IB1 ChemistryHL Energetics

Why do chemical reactions happen?

Topic 15: Energetics (8 hours)

15.1 Standard enthalpy changes of reaction15.1.1 Define and apply the terms standard state, standard enthalpy change of formation (¬H ) fÖ and standard enthalpy change of combustion (¬H ) c Ö .15.1.2 Determine the enthalpy change of a reaction using standard enthalpy changes of formation and combustion.15.2 Born–Haber cycle15.2.1 Define and apply the terms lattice enthalpy and electron affinity.15.2.2 Explain how the relative sizes and the charges of ions affect the lattice enthalpies of different ionic compounds.15.2.3 Construct a Born–Haber cycle for group 1 and 2 oxides and chlorides, and use it to calculate an enthalpy change.15.2.4 Discuss the difference between theoretical and experimental lattice enthalpy values of ionic compounds in terms of their covalent character.

15.3 Entropy15.3.1 State and explain the factors that increase the entropy in a system.15.3.2 Predict whether the entropy change (ΔS) for a given reaction or process is positive or negative.15.3.3 Calculate the standard entropy change for a reaction (¬S Ö ) using standard entropy values (S Ö ) .15.4 Spontaneity2.5 hours15.4.1 Predict whether a reaction or process will be spontaneous by using the sign of ¬GÖ .15.4.2 Calculate ¬GÖ for a reaction using the equation¬GÖ = ¬H Ö− T¬S Ö and by using values of the standard free energy change of formation, ΔGf Ö .15.4.3 Predict the effect of a change in temperature on the spontaneity of a reaction using standard entropy and enthalpy changes and the equation¬GÖ= ¬H Ö− T¬S Ö .

Standard enthalpy change of reaction

in standard state at 101kPa, 298K

(PV = nRT)

Standard enthalpy of combustion: DHc

q

When a substance is fully burned in oxygenfor example CH4 + 2O2 CO2 + 2H2O

Standard enthalpy of formation: DHf

q

DHfq : The energy absorbed or evolved when 1

mol of the substance is formed from its elements in their standard states. The enthalpy of formation of any element is zero. H2(g) + ½O2(g) H2O(l) DHf

q = -285 kJ/mol

DH = SDHf(products) - SDHf(reactants)

Ionic compound consist of ions arranged in a lattice

Two or more electrons can be transferred

Different sized atoms give different mineral structures as they pack in a different way

Hexagonal Beryl crystal; Image Wikipedia

Lattice enthalpy, DHlattice

Relates to the endothermic process MX(s) M+

(g) + X-(g)

in which the gaseous ions of a crystal are separated to an infinite distance from each other.

NaCl(s) Na+(g) + Cl-(g) DHlattice= 771kJ/mol

Endothermic reactions.

Lattice enthalpy depends on charge on an ion size of an ion packing arrangement

MgO -3791kJ/molNaCl -790kJ/mol

Standard enthalpy of atomization: DHat

q

DHatq : The energy required to atomize one mole

of an element. Na(s) Na(g) + e- DHat

q = +108 kJ/mol

(physics: related to latent heat of vaporization)

Electron affinityThe enthalpy change per mole when an atom gains one electron in the gaseous phaseCl(g) + e-

(g) Cl-(g) DHea = -351 kJ/mol.

Electron affinity can be both exothermic and endothermic depending on element.

How does the electron affinity change across a period?

Born-Haber cycle

http://en.wikipedia.org/wiki/File:Born-haber_cycle_LiF.svg

Born-Haber cycles used to calculate lattice enthalpies

Can be used to find out if a bond is more or less ionic

Born Haber cycle for sodium chloride Enthalpy of formation of NaCl = -411kJ/mol Enthalpy of atomisation of Na = +103 kJ/mol Enthalpy of atomisation of Cl = +121 kJ/mol

(½ energy of Cl-Cl bond) Electron affinity of Cl = -364 kJ/mol Ionisation energy of Na = + 500 kJ/mol

Lattice enthalpy for sodium chlorideEnergies of atomisation + Electron affinity +

Ionisation energy = = Enthalpy of formation + Lattice enthalpyLattice energy = 771 kJ/mol

How ionic is the ionic lattice?

Chemistry Data Booklet gives lattice enthalpies as: Experimental values (obtained from Born-Haber

cycle) Theoretical values (calculated using electrostatic

calculations)

Greater difference between theoretical and experimental values

more covalent character of the bond.

Decomposition of ammonium nitrate

NH4NO3(s) N2O(g) + 2 H2O(l)

  NH4NO3(s) DHf

q= -366 kJ/mol DSq = 151 J/K*mol

N2O(g) DHfq= +82 kJ/mol DSq = 220 J/K*mol

H2O(l) DHfq= -285 kJ/mol DSq = 70 J/K*mol

DH = [DHf (N2O(g)) + DHf (H2O(l))] – [DHf(NH4NO3(g)] =

=[82 + 2(-285)] - [-366] = -122 kJ/mol

Entropy: disorder

Which is more likely if the particles are in constant random motion?

Entropy

Entropy, S = Disorder Unit: JK-1mol-1

DS = change in disorder DS = Sp - Sr

Absolute value of S can be measured

Entropy and changes of stae

Solid Liquid GasH2O Ice Water SteamJK-1mol-1 48.0 69.9 188.7

Increasing entropy

Will a reaction happen? Spontaneity

All reactions involve changes in H and S

DS is probably positive if moles of gas increaseand moles of solid or liquid decrease.

NH4Cl(s) NH3(g) + HCl(g) DS = + 285JK-

1mol-1

Pb2+(aq) + 2 I- PbI2(s) DS = - 70 JK-

1mol-1

Spontaneity of a reaction

Nature likes low internal energy (DH to decrease) and high disorder (DS to increase) A reaction will occur if the final state is more

probable than the initial state.Decrease in DH

Increase in DS

Gibbs free energy, DGq = DHq -

TDSq

Temperature dependent

Spontaneous: DG negative (DGq < 0)Equilibrium: DGq = 0 Not spontaneous: DGq positive (DGq > 0)

DH DS DG Spontaneity

Negative(Exothermic)

Positive(More

random)

DG < 0 Always

negative

Always

spontaneousPositive

(Endothermic)

Negative(More order)

DG > 0Always positive

Never spontaneous

Negative(Exothermic)

Negative(More order)

Depends on T

Spontaneous at low Temp

Positive (Endothermi

c)

Positive(More

random)

Depends on T

Spontaneous at high

Temp

Activation energy is also important Just the fact that a reaction is spontaneous

doesn’t mean that it will occur at once. It also depends on activation energy. And we will

deal with that later on in topic 7.

Links Ionic bonding

http://www.teachersdomain.org/asset/lsps07_int_ionicbonding/

Covalent bonding http://www.teachersdomain.org/asset/lsps07_int_covalentbond/