thermodynamics

Question 1

define hess’ law

Answer 1

enthalpy change us independent of the route taken

Question 2

enthalpy of formation

Answer 2

enthalpy change when one mole of compound is formed from elements in their standard states and conditions

Question 3

enthalpy of combustion

Answer 3

enthalpy change when one mole of compound is burnt completely in excess oxygen in standard state and conditions

Question 4

electron affinity

Answer 4

enthalpy change when one mole of gaseous atoms gain a mole of electrons to become a 1- gaseous ion

Question 5

lattice formation

Answer 5

enthalpy change when one mole of solid ionic compound formed from its gaseous ions

Question 6

lattice dissociation

Answer 6

enthalpy change when one mole or solid ionic compound dissociates into its gaseous ions

Question 7

enthalpy of atomisation

Answer 7

enthalpy change when one mile
or gaseous atoms formed from element in their standard state

Question 8

mean bond enthalpy

Answer 8

enthalpy change when one mole of gaseous molecules each breaks a covalent bond to form 2 free radicals averaged over a range of compounds (diatomic) 2x atomisation

Question 9

first enthalpy of ionisation

Answer 9

enthalpy change when one mole or electrons is removed from one mole gaseous atoms

Question 10

what determines which is more exo (lattice enthalpy)

Answer 10

radius
attraction
more or less
higher lattice enthalpies

smaller ions
larger ions

ions increase down group a group in size more exo = stronger ionic bonding

small +ve ion
large -ve ion

more exothermic than perfect ionic model (no covalent character and perfect sphere)

Question 11

enthalpy of solution

Answer 11

one mole of solute dissolves to form its aqueous ions

Question 12

enthalpy of hydration

Answer 12

one mole of gaseous ions are converted to one mole of aqueous ions
k+(g) + aq —> k+(aq)

Question 13

what is perfect ionic model

Answer 13

ions are perfect point spheres

Question 14

what is entropy and what affects and equation

Answer 14

measures disorder

more disorder +ve entropy

more moles of gas formed
dissolving

entropy = sum of products - sum of reactants

Question 15

gibbs free when feasible

Answer 15

^G = ^H - T^S

feasible when ^G<0

^H
——
^S

Question 16

enthalpy of solution of MgCl2

Answer 16

MgCl2 + aq —> Mg2+(aq) + 2-(aq)

Question 17

why is the first electron affinity of oxygen exothermic

Answer 17

strong attraction between nucleus of O atom and outer e-

Question 18

which has the highest enthalpy of lattice and why
MgCl2 -2018
MgO -2443
BaCl2 - 3889

Answer 18

MgCl2 = -2443
MgO= -3889
BaCl2 = -2018

small +ve and large -ve

Ba2+ is large so weakest

O2- larger -Ve than Cl-

so strongest attraction therefore most exothermic

Question 19

if tell increases solubility of
MgCl2 —> Mg2+ + 2Cl-

Answer 19

as temp increases
equilibrium shifts to the left
less moles
more
MgCl2 undissolved
therefore less soluble

Question 20

explain why hydration of chloride is exothermic

Answer 20

chloride ion is strongly attracted to s+ H on H2O

when chloride binds to s+ H energy released

Question 21

why does enthalpy of hydration become
less negative from Li+ to K+

Answer 21

size of ion increases
weaker attraction between +ve ion and if -O in water

Question 22

units and conversions of gibs free:

Answer 22

T=O degrees = 273K

convert ^s to kjmol-1 by dividing by 1000

^G = Kjmol-1

Question 23

why might a feasible reaction not be observed

Answer 23

activation energy too high or reaction happening very slowly

Question 24

explain why the bond enthalpy of a cl-cl bond is greater than Br-Br bond

Answer 24

chlorine is a smaller atom than bromine - stronger attraction between nucleus and outer electron is stronger

Question 25

freezing of water is an exothermic process give one reason why temp of water can stay at a constant value of 0 degrees when it freezes

Answer 25

heat given out escapes

Question 26

explain why electronegativity of fluorine is greater than the electronegativity of chlorine

Answer 26

fluorine atom smaller than chlorine so stronger attraction, electrons attract to the nucleus more strongly

Question 27

explain why the hydration enthalpy of the fluorine ions is more negative than chorine ions

Answer 27

F- ion smaller than Cl- ions so negative charge attacks detail
positive of hydrogen on water more strongly