Chemistry 考试技巧

计算器程序

常见错误

1. 1high涉及分数: 2Energy cycles in reactions

   Mismatched units in Thermodynamic calculations, substituting entropy in Joules directly into Gibbs Free Energy calculations.

   如何避免: Always divide entropy values (typically in J K-1 mol-1) by 1000 to convert them to kJ K-1 mol-1 before using them in the equation: Delta G = Delta H - T * Delta S.
2. 2high涉及分数: 3Electron sharing reactions

   Drawing sloppy curly arrows in reaction mechanisms that start or end in empty spaces.

   如何避免: Ensure curly arrows start precisely on a lone pair of electrons, a formal negative charge, or a covalent/pi bond, and point directly to the nucleus of the electron-deficient atom.
3. 3medium涉及分数: 3Ideal gases

   Failing to convert volume from dm3 or cm3 to m3 when using the Ideal Gas Equation.

   如何避免: When using the gas constant R = 8.31 J K-1 mol-1, always convert volume to cubic meters (m3) by dividing dm3 by 1000, or dividing cm3 by 1,000,000.
4. 4medium涉及分数: 1Green Chemistry, Experimental Methods and Spectroscopy

   Failing to double the absolute uncertainty of a balance or burette when calculating total experimental uncertainty.

   如何避免: For subtraction-based measurements (like mass differences or final/initial burette volumes), you must sum the absolute uncertainties of both measurements (e.g., 2 x +/- 0.05 cm3 = +/- 0.10 cm3).
5. 5medium涉及分数: 1Green Chemistry, Experimental Methods and Spectroscopy

   Omitting charges on fragments in mass spectrometry questions.

   如何避免: Always include a positive charge symbol (e.g., CH3+ or COOH+) on chemical formulas representing molecular or fragment ions in mass spectra.
6. 6medium涉及分数: 1Functional groups: Classification of organic compounds

   Drawing incorrect connectivity for functional groups, particularly linking a bond directly to the hydrogen atom of a hydroxyl or carboxyl group.

   如何避免: Explicitly draw bonds to show carbon-oxygen connectivity (C-O-H), ensuring that lines do not end on hydrogen atoms.