Data section reference
- Syllabus
- 9701–2028–2029
- Topic
- —
- Level
- A2
Learning objectives
—Data reference—Important constants and standardsUse supplied constants and standards including R, F, Avogadro constant, electronic charge, molar gas volume at s.t.p. and room conditions, Kw and specific heat capacity of water.—Data reference—Ionisation energies, electronegativity and Periodic TableUse ionisation energy tables, Pauling electronegativity values and the Periodic Table to support explanations of trends, bonding, periodicity and chemical behaviour.—Data reference—Bond energiesUse exact and average bond energy tables to calculate or interpret enthalpy changes and compare bond strengths, remembering that average bond energies are approximations.—Data reference—Standard electrode potentialsUse standard electrode potential tables to calculate cell potentials, predict feasibility, compare oxidising/reducing strength and interpret redox behaviour.—Data reference—Proton and carbon-13 NMR chemical shiftsUse proton NMR and carbon-13 NMR chemical shift ranges to identify chemical environments and support structural deduction.—Data reference—Infrared absorption frequenciesUse characteristic infrared absorption frequency ranges to identify selected bonds and functional groups, especially in combination with other analytical evidence.
Match every supplied constant to its equation, unit system and conditions
The data section supplies values such as R, F, N_A, electronic charge, molar gas volume at s.t.p. and room conditions, K_w and the specific heat capacity of water. Select the value only after identifying the required equation and conditions.
Check units before substitution: R must match the pressure-volume unit system, molar gas volume depends on stated conditions, and the heat-capacity value must be combined with compatible mass and temperature-change units.
Use F in Q = n(electrons)F; use the room-condition molar volume rather than the s.t.p. value when the question explicitly states room conditions.
A supplied number is not unit-free or condition-free. Copying a familiar constant without checking the table can produce a systematic error even when the arithmetic is correct.
Turn periodic data into a chemical explanation
Use ionisation-energy tables, Pauling electronegativities and the Periodic Table as evidence. Quote the relevant comparison, then connect it to nuclear charge, shielding, distance, electron attraction, shell structure or bond polarity as the question requires.
A large electronegativity difference supports greater bond polarity. A sudden jump in successive ionisation energies indicates that the next electron is removed from a more strongly attracted inner shell.
A table value is evidence, not the explanation itself. Confirm the element, ion, successive-ionisation step and quantity: electronegativity and ionisation energy are not interchangeable.
Use exact and average bond energies with the correct sign convention
Bond energy is the enthalpy required to break a specified covalent bond in gaseous molecules. An exact value belongs to a specified molecular environment; an average value represents several environments and therefore gives an estimate.
ΔH≈∑E(bonds broken)−∑E(bonds formed)
Draw or count every changing bond, apply broken minus formed, retain the sign, and state that a result based on average bond energies is approximate.
Do not reverse the subtraction, count unchanged bonds unnecessarily, or treat one average table value as exact for every molecule.
Read reduction potentials to analyse a redox direction
Standard electrode-potential tables are written as reductions under standard conditions. Select the cathode reduction, reverse the anode half-equation when writing the reaction, and calculate the cell potential from the tabulated reduction values.
Ecell∘=Ecathode∘−Eanode∘
A positive E standard cell supports thermodynamic feasibility in that direction under standard conditions. More positive reduction potentials identify stronger oxidising agents; reverse-couple reasoning identifies reducing strength.
Do not multiply E standard by stoichiometric coefficients. A positive standard potential does not guarantee a fast reaction or the same direction under arbitrary concentrations.
Use NMR shift ranges as one constraint on structure
Proton and carbon-13 chemical-shift ranges indicate electronic environments. Match each proposed environment to a plausible range, then combine shifts with signal count and, for proton NMR, integration and splitting.
Electronegative atoms, carbonyl groups and aromatic pi systems can deshield nearby nuclei and shift signals downfield. Check the complete predicted environment and atom count against the spectrum.
A proton on a carbon next to oxygen is usually downfield from a simple alkyl proton; a carbonyl carbon lies in a distinctive downfield carbon-13 region.
Shift ranges overlap and depend on molecular environment and conditions. One peak position alone rarely proves a unique complete structure.
Use IR ranges with the rest of the structural evidence
An infrared spectrum shows absorptions associated with bond vibrations. Match significant bands to the supplied characteristic-frequency ranges and use band position, shape and strength to support the presence of selected bonds or functional groups.
A broad O-H absorption plus a strong C=O absorption supports a carboxylic acid. Combine IR with molecular formula, mass spectrometry and NMR to distinguish alternatives; use the fingerprint region mainly for comparison with a known spectrum.
One absorption rarely identifies a whole molecule, and overlapping or weak bands require cautious interpretation. The complete evidence set must fit the proposed structure.