Thermodynamics Lecture

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Physical chemistry for Textile

6. Thermodynamics

L10: State of system, First law

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System and Surrounding

Any part of the world which is under our observation is called a system and whatever around it is called surrounding. The system has a definite boundary which separates it from the surrounding.


A bird is a system which is separated from its surrounding by its skin and fethers.

It exchanges both energy and matter with its environment.

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If earth is considered as a system, everything around its surrounding.

It exchanges energy with the environment but not matter.

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A flask containing some material can also be considered as a system where the glass body of flask is system boundary.

Everything outside the flask is it’s surrounding.

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Sometimes the system may not have a definite boundary. For example: a cube of dry ice. The dry ice diffuses into the air without melting at a temperature of -78.5 °C.

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Similarly, naphthalene converts directly from solid to vapor phase.

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What is the phenomenon by which a solid converts directly into vapor without liquifaction?

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Energy and or matter may or may not exchange between system and surrounding depending on the type of system boundary.

Definitions of System

Depending on the properties of system boundary the systems may be divided in three different kinds.

Open systemClosed systemIsolated system

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Open system Open system is the

system in which both energy and matter can move across the boundary of system.

Winch dyeing

In an open winch the material can move in or out of it and energy too.

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Closed system

Closed system is that in which only energy can cross the boundary of system but matter cannot.

Closed system

The boundary of a closed system may expand.

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Jet dyeing

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Jet is a closed machine with high pressure and temperature. Here no material flows in or out of the machine but heat can flow in and out.

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Isolated system Isolated system the

system in which neither matter nor energy can cross the system boundary.

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Comparison of systems

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State of a system

State of a system is the condition of the system defined by certain set of properties. For instance the volume, pressure and temperature of a system defines a certain state of that system. If for example the pressure on a system is reduced, the volume will increase and the system is said to be in a new state.

Similarly if the temperature of the system is increased the volume will increase and the state of system will change.

When the set of properties defining the state of a system are known, the system is said to be in a definite state.

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Properties of state of system

The properties which determine the state of a system are called state properties.

State properties such as temperature, pressure and volume have major significance. However there are other state properties also such as it’s composition.

If the pressure, temperature , volume or other state properties of a system are known then the system is said to be in a definite state.

Properties of state of system

A dyeing system consisting of dye bath containing 10 kg of aqueous dye solution and 1 kg fabric at 25 °C at atmospheric pressure is in a definite state.

We raise the temperature to 60 °C and the system changes to a new state.

When the dye transfers to fabric that is reactants are consumed and products are formed, again the state of system changes.

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Law of conservation of energy

This law states that the total energy in the universe is conserved. That is, it cannot be created or destroyed, however, it can change from form to the other.

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Internal energy

The internal energy of a system is the sum of all types of energy in it.

When a system passes from one state to another state, there is a corresponding change in the internal energy of the system.

ΔE = E2 – E1 E is called internal energy and

ΔE is the change in internal energy.

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Internal energy A system containing calcium oxide (CaO) and water

(H2O) has some internal energy in the form of chemical energy at the beginning of reaction. As the reaction proceeds and reactants change into Ca(OH)2, a large amount of heat energy is evolved. As a result the internal energy of the system reduces. The change in energy can be calculated simply by subtracting the initial

state from final state.

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CaO + H2O = Ca(OH)2 + heat

Internal energy – state function State functions are those properties of a

system which depend only on the state of the system and not on the path they follow to reach the final state.

Since internal energy is a state function, hence depends only on the state of the system. Therefore any change in the internal energy depends only on the initial and final states of the system and is represented by:

ΔE = E2 – E1

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Internal energy – state function

For instance, we consider the following example: If we have two identical containers having equal amounts of water in them.

We heat one of the containers to raise it’s temperature through 1˚C and simply stir the water in other container so that it’s temperature rises through 1˚C.

At the end, the change in both systems is exactly same and independent of the fact how the change was brought up.

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Internal energy

This change in internal energy is independent of the path of change and depends on the state of the system.

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Internal energy

Therefore the internal energy of a system depends on the state of the system and not on the path or the way how it was brought about, that is:

ΔE = E2 – E1 The properties of a system which depend

on the state of the system and not on the path are called state functions.

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Path dependence - Work Those properties of a system which

vary with the path taken to change the state of system are path functions.

The work, for example, vary with the path taken to reach the final state

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Change of state When a

system passes from one state to the other state, it may lose or gain energy as heat or work.

CaO + H2O = Ca(OH)2 + (-ΔH)

In the above instance the heat content of product is less than the reactants.

NaNO2 + H2O = Na++NO-2 +

(+ΔH)

In this example the heat content of product is less than the reactants. 28

Change of state Consider a gas confined in

a cylinder with a movable piston. Now some heat is transferred from surrounding to the system. This absorption of heat energy may trigger two processes: 1. It may increase the

internal energy of the system.

2. Some of the heat absorbed may be consumed in doing P ΔV work against external pressure.

q = ΔE + w29

Change of state

A dyeing bath is heated from room temperature to 100ºC. What changes will occur in the final state if it is an open system?

