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

14. Latent heats, state changes related to chemical structure

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 in terms of their latent heat of melting/fusion and their latent heat of vapourisation/evaporation/boiling.

GCSE (~US grades 8-10) level multiple choice QUIZ on the states of matter: gases, liquids & solids

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, but also mentioned in GCSE physics) 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. Covalent Bonding – small simple molecules 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.

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Key Points about Latent Heat Revision Notes (GCSE/IGCSE Chemistry)

Key Definitions

• Latent Heat of Fusion
  • The energy required to change 1 kg of a substance from solid to liquid at its melting point without a change in temperature.
  • Example: Ice → Water at 0 °C.
• Latent Heat of Vaporisation
  • The energy required to change 1 kg of a substance from liquid to gas at its boiling point without a change in temperature.
  • Example: Water → Steam at 100 °C.

• See sections in GCSE Physics Notes

• 4.3-4.4 Energy transfer in state changes, conservation of mass, no chemical change AND introduction to latent heat and physical changes of state

• 4.5-4.6 A heating curve - state changes on increasing the internal energy of a system AND the cooling curve - state changes on decreasing the internal energy of a system

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Laboratory Experiment Context

• Calorimetry for Fusion:
  • Place crushed ice in a calorimeter, measure energy supplied until all melts.
  • Plot temperature versus time → plateau at 0 °C shows latent heat of fusion.
• Calorimetry for Vaporisation:
  • Heat water in a kettle, measure mass lost as steam.
  • Plateau at 100 °C shows latent heat of vaporisation.
• See Part 13 for all the details of heating curves and cooling curves

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Chemical Structure and Differences

• Fusion (solid → liquid)
  • Energy breaks intermolecular forces holding particles in fixed positions.
  • Example: Sodium chloride (ionic lattice) needs much higher fusion energy than ice (hydrogen bonds).
• Vaporisation (liquid → gas)
  • Energy must overcome all intermolecular forces, separating particles completely.
  • Example:
    • Water (hydrogen bonding) → very high latent heat of vaporisation.
    • Ethanol (weaker hydrogen bonding) → lower latent heat.
    • Methane (London dispersion forces only) → very low latent heat.

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Graphical Representation

• Heating Curve:
  • Temperature rises → plateaus at melting/boiling points → latent heat absorbed.
• Cooling Curve:
  • Temperature falls → plateaus at freezing/condensation points → latent heat released.
• See Part 13 for all the details of heating curves and cooling curves

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Exam Tips

• Always state “no temperature change” during latent heat processes.
• Use units correctly: J/kg for specific latent heat.
• In calculations for energy transfer:
  Q = m x L ]
  where (Q) = energy (J), (m) = mass (kg), (L) = latent heat (J/kg).
• Link to particle theory: fusion = loosen bonds, vaporisation = break bonds completely.
• Compare substances: ionic versus covalent, hydrogen bonding versus van der Waals.

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Common Misconceptions

• Thinking temperature rises during melting/boiling → wrong (energy goes into breaking forces, not raising kinetic energy).
• Confusing latent heat with specific heat capacity (latent heat = phase change, specific heat = temperature change).
• Assuming all substances have similar latent heats → false (depends on bonding strength).
• Forgetting to convert grams → kilograms in calculations.

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Quick Examples

• Ice melting: Latent heat of fusion ≈ 334 kJ/kg.
• Water boiling: Latent heat of vaporisation ≈ 2260 kJ/kg.
• Ethanol boiling: Much lower than water due to weaker hydrogen bonding.
• Liquid nitrogen vaporisation: Very low latent heat due to very weak intermolecular bonding forces.

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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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Website content © Dr Phil Brown 2000+. All copyrights reserved on Doc Brown's Chemistry revision notes, images, quizzes, worksheets etc. Copying of website material is NOT permitted. GCSE level and advanced pre-university level revision notes. Detailed notes on the states of matter and their properties. Based on the syllabus-specifications for students taking the IGCSE/GCSE level physics examinations summary revision notes and key points about relating latent heats of melting and boiling to the chemical structure of the substance, for students taking the WJEC gcse chemistry/physics, CCEA gcse chemistry/physics, CIE igcse chemistry/physics, AQA igcse/gcse physics, Edexcel gcse chemistry/physics, OCR 21st century chemistry/physics, OCR gateway chemistry/physics or any other GCSE or IGCSE level chemistry/physics exams e.g. US grade 9-10 physics courses INDEX of all my notes on the states of matter GCSE (~US grades 8-10) level multiple choice QUIZ on the states of matter: gases, liquids & solids