2B.26-2B.27 - Structure And Physical Properties

Structure From Physical Evidence

A sample arrives with no name on the bottle. You are told only that it is a white solid, it melts at a high temperature, it does not conduct electricity as a solid, but it does conduct when molten and when dissolved in water. A good answer is not "it is probably salt" and then a memory list. The answer has to show why the evidence points to a particle model.

Use this order whenever the substance is unfamiliar:

The important habit is to avoid jumping from one clue to one label. High melting temperature alone does not prove "ionic"; metals and giant covalent substances can also have high melting temperatures. Conductivity alone does not prove "metallic"; graphite conducts because it contains delocalised electrons, and molten ionic compounds conduct because ions are mobile.

Structure Families

Most deductions in this part of Topic 2 use four structure families. The word "bonding" means different things in each family, so name the particles before naming the force.

Notice the phrase "where relevant" for intermolecular forces. Intermolecular forces are relevant for simple molecular substances because there are molecules with attractions between them. They are not the explanation for the high melting temperature of sodium chloride, magnesium oxide, diamond or copper.

Carbon needs a phase-condition caveat: at ordinary pressure it sublimes rather than melting, and its allotropes may transform before vaporising. The useful comparison is the very large energy needed to disrupt the giant covalent network, not a memorised ordinary-pressure diamond melting point.

Deducing Structure From Data

When data are supplied, treat each value as evidence. A useful deduction normally combines at least two of melting or boiling temperature, conductivity, solubility and composition.

Worked example: deducing an unknown

An unknown crystalline solid has these properties.

Deduce the type of structure and bonding present.

Route

1. The high melting temperature suggests a giant structure, not a simple molecular substance.
2. The solid does not conduct, so there are no mobile charged particles in the solid. That rules against a metal and against graphite-like carbon conductivity.
3. The molten substance conducts, so charged particles become mobile after melting. That is typical of ions in an ionic lattice.
4. The solution conducts, which is consistent with ions moving in water.

Conclusion

The substance is likely to have a giant ionic lattice. The particles are positive and negative ions, held by strong electrostatic attractions in all directions. The solid does not conduct because the ions are fixed in position; the molten liquid and aqueous solution conduct because the ions are mobile.

Guided practice

An unknown solid melts above 1600 ${}^\circ\mathrm{C}$, does not conduct electricity as a solid or when molten, and is insoluble in water. The formula contains only non-metal atoms.

Complete the deduction:

• The very high melting temperature suggests ...
• The lack of conductivity suggests ...
• The non-metal composition supports ...
• The most likely structure is ...

Answer

The very high melting temperature suggests a giant structure. The lack of conductivity suggests there are no mobile ions or delocalised electrons. The non-metal composition supports covalent bonding rather than metallic bonding. The most likely structure is giant covalent.

Predicting Physical Properties

Prediction means giving the expected property and supporting it with the model. The support must mention the relevant particles, structure and bonding or intermolecular forces.

Melting and boiling temperature

For a simple molecular substance, melting and boiling overcome intermolecular forces between molecules. The covalent bonds inside the molecules usually remain intact. Stronger intermolecular forces give higher boiling temperatures, but this is still a different scale of explanation from giant lattices.

For a giant ionic lattice, melting requires enough energy to overcome many strong electrostatic attractions between oppositely charged ions. Higher ionic charge and smaller ionic radius can make ionic attractions stronger, but in this lesson the usual deduction is simply "giant ionic lattice, strong electrostatic attractions, high melting temperature."

For a metal, melting disrupts strong electrostatic attractions between positive metal ions and delocalised electrons. For a giant covalent substance, melting or subliming requires disruption of many strong covalent bonds in the network, so the melting temperature is very high.

Electrical conductivity

Conductivity requires mobile charged particles. Metals conduct because delocalised electrons move through the structure. Graphite and graphene conduct because delocalised electrons can move along the carbon layers or sheet. Ionic solids do not conduct because their ions are fixed in the lattice, but molten ionic compounds and aqueous ionic solutions conduct because ions can move. Simple molecular substances usually do not conduct because their molecules are neutral.

Solubility in water

Water is polar and can form strong attractions with ions and with some polar molecules. Many ionic compounds dissolve when attractions between water molecules and separated ions compensate for the attractions in the ionic lattice. Some simple molecular substances dissolve if they can form hydrogen bonds with water, such as small alcohols; many non-polar molecular substances do not. Metals and giant covalent lattices are usually insoluble because water cannot pull the extended structure apart under ordinary conditions.

Worked example: predicting properties from a structure

Predict three physical properties of magnesium oxide, $\ce{MgO}$, from its structure.

Route

$\ce{MgO}$ contains $\ce{Mg^2+}$ and $\ce{O^2-}$ ions arranged in a giant ionic lattice. The ions are held by strong electrostatic attractions in all directions.

Predictions

• Melting temperature: high, because many strong electrostatic attractions between oppositely charged ions must be overcome.
• Electrical conductivity: solid $\ce{MgO}$ does not conduct because ions are fixed in position; molten $\ce{MgO}$ conducts because ions can move.
• Behaviour in water: the giant ionic label alone does not determine solubility. $\ce{MgO}$ has strong lattice attractions and interacts with water to form sparingly soluble magnesium hydroxide. Do not describe stable $\ce{O^2-}$ ions in aqueous solution; any dissolved ions are mobile and can conduct.

Common-error contrast

Do not write "$\ce{MgO}$ has strong intermolecular forces." $\ce{MgO}$ is not made of molecules. The key words are ions, giant ionic lattice, strong electrostatic attractions and mobile ions when molten; dissolved ionic compounds also conduct through mobile ions.

Ambiguous Evidence

Real substances do not always fit a one-clue shortcut. Questions can give enough evidence for a justified prediction, so use all of it.

Small molecular substances can have surprisingly high boiling temperatures

Water, ammonia, hydrogen fluoride and small alcohols can hydrogen bond. Their boiling temperatures are higher than similar-sized molecules with only London forces, but they are still simple molecular substances. During boiling, intermolecular forces are overcome; covalent bonds within the molecules are not broken.

Some covalent structures conduct

Diamond does not conduct because all four outer electrons on each carbon atom are used in covalent bonds. Graphite and graphene conduct because each carbon atom has one electron that becomes delocalised. Therefore, "contains carbon" and "covalent" do not automatically predict the same electrical behaviour.

Water solubility needs a reason

Ionic substances may dissolve if hydration of the ions is favourable, and the resulting solution conducts. Small polar molecules may dissolve through hydrogen bonding or permanent dipole interactions with water, but if they remain as neutral molecules the solution does not conduct. Non-polar molecular substances, metals and giant covalent lattices are usually poorly soluble in water.

Selecting Materials

Material selection is an application of structure-property reasoning. First list every requirement, then reject a material that fails any required condition. A material that conducts only when molten is not suitable if the device must operate as a solid; a material that dissolves in water is unsuitable for a water-resistant support even if it conducts.

For a solid electrode support, check both solid conductivity and water resistance. Metals and graphite-like carbon can supply mobile delocalised electrons in the solid. A giant carbon network is difficult for water to separate because that would require disrupting covalent bonds; an ionic compound may dissolve if water hydrates its ions. Use actual supplied observations rather than assuming every ionic compound is soluble or every carbon material conducts.

A comparison should explain the passing material and the failing material. A supported choice links the operating state to the charge carrier and links water resistance to the structure; it does not simply repeat the word suitable.

Quick Recap

Combine the evidence before assigning a structure. Explain melting or boiling using the attractions disrupted, and conductivity using mobile charged particles. Apply all operating requirements when choosing a material.