THEME: Theoretic bases of bioenergetics. LECTURE 6 ass. prof. Yeugenia B. Dmukhalska

PLAN  Introduction of thermodynamics. Main concepts.  Internal energy; work; heat.  First law of the thermodynamics.  Enthalpy. The Hess’s law. Standard enthalpy changes.  Second law of the thermodynamics. Entropy. Gibbs’ energy.

Definition The branch of science, which deals with the study of different forms of energy and the quantitative relationships between them is known as thermodynamics.

System and Surroundings systemThe part of the universe chosen for thermodynamic consideration (to study the effect of temperature, pressure etc.) is called а system. surroundings.The remaining portion of the universe, excluding the system, is called surroundings. А system usually consists of а definite amount of one or more substances and is separated from the surroundings by а real or imaginary boundary through which matter and energy can flow from the system to the surroundings or vice versa.

Types of systems OPEN SYSTEMA system is said to be an OPEN SYSTEM if it can exchange both matter and energy with the surroundings. CLOSEDSYSTEMIf а system can exchange only energy with the surroundings but not matter, it is called а CLOSED SYSTEM. ISOLATED SYSTEMIf а system can neither exchange matter nor energy with the surroundings, it is called an ISOLATED SYSTEM.

State of а system and state variables. observableThe state of а system means the condition of the system, which is described in terms of certain observable (measurable) properties such as temperature (Т), pressure (P), volume (V) etc. of the system. variables If any of these properties of the system changes, the system is said to be in different state i.е. the state of the system changes. That is why these properties of а system are called state variables.

Properties of systems. Extensive properties. These are those properties which depend upon the quantity of the matter contained in the system. The common examples of these properties are mass, volume and heat capacity. And some other properties internal energy, enthalpy, entropy, Gibbs free energy etc.

Intensive properties. These are those properties which depend only upon the nature of the substance and are independent of the amount of the substance present in the system. The common examples of these properties are temperature, pressure, refractive index, viscosity, density, surface tension, specific heat, freezing point, boiling point, etc. It is because pressure and temperature are intensive properties, independent of the quantity of the matter present in the system that they are frequently used as variables to describe the state of а system.

Thermodynamic processes. Isothermal process. When а process is carried out in such а manner that the temperature remains constant throughout the process, it is called an isothermal process. Adiabatic process. When a process is carried out in such а manner that no heat can flow from the system to the surroundings or vice versa i.e the system is completely insulated from the surroundings, it is called an adiabatic process. Isochoric process. It is а process during which the volume of the system is kept constant. Isobaric process. It is а process during which the pressure of the system is kept constant. А reversible process is а process which is carried out infinitesimally slowly so that all changes occurring in the direct process can be exactly reversed and the system remains almost in a state of equilibrium with the surroundings at every stage of the process. On the other hand, а process which does not meet the above requirements is called an irreversible process.

INTERNAL ENERGY It is the sum of different types of energies associated with atoms and molecules such as electronic energy (Еe), nuclear energy (Еn), chemical bond energy (Еc), potential energy (Ер) and kinetic energy (Еk) which is further the sum of translational energy (Еt), vibrational energy (Еv) and rotational energy (Еr). Е = Е e + Е n + Е с + Е р + Е k or U = U e + U n + U с + U р + U k The energy thus stored within а substance (or а system) is called its internal energy and is usually denoted by the symbol “Е” or “U”.

The first law of thermodynamics is simply the law of conservation of energy which states that. Energy can neither be created nor destroyed although it may be converted from one form to another The total energy of the universe (i.e. the system and the surroundings) remains constant, although it may undergo transformation from one form to the other.

Е 2 = E 1 + q + W or (U 2 = U 1 + q + A) or Е 2 – E 1 = q + W or (U 2 – U 1 = q + A) or  Е = q + W (or  U = q + A) If W = p  V (or A = p  V ) so  Е = q – P  V (or  U = q – P  V ) or q =  Е + P  V (or q =  U + P  V) E (U)– internal energy; q – heat; W (A) – work; V – volume.

Enthalpy If а process is carried out at constant pressure: W = – P  V; q =  E – W; q p =  E – P  V;  E = E 2 – E 1 ;  V = V 2 – V 1 ; q p = (Е 2 – E 1 ) + Р(V 2 – V 1 ) or q p = (Е 2 + РV 2 ) – (E 1 + РV 1 ) Н=Е + PV; Н 2 = Е 2 + PV 2 ; H 1 = E 1 + PV 1 q p = Н 2 – H 1 or q p =  Н  Н = Н 2 – H 1 is the enthalpy change of the system.  Н =  E + P  V

Hess's law The total amount of heat evolved or absorbed in a reaction depends only upon the nature of the initial reactants and that of the final products and does not depend upon the path by which this change is brought about. In other words, the total amount of heat evolved or absorbed in a reaction is same whether the reaction takes place in one step or a number of steps.

CALORIMETER

Enthalpy of reaction is defined as the amount of heat evolved or absorbed when the number of moles of the reactants as represented by the balanced equation have completely reacted. Its value depends upon the conditions of temperature and pressure. Standard enthalpy change (  Н 0 ) – the enthalpy change are reported under standard conditions which are 1 atm pressure and 298 К. The standard enthalpy of formationThe standard enthalpy of formation (  Н 0 f ) is defined as the enthalpy change that takes place when one mole of the substance under standard conditions is formed from its constituent elements in their most stable form and in their standard state. The enthalpy of formation of any element in the standard state is taken as 'zero'.  Н 0 (formation) = 0 enthalpy of combustionThe enthalpy of combustion of a substance is defined as the amount of heat evolved when 1 mole of the substance is completely burnt or oxidized.

Calculation of enthalpies of reactions.  Н=(Sum of the standard enthalpies of formation of products) – (Sum of the standard enthalpies of formation of reactants)  Н reaction =  Н 0 (Products) –  Н 0 (Reactants);

Bond energy usually means bond dissociation energy.  Н reaction =   Н 0 (Products) –   Н 0 (Reactants)  Н reaction =   Н 0 (Bond Energies of Reactants reaction) -   Н 0 (Bond Energies of Products)

Spontaneous and non-spontaneous processes Spontaneous processes:Spontaneous processes: (i) Tendency for minimum energy (ii) Tendency for maximum randomness. non-spontaneous processes:non-spontaneous processes: Tendency for maximum energy Tendency for minimum randomness.

EntropyEntropy is a measure of randomness or disorder of the system  G =  H – T  S - second law THIRD LAW OF THERMODYNAMICS: The entropy of all perfectly crystalline solids may be taken as zero at the absolute zero of temperature.