module 5: physical chemistry and transition elements

Question 1

what is lattice enthalpy?

Answer 1

the enthalpy change when one mole of an ionic compound is formed from its gaseous ions, under standard conditions

Question 2

is the value for lattice enthalpy change always positive or negative, and why?

Answer 2

negative, lattice enthalpy involves ionic bond formation from separate gaseous ions which is an exothermic change

Question 3

what are the key features about lattice enthalpy?

Answer 3

• lattice enthalpy is an exothermic change, lattice enthalpies have a negative sign because energy is given out when ionic bonds are being formed from gaseous ions
• lattice enthalpy is a measure of the strength of ionic bonding in a giant ionic lattice, a large negative value shows that there are strong electrostatic forces of attraction between the oppositely charged ions in the lattice
• a covalent substance does not have a lattice enthalpy because there are no ions in its structure
• lattice enthalpies cannot be measure directly because it is impossible to form a mole of an ionic lattice from gaseous ions

Question 4

how are lattice enthalpies calculated indirectly?

Answer 4

Born-Haber cycle

Question 5

what is the standard enthalpy change of atomisation?

Answer 5

the enthalpy change when one mole of gaseous atoms are formed from the element in its standard state under standard conditions

Question 6

why is the standard enthalpy change of atomisation always an endothermic process?

Answer 6

energy is taken in for bond breaking to form gaseous atoms

Question 7

what is the difference between electron affinity and ionisation energy?

Answer 7

electron affinity is the opposite of ionisation energy
- electron affinity measures the energy to gain electrons
- ionisation energy measures the energy to lose electrons

Question 8

what is the first electron affinity?

Answer 8

the enthalpy change that takes place when one electron is added to each atom in one mole of gaseous atoms to form one mole of gaseous 1- ions

Question 9

why can lattice enthalpies not be determined directly?

Answer 9

you cannot form one mole of an ionic lattice from gaseous ions

Question 10

what are the equations for making the solid sodium chloride?

Answer 10

• atomisation of sodium
• atomisation of chlorine
• ionisation of sodium
• electron affinity of chlorine

Question 11

what is the standard enthalpy change of formation?

Answer 11

the enthalpy change when one mole of a compound is formed from its elements under standard conditions, with all reactants and products in their standard states

Question 12

what is the standard enthalpy change of atomisation?

Answer 12

the enthalpy change when one mole of gaseous atoms is formed from the element in its standard state under standard conditions

Question 13

what is the first ionisation energy?

Answer 13

the enthalpy change required to remove one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions

Question 14

what is the first electron affinity?

Answer 14

the enthalpy change when one electron is added to one mole of gaseous atoms to form one mole of gaseous 1- ions

Question 15

are second electron affinities exothermic or endothermic, and why?

Answer 15

• second electron affinities are endothermic
• a second electron is being gained by a negative ion, which repels the electron away
• so energy must be put in to force the negatively charged electron onto the negative ion

Question 16

what is the enthalpy change of hydration?

Answer 16

the enthalpy change when one mole of gaseous ions is dissolved in water to form one mole of aqueous ions, under standard conditions

Question 17

what are the properties of ionic compounds?

Answer 17

• high melting and boiling points
• soluble in polar solvents
• conduct electricity when molten or in aqueous solution

Question 18

what are the two factors that affect lattice enthalpy?

Answer 18

ionic size and ionic charge

Question 19

how does ionic size affect lattice enthalpy?

Answer 19

• ionic radius increases
• attraction between ions decreases
• lattice energy becomes less negative (less exothermic)
• melting point decreases

Question 20

how does ionic charge affect lattice enthalpy?

Answer 20

• ionic charge increases
• attraction between ions increases
• lattice enthalpy becomes more negative (more exothermic)
• melting point increases

Question 21

what are the two factors affecting hydration enthalpy?

Answer 21

ionic size and ionic charge

Question 22

how does ionic size affect the hydration enthalpy?

Answer 22

• ionic radius increases
• attraction between ion and water molecules decreases
• hydration energy becomes less negative

Question 23

how does ionic charge affect hydration enthalpy?

Answer 23

• ionic charge increases
• attraction with water molecules increases
• hydration enthalpy becomes more negative

Question 24

what is entropy?

Answer 24

a measure of the dispersal of energy and disorder within the chemicals making up the chemical system

Question 25

how does entropy vary across the different states of matter?

