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Reactions and Periodicity: Common Mistakes to Avoid for AP Chemistry

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By — McGraw-Hill Professional
Updated on Mar 31, 2011

Experiments Strategy

Laboratory experiments involving reactions are usually concerned with both the reaction and the stoichiometry. Typical experiments involving these concepts are experiments 7, 14, 15, and 20.

You need some idea of the balanced chemical equation. In the case of an acid–base reaction, an acid reacts with a base. The acid supplies H+ and the base accepts the H+. If the acid is diprotic, such as H2SO4, it can donate two H+.

The key to any reaction experiment is moles. The numbers of moles may be calculated from various measurements. A sample may be weighed on a balance to give the mass, and the moles calculated with the formula weight. Or the mass of a substance may be determined using a volume measurement combined with the density. The volume of a solution may be measured with a pipet, or calculated from the final and initial readings from a buret. This volume, along with the molarity, can be used to calculate the moles present. The volume, temperature, and pressure of a gas can be measured and used to calculate the moles of a gas. You must be extremely careful on the AP exam to distinguish between those values that you measure and those that you calculate.

The moles of any substance in a reaction may be converted to the moles of any other substance through a calculation using the balanced chemical equation. Other calculations are presented in the stoichiometry chapter.

Common Mistakes to Avoid

  1. In balancing chemical equations don't change the subscripts in the chemical formula, just the coefficients.
  2. Molecular compounds ionize, ionic compounds dissociate.
  3. In writing ionic and net ionic equations, show the chemical species as they actually exist in solution (i.e., strong electrolytes as ions, etc.).
  4. In writing ionic and net ionic equations, don't break apart covalently bonded compounds unless they are strong acids that are ionizing.
  5. Know the solubility rules as guidelines, not explanations.
  6. Oxidizing and reducing agents are reactants, not products.
  7. The products of the complete combustion of a hydrocarbon are carbon dioxide and water. This is also true if oxygen is present as well; but if some other element, like sulfur, is present you will also have something else in addition to carbon dioxide and water.
  8. If a substance that does not contain carbon, like elemental sulfur, undergoes complete combustion, no carbon dioxide can be formed.
  9. If an alcohol like methanol, CH3OH, is dissolved in water, no hydroxide ion, OH, will be formed.
  10. Know the strong acids and bases.
  11. HF is not a strong acid.
  12. In titration calculations, you must consider the reaction stoichiometry.
  13. Be sure to indicate the charges on ions correctly.
  14. The common coordination numbers of complex ions are 2, 4, and 6.
  15. Do not confuse measured values and calculated values.
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