L3 Chemistry Thermochemical principles 3.4

What is electronegativity?

A measure of the attraction of the nucleus for the bonding electrons in a compound. It increases to the right and upwards.

Compare electronegativity, first ionisation energy, and size of atoms across the same row/period

(x and y are in the same row and therefore the same energy level.) This means they have the same amount of shielding from inner energy levels. x is further along the row and so has more protons causing nuclear charge to increase and so it has a greater attraction for valence electrons. This means bonding electrons will be closer to its nucleus.

Compare electronegativity, first ionisation energy, and size of atoms down the same group

(x and y are in the same group) x is further down the group and so has more protons. Despite this, EN and IE decrease as the valence electrons are further from the nucleus as there are extra energy level(s) which shield the electrons from the electrostatic attraction of the nucleus. Nuclear charge decreases and so there is less attraction for valence electrons

Define (first) ionisation energy

Ionisation energy is the energy needed to remove the outermost electron from a mole of gaseous atoms.

Comparing ionisation energies in the same row

x and y are in the same row therefore are in the same energy level so have the same amount of shielding from inner energy levels. x has a greater IE than y as it is further along the row so has more protons and a greater electrostatic attraction as nucelar charge increases. This causes the outermost valence electrons to be closer to the nucleus. Therefore more energy is needed to remove the outermost electron

Comparing ionisation energies in the same group

x and y are in the same group. x has a smaller IE than y, even though it has more protons, because its outermost electrons are further from the nucleus. This is because x has extra energy level(s) than y, which shields the outermost electrons from the electrostatic attraction of the nucleus, decreasing nuclear charge. Therefore, less energy is needed to remove the outermost electron

Comparing sizes of atoms and negative ions

Give electron configurations. The size of the x ion is greater than the y atom even though they have the same number of protons in their nuclei. This is because the added electron(s) cause an increased repulsion between electrons in the outermost shell.

Comparing sizes of atoms and positive ions

Give electron configurations. The size of the x ion is smaller than the y atom even though they have the same number of protons in their nuclei. This is because electron(s) have been lost and the ion has one less energy level than the atom. This will result in the outermost electrons being closer to the nucleus making the atom smaller.

Explaining shapes and bond angles of molecules

There are x regions of negative charge around the central x atom, x bonds and x non-bonding.

These repel each other to maximise separation and minimise repulsion, carrying the bonded atoms into a x shape.

The x regions of negative charge give an angle of x.

Definition of polar bond

a bond with a slightly positive and a slightly negative dipole

Definition of a polar molecule

a molecule with an unequal distribution of charges

Polar molecule shapes

Trigonal pyramidal, bent, T-shaped, square pyramid, seesaw. If the bonded atoms are different then all other shapes.

Non-polar molecule shapes

Tetrahedral, linear, trigonal planar, octahedral, square planar, trigonal pyramid.

Explain polarity if all bonded atoms are the same (non-polar and polar molecules)

The bond between x and the central atom x is polar because these atoms have different electronegativities giving one atom a slightly neg charge and the other slightly pos.

Non-polar: There are identically sized bond dipoles arranged symmetrically around the central atoms giving a x shape. The dipoles cancel out therefore the molecule is non-polar.

Polar: The bond dipoles are arranged asymmetrically around the central x atom giving a x shape. The dipoles don't cancel out therefore the molecule is polar.

Explaining polarity if all bonded atoms are not the same

The bonds between x and x and between x and x are polar because the atoms have different electronegativities. The resulting bond dipoles will have different sizes. Therefore, despite the symmetric x shape of the bond dipoles they won't cancel out and the molecule is polar.

Do non-bonding electrons repel more or less than bonded regions and why?

Non-bonding electrons repel more than bonded regions because they are only attracted to one nucleus therefore are closer to the central atom so their repulsive effect is greater.

Bond angle in shapes with 4 regions of negative charge.

109

Bond angle in shapes with 3 regions of negative charge.

120

Bond angle in shapes with 2 regions of negative charge.

180

How to discuss the melting point.

Strong bonds = lots of (heat) energy is required to break them = high MP.

Explain why the bond angle may be less than expected

If there is a double bond then the region of negative charge occupies more space therefore repelling more than the electrons in the single bonds. This pushes the atoms closer together causing the bond angle to be slightly smaller than expected.

Define an exothermic reaction

A reaction where products have less energy than reactants. Energy is released as bonds form, and the temperature of the environment increases. Enthalpy change is negative.

Define a endothermic reaction

A reaction where products have more energy than reactants. Energy is absorbed as bonds break, and the temperature of the environment decreases. Enthalpy change is positive..

