Energetics Definitions

Question 1

Enthalpy change of reaction

Answer 1

energy change in a chemical rxn when molar quantities of reactants stated in chemical eqn react under standard conditions

Question 2

Enthalpy change of formation (substance)

Mg(s) + 0.5O2(g) –> MgO(s)

Answer 2

energy change when 1 mole of pure substance is formed from its constituent element in their standard state under standard conditions

can be exo or endo

Question 3

Enthalpy change of formation (element)

O2(g) –> O2(g)

Answer 3

energy change when 1 mole of pure element is formed from its constituent element in their standard state under standard conditions is always zero

ALWAYS ZERO

Question 4

Enthalpy change of combustion

Answer 4

energy released when 1 mole of substance is completely burnt in excess oxygen under standard conditions

always exothermic

Question 5

Enthalpy change of neutralisation

usually occurs b/w acid and base

Answer 5

energy change when acid and base react to form 1 mole of water under standard condition

Question 6

Enthalpy change of atomisation (element)

Answer 6

energy adsorbed when 1 mole of gaseous atoms is formed from the element under standard conditons

always exorthermic as bonds are broken

Question 7

Enthalpy change of atomisation (compound)

Answer 7

energy adsorbed when 1 mole of compound is converted to gaseous atoms under standard conditions

always exothermic as bonds are broken

Question 8

Bond dissociation energy

H2 (H-H) (g) —> 2H(g)

Answer 8

energy required to break 1 mole of X-Y bond in XY compound in gaseous state

always endothermic

Question 9

Bond Energy

in data booklet

Answer 9

average energy required to break 1 mole of X-Y bond in gaseous state

Question 10

1st ionisation energy

Ca(g) —> Ca+(g) + e-

Answer 10

energy required to remove 1 mole of e- from 1 mole of gaseous atoms to form 1 mole of singly positively charged gaseous ions

for 2nd IE change singly to doubly

Question 11

1st electron affinity

O(g) + e- —> O-(g)

Answer 11

energy change when 1 mole of gaseous atoms acquires 1 mole of e- to form 1 mole of singly negatively charged gaseous ions

Question 12

Lettice energy

Na+(g) + Cl-(aq) —> NaCl(s)

Answer 12

Energy released when 1 mole of solid ionic compound is formed from its constituent gaseous ions under standard conditions

proportional to (cat charge + anion charge / cat + anion radius)

Question 13

Enthalpy change of hydration

Na+(g) + aq. —> Na+(aq)

Answer 13

energy released when 1 mole of gaseous ions is hydrated under standard conditions

exothermic

Question 14

Enthalpy change of solution

ie. NaCl(s) + aq. —> Na+(aq) + Cl-(aq)

Answer 14

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

ΔH(soln) = −LE+ΔH(hyd)
soluble if ΔH(soln) < 0
insoluble if ΔH(soln) > 0

Question 15

Define entropy

Answer 15

a measure of disorder in the system, the more disordered the system, the larger the entropy

Question 16

factors affecting entropy

Answer 16

entropy increases when:
* temp increases:
broadening of energy distribution of particles, as there are more possible energy states particles can adopt at higher temp
* change in phase: (solid to liquid/gas)
particles more randomly arranged, more disordered, increase in volume (for liquid to gas), entropy increases
* change on no. of particles: (for gaseous system)
more ways to arrange the particles, more ways to distribute energy in the system, creating more disorder, entropy increases
* mixing of particles