Thermochem

Question 0

Describe an orbital

Answer 0

An orbital is a region of space which can hold a maximum of two electrons around a nucleus.

Question 1

Give the electron configuration for the following atoms:

a) C b) Cl c) Sc d) Se e) Cr f) Al

Answer 1

a) C (6e-) 1s2 2s2 2p6
b) Cl (17e-) 1s2 2s2 2p6 3s2 3p5
c) Sc (21e-) 1s2 2s2 2p6 3s2 3p6 3d1 4s2
d) Se (34e-) 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p4
e) Cr (24e-) 1s2 2s2 2p6 3s2 3p6 3d5 4s1
f) Al (13e-) 1s2 2s2 2p6 3s2 3p1

Question 2

State the maximum number of electrons that can occupy an orbital.

Answer 2

Two

Question 3

Describe what is meant by the term subshell.

Answer 3

Groups of orbitals within a shell (or energy level) in which the electrons have the same energy.

Question 4

For each atom identified below, give the subshell(s) in which valance electrons would be found.

a) An atom with a total of 9 electrons.
b) An atom with the electron arrangement 2, 8, 8, 2.

Answer 4

a) 2s and 2p

b) 4s

Question 5

Give the electron configurations for the following ions:

a) Al3+ b)K+ c)F- d)Mn2+ e)Fe2+ f)Cu+

Answer 5

a) Al3+ (13 - 3 = 10e-) 1s2 2s2 2p6
b) K+ (19 - 1 = 18e-) 1s2 2s2 2p6 3s2 3p6
c) F- (9 + 1 = 10e-) 1s2 2s2 2p6
d) Mn2+ (25 - 2 = 23e-) 1s2 2s2 2p6 3s2 3p6 3d5
e) Fe2+ (26 - 2 = 24e-) 1s2 2s2 2p6 3s2 3p6 3d6
f) Cu+ (29 - 1 = 28e-) 1s2 2s2 2p6 3s2 3p6 3d10

Question 6

Name and give the formula of one ion that has the same arrangement of electrons as (is isoelectronic to):

a) Ar 1s2 2s2 2p6 3s2 3p6
b) He 1s2
c) Zn2+ 1s2 2s2 2p6 3s2 3p10

Answer 6

a) Any one of: Phosphide P3-, sulfide S2-, Chloride Cl-, Potassium K+, Calcium Ca2+, Scandium Sc3+
b) Any one of: hydride ion H-, lithium ion Li+, Beryllium ion Be2+
c) Copper(I) ion Cu+

Question 7

State what is meant by the term ‘isoelectronic’

Answer 7

The particles have the same number and same arrangement of electrons

Question 8

Give the electron configurations for the following species:

a) Na+, Mg2+, Al3+
b) O2-, F-, Ne

Answer 8

a) 1s2, 2s2, 2p6

b) 1s2, 2s2, 2p6

Question 9

State why there are no values for the ionic radii of the noble gases

Answer 9

Noble gases do not form ionic compounds

Question 10

State whether the sodium atom is larger than, smaller than, or about the same size as the magnesium atom. Explain your answer.

Answer 10

Larger. Both have three shells of electrons but the sodium atom has 11 protons, 1 fewer than magnesium (12 protons), and a weaker nuclear attraction on the outer electrons

Question 11

Explain why

a) the sodium atom is larger than the lithium atom
b) the sodium ion is smaller than the potassium ion

Answer 11

a) Sodium (2, 8, 1) has three shells of electrons compared with the two shells of lithium (2, 1)
b) The sodium ion (2, 8) has two shells of electrons compared with the three shells of the potassium ion (2, 8, 8)

Question 12

Discuss the relative sizes of the sodium and fluorine atoms and their monoatomic ions, Na+ and F-

Answer 12

The Na atom has an electron configuration of 1s2 2s2 2p6 3s1 which has an extra level compared with Na+ (1s2, 2s2, 2p6) so the radius of Na atom is larger. Both ions Na+ and F- share the same electron configuration, however Na+ has 11 protons and F- has 9. The smaller number of protons means the electrostatic attraction between the nucleus and the valance electrons thereby making the radius larger than that of Na+. F- ion has one more electron in it’s outer shell than compared with the F atom. Increased electrostatic repulsion between valance electrons results in increased radius of the ion.

Question 13

Explain why there is a large increase in size between B3+ and N3-

Answer 13

B3+ has five protons and an electron arrangement of 1s2. N3- has seven protons and an electron arrangement 1s2 2s2 2p6. Although the N3- ion has two more protons, it has an extra shell of electrons, so its radius is much larger.

Question 14

H- and Li+ have the same number of electrons. Explain why H- is bigger than Li+

Answer 14

H- and Li+ both have the same electron arrangement 1s2, but Li+ has three protons and H- only one proton. This means Li+ has a stronger electrostatic attraction between the nucleus and valance electrons, so the radius of Li+ is smaller.

