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STATES OF MATTER - properties of gases and liquids (fluids) and solids

14. Relating and comparing latest heats of state changes to the chemical structure of the substance e.g. looking at differences between metallic, ionic and simple molecular structures

Doc Brown's chemistry revision notes: Basic school chemistry science GCSE level chemistry, IGCSE  chemistry, O level and ~US grades 8, 9 and 10 school science courses or equivalent for ~14-16 year old science students for national examinations in chemistry and also helpful for UK advanced A level chemistry students aged ~16-18 and US grades 11-12 K12 honors courses.

14. More on the latent heat values of physical changes of state related to structure

  • Changes of physical state i.e. gas <==> liquid <==> solid are also accompanied by energy changes.

  • To melt a solid, or boil/evaporate a liquid, heat energy must be absorbed or taken in from the surroundings, so these are endothermic energy changes. The system is heated to effect these changes.

  • To condense a gas, or freeze a solid, heat energy must be removed or given out to the surroundings, so these are exothermic energy changes. The system is cooled to effect these changes.

  • Generally speaking, the greater the forces between the particles, the greater the energy needed to effect the state change AND the higher the melting point and boiling point.

A comparison of energy needed to melt or boil different types of substance

(This is more for advanced level students)

  • The heat energy change involved in a state change can be expressed in kJ/mol of substance for a fair comparison.

    • In the table below

    • ΔHmelt is the energy needed to melt 1 mole of the substance (formula mass in g).

      • Can be called the specific latent heat of fusion.

    • ΔHvap is the energy needed to vaporise by evaporation or boiling 1 mole of the substance (formula mass in g).

      • Can be called the specific latent heat of vapourisation.

    • So, these are the latent heats required to change the physical state of a substance.

  • For simple small covalent molecules, the energy absorbed by the material is relatively small to melt or vaporise the substance and the bigger the molecule the greater the inter–molecular forces.

    • These forces are weak compared to the chemical bonds holding atoms together in a molecule itself.

    • Relatively low energies are needed to melt or vapourise them.

    • These substances have relatively low melting points and boiling points.

  • For strongly bonded 3D networks e.g.

    • (i) an ionically bonded lattice of ions (ionic bonding),

    • (ii) a covalently bonded lattice of atoms (covalent bonding – giant covalent structures),

    • (iii) and a metal lattice of ions and free outer mobile electrons (metallic bonding),

      • the structures are much stronger in a 3D way because of the continuous chemical bonding throughout the structure.

      • Consequently, much greater energies are required to melt or vaporise the material (much larger specific heats of melting/fusion or boiling/vapourisation..

      • This is why they have so much higher melting points and boiling points.

  • The data table below illustrates the points argued above about the values you might expect for latent heats - the thermal energy input to effect a state change solid => liquid => gas and the temperatures at which the substance melts or boils.

    • Small covalent molecules

    • Metallic structures

    • Ionic structures

    • Giant covalent structures

Substance formula Type of bonding, structure and attractive forces operating Melting point K (Kelvin = oC + 273) ΔHmelt = energy to melt the substance Boiling point K (Kelvin = oC + 273) ΔHvap = energy to boil the substance
methane CH4 small covalent molecule – very weak intermolecular forces 91K –182oC

very low

0.94 kJ/mol

very low

112K –161oC

very low

8.2 kJ/mol

 very low

ethanol  ('alcohol') C2H5OH larger covalent molecule than methane, greater, but still weak intermolecular forces 156K –117oC

very low

4.6 kJ/mol

low

352K 79oC

low

43.5 kJ/mol

low

sodium chloride Na+Cl ionic lattice, very strong 3D ionic bonding due to attraction between (+) and (–) ions 1074K 801oC

high

29 kJ/mol

very high

1740K 1467oC

very high

171 kJ/mol

high

iron Fe strong 3D bonding by attraction of metal ions (+) with free outer electrons (–) 1808K 1535oC

very high

15.4 kJ/mol

high

3023K 2750oC

very high

351 kJ/mol

very high

silicon dioxide (silica) SiO2 giant covalent structure, strong continuous 3D bond network of O-Si-O 1883K 1610oC

very high

46.4 kJ/mol

very high

2503K 2230oC

very high

439 kJ/mol

very high

Graphite (carbon) C Layered giant covalent structure >3600oC

it sublimes

715 kJ/mol

very high

>3600oC

it sublimes

715 kJ/mol

very high

Note on graphite

It takes an enormous amount of energy to vapourise carbon in the form of graphite.

Many giant covalent organic compounds (e.g. epoxy resins) carbonise at high temperatures and absorb a lot of heat energy in the process.

In the Apollo space program of the 1970s, the re-entry vehicle has to withstand high temperature from the friction between it and the Earth's atmosphere.

The heat shield consisted of layers of brazed steel, fibreglass and epoxy resin.

Much of the heat energy is absorbed in vaporising the epoxyresin.


Learning objectives for latent heats for different structures

Know and be able to explain and describe that the latent heat of fusion or the latent heat of vapourisation is very dependant on the structure of the substance.

Weak physical structure like small covalent molecules like water or ethanol will have relatively low latent heats of state changes and relatively low melting points and boiling points.

Strong physical structures like ionic compounds, metals or giant covalent structures will tend to have high latent heats of state changes and relatively high melting points or boiling points

Be able to relate relative latent heats of fusion or vapourisation to the structure of the substance.


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INDEX of all my notes on the states of matter

(GCSE level and advanced pre-university level revision notes)

Detailed notes on the states of matter and their properties

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