Energetics Definitions

Question 1

Standard Enthalpy Change of Formation

Answer 1

of a substance is the enthalpy change that occurs when one mole of the substance is formed from its elements in their standard states

Question 2

Bond Enthalpy

Answer 2

is the energy needed to break one mole of bonds in gaseous molecules under standard conditions.

Question 3

First Ionisation Energy

Answer 3

is the minimum energy required to remove one mole of electrons from one mole of gaseous atoms
X(g) —> X+(g) + e
(endothermic) +

Question 4

First Electron Affinity

Answer 4

is the enthalpy change when one mole of gaseous electrons is added to one mole of gaseous atoms.
X(g) + e —> X-(g)
(exothermic) -

Question 5

Lattice Enthalpy

Answer 5

is the enthalpy change that occurs when one mole of a solid ionic compound is separated into gaseous ions under standard conditions.
XY(s) —> X+(g) + Y-(g)
(endothermic)

Question 6

Enthalpy Change of Atomisation

Answer 6

is the enthalpy change that occurs when one mole of gaseous atoms is formed from the element in its standard state
X(s) —> X(g)

Question 7

Enthalpy Change of Solution

Answer 7

is the enthalpy change when one mole of a solute is dissolved in a solvent to infinite dilution under standard conditions of temperature
related to the lattice enthalpy and the hydration enthalpies of the constituent ions

Question 8

Enthalpy of Hydration of an Ion

Answer 8

is the enthalpy change that occurs when one mole of gaseous ions is dissolved to form an infinitely dilute solution of one mole of aqueous ions
(exothermic)

Question 9

Entropy

Answer 9

refers to the distribution of available energy among the particles, the more ways energy can be distributed the higher the entropy
or
a measure of the disorder or randomness of the particles

Question 10

Gibbs Free Energy

Answer 10

relates to the energy that can be obtained from a chemical reaction to the change in enthalpy, change in entropy and absolute temperature

measure of the driving force of a reaction, its tendency to proceed spontaneously

Question 11

Enthalpy

Answer 11

is the heat content of a system

Question 12

Endothermic

Answer 12

heat is added to a system
enthalpy change is positive
graph shows products have higher energy than reactants

Question 13

Exothermic

Answer 13

heat is released to the surroundings
enthalpy change is negative
graph shows products of lower energy than reactants

Question 14

Standard Enthalpy Change of Combustion

Answer 14

the enthalpy change for the complete combustion of one mole of a substance in its standard state in excess oxygen under standard conditions

Question 15

Why do experimental values of standard enthalpy of combustion differ from data booklet ones?

Answer 15

• not all the heat produced by the combustion reaction is transferred to the water, some is needed to heat the calorimeter and some has passed to the surroundings
• the combustion of the ethanol is unlikely to be complete owing to the limited oxygen available, as assumed by the literature value
• some experiments may not be carried out under standard conditions

Question 16

Hess’s Law

Answer 16

the enthalpy change for a reaction that is carried out in a series of steps is equal to the sum of the enthalpy

Question 17

Why are bonds in oxygen and ozone broken by UV of different wavelengths?

Answer 17

O2 has a higher bond enthalpy because it has a double bond (bond order 2) and therefore needs a shorter wavelength of higher energy.
O3 has a lower bond enthalpy (bond order 1.5) due to delocalisation. Therefore it needs UV light of a longer wavelength (less energy)

Question 18

Steps of the natural formation and depletion of Ozone

Answer 18

step 1:
O2 –> 2O.
endothermic
high energy needed
step 2:
O. + O2 (in eq. with) O3
forward is exothermic
lower energy
step 3:
O3 + O. –> 2O2
exothermic

Question 19

Born Haber Cycle

Answer 19

is an energy cycle based on Hess’s Law but shows the formation of an ionic compound

Question 20

What do lattice enthalpies depend on?

Answer 20

depends on the product of the ionic charges and the sum of the ionic radii
- an increase in the ionic radius of one of the ions decreases the attraction between the ions
- an increase in the ionic charge increases the ionic attraction between the ions

Question 21

G and Spontaneity

Answer 21

if G < 0 it is a spontaneous reaction
if G > 0 it is not a spontaneous reaction
if G = 0 the reaction is at equilibrium

Question 22

Determining G and spontaneity based on H and S

Answer 22

H = + and S = + : G depends on T and reaction is only spontaneous at high T
H = + and S = - : G always + so reaction is never spontaneous
H = - and S = + : G is always negative so the reaction is always spontaneous
H = - and S = - : G depends on T and the reaction is only spontaneous at low T

Question 23

What do values of S mean?

Answer 23

if deltaS is + there has been an increase in the disorder of the system
if deltas is - there has been a decrease in disorder (become more ordered)

Question 24

Formula for enthalpy for calorimetry (experimental data)

Answer 24

H = q/n
enthalpy change = heat energy / number of moles

Question 25

Formula to use when directed to enthalpy of formations

Answer 25

H = sum of Hf products - sum of Hf reactants
* remember Hf of elements = 0

Question 26

Formula to use when directed to bond enthalpies

Answer 26

H = H bonds broken - H bonds formed

Question 27

Formula to use when finding change in entropy

Answer 27

S = sum S products - sum of S reactants

Question 28

Formula to use when finding G not using H or T or S

Answer 28

G = G products - G reactants