Question 15

Match the radii 99pm, 137pm and 197pm to the following atoms and ion:
Ca Ca2+ Mn
Justify your answer.

Answer 15

Ca 197pm Ca2+ 99pm Mn 137pm
Justification: Both Ca and Mn have valance electrons in the 4s orbital, but there is a large nuclear charge in the Mn atom. This exerts a stronger electrostatic attraction on valance electrons and the radius of Mn is smaller.
Ca2+ has the smallest radius as its valance electrons are in the 3rd energy level (3p) and are closer to the nucleus.

Question 16

a) write an equation to represent the first ionisation energy of sodium.
b) explain how the reactivity of metals is linked to their ionisation energies.

Answer 16

a) Na(g) -> Na+(g) + e-
b) Metals react chemically by losing electrons. The more reactive metals tend to have lower ionisation energies. (As it is easier for them to lose electrons than other metals based on their electron arrangement)

Question 17

By considering the electron configuration of the atoms, explain the following differences in first ionisation energies:

a) Ar (1350, K (418)
b) Na (494), Mg (736)
c) Li (526), Na (494), K (418)

Answer 17

a) A potassium atom (2,8,8,1) loses its electron from the 4th shell, which is a greater distance from the nucleus with more shielding than in an atom of argon (2, 8, 8) which loses its electron from the 3p sublevel closer to the nucleus.
b) Both atoms lose first electron from the 3s orbital, but magnesium has one more proton and the electron is lost closer to the nucleus so a greater nuclear attraction must be overcome.
c) The decrease in ionisation energies is due to the electron removed from valance shells being more distant from the nucleus and with more shielding due to more inner shells of electrons.

Question 18

Write the equation for the reaction in which the energy change is the first ionisation energy of sulfur.

Answer 18

S(g) -> S+(g) + e-

Question 19

Discuss the factors that cause the first ionisation energy of sulfur to be higher than the first ionisation energies of both sodium and selenium.

Answer 19

The valance electrons of atoms of both Na (2, 8, 1) and S (2, 8, 6), are in the third energy level, but an atom of S has more protons in the nucleus so the electrostatic attraction between the nucleus and the valance electrons is stronger for an atom of S, and the radius is smaller. The valance electrons of an atom of Se are in the 4th energy level, a level further from the nucleus compared to S so there is an increased shielding from inner shells of electrons. The greater distance between the nucleus and valance electrons for an atom of Se means electrostatic attraction is weaker and IE is smaller.

Question 20

Define the term ‘electronegativity’

Answer 20

Electronegativity is a measure of the attraction between the nucleus of an atom and the bonded electrons.

Question 21

Give a reason why there is no electronegativity value in the data books for Ne.

Answer 21

Ne is a group 18 element that does not bond to other atoms (as it has a full outer shell), so it is not assigned a relative electronegativity value.

Question 22

Explain the trend in electronegativity down group 17 of the periodic table.

Answer 22

Going down group 17, the bonded electrons are in energy levels further from the nucleus so there is a decreased attraction between these electrons and the nucleus and a decrease in electronegativity.

Question 23

Explain what is meant by the term ‘chemical bond’.

Answer 23

A strong electrostatic force of attraction between negatively and positively charged species

Question 24

State whether the bond character in the following compounds is most likely to be ionic, covalent, or both ionic and covalent.
a) NaCl b) H2O c) Na2O2 d) CuSO4

Answer 24

a) Ionic bonding - due to the presence of Na+ and Cl- ions
b) Covalent bonding - only non metals involved
c) Ionic bonding - due to the Na+ and O2- ions
d) Ionic bonding between Cu+ and SO42- ions, covalent bonding between atoms in SO42- ions

Question 25

Give reasons for each of the following:

a) the strength of the ionic bond in sodium chloride is less than the strength of the ionic bond in magnesium oxide
b) magnesium oxide has a higher melting point than sodium oxide
c) lithium fluoride has a higher melting point than potassium bromide

Answer 25

a) sodium chloride is composed of 1+ and 1- ions whereas magnesium oxide is composed of 2+ and 2- ions. The higher the charge, the stronger the ionic bond.
b) bonds in MgO are stronger than bonds in Na2O. The bond in MgO require more energy to break, which leads to a higher melting point.
c) lithium fluoride consists of smaller anions and cations with a greater charge density, which cause stronger bonds, resulting in a higher melting point.

Question 26

List the bonds i - iii in order of increasing strength. Explain your answers.

a) i) C_=N ii) C-N iii) C=N
b) i) C-H ii) C-N iii) C-P

Answer 26

a) ii, iii, i Single bonds are weaker than double bonds, which in turn are weaker than triple bonds.
b) iii, ii, i Atoms of a larger radius have weaker bonds to carbon