Answer 25

- solids have the smallest entropies - liquids have greater entropies - gases have the greatest entropies

Question 26

what factors control whether a reaction can occur?

Answer 26

a reaction can happen if the products have a lower overall energy than the reactants

Question 27

what is the free energy change?

Answer 27

the overall change in energy during a chemical reaction, denoted as ΔG

Question 28

what are the two types of energy that make up the free energy change?

Answer 28

the enthalpy change and the entropy change

Question 29

what is the condition for feasibility according to the Gibb's equation?

Answer 29

for a reaction to be feasible, there must be a decrease in free energy: ΔG < 0

Question 30

what are the limitations of predictions made by ΔG about feasibility?

Answer 30

- many reactions have a negative ΔG value but do not seem to take place - a large activation energy can result in a very slow rate of reaction - although the sign indicates thermodynamic feasibility, it takes no account of the kinetics or rate of a reaction

Question 31

what is reduction?

Answer 31

the gaining of electrons, decrease in oxidation number

Question 32

what is oxidation?

Answer 32

the loss of electrons, increase in oxidation number

Question 33

what is an oxidising agent?

Answer 33

takes electrons from the species being oxidised, contains the species being reduced

Question 34

what is a reducing agent?

Answer 34

adds electrons to the species being reduced, contains the species being oxidised

Question 35

what are the standard conditions?

Answer 35

- a temperature of 298K - a pressure of 100kPa - concentration of 1.00 mol dm-3

Question 36

which electrode does oxidation and reduction occur at?

Answer 36

oxidation happens at the anode, reduction happens are the cathode

Question 37

which electrode is the positive terminal?

Answer 37

the cathode

Question 38

which electrode is the negative terminal?

Answer 38

the anode

Question 39

what is the function of the salt bridge?

Answer 39

in electrochemical cells, the salt bridge allows charge balance to be maintained in the cell

Question 40

what is a transition element?

Answer 40

a d-block element that can form at least one ion with a partially filled d-orbital

Question 41

where are the transition elements located in the periodic table?

Answer 41

the d-block

Question 42

what are the two elements in the d-block that are not considered as transition metals?

Answer 42

scandium and zinc

Question 43

why are scandium and zinc not considered as transition metals?

Answer 43

they do not form ions with partially filled d-orbitals - scandium only forms a 3+ ion, where the d-orbitals are empty - zinc only forms a 2+ ion, where the d-orbitals are full

Question 44

which electrons do transition metals lose first when forming ions?

Answer 44

4s electrons

Question 45

what are some characteristic physical properties of transition metals?

Answer 45

- metallic - high density - high melting and boiling point - shiny - good conductors of heat and electricity

Question 46

what are the characteristic chemical properties of transition metals?

Answer 46

- variable oxidation states - coloured compounds/ions in solution - good catalysts - form complex ions

Question 47

what are the two main ways in which transition metals act as an effective catalyst?

Answer 47

- they provide a surface on which reaction can take place - they change oxidation states to form intermediates required for pathways with lower activation energy

Question 48

what is a complex ion?

Answer 48

a transition metal bonded to ligands by coordinate bonds

Question 49

what is a ligand?

Answer 49

a molecule or ion that can donate an electron pair to the central metal ion to form a coordinate bond

Question 50

what is a coordinate bond?

Answer 50

a shared pair of electrons in which the bonded pair has been provided by only one of the bonding atoms

Question 51

what does the coordination number indicate?

Answer 51

the number of coordinate bonds between the metal ion and the ligands

Question 52

what is a monodentate ligand?

Answer 52

a ligand that is able to donate one pair of electrons to the central metal ion to form one coordinate bond

Question 53

what is a bidentate ligand?

Answer 53

a ligand that is able to donate two pairs of electrons to the central metal ion to form two coordinate bonds

Question 54

what is a multidentate ligand?

Answer 54

a ligand that forms three or more coordinate bonds with the central metal ion

Question 55

what are the most common bidentate ligands?

Answer 55

1,2-diaminoethane and the ethanedioate ion

Question 56

EDTA is a chelating agent, what does that mean?

Answer 56

EDTA decreases the concentration of metal ions in the solution by binding to them and forming complex ions

Question 57

why is carbon monoxide toxic?

Answer 57

- CO also coordinately bonds with the central Fe 2+ ion - CO bonds more strongly than oxygen and this binding is irreversible - stops oxygen gas from binding to the haemoglobin - so oxygen cannot be transported around the body