thermodynamics

Question 1

define enthalpy change of formation

Answer 1

the enthalpy change when one mole of a compound is formed from its constituent elements (in their standard state) under standard conditions (100KPa, 298K, 1mol of reactants)

Question 2

define enthalpy change of combustion

Answer 2

the enthalpy change when 1 mole of
a substance is completely burned in O2 under standard conditions with all
reactants and products in their standard states.

Question 3

define bond dissociation enthalpy

Answer 3

the enthalpy change when 1 mole of
covalent bonds are broken under standard conditions in the gaseous state.

Question 4

define enthalpy change of atomisation

Answer 4

the enthalpy change for the formation of 1 mole of gaseous atoms from the element/compound in its standard state.

Question 5

define first ionisation energy

Answer 5

enthalpy change when 1 mole of electrons is removed from 1 mole of gaseous atoms to form one mole of gaseous ions with a +1 charge.

Question 6

define second ionisation energy

Answer 6

enthalpy change when 1 mole of electrons is removed from 1 mole of gaseous ions with a charge of +1 to form one mole of gaseous ions with a charge of +2

Question 7

define first electron affinity

Answer 7

enthalpy change when 1 mole of gaseous
atoms form 1 mole of gaseous ions w/ a charge of -1.
values for first electron affinity are typically negative, as the process is exothermic

Question 8

values of the second electron affinity are…

Answer 8

+ve as the process is
endothermic.
This is due to the repulsion from the negative ion toward the
negative electron being added.

Question 9

define enthalpy of lattice dissociation

Answer 9

the enthalpy change when 1 mole of an ionic compound is separated into its constituent gaseous ions.

Question 10

define enthalpy of lattice formation

Answer 10

the enthalpy change when 1 mole of
an ionic compound is formed from its constituent ions in the gaseous state

Question 11

define enthalpy of hydration

Answer 11

the enthalpy change when 1 mole of gaseous
ions is converted into 1 mole of aqueous ions.

Question 12

define enthalpy of solution

Answer 12

the enthalpy change when 1 mole of an ionic
substance dissolves in enough solvent to form an infinitely dilute
solution

Question 13

how do we work out enthalpy of solution?

Answer 13

enthalpy of lattice dissociation + enthalpy of hydration of positive ions + enthalpy of hydration of negative ions

Question 14

why can ionic substances dissolve in water?

Answer 14

because water is a polar molecule - it has a positive dipole (hydrogens) and negative dipole (oxygen)

Question 15

what is hess’ law?

Answer 15

The enthalpy change for a chemical reaction is independent of the route taken

Question 16

what do arrows pointing up and arrows pointing down represent on a born-haber cycle?

Answer 16

arrow pointing up = endothermic
arrow pointing down = exothermic

Question 17

the greater the value of the lattice enthalpy…

Answer 17

the stronger the ionic bonding

Question 18

what factors cause stronger ionic bonding?

Answer 18

• smaller ions; more closely packed in the lattice, so are more
  attracted to each other, resulting in high lattice enthalpy.
• higher positive charge; stronger electrostatic forces of attraction
  between the ions which results in high lattice enthalpy

Question 19

what two assumptions is the perfect ionic model based upon

Answer 19

• the bonding is completely ionic
• the ions are regarded as perfect spheres and are not distorted (charge is perfectly distributed)

Question 20

what causes covalent character in an ionic compound?

Answer 20

a small and highly charged cation can distort a large anion so
that there is covalent character to the ‘ionic compound’. This means that ions are distorted and not spherical (That IS assumed by the perfect ionic model)

Question 21

what does it mean if the compound has covalent character?

Answer 21

the forces of attraction in the compound are found to be greater than predicted by the perfect ionic model.
*This leads to a difference between the values of the experimental lattice enthalpy and the theoretical lattice enthalpy. The greater the difference in the values, the greater the covalent character in the
compound.

Question 22

what is a feasible reaction?

Answer 22

a reaction which can occur at a particular temperature and, once started, will carry on to completion without any energy being supplied

Question 23

in every feasible reaction, what happens to the entropy of the system?

Answer 23

entropy of the system increases

Question 24

what is entropy?

Answer 24

a numerical value of the disorder within a system

Question 25

what is entropy measured in?

Answer 25

j/kelvin/mol

Question 26

how does physical state affect entropy?

Answer 26

gases are more disordered than liquids and solutions.
Liquids and solutions are more disordered than solids

Question 27

how does dissolution affect entropy?

Answer 27

dissolving a solid increases its entropy as dissolved particles can move freely because they are no longer held in fixed positions.

Question 28

how does number of particles affect entropy?

Answer 28

more moles of particles increased entropy. The
more particles there are, the more ways they, and their energy can be
arranged.

Question 29

what is the formula for entropy change in a reaction?

Answer 29

∆S = [sum of ∆SƟ products] - [sum of ∆SƟ reactants]

Question 30

at what temperature is a crystalline form of a structure a perfect crystal?

Answer 30

absolute zero (0k)

Question 31

what is the formula for gibbs free energy?

Answer 31

∆G = ∆H – T∆S

Question 32

what is gibbs free energy?

Answer 32

a measure used to predict whether a reaction will be feasible at a particular temperature

Question 33

for a reaction to be feasible, what should gibbs be?

Answer 33

equal to or less than zero