ENERGETICS /THERMOCHEMISTRY (AS)

1.Often chemical changes are accompanied by changes in heat content / enthalpy of the materials reacting (H) 2. This change is shown by a change in temperature.

a. heat is lost to the surroundings Temperature of reaction mixture rises / increases H is negative Exothermic reaction

Enthalpy diagram / energy level diagram OR Reaction pathway/reaction coordinate/extent of reaction

b. heat is absorbed from the surroundings , temperature of reaction mixture decreases / falls H is positive Endothermic reaction

3. a. The value of H depends on temp , pressure and concentrations of reactants. b. H are measured under standard conditions : Temperature = 298K ( 250 C ) Pressure = 1 atm / 1.01 x 105 Pa Concentrations = 1 mol dm-3

c. Any H measured under standard conditions is described as standard enthalpy change. Symbol : H

State symbols must be included in equations STANDARD ENTHALPIES Definitions State symbols must be included in equations

STANDARD ENTHALPHY OF FORMATION ( Hf ) Enthalpy change when 1 mole of a compound is formed from its elements in their standard states , under standard conditions temperature 298 K and pressure of gases at 1 atm

Steps in writing equations: 1. write formula of compound formed 2. identify elements required to form the compound 3. balance equation to form 1 mole of the compound

Examples Mg (s) + ½O2 (g) → 1MgO (s) -602 kJ mol-1 602 kJ evolved for every 1 mole of MgO formed ½ H2 (g) + ½ Cl2 (g) → 1HCl (g) - 92.3 kJ mol-1

Examples Notes : 1) Hf of elements is zero Eg : Cu(s)  Cu(s) , Hf = 0 2) Hf are often theoretical only

Exercise : Write the equation for enthalpy of formation for : KMnO4 (s) and H2O (l) K(s) + Mn(s) + 2O2(g) 1KMnO4(s) H2(g) + ½O2(g)  1H2O (l)

STANDARD ENTHALPY CHANGE OF COMBUSTION ( Hc ) Enthalpy evolved when 1 mole of the element or compound is completely burned in excess oxygen , under standard conditions

Steps in writing equations : 1. identify products formed from burning of compound in excess oxygen 2. balance equation for 1 mole of compound burnt

Examples 1 C (s) + O2 (g) → CO2 (g) Hc = - 394 kJ mol-1 394 kJ evolved for every 1 mole of carbon burnt 1 H2 (g) + ½O2 (g) → H2O (l)

Exercise : Write an equation for the enthalpy of combustion for C3H6 and CH3OH

1C3H6 (g) + 9/2 O2 (g)  3CO2 (g) + 3H2O (l) 1CH3OH (l) + 3/2 O2 (g)  CO2 (g) + 2H2O (l)

STANDARD ENTHALPY OF ATOMISATION ( Hat ) Enthalpy required/absorbed 1 mole of gaseous atoms formed From the element in its standard state under standard conditions Examples : Fe (s)  1 Fe (g)

Example : ½Cl2(g)  1 Cl (g) + 122 kJ mol-1 molecules atoms Note : from Data Booklet Bond energy Cl-Cl = + 244 kJ mol-1 Cl-Cl  2 Cl (g) 2 moles atoms Hat of Cl = ½ x bond energy Cl-Cl

STANDARD BOND DISSOCIATION ENTHALPY Also called bond energy Energy absorbed Separate the 2 atoms in a covalent bond in gaseous state under standard conditions , per mole of bond.

Examples 1) HCl (g) → H (g) + Cl (g) , +431 kJmol-1 2) CH4 contains 4 x C-H bond Total energy required to break all bonds in CH4 = 1640 kJ ¼ CH4 = ¼ ( 4 C-H bond ) = one C-H bond Average bond energy of one C-H bond = ¼ ( 1640 ) = +410 kJ ¼ CH4 (g)→¼ C (g) + H (g),+ 410 kJmol-1

STANDARD ENTHALPY OF NEUTRALIZATION Energy evolved Acid reacts with base to form 1 mole of water , under standard conditions Examples : NaOH + HCl → NaCl + 1 H2O KOH + HCl → KCl + 1 H2O Ionic equation(strong acid + strong base: H+(aq) + OH-(aq)  1 H2O (l), H neutralisation = -57 kJ mol-1

Generally, 1) Strong acid / strong alkali : H = - 57 kJ mol-1 2) Weak acid / base : H = -(<57) kJ mol-1 (eg. -54 kJmol-1) Reason : Certain amount of energy required to ionise the weak acid or base first

STANDARD ENTHALPY OF HYDRATION ( Hhyd ) Energy evolved 1 mole of separate gaseous ions dissolved in water under standard conditions Exothermic as attraction/bond forms between the ions and polar water molecules Called ion dipole attraction

Examples : 1 Na+ (g) → Na+ (aq) -406 kJmol-1 1 Cl- (g) → Cl- (aq) - 381 kJmol-1

Hhyd for compounds = sum of Hhyd of constituent ions Eg : Hhyd MgCl2 = Hhyd Mg2+ + 2 x Hhyd Cl- = -1891 + 2(-381) = - 2653 kJ

H (hyd)  charge density of ions Charge density = charge/size Higher charge density , stronger ion dipole attraction , more exothermic H(hyd) Eg : H(hyd) Cl- > H(hyd) Br – -381 -351

STANDARD ENTHALPY OF SOLUTION ( Hsolution ) Enthalpy change when 1 mole of a substance dissolved in a stated amount of solvent under standard conditions Example : 1 KOH (s)  K+ (aq) + OH- (aq) or KOH (aq) - 57 kJ mol-1

STANDARD ENTHALPY CHANGE OF REACTION ( Hr ) Enthalpy change in a chemical reaction , for the number of moles of reactants as shown in a balanced chemical equation under standard conditions Example : 4 H2O + 3 Fe  Fe3O4 + 4 H2  Hr = x kJ when 4 moles H2O reacts with 3 moles Fe