AVOIDING “STUPID MISTAKES” ON THE FREE-RESPONSE SECTION - Appendixes - 5 Steps to a 5 AP Chemistry (2015)

5 Steps to a 5 AP Chemistry (2015)

Appendixes

AVOIDING “STUPID MISTAKES” ON THE FREE-RESPONSE SECTION

We (the authors) have been grading the free-response part of the AP Chemistry exam for quite a while. Between the two of us, we have nearly 20 years of grading experience—that’s more than 100,000 exams! Over the years, we have seen quite a number of careless mistakes made by students. These mistakes resulted from not being careful rather than not being prepared for the exam. Here are some practical tips to avoid the most common careless errors.

• Don’t forget to state the units of measurement. Many students would have gotten more credit if they had shown the units, both in the calculations and in the final answer. The units help you stay on the right track and help the grader determine if (or where) you went wrong.

• Use the formula given. If the exam gives you a chemical formula, don’t use a different formula in your answer. In general, do not alter anything given to you on the exam. For example, we have seen Ba(NO3)2 become Ba(NO2)2.

• Be careful with the math. We have seen many errors involving the simplest math such as 12 mL + 3 mL = 0.042 L (rather than 0.015 L).

• Don’t confuse molarity and moles. The units M and [ ] are identical (molarity) and are completely different from moles.

• Show your work for conversions. For example, if you are changing grams to moles and make a simple mistake, showing your work (labeled) may get you partial credit.

• Don’t argue with the test. This is an argument you cannot win. For example, if the question asks for calculations, you are unlikely to get full credit without any calculations even if you have the right answer. It won’t help to write that you feel the calculations are unnecessary.

• Be careful in applying gas laws. Gas laws can be very useful. However, they should never be used when there is not a gas in the problem. Having a volume included in the question information doesn’t necessarily mean you are dealing with a gas.

• Be careful making comparisons. We have seen many students incorrectly say that 10–8 is smaller than 10–12 and actually write 10–8 < 10–12. We have even seen students write the relationship correctly (10–8 > 10–12) but still state that 10–8 is smaller.

• Be careful using 22.4 L/mol. You will probably not need to use this on the exam. But if you do want to use this value, you must have a gas and this gas must be at 0°C (273 K) and 1 atm (STP). If you forget the values for STP, they can be found on the exam. We have seen quite a few students incorrectly use this value at 298 K.

• There are no trick questions on the exam. If you think you have found a trick question, you need to reevaluate your thinking and reread the question.

• Don’t confuse solutions and precipitates in solution. They are different phases and are not interchangeable. The color of one is not necessarily the color of the other.

• Be careful describing reactions. If the problem gives you, for example, a sodium nitrate solution, part of your answer describing a reaction cannot be “the sodium nitrate dissolves.” You already have a solution, so the process of dissolving happened before you got to the problem. Furthermore, dissolving should not be treated as a reaction.

• Be careful using positive and negative charges. In the following equation, each reactant and product is wrong: Image, and will not substitute for the correct Image. Remember, ionic equations, of any type, have ions (with charges) on one or both sides of the reaction arrow.

• Don’t do a calculator dump (write down every number displayed by your calculator). For example, your final answer will not be 3.27584827 g.

• Keep in mind the meaning of “observe.” If the problem asks about observation, tell what you would actually observe (see, hear, or smell). You will not see a compound separating into ions; usually you will not see the excess reagent, and you will not see the atoms forming bonds. In contrast, you might observe a compound dissolving.

• Remember a solvent is usually not a reactant. Therefore changing the grams of solvent to moles is probably wrong. (However, you will need to know the moles of solvent if you are looking for a mole fraction.)

• Think before creating mole ratios. Since the solvent is not a reactant, a mole ratio relating the solvent to anything else in the problem is most likely wrong. We have seen many students change the grams of water to moles and then use these moles in a mole ratio to relate to some other substance in the problem.

• Don’t go off on a tangent. Stay focused on answering the original question.

• Double-check the numbers you use. We have seen many cases where the problem gave a number like 2.75 × 10–18, and the student worked the problem with 2.75 × 10–8. If you show your work, it will be obvious to the grader that you miscopied the value and you might pick up some points; otherwise, you just have a wrong answer.

• Remember that sometimes not all of the information given is needed to solve the problem. For example, in the equilibrium problem, many times the temperature is given but it is not actually part of the calculations.

• Only round your final answer. Don’t round off the results of intermediate calculations; only use rounding after you’ve gotten your final answer.

• Be careful in reading graphs. Especially take care in reading the scales. We have seen students write down that 0.5 is between 1.0 and 2.0.

• Don’t confuse intermolecular and intramolecular forces. These are two different concepts and are not interchangeable.

In addition to avoiding the careless mistakes mentioned above, here are some easy ways to help improve your score on the free response questions:

• Show your work. In most cases, no work, no credit.

• Use the space provided for answers. It helps you and the grader if you answer the question in the space provided instead of crowding the answers between the questions. You will have more than enough room on the following page(s). It also helps to label the parts (a, b, etc.) and to answer the parts in order.

• Make sure your answer can be easily read. It will really help the grader—and your score—if you write legibly, in a normal size (not too small, please), and use a pencil or pen that writes dark enough to be easily read.

• Don’t use periodic trends and general rules as explanations. General rules such as “like dissolves like” are never explanations. They may help you in answering the multiple-choice part of the exam, but will be of little benefit by themselves in the free-response section.

• Don’t confuse “define” and “describe.” They are two different processes. If you are asked to describe or explain, simply giving a definition will earn you very few points.

• Use only standard abbreviations. Your instructor may understand your abbreviations, but the grader may not. If you want to use abbreviations in a response, be sure to define them.

• Don’t ramble. Normally an explanation or justification can be done in five sentences or less. Your answers should be clear, concise, and to the point.