Unit for energy released/absorbed

kJ

Unit for bond energy

kJ mol^-1

equations for number of moles

n = energy released/change in enthalpy

n = mass/molar mass

Explain the relationship between orbitals, subshells, and electron shells

Electrons occupy energy shells which surround the nucleus. These electron shells are divided into sublevels (s, p, d, f). These consist of orbitals, each of which can hold up to 2 electrons. For example, 1S and 2S are in the s sub-level. 2S and 2P will be in the same electron shell.

Orbital order

1s², 2s², 2p⁶, 3s², 3p⁶, 4s², 3d¹⁰, 4p³

in electron configurations, [Ar] replaces how many electrons?

the first 18

Electron configuration exceptions and why

Cr [Ar] 4s¹ 3d⁵ and Cu [Ar] 4s¹ 3d¹⁰

Cr: The 3d and 4s orbitals are very close in energy. It is more stable to have a set of half filled orbitals than the 4s orbital completely filled and the 3d with one less electron as the electrons repel.

Where are electrons lost from first in EC (ions)?

from 4s. For example CO is [Ar] 4s² 3d⁷ and CO²⁺ is [Ar] 3d⁷

Define intermolecular forces

Forces between molecules. The strength of an IMF id dependant on how close molecules can get to each other. The closer they are the more energy needed to break the molecules apart from each other and the higher the MP, BP, etc. IMF include attractive forces like hydrogen bonds, temporary dipole-dipole attractions, and permanent dipole-diple attractions.

Discussing intermolecular forces

Define the property. Link the property to the strength of IMF between molecules. Give the types of attractive forces. Link the temp dipole attractions to the size of electron cloud (electron number). If only temp dipoles, then it is non-polar, if perm, it is polar. If hydrogen bonds then O, N, or F (with unbonded pairs of electrons bonded to H). Consider structure if all attractive forces are equal - may not fit closely together so IMF is weaker. Mention IMF are between molecules.

Hydrogen bonds

A molecule can form H bonds if it has a small and very EN atom (N, O or F) with unbonded pairs of electrons, that is bonded to H. A strong dipole-dipole attraction forms between molecules pulling them very closely to each other. These are the stongest type of IMF

Temporary dipole-dipole attractions

The strength of these is dependent on the size of its electron cloud (total number of electrons in the molecule). The molar mass gives a rough idea of this. All molecules form temp dipoles.

Permanent dipole-dipole attractions

Only polar molecules form these

Explaining spontaneity of reactions

If a reaction is spontaneous it needs no external energy input to keep it going (in addition to activation energy). Whether a reaction is spontaneous or not is determined by the total entropy change of the system and the surroundings.

The reaction is (exothermic/endothermic) because heat energy is (released/form) (∆rH is negative/positive) into/from the surroundings. As a result there is an (increase/decrease) in the dispersal of energy, so the entropy of the surroundings (increases/decreases).

The reaction has a positive/negative entropy because…. (more moles of products form than reactants/more moles ofgas or liquid form/ions are produced in a solution) This increase/decreases entropy as dispersal of matter and energy increases.

If both the entropy of the surroundings and the system increase it is spontaneous, if both decrease then it is non-spontaneous.

Define ∆vapH

The enthalpy of vaporisation and is the energy absorbed when 1 mole of a substance is changed from a liquid to a gas at its boiling point. (kJmol⁻¹)

Define ∆fusH

The enthalpy of fusion and is the energy required to change 1 mole of a solid into a liquid at its melting point.

Define ∆subH

The latent heat of sublimation is the energy required to convert one mole of the solid to a gas.

Define the standard enthalphy of formation

The standard reaction enthalpy for the formation of one mole of substance from its elements (in their most stable forms) under standard conditions. It is 0 for any element

Define ∆cH

The standard enthalpy of conduction. The energy released in the complete combustion in oxygen of 1 mole of the substance in its standard state and where the products of combustion return to their standard state.

Pos and negative enthalpies of vaporisation and fusion

Values will be positive if bonds are broken and energy is absorbed (melting and boiling) and will be negative if bonds form and energy is released (freezing and condensation)

what is q measured in in q = mc∆T

Joules

Possible errors

• Not done under standard conditions (100KPa and 25C)

• errors when making measurements

• some incomplete combustion occuring

bond angles of square planar

90°

bond angles of trigonal bipyramid

120, 90, and 180

Endothermic

Energy is absorbed as intermolecular forces are broken (pos enthalpy change).

Exothermic

Energy is released as intermolecular forces form (neg enthalpy change).

1st ionisation energy equation

X (g) → X⁺¹(g) + e⁻