Thermodynamics

Thermodynamics
Content

• Terminology related with thermodynamics
• First law of thermodynamics
• Enthalpy
• Heat capacity
• Spontaneous process
• Gibbs free energy

• A branch of chemistry which deals with the heat changes in the chemical reactions is known as thermodynamics.
• The part of the universe in which observations are made is called a system in thermodynamics and the remaining universe constitutes the surroundings. Hence the system and surrounding together constitutes the universe.
• There are three types of system:
  1. Open system is one in which transfer of energy and matter takes place between the system and surrounding.
  2. Closed system is one in which only the transfer of energy takes place between the system and surrounding.
  3. Isolated system is one which does not exchange matter and energy with the surrounding.
  4. State functions are those thermodynamic functions which depends only on the initial and final state of system. e.g.internal energy (U), Enthalpy (H), etc.
  5. Extensive variable are mass dependent variables depends on the size of the systems. e.g. H, U, V, etc.
  6. Intensive variables are mass independent variables & independent on the size of the system. Examples temperature, pressure, etc.
  • Path functions depends only on the path followed/taken by a process. Heat (q) and work (w) are path dependent functions Work is a mode of energy transfer to or from a system with reference to the surroundings.
    
    Note: Electrical work W = -nFE⁰
    where n = number of moles of electron involved in redox reaction, F = faraday constant, E⁰ = cell potential.
  
  
  • Heat may be defined as the amount of energy that flows between system and surroundings due to difference in temperature Q = m x c x t, where m = mass of the substance, c = specific heat, Dt = temperature difference.
  • The Q is positive, when heat is transferred from the surrounding to the system and Q is negative when heat is transferred from system to the surroundings.
  • Internal energy (U) It is the sum of total kinetic, potential and rotational energy etc. These are called the components of internal energy.
    
    U = U_trans + U_rot + U_vib + U_bonding + U_electronic
  • First law of thermodynamics states that the change in internal energy (U) is equal to the sum of the work done on the system and heat supplied to the system
    
    U = q + w but w = -Pv
    U = q - Pv , or q = U + Pv
  
  
  • Enthalpy or heat content (H)- H = u + PV
  • At constant pressure, heat change H = u + P V
  • For exothermic reactions, H <0 while for endothermic reactions, H > 0
  • Heat capacity: Heat capacity is the amount of heat required to raise the temperature of a given substance by 1^oc.
  • Molar heat capacity is the amount of heat required to raise temperature of 1 mol of the substance by 1^oc and specific heat is the amount of heat required to raise the temperature of 1 unit mass of a substance by 1^oC
  • There are two types of heat capacities
  • Heat capacity at constant volume (C_v) and heat capacity at constant pressure (C_P) and both are related as:
    
    C_P -C_v = nR for n mole of an ideal gas for 1 mole of an ideal gas-
    C_P -C_v = R
  
  Note: The standard state of any substance is its most stable form at a specified temperature and at 1 atm pressure.
• The enthalpy change when 1mole of a product is formed from its elements in their standard state is known as standard elthalpy of formation (H⁰_f) e.g.
  
  C + O₂ CO₂
  H⁰_f of CO₂ = -393 kj/mole, H⁰_f can be zero, positive or negative.
• The enthalpy change when one mole of a substance is completely burned in excess of oxygen is known as standard enthalpy of combustion (H⁰_c). Combustion reaction are exothermic and redox reaction e.g.
  
  CH₄ + 2O₂(g) CO₂(g) + 2H₂O(l)
  H⁰_c of CH₄ (g) = - 891 kj/mole
• The enthalpy change when one gram equivalent of an acid is neutralized by one gram equivalent of a base and vice-versa in a dilute solution of a given temperature is known as standard enthalpy of neutralization e.g.
  
  H⁺ (aq) + OH ^- (aq) H₂O(l), H⁰_neut = - 57.1 kj/mole
  H⁰_neut of strong acid and strong base reaction is always - 57.1 kj/mole and H0neut of weak acid-strong base reaction is less than 57.1 kj/mole.
• The enthalpy change when one mole of a substance is dissolved in excess of solvent at a given temperature so that no further dilution has any effect on H is known as standard enthalpy of solution (H⁰_sol)
• The enthalpy change when one mole of a solid (or liquid) substance converted into liquid (or vapour) at its melting (or boiling point) is known as standard enthalpy of fusion (or vaporization).
  
  
  The total enthalpy change remains same whether the reaction takes place in one or several steps. This is known as Hess's law of heat summation.
  
  
  
• The enthalpy change when the reactants have been combined to form products as indicated by the balanced chemical equation is known as standard enthalpy of reaction (H⁰_r)
• The amount of energy required to break 1 mole of a bond of particular type in gaseous state is known as bond enthalpy. Standard heat of reaction
  (H⁰_r ) can be calculated from the bond enthalpy data as
  
  H⁰_r = BE (reactants) - BE (products)
  Spontaneity
• Spontaneous process are those which occurs on their own without any external help. All material processes proceeds spontaneously and are thermodynamically irreversible.
• The thermodynamic function which is a measure of randomness or disorder is called entropy (s). The change in entropy is defined as
  
  
• Entropy is a measure of disorders. For a given substance, the order of increasing randomness is, S(solid) < S(liquid) S_total = + ve (increase in randomness and spontaneous).
  
  S_total = -ve (decrease in randomness)
  
  where S_total = S_sys + S_surr
• The entropy change S for any phase transition reaction can be calculated as  ,
  
• Free energy or Gibbs free energy, G is a measure of useful work. It is defined as G = H -TS and the change in Gibbs energy at constant T is
  G = H -TS
  
  This equation is known as Gibb's equation. Gibb's energy is the maximum energy available to a system during a process which can be converted into useful work.
• Criteria of spontaneity at constant T and P is
• If G < 0 (spontaneous)
• If G >0 (Non-spontaneous in a given direction).
• If G = 0, equilibrium state and there is no net reaction in either direction..
• The standard Gibb's energy change is defined as the Gibb's energy change when 1 mole of a product is formed from its elements in their standard state. G⁰_f of an element in its standard state is zero.
  
  
  Efficiency of a Heat Engine
• The relationship between W, the net work done by the system and q₂, the quantity of heat absorbed at the higher temperature T₂, in case of the cyclic process (i.e. the carnot cycle), can be obtained from the following two equations.
  W = R (T₂ - T₁)
  q₂ = RT₂
  W = q₂
  
  The fraction of the heat absorbed by an engine which it can convert into work gives the efficiency().
  =
  The net heat absorbed by the system, q is equal to q₂ - q₁ and according to the first law of thermodynamics, this must be equivalent to the net work done by the system. Thus,
  W = q₂ - q₁
  or = = . Since is invariably less than 1, the efficiency of a heat engine is always less than 1.