5.2 Energy

Question 1

bond enthalpy definition

Answer 1

(average) energy required to break 1 mole of bonds in gaseous particles

Question 2

enthalpy change of combustion definition

Answer 2

enthalpy change when 1 mole of substance is burnt completely, in excess oxygen

Question 3

enthalpy change of reaction definition

Answer 3

energy change when the amount in moles of the substances as written react

Question 4

enthalpy change of formation definition

Answer 4

enthalpy change when 1 mole of substance is formed from its elements in their standard states

Question 5

enthalpy change of neutralisation definition

Answer 5

enthalpy change when 1 mole of water is formed in a reaction between an acid and a base

Question 6

lattice enthalpy definition

Answer 6

enthalpy change when 1 mole of ionic solid is formed from its gaseous ions

Question 7

factor affecting lattice bond enthalpy

Answer 7

size of ions (larger ions = lower charge density = less energy released during formation / absorbed during break down due to weaker bond formed = less negative lattice enthalpy)

Question 8

enthalpy change of atomisation definition

Answer 8

enthalpy change when 1 mole of gaseous atom is formed from an element in its standard state

Question 9

enthalpy change of electron affinity

Answer 9

enthalpy change when each atom in 1 mole of gaseous atoms takes up 1 electron to form 1 mole of gaseous 1- ions

Question 10

enthalpy of solution definition

Answer 10

enthalpy change when 1 mole of a solid compound is dissolved in water to form an infinitely dilute solution

Question 11

enthalpy change of hydration definition

Answer 11

enthalpy change when 1 mole of gaseous ions is dissolved in water to form an infinitely dilute solution
e.g. K+(g) + aq -> K+(aq)

Question 12

relationship between lattice enthalpy and breaking of lattice

Answer 12

energy involved in breaking the lattice is directly involved in forming the lattice
opposites

Question 13

enthalpy of solution exo or endothermic

Answer 13

depends on balance between magnitudes of lattice enthalpy and enthalpy of hydration

Question 14

entropy definition

Answer 14

measure of dispersal of energy in a system

Question 15

entropy organised or disorganised

Answer 15

the greater the entropy, the more disordered the system

Question 16

standard entropy definition

Answer 16

entropy content of one mole of substance under standard conditions
measured in J K^-1 mol^-1

Question 17

how reaction is feasible

Answer 17

overall energy of products is lower than overall energy of reactants
negative ∆G

Question 18

Gibbs free energy change formula

Answer 18

∆G = ∆H - T∆S

Question 19

entropy change of surroundings formula

Answer 19

∆S (surroundings) = ∆H(reactions)/T

Question 20

why feasible reactions may not happen

Answer 20

activation energy may be too high

rate of reaction may be very slow

Question 21

free energy and feasability when -ve enthalpy, -ve entropy

Answer 21

-ve ∆G at low temperatures

only feasible at low temperatures

Question 22

free energy and feasability when -ve enthalpy, +ve entropy

Answer 22

always -ve ∆G

always feasible

Question 23

free energy and feasability when +ve enthalpy, -ve entropy

Answer 23

always +ve ∆G

never feasible

Question 24

free energy and feasability when +ve enthalpy, +ve entropy

Answer 24

-ve ∆G at high temperatures

feasible at high temperatures

Question 25

calculating enthalpy/entropy change of reaction from given enthalpy change of formation/entropy values

Answer 25

products - reactants

Question 26

why is it difficult to predict whether enthalpy change of solution becomes more exothermic or less endothermic down group 7

Answer 26

ionic radius increases going down the group
lattice enthalpy becomes less exothermic when going down the group (enthalpy change of solution would be more exothermic)
enthalpy of hydration becomes less exothermic going down the group (enthalpy change of solution would be less exothermic)
difficult to predict whether enthalpy change of solution is exothermic or endothermic

Question 27

why second electron affinity is endothermic

Answer 27

X- ion repels the electron being added

requires heat energy to overcome repulsion