2.2.4Unit 2. Energetics, Group Chemistry, Halogenoalkanes and Alcohols
Solvent Choice — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 4 parts Lesson 20 of 53 Unit quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 4Like Dissolves Like
A substance dissolves when the new attractions between solute and solvent compensate for the attractions broken in each.
Non-polar in non-polar
- Non-polar in non-polar Hydrocarbons dissolve in non-polar solvents because London forces form between solute and solvent particles of similar strength.
Polar in polar
- Polar in polar Polar molecules dissolve in polar solvents through permanent dipole-dipole forces or hydrogen bonds with the solvent.
Mismatch
- Mismatch Polar substances dissolve poorly in non-polar solvents because their dipole attractions cannot be replaced by anything comparable.
Giant covalent
- Giant covalent Diamond and silicon dioxide do not dissolve in common solvents because the covalent lattice is far too strong to break.
Size effect
- Size effect As a molecule gets larger the polar part is a smaller fraction, so solubility in water falls.
Part 2 of 4Alcohols and Halogenoalkanes in Water
Alcohols and Halogenoalkanes in Water
Ethanol
- Ethanol Ethanol is miscible with water because its O-H group forms hydrogen bonds with water molecules.
Hexan-1-ol
- Hexan-1-ol With six carbons the non-polar chain dominates, so hexan-1-ol is only slightly soluble in water despite its O-H group.
Halogenoalkanes
- Halogenoalkanes The C-X bond is polar, but there is no H-O, H-N or H-F, so they cannot hydrogen bond and are only slightly soluble in water.
Organic solvents
- Organic solvents Halogenoalkanes dissolve readily in non-polar solvents such as hexane because London forces replace those between solute molecules.
Part 3 of 4Dissolving Ionic Compounds
Water is a good solvent for many ionic compounds because it can replace the lattice attractions with ion-dipole attractions.
1
The δ⁻ oxygen ends of water molecules are attracted to the positive cations at the surface of the crystal.
2
The δ⁺ hydrogen ends of water molecules are attracted to the negative anions at the surface of the crystal.
3
These attractions pull ions out of the lattice, and energy released by forming them offsets the energy needed to break the lattice.
4
Each dissolved ion becomes surrounded by water molecules, which stops it returning to the lattice.
5
Solubility is low when the lattice attractions are much stronger than the ion-water attractions, as with highly charged ions.
Part 4 of 4Exam Checklist & Tips
2.2.4 – Key Points for the Exam
- Solute The substance that dissolves in a solvent.
- Solvent The liquid in which a solute dissolves, often water.
- Miscible Describes two liquids that mix completely in all proportions.
- Polar solvent A solvent whose molecules have permanent dipoles, such as water or ethanol.
Common Mistakes
- Writing that water dissolves ionic compounds by breaking the covalent bonds in water;
- it is the ionic lattice that breaks.
- Assuming all polar molecules dissolve in water;
- the molecule must also be able to form favourable interactions with water.
- Forgetting to mention that hydrogen bonds between alcohol and water are needed to explain ethanol's solubility.
- Saying non-polar substances are insoluble in everything; they dissolve well in non-polar solvents.
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