AS91164: Bonding, Structure, Properties, and Energy Changes

Comprehensive practice flashcards covering all key concepts, definitions, structures, properties, shapes, and thermochemical calculations from the AS91164 chemistry lecture notes.

Between what types of elements does ionic bonding typically occur?

Between a metal and a non-metal.

Which compound is noted as an exception that behaves as a covalent molecular substance rather than an ionic solid?

$$AlCl_3$$

What particles are present in an ionic solid?

Positive cations and negative anions (ions).

What attractive force holds ionic solids together?

An ionic bond, which is a strong electrostatic attraction between oppositely charged ions.

What structural arrangement is formed by ionic solids?

A 3-D lattice structure of alternating cations and anions.

Are ionic bonds directional or non-directional?

Ionic bonds are directional.

What particles make up a metallic substance?

Metal atoms, described as metal cations in a sea of delocalised valence electrons.

How is a metallic bond defined?

The attraction between loosely held valence electrons and the positively charged nuclei of neighbouring atoms.

What structural arrangement do metal atoms form?

A 3-D lattice.

Are metallic bonds directional or non-directional?

Metallic bonds are non-directional.

Between what types of elements are covalent network solids formed?

Between non-metal and non-metal atoms.

What particles make up covalent network solids?

Atoms.

What attractive forces hold covalent network solids together?

Strong covalent bonds extending across a 2-D or 3-D network.

What are the three key examples of covalent network solids given in the notes?

Diamond, graphite, and silicon dioxide ($SiO_2$).

Describe the arrangement of carbon atoms in diamond.

Each carbon atom is covalently bonded to 4 other carbon atoms in a tetrahedral arrangement.

Describe the bonding and structure of silicon dioxide ($SiO_2$).

Silicon and oxygen atoms are held together by covalent bonds in a tetrahedral arrangement, creating a 3-D network.

Describe the bonding arrangement of carbon atoms within each layer of graphite.

Carbon atoms are each covalently bonded to 3 other carbon atoms in a trigonal planar shape, forming 2-D hexagonal layers.

What forces hold the 2-D layers together in graphite?

Weak van der Waals forces.

Why does graphite have delocalised electrons?

Because each carbon atom forms 3 covalent bonds, leaving one delocalised electron free to move between the layers.

What particles are present in covalent molecular substances?

Discrete molecules.

List the eight examples of covalent molecular substances given in the notes.

$H_2O$, $I_2$, $CO_2$, $S_8$, $PCl_3$, $SCl_2$, $SiCl_4$, and $SO_3$.

What forces exist between separate molecules in a covalent molecular substance?

Weak intermolecular forces (or van der Waals forces).

What forces exist within the atoms inside a molecule?

Strong covalent bonds (intramolecular forces).

How is a covalent bond defined?

A bond in which one or more pairs of electrons are shared by two atoms.

What does the term 'intramolecular' mean?

Acting between atoms within the molecule.

What does the prefix 'inter' mean in the term intermolecular forces?

It means 'between' molecules.

Which types of solids possess high melting and boiling points?

Ionic solids, covalent network solids, and metals.

Why do ionic solids, covalent network solids, and metals have high melting and boiling points?

Because they have strong attractions (ionic bonds, strong covalent bonds, or metallic bonds) between particles, requiring a large amount of heat energy to break them.

Why do covalent molecular substances have low melting and boiling points?

Because weak intermolecular forces (van der Waals forces) exist between molecules, requiring only a small amount of energy to overcome.

What happens to the internal covalent bonds when a covalent molecular substance melts or boils?

The strong covalent bonds inside the molecules are NOT broken; only the weak intermolecular forces are overcome.

Which two molecular substances are explicitly noted to sublime?

$I_2$ and $CO_2$.

What are the two mobile charge carriers that can enable electrical conductivity?

Free-moving delocalised electrons or free-moving ions.

In which states do metals conduct electricity and why?

In both solid and liquid states, because their delocalised valence electrons are free to move throughout the structure.

Why does graphite conduct electricity?

Because each carbon atom forms only 3 bonds, leaving one delocalised electron per atom free to move between layers.

Why can solid ionic substances not conduct electricity?

Because their ions are held in fixed positions in the 3-D lattice and cannot move.

Under what two conditions do ionic substances conduct electricity?

When melted (molten) or dissolved in water (aqueous), because the ions become free to move.

