Lower Six Year End Assignment 2009

Latihan Pakej Percutian Bulan Ogos 2009 L6 CHEMISTRY

Org_ex1.0_Ans2/2 5/8/09

Org_ex1.0_Ans1/2 5/8/09

Organic Ex1.0 4/8/09

Atomic Structure

Protons, electrons, neutrons.

particle	relative charge	relative mass
electron	-1	negligible mass
proton	+1	1 atomic mass unit
neutron	0	1 atomic mass unit

 

Isotopes

Isotopes are atoms of the same element that have the same number of protons but a different number of neutrons. Their chemical reactions are the same, physical properties different such as density.

The nucleon number(A) is the sum of protons and neutrons in the nucleus

The proton number (Z) is the number of protons in the nucleus.

Atomic mass units and relative isotopic mass.

1 a.m.u is equal to one-twelfth the mass of a ¹²C atom.

Relative isotopic mass is the mass of an atom of an individual isotope, relative to one-twelfth the mass of a ¹²C atom.

Relative atomic mass is the average mass of an atom, taking the natural abundances

of its isotopes into account, relative to one-twelfth the mass of a ¹²C atom.

Relative molecular mass is the mass of a molecule of an element or compound, relative to one-twelfth the mass of a ¹²C atom.

Various Types of Enthalpy Changes.

Standard Enthalpy Change of Atomisation, ∆H^θat.

∆H^θat of an element is the energy absorbed when 1 of gaseous atoms are formed from its elements under standard conditions.

1/2Cl2(g) → Cl(g) ∆H^θat = +121 kJ mol^-1.

Na(s) → Na(g) ∆H^θat = +108 kJ mol^-1.

For diatomic gases, H2, Cl2 etc; Cl2(g) → 2Cl(g) ∆H = +242 kJ mol^-1.

Therefore: ∆H^θat = ½ x bond energy

= ½ x +242 = +121 kJ mol^-1.

Standard Enthalpy Change of Electron Affinty. ∆H^θEA.

The first electron affinity (EA) of an element is the enthalpy change when 1 mole of gaseous atoms accept a mole of electrons to form a mole of uninegatively charged ions.

Cl(g) + e → Cl^̄ (g) ∆H^θea = ˗ 364 kJ mol^-1.

The first electron affinity (EA) is always negative.

The second and successive electron affinity of any element is always positive because energy absorbed to overcome repulsive forces between the negatively charged anions and the added electron.

O(g) + e → O^¬(g) ∆H^θea1 = ˗ 142 kJ mol^-1.

O^¬(g) + e → O^2¬(g) ∆H^θea2 = + 844 kJ mol^-1.

Lattice Energy : Part I and II

Lattice Energy, ∆H^Θl_at.

∆H^Θ_lat. is the energy released when 1 mol of an ionic compound in the solid state is formed from its gaseous ions under standard conditions.

Na+(g) + Cl-(g) → NaCl(s) ∆H^Θ_lat.= -788 kJ mol^-1.

Magnitude of lattice energy is proportional to the charge on the ion and is inversely proportional to the interionic distance (r+ + r-) .

Standard Enthalpy Change of Neutralization.

∆Hneut is the enthalpy change when one mole of H⁺ ions from an acid reacts with one mole of OH^- ions from an alkali to form 1 mol of water under standard conditions.

H⁺ (aq) + OH^-(aq) → H2O(l)

Hess Law

Hess law states that the enthalpy change is independent of the route during a chemical reaction, provided initial and final conditions are the same.

Standard Enthalpy Change of Combustion.

∆H^Θc is the standard enthalpy change when 1 mole of the substance is completely burned in excess oxygen under standard conditions.

Example: C(s) + O2(g) → CO2(g) ∆H^Θ = - x kJmol^-1

If 2C(s) +2O2(g) → 2CO2(g) ∆H^Θ = -2 x kJmol^-1

Standard Enthalpy Change of Formation.

∆Hf : s the enthalpy change when 1 mol of a compound is formed from it's elements under standard conditions.

Example: standard enthalpy change of formation of water: H2(g) +⅟2O2(g) → H2O(l) :∆Hf = -285.5 kJ mol^-1.

 

 

 

Thermochemistry.

Enthalpy, symbol H means heat content.

Cannot measure absolute value of enthalpy of system but can measure enthalpy change (∆H).

∆Hrxn = total enthalpy of products – total enthalpy of reactants

= ∑ Hproducts - ∑ Hreactants

Exothermic reactions : ∆Hrxn = -ve because ∆Hproducts < ∆Hreactants

Endothermic reactions : ∆Hrxn = +ve because ∆Hproducts > ∆Hreactants

Calorimetry: experimental technique to measure ∆Hrxn.

Molar heat capacity: energy required to raise the temperature of 1 mole of the substance by one Kelvin.

Specific heat capacity: energy required to raise the temperature of 1 gram of the substance by one Kelvin, eg specific heat capacity of water is 4.2 J K^-1 g^-1.

∆H = mc ∆T where c = specific heat capacity, m= mass of solution, ∆T = change in temp.

Application of electrolysis

1. Manufacturing of chlorine.
Cathode
a) Mercury cathode Electrolyte : cell brine/salt water
Anode (graphite): Cl- -> 1/2 Cl2 + e
Cathode(mercury): Na+ +e --> Na
Na +Hg --> Na-Hg
Na-Hg +H2O -->NaOH+1/2H2 +Hg

b) Diaphragm cell Electrolyte: brine/conc NaCl
Anode(graphite): Cl- -> 1/2Cl2 +e
Cathode(steel): H2O + e --> OH- + 1/2H2
Besides chlorine, side product NaOH and H2.
For same quantity of electricity, say 1F,
ratio Cl2 : NaOH : H2
ratio 1/2 : 1 : 1/2