Reactions and Periodicity: Common Mistakes to Avoid for AP Chemistry
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
- In balancing chemical equations don't change the subscripts in the chemical formula, just the coefficients.
- Molecular compounds ionize, ionic compounds dissociate.
- In writing ionic and net ionic equations, show the chemical species as they actually exist in solution (i.e., strong electrolytes as ions, etc.).
- In writing ionic and net ionic equations, don't break apart covalently bonded compounds unless they are strong acids that are ionizing.
- Know the solubility rules as guidelines, not explanations.
- Oxidizing and reducing agents are reactants, not products.
- 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.
- If a substance that does not contain carbon, like elemental sulfur, undergoes complete combustion, no carbon dioxide can be formed.
- If an alcohol like methanol, CH3OH, is dissolved in water, no hydroxide ion, OH–, will be formed.
- Know the strong acids and bases.
- HF is not a strong acid.
- In titration calculations, you must consider the reaction stoichiometry.
- Be sure to indicate the charges on ions correctly.
- The common coordination numbers of complex ions are 2, 4, and 6.
- Do not confuse measured values and calculated values.
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