Why do diamond and silicon dioxide ($SiO_2$) not conduct electricity in any state?

Because all valence electrons are tied up in covalent bonds, leaving no free-moving charged particles.

Why do covalent molecular substances not conduct electricity in any state?

Because all valence electrons are localized or tied up in covalent bonds, leaving no free-moving charged particles (no free ions or electrons).

What solubility principle explains why non-polar substances dissolve in non-polar solvents?

'Like dissolves like'.

Why does non-polar iodine ($I_2$) dissolve in non-polar cyclohexane?

Because similar weak intermolecular forces exist between the different molecules, allowing weak forces to form between them.

Why do ionic solids dissolve in polar water ($H_2O$)?

Polar water molecules are attracted to the positive and negative ions with enough force to pull them away from the lattice and hydrate them.

Why are ionic solids insoluble in non-polar solvents like cyclohexane?

Because there is no attraction between the non-polar solvent molecules and the ions.

Why are metals insoluble in water?

Because their metallic bonds are too strong to be broken by attraction to water molecules.

Why are metals malleable and ductile?

Because metallic bonds are non-directional, allowing layers of metal atoms to slide past each other without breaking the bond or disrupting the overall lattice structure.

What does the property 'malleable' mean for metals?

They can be shaped.

What does the property 'ductile' mean for metals?

They can be drawn into wires.

Why are ionic solids brittle?

Bonds are directional; a sharp force forces ions to shift position, bringing like-charged ions next to each other, which repel and cause the crystal to shatter.

Why are molecular solids soft or brittle?

Because the weak van der Waals forces between molecules allow them to be easily separated.

Are students required to learn different types of intermolecular forces for standard AS91164?

No, different types of intermolecular forces are not required to be learned for this standard.

What do dots or crosses represent in a Lewis structure?

Outer shell valence electrons.

How can the number of valence electrons for group 13 element aluminium ($Al$) be determined?

Group number minus 10 ($13 - 10 = 3$ valence electrons).

What is the restriction regarding electron pairs about any atom in simple molecules for this standard?

Restricted to simple molecules with no more than four electron pairs about any atom.

Name two electron-deficient species mentioned in the notes that do not have a full octet around the central atom.

$Be$ in $BeCl_2$ and $B$ in $BCl_3$.

What is the core principle of Valence Shell Electron Pair Repulsion (VSEPR) Theory?

Regions of negative charge (electron density/clouds) around the central atom arrange themselves to be as far apart as possible to minimize repulsion.

In VSEPR theory, how are double and triple bonds counted?

Double and triple bonds count as a single region of negative charge.

What is the molecular shape and bond angle for a central atom with 2 regions of negative charge and 0 lone pairs?

Linear ($180^\circ$).

Give an example of a linear molecule with a bond angle of $180^\circ$.

$$CO_2$$

What is the molecular shape and bond angle for a central atom with 3 regions of negative charge and 0 lone pairs?

Trigonal Planar ($120^\circ$).

Give two examples of trigonal planar molecules with bond angles of $120^\circ$.

$BF_3$ and $H_2CO$.

What is the molecular shape and bond angle for a central atom with 4 regions of negative charge and 0 lone pairs?

Tetrahedral ($109.5^\circ$ / $109^\circ$).

Give two examples of tetrahedral molecules.

$CH_4$ and $CCl_4$.

What is the molecular shape and bond angle for a central atom with 4 regions of negative charge: 3 bonding regions and 1 lone pair?

Trigonal Pyramidal (approx $107^\circ$ / $109^\circ$).

Give two examples of trigonal pyramidal molecules.

$NH_3$ and $PCl_3$.

What is the molecular shape and approximate bond angle for a central atom with 4 regions of negative charge: 2 bonding regions and 2 lone pairs?

V-shaped / Angular / Bent (approx $105^\circ$).

Give an example of a bent molecule with a bond angle of approximately $105^\circ$.

$$H_2O$$

What is the molecular shape and approximate bond angle for a central atom with 3 regions of negative charge: 2 bonding regions and 1 lone pair?

V-shaped / Angular / Bent (approx $120^\circ$).

Give an example of a bent molecule with a bond angle of approximately $120^\circ$.

$$SO_2$$

State the order of repulsive force among lone pairs and bonding pairs.

lone pair–lone pair > lone pair–bonding pair > bonding pair–bonding pair.

Why is the bond angle in $NH_3$ compressed to approximately $107^\circ$?

