~THermoCHemistrY~

Concept of entalphy

exothermic:heat release from the system to surrounding.

                   H₂O(g)→H₂O(l)+energy

endothermic:heat absorb from the surrounding to the system

                     Energy+H₂O(s)→H₂O(l)

formula for enthalphy change(∆H) =(product) - (reactant)

Energy profile diagram

Enthalpy of combustion is heat released when 1 mole of a substance completely combusted in oxygen gas

C₄H₁₀(l) + 13/2 O₂ → 4CO₂(g) + 5H₂O(l)

Enthalpy of formation is heat change when 1 mole of a compound is produce from its element

K(s) + 1/2 Br2(l) → KBr(s)

Enthalpy of atomization is heat absorb when 1 mole of gaseous atom is formed from its element

Na(s) → Na(g)

Enthalpy of neutralisation is heat released when 1 mole of water formed when acid react with base

HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)

Enthalpy of solution is heat change when 1 mole of solute dissolve in solvent

NaCl(S) + H₂O(l) → Na⁺(aq) + Cl^-(aq)

Enthalpy of hydration is heat release when 1 mole of gaseous ion is hydrated in aqueous solution

Na⁺(g) + H₂O → Na⁺(aq)

Enthalpy of sublimation is heat absorbed when 1 mole of substance in solid state sublimed

I₂(s) → I₂(g)

Specific heat capacity is amount of heat required to rise temperature of 1 gram of a substance by 1 kelvin

                                                          C=q/m∆T
   q = amount of heat         T = different temperature          m = mass of water

Heat capacity is amount of heat require to rise temperature of given quantity of substance by 1 kelvin


C = mc        c=q/∆T

Calorimeter is device used to measure the heat released(or absorb) by a physical or chemical process

• coffee cup calorimeter (constant pressure)      q_rxn= -(q_cal +q_solution)
• Bomb calorimeter (constant volume)              q_rxn= -(q_cal +q_water)    

   bomb calorimeter 

  









               coffee cup calorimeter

Bomb-Haber cycle


1.Ionization energy is minimum amount energy required for 1 mole of gaseous atom to remove completely 1 mole of electron from 1 gaseous atom
              Li(g) → Li⁺(g) + e^{-
  
2.Electron affinity is energy change when 1 mole of electron is added to 1 mole of gaseous atom}
               F(g) + e^-  → F^-(g)

3.Lattice energy is energy change when 1 mole of solid ionic compound formed from it gaseous ion
                Li⁺(g) + F^-(g) → LiF(s)

 magnitude lattice energy

• indicate the strength of ionic bond
• more negative,more stronger ionic bonding

   factor affect lattice energy

• ionic bond strength
• charge(Q)
• size of ion(d)

Bond Haber Cycle is a series of chosen step from element to ionic compound which entalpy are known except lattice energy

Born Haber Steps

1. atomization of element
2. formation of cation:Ionisation energy
3. Formation of anion:Electron affinity
4. formation of ionic compound

Hess`s Law of heat summation is when reactant are converted to product the change in enthalpy is the same whether the reaction take step in one step or in a series of step.There are 2 method calculation using Hess`s law

• algebraic method

         -identify the target equation
         -rearange the equation given: mole of reactant and product are on the correct side
         -add the equation to obtain the target equation

• energy cycle method

      

             C(s)           +          2H₂(g)             →          CH₄(g)

              ↓-393                 ↓2(-286)                          -891

             CO₂(g)      +          2H₂O(l)                         ↙

           

             -393 + 2(-286) - (-819) = -74KJ mol^-1
QUESTION

1.State Hess law.

a.Determine the enthalpy change ∆f  for the reaction

  C(graphite)+2H₂(g)→CH₄(g) .Given:

  C(graphite)+O₂(g)→CO₂(g)                             ∆H=-393.5 kj

  2H₂(g)+O₂(l)→2H₂O(l)                                     ∆H=-571.6 kj

  CH₄(g)+2O₂(g)→2H₂O(l)                                  ∆H=-890.4 kj

2.Draw the born haber cycle for the formation of magnesium chloride,MgCl₂ from magnesium metal         and   chlorine gas.Calculate the enthalpy of formation of MgCl₂.Given

  Heat of sublimation of magnesium  ∆H=149 kj mol^-1

  Heat ionization energy of magnesium ∆H=740 kj mol^-1

  Second  ionization energy of magnesium ∆H=1456 kj mol^-1

  Heat of atomization of chlorine ∆H=240 kj mol^-1

  Electron affinity of chlorine ∆H=-369 kj mol^-1

  Lattice energy of MgCl₂ ∆H=-3933 kj mol^-1

3.Define standard enthalpy of formation.The combustion of 4.6g of ethanol has evolve 136.7kj heat       at  25^oc and 1 atm.Calculate the standard enthalpy of formation of ethanol

[∆H_f CO₂(g)=-393.5 kj mol^-1 and ∆f₂ H₂O(l)=-285.8 kj mol^-1]

b.The following equation show the combustion reaction of ethyne and propane

   2C₂H₂(g)+5O₂(g)→4CO₂(g)+2H₂O(l)          ∆H=-2598.8 kj

   C₃H₈(g)+5O₂(g)→3CO₂(g)+4H₂O(l)            ∆ H=-2220.0 kj

  A mixture of 5 mol gases consisting  of ethyne and propane is burnt and 7647.8 kj heat is evolve.What is the  mole ratio of ethyne to propane in the mixture?

4.(i) Define standard enthalpy of combustion

  (ii) Calculate the heat of combustion of methane using standard heat of formation below

      ∆ H_f(CO₂(g))=-394 kj mol^-1,∆H_f(H₂O(l))=-286 kj mol^-1,∆H_f(CH₄(g))=-75 kj mol^-1

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