chem energetics

Question 1

standard enthalpy change of combustion

Answer 1

energy evolved when one mole of the substance is completely burnt in oxygen under standard conditions

Question 2

define standard enthalpy change of hydration

Answer 2

energy evolved when one mole of gaseous ions is hydrated under standard conditions

Question 3

define standard enthalpy change of solution

Answer 3

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

Question 4

standard enthalpy change of neutralisation

Answer 4

energy evolved when one mole of water is formed from the neutralisation between acid and base under standard conditions

Question 5

define standard enthalpy change of atomisation

Answer 5

energy absorbed when one mole of gaseous atoms is formed from the element under standard conditoins

Question 6

define bond energy

Answer 6

average energy adsorbed to break one mole of covalent bonds between 2 atoms in the gaseous state to form gaseous atoms under standard conditions

Question 6

define lattice energy

Answer 6

energy evolved when one mole of the solid ionic compound is formed from its constituent gaseous ions under standard conditions

Question 6

effect of ionic charge and of ionic radius on the numerical magnitude of a
lattice energy

Answer 6

|△Hlatt|∝ (q+)(q-) /(r+)+(r-)

Question 7

define entropy

Answer 7

measures the degree of disorder/randomness in a system

Question 8

effects on the entropy of a chemical system

Answer 8

(i) change in temperature
- more energy to distribute the particles

(ii) change in phase (solid < liquid < gas)
-move more freely and with greater speeds, more ways..

(iii) change in the number of particles (especially for gaseous systems)
- arrangement of molecules more disordered, more gasesous particles moving randomly, more ways..)

(iv) mixing of particles(expansion of volume for gaseous system)
- more ways @ larger volume!!!

more ways to distribute the particles and energy

Question 9

limitations in the use of ΔG⦵
to predict the spontaneity of a reaction

Answer 9

a) only valid for STANDARD CONDITIONS
b) kinetic feasibility not considered

Question 10

define standard enthalpy change of formation

Answer 10

energy change when one mole of the substance is formed from its constituent elements under standard conditions

Question 10

define 1st ionisation energy

Answer 10

energy absorbed when one mole of gaseous atoms loses one mole of electrons to form one mole of singly positively charged gaseous ions

Question 11

define 2nd ionisation energy

Answer 11

energy absorbed when one mole of singly positively charged gaseous ions loses one mole of electrons to form one mole of doubly positively charged gaseous ions

Question 12

define first electron affinity

Answer 12

energy evolved when on mole of gaseous atoms acquires one mole of electrons to give one mole of singly negatively change gaseous ions

Question 13

ΔHr =

Answer 13

ΔHf products - ΔHf reactants

ΔHc reactants - ΔHc products

BE reactants - BE products

Question 14

why ΔHn (weak acid/base) < 57.0?

Answer 14

• weak acids/bases are only sightly dissociated in aq solution. ionisation, which involves bond breaking, is an endo process.
• some energy evolved from neutralisation process is used to further dissociate the weak acid/base completely
  -thus lest exo

Question 15

why 2d E.A is endo while 1st E.A is exo?

Answer 15

2nd EA also endo since e is added to -vely charged ion.
energy has to be supplied to overcome repulsive forces between 2 -vely charged species

Question 16

ΔHf of an element at 25C/298K is always

Answer 16

zero

Question 17

manyy enthalpy changes cannot be determined directly by experiments due to :

Answer 17

1. reaction does not take place at standard conditions upon mixing reactants
2. ΔHrxn is just a theoretical value as reaction does not take place in practice

Question 18

assumptions of conducting experiment in calorimeter: (2)

Answer 18

1. negligible heat lose to or gained from the surroundings due to insulation -> 100% efficiency
2. specific heat capacity of solution = specific heat capacity of water = 4.18

density of solution = desity of water = 1.00gcm3

Question 19

theoretical vs experimental lattice energies differs due to

Answer 19

partial covalent character in the ionic bond has not been accounted for by the theoretical ionic compound model.

this results in discrepancy between ..

more exo experimental lattice energies = stronger ionic bonding

Question 20

salt is likely to be soluble when

Answer 20

ΔHsoln < 0. |ΔHhyd|(enough to compensate for latt energy) > |L.E.|

and vice versa

Question 21

At ΔG =0,

Answer 21

reaction is at equilibrium (PHASE CHANGES (melting, freezing, vapourisation, condensation, sublimation) OCCURS AT EQUILIBRIUM!!!)

Question 22

why there is a diff in value for theoretical and experimental values for bond energy?

Answer 22

bond energy values taken from data booklet for C-H and O-H bonds are only average values obtained from polyatomic molecules

Question 23

ΔHsoln =

Answer 23

+|ΔHlatt| - |ΔHhyd|

Question 24

Energetic stability of a substance arises when

Answer 24

its enthalpy content is relatively lower than that of the product of a chemical reaction

Question 25

Kinetic stability of a substance arises when

Answer 25

the rate of chemical reaction is extremely slow, or negligible. this occurs when Ea is extremely high

Question 26

spontaneous reaction

Answer 26

can occur without any external assistance. it is irreversible as it cannot be brought back to its original state again under the same conditions