Because the lone pair has a greater repulsive force than bonding pairs, which pushes the bonding pairs down.

Are lone pairs drawn as part of the final molecular shape?

No, lone pairs determine the shape and angles, but are not drawn as part of the final molecular shape.

What is electronegativity?

A measure of an atom's attraction for the electron pair in a covalent bond.

How does electronegativity change across a period and down a group?

It increases across a period and decreases down a group.

Which element is the most electronegative?

Fluorine ($F$).

State the electronegativity trend ranking given in the notes.

$$F > O > N \approx Cl > S \approx C > P \approx H$$

What is a non-polar covalent bond?

A bond where identical atoms share the electron pair equally (e.g., $Cl\text{–}Cl$).

What occurs when different atoms with different electronegativities are covalently bonded?

The more electronegative atom attracts the electrons more strongly, creating a polar covalent bond with partial charges ($\delta^+$ and $\delta^-$).

What two conditions make a molecule polar overall?

It contains polar bonds AND has an asymmetrical molecular shape (so bond dipoles do not cancel out).

Give four examples of polar molecules from the notes.

$H_2O$, $NH_3$, $SO_2$, and $PCl_3$.

Why is a molecule non-polar if it contains polar bonds in a symmetrical arrangement?

Because the bond dipoles cancel out symmetrically (the centres of positive and negative charge coincide).

Give four examples of non-polar molecules from the notes.

$CO_2$, $CH_4$, $CCl_4$, and $Cl_2$.

What is enthalpy ($H$)?

The 'heat content' or chemical potential energy within a substance.

What does $\Delta H$ represent?

The change in enthalpy during a reaction.

What is an exothermic reaction?

A reaction that gives out heat energy to the surroundings, causing the reaction mixture to warm up or feel hot.

What is the sign of $\Delta_r H$ for an exothermic reaction?

Negative ($-\Delta H$).

In an exothermic reaction, how do the energy levels of reactants and products compare?

Reactants are at a higher energy level than the products; products have less enthalpy and are more stable than the reactants.

Name five chemical or physical examples of exothermic reactions given in the notes.

Combustion, neutralisation, respiration, rusting iron, and mixing water with an anhydrous salt (or hand warmers, adding solid $NaOH$ to water, steam condensing, water freezing, gas condensing).

What is an endothermic reaction?

A reaction that absorbs heat energy from the surroundings, causing the reaction mixture to cool down or feel cold.

What is the sign of $\Delta_r H$ for an endothermic reaction?

Positive ($+\Delta H$).

In an endothermic reaction, how do the energy levels of reactants and products compare?

Reactants are at a lower energy level than the products; products have more enthalpy and are less stable than the reactants.

Name five chemical or physical examples of endothermic reactions given in the notes.

Photosynthesis, dissolving ammonium nitrate ($NH_4NO_3$) in water, ice melting, water boiling, and sweating/evaporation of water (or making an anhydrous salt from a hydrate).

Is bond breaking endothermic or exothermic, and what is the sign of its enthalpy change?

Endothermic; $+\Delta H$ (energy must be absorbed/put in to break a chemical bond).

Is bond making endothermic or exothermic, and what is the sign of its enthalpy change?

Exothermic; $-\Delta H$ (energy is released when chemical bonds are formed).

What is the definition and unit of average bond enthalpy?

The average energy required to break 1 mole of a particular bond, with units of $kJ\,mol^{-1}$.

Are average bond enthalpies listed as positive or negative numbers?

They are always listed as positive numbers.

What formula calculates reaction enthalpy using bond enthalpies?

$$\Delta_r H^\circ = \sum(\text{bonds broken}) - \sum(\text{bonds made})$$

What is the shortcut rule for bond enthalpy calculations?

You only need to break the bonds that change and make the new bonds formed; you do not need to break every bond if a large part of the structure remains unchanged.

What is the relationship between the amount of substance reacting in a balanced equation and the enthalpy change ($\Delta H$)?

There is a direct proportionality between them.

What is the formula linking amount in moles ($n$), mass ($m$), and molar mass ($M$)?

$$n = \frac{m}{M}$$

What two elements must be included when reporting an enthalpy change ($\Delta_r H$)?

The correct sign ($+\text{ or }-\text{}$) and units ($kJ\,mol^{-1}$ or $kJ$).

To how many significant figures should final numerical calculation answers usually be rounded?

Strictly three significant figures.