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Writing Chemical Equations on the AP Chemistry Exam

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  • 0:02 What to Expect
  • 2:10 Types of Chemical Reactions
  • 5:09 Tips for Writing Reactions
  • 6:58 Practice
  • 9:23 Lesson Summary
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Lesson Transcript
Instructor: Elizabeth (Nikki) Wyman

Nikki has a master's degree in teaching chemistry and has taught high school chemistry, biology and astronomy.

Learn what kinds of equations you will be expected to write on the AP Chemistry exam. Discover some tips and tricks that will help you prepare for writing chemical equations on the free response portion of the AP Chemistry Test.

What to Expect

Here we are in AP Chemistry lab, excitedly performing an experiment involving chemical reactions. 5 mL of a 0.10 Molar solution of Calcium Nitrate is mixed with 10 mL 0.20 Molar solution of Sodium Sulfate. A white precipitate forms.

What is the identity of this precipitate? Could you write a balanced chemical equation for the reaction observed? What evidence do we have that a reaction occurred? Can you answer these questions on the spot, right now?

If so, nice work! You're in good shape for tackling the portion of the AP Chemistry test that involves writing chemical equations. Chemical reactions are a big deal in AP Chemistry, so it goes without saying that they will be on the AP Test. Your ability to write equations in the Free Response section of the exam will be extremely helpful when taking the test.

The free response section consists of seven multi-part free response questions. Many of these questions contain prompts like the one I introduced this video with. They may ask you to write chemical equations given data, observations or particle views. These may be represented by molecular, ionic or net ionic equations.

Here's a cool thing about the AP Chemistry exam: You can get points for logical answers, even if they aren't right. So if you find yourself unsure if the equation you wrote down is right, it's worth a try!

Writing an equation down - whether right or wrong - is especially important because the new AP format may require that you use your equation for many parts of each Free Response question.

After you've written equations, of course you have to balance them! You'll likely have to apply balanced equations to a scenario. This could involve finding the amount of product created or the amount of reactant needed for the reaction, finding the limiting reactant or excess reactant, or finding the percent yield or theoretical yield. I said it once, but I'll say it again - you will be rewarded for logical thinking, even if your answer isn't entirely right!

Types of Chemical Reactions

It's good to know your types of reactions. If you can identify how two reactants are likely to react, then you can predict products. The reverse is true, too - if you know the products and the type of reaction, you can likely predict the reactants. Many of the equations you'll have to write involve this type of thinking. There are four main types of reactions.

Synthesis Reactions: Two reactants combine to make a product. Chances are, you've done a synthesis reaction in lab, like the formation of magnesium oxide from magnesium metal and oxygen gas:

2Mg(s) + O2 (g) --> 2MgO

Decomposition: A reactant breaks down into two or more products. It's the opposite of a synthesis reaction. You might have encountered a reaction like this in lab, too.

Perhaps you decomposed silver (I) oxide into silver and oxygen gas?

2Ag2O (s) --> 4Ag(s) + O2(g)

Acid-Base (Bronsted-Lowry Theory): Protons are transferred from a proton donor (an acid) to a proton acceptor (a base).

Most of the time acids are identifiable by the protons displayed at the front of their chemical formula or by their name. Careful though, sometimes acids (especially weak ones) will have their detaching proton at the end of their formula or embedded within. Tricky acids like these are usually accompanied by a little hint from the AP Test.

Acids will always lose a proton to form a conjugate base. The conjugate base has the same formula as the acid, except it is missing one proton and has a negative charge.

Bases are often hydroxides or amines. These compounds will gain a proton to become conjugate acids. In the case of weak bases, the conjugate acid has the same formula as the base, except that it has gained a proton and now has a positive charge.

Acetic acid (which can be written as HC2H3O2 or CH3COOH) reacts with aniline, a weak base.

HC2H3O2 + C6H5NH2 --> C2H3O2- + C6H5NH3+

On the products side, acetic acid has lost a proton and now is negatively charged. Aniline gained that proton on its amine group and is now positively charged.

Strong bases will ionize into hydroxides and a positive cation. The hydroxides react with the protons from the acid to make water. The remaining ions form a salt.

HCl + NaOH --> H2O + NaCl

Neutralization reactions like these are best represented by a net ionic equation.

H+ + OH- --> H2O

We'll go over net ionic equations more in the next section.

Oxidation-Reduction: Involves a net transfer of electrons. The species that loses electrons is oxidized, the species that gains electrons is reduced. Most single replacement reactions fall into this category. Any electrochemistry involves oxidation-reduction reactions.

Perhaps you've put a piece of metal in acid? Like magnesium metal into hydrochloric acid?

Mg + HCl --> H2 + MgCl2

In this reaction, magnesium loses two valence electrons to hydrogen, becoming oxidized. Two hydrogen atoms each gain an electron and become reduced.

Tips for Writing Chemical Reactions

The first and best tactics for writing chemical reactions include being able to identify the type of compound present and being familiar with the rules for naming compounds.

If you haven't already, memorize the names and formulas of common polyatomic ions. Understand how to determine the charges on the ionic forms of representative elements.

Remember that hydrogen, oxygen, fluorine, bromine, iodine, nitrogen, and chlorine form gaseous, diatomic compounds with themselves if they are not bonded to any other element. We even name them as if they existed monatomically. For example, when a prompt says hydrogen gas, the formula is H2(g). It's helpful to remember these elements as the HOFBrINCl series.

Study solubility rules. It's good to have a firm understanding of what compounds are likely to be soluble or insoluble. For example, know that compounds containing nitrates are always soluble.

Be familiar with the activity series. What metals oxidize easiest? By all means, don't memorize this, but know that metals with low ionization energy will oxidize easier than with relatively high ionization energy. This kind of knowledge will be helpful with writing equations for oxidation-reduction reactions.

State signs can be helpful, but points are generally not awarded for them. When writing equations, it's okay to write them down; however, unless specified, they aren't necessary.

Balance equations. Writing the equation is often difficult, but balancing an equation should be easy.

You can practice with real AP questions. Go onto the AP Central Chemistry Exam website and access Free Response questions and their scoring rubrics from previous years. I recommend doing these problems and checking your work against the key. You'll get an idea of what graders are looking for.

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