ENERGETICS

Question 1

exothermic

Answer 1

-heat rel to surrounding
-temp of surrounding ↑
-energy taken in for BB > energy given out for BF

Question 2

endothermic

Answer 2

-heat absorbed from surrounding
-temp of surrounding ↓
-energy taken in for BB>energy released for BF

Question 3

examples of exothermic

Answer 3

neutralisation, combustion, acid + metal, g->l->s

Question 4

examples of endothermic

Answer 4

thermal decomp, dissolving ionic cmpds, s->l->g

Question 5

∆Hf

Answer 5

the energy change when 1 mol of substance is formed from its constituent elements under std conditions

Question 6

∆Hc

Answer 6

energy evolved when 1 mol of subst is completely burnt in EXCESS oxygen under std conditions

Question 7

∆Hneut + value for strong acid and base

Answer 7

energy evolved when 1 mol of WATER is formed from the neutralisation b/w acid and base under std conditions

∆Hneut = -57.3kJ mol-1

Question 8

∆Hatomization

Answer 8

energy akbsorbed when 1 mol of gaseous atoms is formed from the element under std condition

Question 9

EA(electron affinity)

Answer 9

the enthalpy change when 1 mol of gaseous atoms gain 1 mol of electron to give 1 mol of singly -ve charged gaseous ions

Question 10

∆Hhyd

Answer 10

the energy evolved when 1 mol of gaseous ions is hydrated under std conditions

Question 11

∆Hsol

Answer 11

energy change when 1 mol of substance is completely dissolved in a solvent to form an infinitely dilute solution under std conditions

Question 12

eqn for hyd le sol

Answer 12

ΔHhyd - L.E. = ΔHsol

Question 13

ΔHr(combustion)

Answer 13

ΔHc(rxt) - ΔHc(pdt) / CRP

Question 14

ΔHr(formation)

Answer 14

ΔHf(pdt) - ΔHf(rxt) FPR

Question 15

ΔHr(all gas)

Answer 15

BE(rxt) - BE(pdt) / FPR

Question 16

qsol abs

Answer 16

mass(sol) x C(4.18) x ΔT OR
efficiency x qrel

Question 17

T fall(ΔH)

Answer 17

+ve

Question 18

T rise(ΔH)

Answer 18

-ve

Question 19

ΔH

Answer 19

+- qsol/nref

Question 20

qabs(if ∆H given)

Answer 20

n/100 x qrel

Question 20

qrel(if mct given)

Answer 20

qabs/n x100

Question 21

sources of error for calorimetric

Answer 21

heat loss to surrounding unaccounted for(use heat shield)

heat abs by calorimeter

incomplete combustion of subst

Question 22

BE

Answer 22

Avg energy absorbed to break 1 mol of covalent bond in the gaseous state to form gaseous atoms under std conditions

Question 23

why is 1st EA negative

Answer 23

energy released when nucleus attracts an electron is more than energy taken in to overcome interelectronic repulsion

Question 24

Why is 2nd EA positive

Answer 24

energy has to be supplied to overcome the repulsive forces between the 2 negatively charged species

Question 25

L.E.

Answer 25

energy evolved when 1 mol of solid ionic compound is formed from its constituent gaseous ions under std conditions

Question 26

Limitations w/ L.E.

Answer 26

assuming 100% ionic character as cations may have polarising power or anions can be polarisable

Question 27

Why is B.E. of (C-C) lower in cubane than calculated B.E.?

Answer 27

BE(C-C) in cubane is less endothermic than that in the data booklet due to the angle strain in cubane since tis bond angles are 90 degrees instead of 109.5 degrees -> causes less effective overlap of hybrid orbitals

Question 28

limitations in using ΔG to predict spontaneity

Answer 28

ΔG predicts spontaneity only under std conditions, and spontaneity may Δ at other temps ΔG only gives an indication of the thermodynamic feasibility of rxn but not its kinetic feasibility. A rxn energetically favourable may not proceed if it is too slow.

Question 29

factors affecting ΔS/explain the significance of the sign of ΔS

Answer 29

1. temp(↑,↑ avg KE of molecules) 2. ∆ in state/phase(melting/vaporisation) 3.mixing (g) and (l) 4.∆ in no. of particles 5.↓ in pressure(inversely proportionate to volume), gas will spontaneously expand,↑Vgas at same temp ↑ in no. of ways energy can be distributed among molecules, increasing ΔS

Question 30

why are reactions involving weak acids/bases less exothermic?

Answer 30

The weak acids/bases do not ionise completely in dilute aqueous solutions and energy has to be used to completely ionise the weak acid/base

Question 31

ΔS definition

Answer 31

The no. of ways the energy of a system can be dispersed through the motion of its particles

Question 32

Explain why (rxn) is feasible

Answer 32

↑entropy from the production of gas/anyth and the elevated temperature readily offsets the slightly endothermic enthalpy change causing ΔG to be -ve, hence the reaction is feasible

Question 33

limitations with B.E.

Answer 33

The ΔHc calculated is less exothermic than that calculated in __ as bond energies given in data booklet are averge values and do not relate to any specific compound