Module 5 - (chapter 22) Enthalpy and Entropy

Question 1

lattice enthalpy

Answer 1

• the enthalpy change the accompanies the formation of one mole of an ionic compound from its gaseous ions under standard conditions
• it is exothermic and the value will always be negative

Question 2

standard enthalpy change of formation

Answer 2

• enthalpy change that takes place when one mole of a compound is formed from its constituent elements under standard states
• the compound will always be an ionic compound in its solid lattice

Question 3

standard enthalpy change of atomisation

Answer 3

• the enthalpy change that takes place for the formation of one mole of gaseous atoms from the element in its standard state under standard conditions
• always endothermic because bonds are broken to form gaseous atoms

Question 4

first ionisation energy

Answer 4

the enthalpy change required to remove one electron from each atom in one mole of gaseous atoms to for one mole of gaseous 1+ ions
-endothermic because energy is required to overcome the attraction between negative electron and positive nucleus

Question 5

electron affinity

Answer 5

• the enthalpy change that takes place when one electron is added to each atom in one mole of gaseous atoms to form one moles of gaseous 1- ions
• exothermic because the electron being added is attracted towards the nucleus

Question 6

successive electron affinities

Answer 6

• occurs when an anion has a greater charge than 1- such as O2-, successive electron affinities are required
• they are endothermic as a second electron is being gained by a negative ion, which repels the electron away. energy must be put in to force the negatively charged electron onto the negative ion

Question 7

standard enthalpy change of solution

Answer 7

• the enthalpy change that takes place when one mole of a solute dissolves in a solvent
• if the solvent is water, the ion from the ionic lattice finish surrounded with water molecules as AQ ions
• can be exothermic or endothermic

Question 8

how does something dissolve? (e.g. sodium chloride)

Answer 8

• the partial + and - shares in the water molecules are attracted towards the positive and negative ions
• the delta negative oxygen atom is attracted to the positive sodium ion
• the delta positive hydrogen atom is attracted to the negative chloride ion

Question 9

what is important to note in MCAT

Answer 9

• make sure you consider what is changing temperature
• e.g. solution may be what is dissolved plus water
• if you don’t use what is dissolved you would have obtained finally enthalpy charge

Question 10

what two types of energy are involved in the dissolving process

Answer 10

• the ionic late breaks up
• water molecules are attracted to and surround the ions (hydrated)
• the ionic lattice is broken up forming separate gaseous ions. this is the opposite energy change from lattice energy, which forms the ionic lattice from gaseous ions
• the separate gaseous ions interact with polar water molecules to form hydrated aqueous ions. (ECoH)

Question 11

enthalpy change of hydration

Answer 11

the enthalpy change that accompanies the dissolving of gaseous ions in water to form one moles of aqueous ions.
exothermic as we are forming bonds and energy is given out as water molecules bond to the metal ions

Question 12

enthalpy change of solution + equation

Answer 12

can be exothermic or endothermic, depending on the relative sizes of the lattice enthalpy and the enthalpy change of hydration
is equal to (-lattice enthalpy) + hydration

Question 13

borne- harbour route for dissolving

Answer 13

lattice enthalpy down arrow from gaseous to lattice

hydration down from gaseous to aqueous

dissolving up from lattice to aqueous

Question 14

general properties of ionic compounds

Answer 14

-high melting and boiling points
some can be melted using a bunsen burned, others are so high that they can be used to coat the inside of furnaces
-soluble in polar solvents
-conduct electricity when molten or in aq solution
-some are soluble in polar solvents (e.g. water), others are insoluble

Question 15

how does ionic size effect lattice enthalpy

Answer 15

• as you go down group 1 ionic radius increases
• attraction between ions decreases
• less energy needed to overcome forces
• lattice energy less exothermic (less negative9
• melting point decreases.

Question 16

how does ionic charge impact lattice enthalpy

Answer 16

• as the ionic change increases
• the attraction between ions increases
• lattice energy becomes more negative (more exothermic)
• melting point increases

Question 17

Na2+ to Al3+

Answer 17

• two supporting effects
• increasing charge gives more attraction
• decreasing size gives more attraction

Question 18

cl- to S2-

Answer 18

• two opposing effects
• increase change gives more attraction
• increasing size gives less attraction

Question 19

predicting melting points

Answer 19

• magnitude of lattice enthalpy indicated melting point
• MgO for example has very exothermic lattice enthalpies and very high mps
• these so state used to coat indie of furnaces

Question 20

predicting solubility

Answer 20

• the attraction between the ions in the lattice must be overcome
• water molecules are attracted to the positive and negative ions, surrounding them and releasing energy equal to hydration enthalpy
• yet, compounds with + enthalpy changes are also soluble

Question 21

entropy

Answer 21

used to describe the dispersal of energy within e chemical system which is greater the more disordered it is

• the greater the entropy, the greater the dispersal of energy and the greater the disorder and the greater the that energy is spread out per kelvin per mole
• units are JK-1mol-1

Question 22

general entropy rules

Answer 22

• solids have the smallest entropies
• liquids have greater entropies
• gases have the greatest entropies
• this is because melting and boiling increase the randomness of particles meaning energy is spread out more and is thus more positive
• however even states have substances thatch have very different entropies

Question 23

predicting entropy values

Answer 23

• at 0K there would be no energy and all substances would have zero entropy
• above 0K energy becomes dispersed
• if a system changes to become more random, energy can be spread out more and the entropy change will be positive
• if a system changes to become less random, energy becomes more concentrated meaning the entropy change will be negative

Question 24

how does moles affect entropy

Answer 24

• if the products have more gas molecules entropy increases

- disorder increases so energy more spread out and more positive

Question 25

standard entropies

Answer 25

entropy of one mole of a substance under standard conditions -always positive

Question 26

standard entry change

Answer 26

sum of standard entropy of products - sum of standard entropy of reactants

Question 27

feasibility

Answer 27

used to describe whether a reaction is able to happen and is energetically feasible -also known as spontaneous -for a reaction to be feasible deltaG must be less than 0 -overall energy of the products must be lower than reactants -just because reaction is feasible does t mean it reacts spontaneously at room temp, activation energy must be put in

Question 28

free energy

Answer 28

- the overall change in energy during a reaction - delta G - made of the enthalpy change (the heat transferred between chemical system and the surroundings) and entropy change (the dispersal of energy within a chemical system)

Question 29

the Gibbs equation

Answer 29

deltaG = deltaH - Tx deltaS

Question 30

conditions for feasibility

Answer 30

- the value of deltaH is usually much larger than deltaS so dominated the equation (usually given in KJ-1mol-1) - as temperature increases the T x deltaS term becomes more important

Question 31

delta H Negative | delta S positive

Answer 31

delta G is negative if T is low or high | reaction is feasible

Question 32

delta H positive | delta S negative

Answer 32

dealt G positive if T is low or high | not feasible

Question 33

delta H Negative | delta S negative

Answer 33

when T is low delta G is negative, so feasible | when T is high delta G is positive, so not feasible

Question 34

delta H positive | delta S positive

Answer 34

when T is low delta G is positive, so not feasible | when T is high delta G is negative, so feasible

Question 35

which reactions tend to be more feasible

Answer 35

exothermic - delta H will be negative meaning delta G is ore likely to be negative - if endothermic, the delta S or T must be very high to make delta G negative

Question 36

limitations of Delta G

Answer 36

- indicated thermodynamic feasibility but doesn't account for kinetics or rate of reaction 0e. g. hydrogen peroxide is negative but large activation energy so reaction very sow. MnO2 is added as a catalyst