2 First principle of TD

Question 1

Thermodynamics. First Principle of Thermodynamics. Enthalpy.

Answer 1

Basic thermodynamic terms.
First principle of thermodynamics.
Enthalpy.
Hess’ law.
Work in the human body.

Question 2

2 Thermodynamics

Answer 2

2 Developed during the 1800’s to explain how
engines (particularly steam engines)
converted heat into work.
• The first steam engines were very, very
inefficient, converting perhaps 2 % of their
fuel into useful work.
• The word “Thermodynamics” was coined by
Lord Kelvin in 1849.

Question 3

3 Subject Of Thermodynamics

Answer 3

3 Thermodynamics - studies the conversions of
energy exchanged between systems as heat and
work
Biological thermodynamics - studies the general
laws of energy conversion in living systems

Question 4

4 Thermodynamic Bodies And Systems

Answer 4

4 • Thermodynamic body - a very large ensemble of particles
• Thermodynamic system - a set of thermodynamic bodies exchanging energy
Characteristics of thermodynamic bodies and systems:
volume, pressure, temperature, energy, etc.

Question 5

5 Types Of Thermodynamic Systems

Answer 5

5 • lsolated - do not exchange matter and energy with the environment
• Closed - exchange energy but do not exchange matter
• Open - exchange both matter and energy with the environment

Question 6

6 Thermodynamic Variables

Answer 6

6 Thermodynamic variables - Properties characterizing the thermodynamic systems
Examples: mass, volume, pressure, etc.

Question 7

7 Extensive Variables

Answer 7

7 Chraracterize the system as a whole (directly proportional to the amount of substance in the system)
• An extensive variable can be calculated as the sum of the values of the variable for the separate subsystems that compose the system
Examples: mass, volume, energy, enthalpy, entropy, etc

Question 8

8 Intensive Variables

Answer 8

8 May have different values at different points in the system
• Do not depend on the amount of substance in the system
• Can not be summed
Examples: temperature, density, etc.

Question 9

9 Thermodynamic State

Answer 9

9 • Thermodynamic state - the set of instantaneous values of the thermodynamic variables of a system
• The state changes if any of the variables changes its value

Question 10

10 Types Of Thermodynamic States

Answer 10

10 • Equilibrium state – all the thermodynamic variables of the system remain constant over time and have the
same values at all points in the system
• Non-equilibrium state - the variables have different values at different points in the system and change over time

Question 11

11 Equation Of State

Answer 11

11 • An equilibrium state is determined by the pressure (p), volume (V), temperature (T) and amount of substance of the system (n).
• Equation of state - a relationship between the variables of the equilibrium state, describing the properties of the system: f(p, V, T, n) = 0

Question 12

12 Thermodynamic States – Graphic Representation

Answer 12

12 [graph]
Equilibrium states can be represented by a point
on a pressure-volume diagram (pV-diagram).
Non-equilibrium states can not be represented
on a pV-diagram.

Question 13

13 Thermodynamic Process

Answer 13

13 • Thermodynamic Process:
The transition of a thermodynamic system from one state to another
• Types of thermodynamic processes:
—Equilibrium (quasistatic) process – the system is always in equilibrium state (the process runs infinitely slow)
—Non-equilibrium process – the system passes through non-equilibrium states (the process runs with finite speed)

Question 14

14 Thermodynamic Process – Graphic Representation

Answer 14

14 [graph]
Equilibrium processes can be represented by a curve
on a pV-diagram.
Non-equilibrium processes can not be represented on
a pV-diagram

Question 15

15 Thermodynamic Potential (State Function)

Answer 15

15 Variable of a thermodynamic system whose changes
depend on the initial and final states only, and do not
depend on the path taken by the process

Question 16

16 lnternal Energy

Answer 16

16 lnternal energy is a thermodynamic potential.
• The total potential energy associated with the inter-atomic and inter-molecular forces and the kinetic energy of the thermal motion of the atoms and molecules in the
thermodynamic system.
• Does not depend on the position and movement of the system as a whole.

Question 17

17 Work

Answer 17

17 Work is a form of energy transfer which can described by changes of macroscopic system variables
Example: a gas can perform work when expanding (volume change) against an external force

Question 18

18 Heat

Answer 18

18 Heat is a form of energy transfer by changing the
microscopic thermal motions of particles
Example: when a hot and cold body are in contact,
their molecules exchange energy

Question 19

19 Energy Changes

Answer 19

19 Performing work changes the kinetic, potential or internal energy of a thermodynamic system
• Heat exchange alters only the internal energy

Question 20

20 First Principle Of Thermodynamics

Answer 20

20 The amount of heat δQ transferred to a thermodynamic system is used to increase the internal energy of the system by dU and to perform work δA on the environment:
δQ = dU + δA
This is essentially a law about energy conservation in thermodynamics.

Question 21

21 Enthalpy

Answer 21

21 When a system changes its volume by dV at constant pressure p, the work performed by the system is:
δA = p.dV
When a system changes its volume at constant pressure, the quantity Q is a thermodynamic
potential - enthalpy (H):
δQ = dU + p.dV = dH