AS 91164 Chemistry 2.4 – Bonding, Structure & Enthalpy

A comprehensive set of Q&A flashcards covering ionic and covalent bonding, Lewis structures, electronegativity, molecular shapes and polarity, solid structures and properties, and enthalpy/energy calculations for AS 91164 Chemistry 2.4.

What type of bond forms between a metal and a non-metal?

An ionic bond.

What causes an ionic bond to form?

Electrostatic attraction between positive and negative ions created by electron transfer.

How do covalent bonds form?

By sharing pairs of electrons between two non-metal atoms.

What does a Lewis (electron-dot) diagram show?

Only the valence-shell electrons around the atoms in a molecule.

In a Lewis diagram, how many electrons satisfy H and Li?

2 electrons (duet).

State Step 1 when drawing a Lewis diagram.

Count total valence electrons for all atoms.

Give the periodic trend for electronegativity.

It increases across a period (left→right) and up a group (bottom→top).

Define electronegativity.

The ability of an atom’s nucleus to attract shared electrons.

What EN difference (≈ΔEN) produces a non-polar covalent bond?

ΔEN ≤ 0.5.

What EN difference produces an ionic bond?

ΔEN > 1.6.

What EN difference gives a polar covalent bond?

0.5 < ΔEN < 1.7.

How many areas of electronegativity give bond angles of 109°?

Four areas.

Base shape and bond angle for three areas of electron density?

Trigonal planar, 120°.

Base shape and bond angle for two areas of electron density?

Linear, 180°.

Name the molecular shape for 4 regions / 4 bonds.

Tetrahedral.

Name the molecular shape for 4 regions / 3 bonds.

Trigonal pyramidal.

Name the molecular shape for 4 regions / 2 bonds.

Bent (V-shaped).

Name the molecular shape for 3 regions / 2 bonds.

Bent (120°).

Outline the four steps to determine molecular shape (VSEPR).

1) Draw Lewis diagram; 2) Count regions of electron density; 3) Use regions to find base angles; 4) Count bonded atoms to name shape.

What two factors determine molecular polarity?

Individual bond polarities and the symmetry (arrangement) of those bonds.

What is indicated by δ+ and δ- on a structural diagram?

Partial positive and partial negative charges showing bond polarity.

Rule for solubility based on polarity?

‘Like dissolves like’: polar substances dissolve in polar solvents; non-polar dissolves in non-polar.

What structural feature makes a substance electrically conductive as a solid?

Presence of mobile charged particles (delocalised electrons or mobile ions).

Describe the structure of metallic solids.

Positive nuclei in a lattice surrounded by a sea of delocalised electrons.

Why are metals malleable and ductile?

Mobile electrons allow layers of ions to slide while maintaining metallic bonding.

When do ionic solids conduct electricity?

When molten or dissolved, because ions become mobile.

Why are ionic solids brittle?

Shifting layers can align like charges, causing repulsion and lattice fracture.

What holds molecular solids together?

Weak intermolecular forces (e.g., dispersion, dipole-dipole, hydrogen bonding).

Compare melting points: molecular vs covalent network solids.

Molecular solids: low; covalent networks (e.g., diamond) : very high.

Which covalent network form of carbon conducts electricity and why?

Graphite; it has delocalised electrons between its 2-D layers.

State the specific heat capacity of water.

4.2 J g⁻¹ °C⁻¹.

Define enthalpy (H).

The total heat energy content of a substance.

What sign of ΔH denotes an exothermic reaction?

Negative (-ΔH).

What is activation energy?

The minimum energy required to initiate a reaction.

Energy equation using bond energies to find ΔH.

ΔH = Σ(bonds broken) – Σ(bonds formed).

Explain why breaking bonds is endothermic.

Energy must be absorbed to overcome attractive forces within a bond.

Describe the energy profile of an exothermic reaction.

Products lie lower in energy than reactants; overall energy is released (-ΔH).

Calculation step: How is ΔH per mole adjusted for a coefficient in the balanced equation?

Divide the tabulated ΔH by the stoichiometric coefficient for that species.

Endothermic vs exothermic: which feels cold to the touch?

Endothermic reactions, because they absorb heat from surroundings.

What particles carry charge in molten ionic compounds?

Mobile ions (cations and anions).

Why don’t molecular solids conduct electricity?

They lack free ions or delocalised electrons.

Relationship between bond order and bond length.

Higher bond order (double/triple) → shorter bond length and higher bond energy.

Key requirement for a substance to be soluble in water.

It must be polar or able to form strong interactions (e.g., hydrogen bonds) with water.

What is meant by ‘like dissolves like’?

Substances dissolve best in solvents with similar polarity.

How does increased bond strength affect melting point?

Stronger bonds require more energy to break, thus higher melting points.

Give the enthalpy change for forming 1 mol NH₃ when ΔrH = –92 kJ for 2 mol.

–46 kJ mol⁻¹.

State the two main ways to determine ΔH experimentally.

Calorimetry using measured temperature change, or using known ΔrH and stoichiometry.