5.5 Enthalpy and Entropy

Question 1

what is enthalpy change of atomisation

Answer 1

the enthalpy change when 1 mole of gaseous atoms are formed from an element in its standard state
Na(s) -> Na(g)

Question 2

what is first ionisation energy

Answer 2

energy required to remove the 1st electron from each atom in 1 mole of atoms in its gaseous state
Na(g) -> Na+(g) + e-

Question 3

what is electron affinity

Answer 3

enthalpy change when every atom in 1 mol of gaseous atom acquires an electron to form 1 mol of gaseous ions

Question 4

what does first electron affinity involve?

Answer 4

the electron being gained experiencing an attraction to the nucleus
is an exothermic reaction

Question 5

what does second electron affinity involve?

Answer 5

the electron being gained experiencing repulsion by the negatively charged ions

Question 6

what is enthalpy of formation

Answer 6

the enthalpy change when 1 mole of a compound is made from its elements in their standard states
H2(g) + 1/2O2(g) -> H2O(l)

Question 7

what does Hess’ law state

Answer 7

the enthalpy change for a chemical reaction depends only on the initial and final states and is independent on the path followed.
or
whatever route in which the reaction proceeds the overall enthalpy change for the reaction will be the same.

Question 8

Born-Haber cycles

Answer 8

used primarily as a means of calculating lattice enthalpies which cannot otherwise be measured directly
made of many separate sub-processes each with their own enthalpies

Question 9

what is the process of the Born-Haber cycle

Answer 9

1. atomisation
2. 1st ionisation
3. atomisation
4. electron affinity
5. lattice enthalpy
6. formation

Question 10

what are exothermic reactions

Answer 10

release energy (get hot)
makes bonds

Question 11

what are endothermic reactions

Answer 11

absorb energy (get cold) 
breaks bond

Question 12

why do solid ionic lattices form

Answer 12

not only as a result of the transfer of electrons, but also the huge release in energy that occurs when the 2 ions of opposite charge combine to form a solid

Question 13

what is lattice enthalpy

Answer 13

enthalpy change when 1 mole of an ionic compound is formed from its gaseous ions under standard conditions
values are used as a measure of the strength of ionic bonding in giant ionic lattices

Question 14

how do you calculate standard enthalpy change of solution

Answer 14

1. calculate energy change (q) in kJ using q=mcT
2. calculate amount in moles of the thing that’s being dissolved
3. calculate standard enthalpy change of solution using △H=q/n

Question 15

what is enthalpy change of solution

Answer 15

the enthalpy change for 1 mole of solute dissolving

the process of dissolving can be endo or exothermic

Question 16

what happens when a solid dissolves- breakdown

Answer 16

1. breakdown of the ionic lattice into monatomic gas ion
   opposite to lattice enthalpy
   K+ + Cl- -> KCl(s)
   KCl(s) -> K+ + Cl-

Question 17

what happens when a solid dissolves- hydration

Answer 17

hydration of gaseous ions by water molecules
ions bond to water
is exothermic
slight + ion are attracted to slight - oxygen
slight - ion are attracted to slight + hydrogen

Question 18

what is enthalpy change of hydration

Answer 18

enthalpy change for 1 mole of gaseous ions dissolving in water
energy released when the ions form bonds with water molecules
different ions have different enthalpy changes

Question 19

what happens when a solid dissolves

Answer 19

breaking the lattice is endothermic

water interacting with the gaseous ions is exothermic

Question 20

what happens if the lattice is….

Answer 20

if lattice enthalpy > enthalpy of hydration, then the enthalpy of solution will be endothermic
if lattice enthalpy < enthalpy of hydration, then the enthalpy of solution will be exothermic

Question 21

what is the relation between ionic size of atoms and lattice enthalpy

Answer 21

as the radius gets smaller, there is an increased attraction, as the ions closer together.
this means that lattice enthalpy increases

Question 22

what is the relation between charge and lattice enthalpy

Answer 22

as charge increases, there is more lattice enthalpy. this is because it can form stronger bonds and so a larger attraction
ion charge is more important than ionic size

Question 23

enthalpy change of hydration

Answer 23

as ionic radius decreases, the enthalpy change of hydration becomes more negative
hydration depends on the ability of an ion to attract and bond with water. smaller molecules exert more attraction

Question 24

what is the relation of lattice enthalpy, enthalpy change of solution and enthalpy change of hydration

Answer 24

enthalpy change of solution is more affected by lattice enthalpy than enthalpy change of hydration
if lattice enthalpy is smaller than hydration, the enthalpy of solution is exothermic
if lattice enthalpy is greater than hydration, the enthalpy of solution is endothermic

Question 25

entropy

Answer 25

measure of the dispersal of energy in a system, which is greater the more disordered the system has an irregular arrangement of particles and so has a high entropy. has a regular arrangement of particles and so has a low entropy symbol is S units is JK-1mol-1

Question 26

what is an idea of entropy

Answer 26

nature tends to move from an ordered state to one of disorder a system becomes energetically more stable when it becomes less ordered

Question 27

what are factors that affect entropy

Answer 27

- when a solid becomes a gas - when a solid lattice dissolves - when the number of gaseous molecules increases

Question 28

explanation- when a solid becomes a gas

Answer 28

solids wobble around a fixed point | gas has high randomness

Question 29

explanation- solid lattice dissolves

Answer 29

dissolving increases entropy | moves freely

Question 30

explanation- more gas

Answer 30

more particles means there's more ways they and their energy can be arranged

Question 31

what is the equations for entropy

Answer 31

entropy= sum of products- sum of reactants

Question 32

spontaneous changes

Answer 32

occurs in 1 particular direction and takes place without the need for work exothermic reactions are usually spontaneous

Question 33

what is free energy

Answer 33

a reaction is only spontaneous if it can do work and must generate a free potential - free energy (△G) indicates a reaction capable of proceeding of its own accord

Question 34

free energy and spontaneity

Answer 34

if free energy is < 0, it is negative and a spontaneous reaction if free energy is = 0, the system is in equilibrium if free energy is > 0, it is positive and a non-spontaneous reaction

Question 35

what is the 2nd law of thermodynamics

Answer 35

based on entropy entropy tends to a maximum infers that all chemical and physical changes involve an overall increase in entropy

Question 36

how to increase entropy

Answer 36

``` solids melt liquids boil ionic substances dissolve in water number of gas molecules increases temperature increases ```

Question 37

what is the free energy equation

Answer 37

△G= △H-T△S free energy = enthalpy change - (entropy X temperature) if free energy in a reversible reaction at equilibrium is 0, △S= △H/T or △H=T△S