A combination reaction has two or more reactants forming one product. The formation of sulfur dioxide by burning sulfur in air is an example of a combination reaction.
S (s) + O2 (g)
Key Concept
Combination reactions generally have more reactants than products.
A + B
DECOMPOSITION REACTIONS
A decomposition reaction is the opposite of a combination reaction: A single compound reactant breaks down into two or more products, usually as a result of heating or electrolysis. An example of decomposition is the breakdown of mercury (II) oxide. (The
SINGLE-DISPLACEMENT REACTIONS
Sometimes one atom in a molecule gets tired of the other (“I love you, but I’m not in love with you…”), and divorce becomes inevitable when something better comes along. This is a single-displacement reaction: an atom (or ion) of one compound is replaced by an atom of another element. For example, zinc metal will displace copper ions in a copper sulfate solution to form zinc sulfate.
Zn (s) + CuSO4 (aq)
Single-displacement reactions are often further classified as redox reactions, which will be discussed in great detail in Chapter 11, Redox Reactions and Electrochemistry. Not to carry the analogy to the point of ridiculousness, but just to make the point: Cu in CuSO4 has an oxidation state of +2, but when it leaves the compound, it gains two electrons (the Cu+2 is reduced to Cu)—you may think of this as Cu+2 getting an alimony settlement or gaining back its dignity for leaving that no-good cheater, SO42-. On the other side, Zn loses its dignity (in the form of two electrons) when it desperately throws itself into the arms of SO42-.
DOUBLE-DISPLACEMENT REACTIONS
Well, if you think atoms are acting scandalously in single-displacement reactions, just wait until you get a load of the atomic depravity in double-displacement reactions. In double-displacement reactions, also called metathesis, elements from two different compounds swap places with each other (hence, the name double-displacement) to form two new compounds. This type of reaction occurs when one of the products is removed from the solution as a precipitate or gas or when two of the original species combine to form a weak electrolyte that remains undissociated in solution. For example, when solutions of calcium chloride and silver nitrate are combined, insoluble silver chloride forms in a solution of calcium nitrate.
CaCl2 (aq) + 2AgNO3 (aq)
NEUTRALIZATION REACTIONS
Neutralization reactions are a specific type of double displacement in which an acid reacts with a base to produce a salt. For example, hydrochloric acid and sodium, hydroxide will react to form sodium chloride and water.
HCl (aq) + NaOH
Key Concept
Decomposition reactions generally have more products than reactants.
C
Bridge
Acids and bases (which we will see in Chapter 10) combine in neutralization reactions to produce salts and water.
Net Ionic Equations
Just as in our discussion of equivalents and normality, in which we admitted that we really care about the acid and the base functionalities themselves rather than the compounds that are donating the hydrogen protons and hydroxide ions, here we have another opportunity to confess our little secrets. Come on, say it with us—you’ll feel better getting this off your chest: When it comes to many reactions, there are certain species that we find boring and want to ignore. For example, in many reactions, such as displacements, the ionic constituents of the compounds are in solution, so we can write the chemical reaction in ionic form. In the previous example involving the reaction between zinc and copper (II) sulfate, the ionic equation would be
Zn (s) + Cu2+ (aq) + SO42- (aq)
You’ll notice that the SO42- (aq) is just hanging, not really doing anything. It’s not taking part in the overall reaction but simply remains in the solution unchanged. We call such species spectator ions. They’re like boring people who go to parties and just stand around, taking up space. Because the SO42- ion isn’t doing anything of interest, we can ignore it and write a net ionic reaction showing only the species that actually participate in the reaction:
Zn (s) + Cu2+ (aq)
Net ionic equations list only the cool people at the party who are actually doing fun stuff. They are important for demonstrating the actual reaction that occurs during a displacement reaction.
Balancing Equations
MCAT Expertise
It is unlikely that you will come across a question that explicitly asks you to balance an equation. However, you will need to recognize unbalanced reactions and quickly add the necessary coefficients. Look at the
1) charge on each side; and
2) number of atoms of each element.
Balancing a checkbook is a dying art. In this age of automated and computer banking, it’s easy enough to monitor the account balance without taking the time to record by hand each and every transaction. Nevertheless, the art of balancing chemical equations is one in which you must be skilled, and you ought to expect the MCAT to test your understanding of the steps involved. Because chemical equations express how much and what type of reactants must be used to obtain a given quantity of product, it is of utmost importance that the reaction be balanced so as to reflect the laws of conservation of mass and charge. The mass of the reactants consumed must equal the mass of products generated. More specifically, you must ensure that the number of atoms on the reactant side equals the number of atoms on the product side. Stoichiometric coefficients, which are placed in front of the compound, are used to indicate the number of moles of a given species involved in the reaction. For example, the balanced equation expressing the formation of water is
2 H2 (g) + O2 (g)
The coefficients indicate that two moles of H2 gas must be reacted with one mole of O2 gas to produce two moles of water. In general, stoichiometric coefficients are given as whole numbers.