 8.314 J/K/molT = absolute temperature = (t oC + 273) K

PROOF

2.Work done during free expansionW= 0

3.Work done during irreversible process.

W= -p∆V

Different equations for 1st law of thermodynamics1.A process carried out at constant volume

∆U = qv

2.Isothermal processq = -w

3.Isothermal reversible processq = 2.303nRTlog (Vf/Vi)

4.Isothermal irreversible processq = Pex(Vf-Vi)

5.Adiabatic process∆U= Wad

ENTHALPY (H)∆H = ∆U +P∆V

1. Change in enthalpy is the sum of internal energy change and the pressure volume work in a system

2.Change in enthalpy is the heat absorbed by the system at constant pressure.

∆H = qp

ENTHALPY (H)

1.It is an extensive property2.It is a state function3.Its unit is Joule

∆H = ∆U +∆ngRTFor a gaseous reaction

Where ∆ng = (Number of moles of gaseous products – number of moles of gaseous reactants)

For Exothermic process ∆ H = -VeFor endothermic process ∆ H = +Ve

Sign of Enthalpy ?