Topic 5 &15 Energetics

Question 1

Enthalpy of Solution

Answer 1

enthalpy change when one mole of solid is dissolved in sufficient solvent to give an infinitely dilute solution of aqueous ions
e.g. NaCl(s) + aq —> Na+(aq) + Cl- (aq)

Question 2

enthalpy of hydration is always? (endo or exo)

Answer 2

exothermic because attractions are being formed between cations and partially negative oxygen in water and partially positive H+ in water (bond forming will always be exothermic!)

Question 3

enthalpy of hydration

Answer 3

enthalpy change when one mole of GASEOUS ions is dissolved in sufficient solvent to give an infinitely dilute solution of aqueous ions
e.g. Na+ (g) + aq —> Na+ (aq)

Question 4

why is the enthalpy of hydration of Cl- less exothermic than Na+

Answer 4

Cl- has a larger atomic radius than Na+ and therefore weaker attractions with water, so it is less exothermic

Question 5

lattices with ions of (a) charge and (b) radii have more endothermic lattice energy values than those with ions of __(c)__ charge and _(d)__ radii

Answer 5

a) higher
b) smaller
c) lower
d) larger

Question 6

stronger ionic bonding means a more _(endo or exotheric)__ lattice energy

Answer 6

endothermic,because it requires more energy to break the lattice

Question 7

standard enthalpy of formation

Answer 7

enthalpy change that occurs when one mole of substance is formed from its elements in their standard states under standard conditions of 100kPa and 298K

Question 8

standard enthalpy change of formation of an element in its most stable form is equal to:

Answer 8

zero

Question 9

enthalpy of combustion formula

Answer 9

= sum of combustion of reactants - sum of combustion of products

Question 10

Hess’s law

Answer 10

enthalpy change for any reaction is independent of the route by which the rxn takes place, provided the starting and final conditions, and rxns and products, are the same

Question 11

enthalpy of formation formula

Answer 11

sum of formation of products - sum of formation of reactants

Question 12

entropy

Answer 12

measure of disorder or randomness

Question 13

molecules with more atoms have ___ standard entropies than those with fewer atoms

Answer 13

higher

Question 14

as temperature increases, the entropy of the chemical

Answer 14

increases

Question 15

entropy change of a system formula

Answer 15

= sum of entropy of products - sum of entropy of reactants

Question 16

why is breaking bonds endothermic

Answer 16

energy is required to overcome electrostatic attraction between shared pair of electrons and positive nuclei of bonded atoms

Question 17

average bond enthalpy

Answer 17

energy needed to break one mole of bonds of gaseous molecules under standard conditions, averaged from values of bond enthalpy in a range of similar molecules

Question 18

first electron affinity

Answer 18

energy change associated when 1 mole of electrons is added/gained by 1 mole of gaseous atoms to form 1 mole of gaseous 1- ions

Question 19

how does first electron affinity change down a group

Answer 19

gets less exothermic because incoming e- and nucleus is weaker as the atomic radius increases

Question 20

standard enthalpy of atomization

Answer 20

enthalpy change when one mole of gaseous atoms is formed from element in its standard state under standard conditions
e.g. Na(s) -> Na(g)
or for a diatomic molecule:
e.g. chlorine: 1/2Cl2(g) -> 1 Cl (g)

Question 21

lattice energy

Answer 21

energy change when one mole of an ionic solid is broken down into its constituent gaseous ions under standard conditions

Question 22

first ionization energy

Answer 22

amount of energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of singly charged positive gaseous ions

Question 23

which two pieces of information do you need to calculate the enthalpy of solution

Answer 23

1. lattice enthalpy i.e. NaCl (s) -> Na+ (g) + Cl-(g)
2. enthalpy of hydration i.e. (g) -> (aq)

Question 24

how would a larger ionic radius affect the enthalpy of hydration

Answer 24

larger ionic radius = weaker attractions with water therefore less exothermic

Question 25

which reactions are exothermic

Answer 25

neutralization, combustion, condensation

Question 26

is first electron affinity generally endo or exothermic and why

Answer 26

exothermic, because attractions are forming between the positive charge of the atom’s nucleus and the electron

Question 27

why is the second electron affinity always endothermic

Answer 27

an electron is being added to an ion which is already negative resulting in repulsion, which requires energy to overcome

Question 28

which equation corresponds to the bond enthalpy of H-I bond

Answer 28

HI(g) -> H(g) + I(g)

Question 29

if Kc > Q what does this mean for the equilibrium and the value of delta G

Answer 29

reaction proceeds in the forward direction, so delta G is < 0

Question 30

if Kc < Q what does it mean for the eqm and value of delta G

Answer 30

reaction proceeds in the reverse direction, so delta G is > 0

Question 31

if Kc = Q what does it mean for eqm and value of delta G

Answer 31

rxn @ eqm; delta G = 0

Question 32

homogenous catalyst

Answer 32

in the same state (phase) as the reactant

Question 33

in what region of the em spectrum does ozone absorb

Answer 33

infrared, which raises the temperature of the stratosphere, contributing to global warming

Question 34

why does oxygen have a stronger bond strength than ozone

Answer 34

it has a double bond and bond order of two compared to ozone, which has a bond order of 1.5, so it absorbs shorter wavelengths of light because it requires more energy to break the bonds in oxygen than ozone

Question 35

standard enthalpy of combustion

Answer 35

enthalpy change that occurs when one mole of substance burns completely under standard conditions

Question 36

what is combustion always (endo or exo)

Answer 36

exothermic

Question 37

what is the significance of Q

Answer 37

reaction quotient; measure of relative amounts of products or reactants for a reaction that is not yet at eqm (also indicates in which direction the rxn is likely to proceed)