Energetics I
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Standard Conditions - temp pressure and conc
Temp: 25 degrees celsius / 298K Pressure: 1 bar / 10^5 Pa Conc of aq solute: 1mol/dm^3
Defn of enthalpy and symbol High enthalpy = less/more stable?
Enthalpy (H) is a measure of energy content of a sub, hence reflecting its stability High enthalpy = less stable
Eqn for enthalpy
ΔHr = Σ (H prod / final) - Σ (H reactants / initial)
ΔH > 0 • meaning • energy profile diagram • energy lvl diagram • prod or reactants are more stable?
Endothermic - energy absorbed from surroundings & temp of surrounding decrease H prod > H rcts Products are less stable than rcts
ΔH < 0 • meaning • energy profile diagram • energy lvl diagram • prod or reactants are more stable?
Exothermic - energy released to surroundings & temp of surrounding increases H prod < H rcts Products are more stable than rcts
Defn of activation energy & symbol
Activation energy (Ea) is the min. energy the rct molecules must possess in order to collide successfully to form prod. It affects the speed of the rctn
Standard enthalpy change of rctn & symbol
ΔHrΘ The energy change when molar quantities of rcts specified in the chemical eqn reacts to form prod at 1 bar & 298K
All chem eqn must have …
State symbols
Measurement of Standard enthalpy change of rctn depends on (2)
• Amt of rcts and prod - if they are X2, then ΔH X2 - order is reversed then sign is opp • Phy state of the sub - Prod (l) is more stable than prod (g) so lower - Diamond is also more stable than graphite so lower
Defn of Hess’ Law
The enthalpy change accompanying a chem rctn is the same regardless of the route by which the chem change occurs, provided the initial and final conditions are the same.
5 steps of Hess’ Law
• Write balanced eqn • start energy cycle with most complicated eqn • complete cycle by adding in eqns…. • Write the corresponding ΔH values next to the arrows • Sum of CW arrows = sum of ACW arrows
Defn of standard enthalpy change of formation symbol eqn Common elements* For an element at std state … Stability of a sub relative to the constituent elements
ΔHfΘ is the enthalpy change when 1 mole of a species is formed from its constituent elements in their std states at 298K and 1 bar ΔHrΘ = Σ HfΘ (prod) - Σ HfΘ (reactants) FLAPPER Common elements: P4 (s), Br2 (l), S8 (s) For an element at std state, ΔHfΘ = 0 If ΔHfΘ < 0, then sub is more stable than the elements & vice versa
Defn of standard enthalpy change of combustion symbol eqn For org compounds with only C / H / O, prod are always…
ΔHcΘ is the heat evolved when 1 mole of a sub is completely burned in excess oxygen under std conditions of 298K and 1 bar ΔHrΘ = Σ HcΘ (rcts) - Σ HcΘ (prod) CRAP Prod are always CO2 (g) + H2O (l) Always exo
Defn of bond energies symbol eqn
Bond energies is the ave energy absorbed when breaking 1 mole of covalent bond in the gaseous state ΔHrΘ = Σ BE (rcts / bond broken) - Σ BE (prod / bonds formed) BURP
Requirement to use bond energies in calc
Rctn must take place in the gaseous state
Why might there be discrepancies in BE in DB
BE in DB are averaged across a large sample of molecules containing the covalent bond
Qn eg: Using relevant BE values in DB calc… state and explain if the and in (a) is an estimate/exact value
Exact value as the BE used in calc are exact values obtained from the DB as HCL is a heteronuclear molecules, while H2 and CO2 are homonuclear molecules and all in gaseous state
Defn of bond dissociation energy of a X-Y bond
Energy required to break 1 mole of that particular X-Y bond in a particular compound in gaseous state
Defn of standard enthalpy change of hydration symbol Mag depends on eqn eg
ΔHhydΘ is the energy released when 1 mole of a gaseous ion is dissolved in a large amt of water at 298K and 1 bar |ΔHhydΘ| ∝ |charge of ion/size of ion| (charge density) Na+ (g) -> Na+ (aq)
Why is std enthalpy change of hydration is exo
Energy is released during the formation of ion-dipole interactions Larger charge density - more exo ΔHhydΘ - stronger ion-dipole interactions
Defn of std enthalpy change of soln symbol soluble/insoluble
ΔHsolΘ is the enthalpy change when 1 mole of solute is completely dissolved in a solvent to form an infinitely dilute soln under std conditions of 298K and 1 bar (usually a solid ionic compound) ΔHsolΘ»_space; insoluble in water ΔHsolΘ < soluble in water
Defn of lattice energy Symbol Mag depends on
ΔHlattΘ is the heat evolved when 1 mole of a solid ionic compound is formed from its constituent gaseous ions under std conditions of 298K and 1 bar |ΔHlattΘ| ∝ | q+ x q- / r+ + r- |
Eqn for ΔHsolΘ
ΔHsolΘ = Σ ΔHhydΘ - Σ ΔHlattΘ fav soln is hyd latte
Is Lattice energy exo/endo and why
Exo as Energy is released when strong electrostatic forces of attraction are formed b/w cations and anions