What if the temperature of dyeing bath is raised to 30ºC only?

q = ΔE + w

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Change of state

In chemical reactions if some gaseous product is formed or gaseous reactants change to liquid product work is done by the system or on the system.

HCl + Zn = ZnCl2 + H2

HCl + H2O = H3O+

+Cl-

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Change of state

Condensation of water vapor?

Changing of water into steam?

Work done on or by

the system?

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Relationship between internal energy, heat and work The change in internal energy is given by

the equation: q = ΔE + w ΔE = q – w Or = q – P ΔV

There are certain conditions under which heat and work terms become definite: Constant pressure and Constant volume

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Enthalpy The term E + PV is called enthalpy or

heat content of the system and represented by H, therefore:

H = E+PV and

qp = H2 - H1 = ΔH

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The First law The first law of thermodynamics is

simply the statement of law of conservation of energy, that is: The energy cannot be created or destroyed.

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Specific heat

If a system consists of 1g of a substance and a definite amount of heat is transferred to it so that it’s temperature rises by 1°C, the amount of heat thus transferred is called it’s specific heat.

The specific heat or specific heat capacity of a substance is the amount of heat required to raise the temperature of 1 gram of a substance by 1°C.

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Heat capacity

Heat capacity of a system is defined as the amount of heat required to raise the temperature of the system by 1˚.

The heat capacity of a system is given by the following equation:

C = specific heat x mass (g)

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Heat capacity

For e.g. if the temperature of 1000 kg of water is to be raised by 1°C, the amount of heat absorbed will be:

C = specific heat x mass (g)

= 1 cal/g °C x 1000 Kg x 1000

= 1000,000 cal/°C

= 1000 kcal/°C

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Molar heat capacity

If the system consists of 1 mole of a substance it is called molar heat capacity.

The molar heat capacity is the amount of heat required to raise the temperature of 1 mole of a substance through 1°.

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Molar heat capacity

Find the mole of each of following.

Oxygen = ? Carbon dioxide = ? Methanol = ? Mole of ethanol = ? Mole of glycerine = ? Water = ? Copper = ? Iron = ?

Calculate the molar heat capacity of each of these from specific heat data.

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Heat capacity Since the heat capacity C may vary with

temperature, it is written in the form: C = q/dT

Here q is an infinitesimally small amount of heat absorbed by the system when the temperature is raised by dT degrees.

Since q is an indefinite quantity unless certain conditions are defined, hence, it is normally defined at constant pressure or constant volume.

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Heat capacity at constant pressure - Cp

Heat capacity at constant pressure is equal to rate of change of the energy of the system with temperature at constant pressure.

Cp = qp/dT

qp = ΔH

Cp = dH/dT

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Heat capacity at constant volume - Cv

Heat capacity at constant volume is equal to rate of change of the energy of the system with temperature at constant volume.

Cv = qv/dT

qv = ΔE

Cv = dE/dT

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Difference of Cv and Cp

In case of Cp, some of the energy absorbed will be consumed in doing work against the surrounding and rest of it will raise the internal energy.

However in case of Cv, no work is done by the system.

Hence for a similar system the energy required in case of Cp will be greater than in Cv.

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Importance of thermal data

We can calculate the amount of heat required to gain the required temperature of system of definite properties.

We can calculate the cost of heating.

How the heat of dyeing can be calculated?How the energy cost of dyeing can be calculated?

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Different materials have different specific heat capacity. Hence two materials having different specific heat capacities on absorbing the same amount under the same condition achieve different temperatures.

Observe temperatures of different objects placed in sunlight.

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WoodIron body of a carWaterA plastic bottleWoolCottonpolyester


If cotton and polyester fabrics are placed under sun under the same conditions, why the temperature of polyester rises above that of cotton.

Why it happens so?

This is one of the reasons why wearing pure polyester in hot weather is uncomfortable while cotton is more comfortable.

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Problem 1 Determine the heat capacity of 100 kg

of water in kJ/kg-K. The specific heat of water is 1 cal/g. The conversion factor to convert cal to Joule is 4.184.

What difference in Cp and Cv for the above system do you expect.

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Problem 2 If a piece of wood and that of iron of

same mass are put in environment under the same condition, iron will gain higher temperature than wood. What is the reason of this phenomenon.

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Problem 3 How many calories are required to

heat each of the following from 15 to 65 ˚C:550 gram of water1 kg of Polyester1 kg of Nylon1 kg of cotton

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Which system?

A classroom? Open Closed isolated

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Which system?

An open glass of water kept on table?

Open Closed isolated

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Which system?

A pressure cooker with food inside?


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Which system?

A water distillation unit?


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Which system?

The earth with its atmosphere?


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Which system?

A volcano with magma inside it?


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Which system?

A pod of peas? Open Closed isolated

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Which system?

A tomato? Open Closed isolated

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Which system?

A thermos? Open Closed isolated

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Thermodynamics Lecture

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Transcript of Thermodynamics Lecture