Using English to Predict in Chemical Reaction

              The first step  identifying the type of reaction involved is usually the most difficult. The primary reaction types students encounter are displacement, acid-base and combustion. They are easily identified if the tell-tale signs are known. Displacement reactions involve two ionic compounds with cations and anions, such as sodium sulfate, in which sodium (Na?) is the cation and sulfate (SO?²?) is the anion. Ionic compounds always consist of a metal and a nonmetal or polyatomic (multiple-atom) anion. Decomposition reactions involve a single compound breaking into two or more compounds. Acid-base reactions must involve an acid (identified by its chemical formula that begins with “H,” such as HCl). Combustion reactions involve hydrogen or a hydrocarbon (such as CH?) reacting with oxygen (O?).

Displacement Reactions

            Identify the cation and anion of the compounds involved in the reaction, as well as their charges. If necessary, refer to tables of cations and anions, such as the one available at Penn State University’s website (See Resources). Sodium chloride (NaCl), for example, consists of a sodium ion (Na?) and a chloride ion (Cl?).

Exchange the anions of the two reactants to determine the products of the reaction. Displacement reactions take this general form:

AB + CD ? AD + CB

Thus, for a reaction between sodium chloride (NaCl) and silver nitrate (AgNO?):

NaCl + AgNO? ? NaNO? + AgCl

Determine whether the products are soluble. This may require referring to a list of “solubility rules,” such as that at Southern Methodist University (see Resources). In the example from Step 2, NaNO? is soluble and thus remains in solution, but AgCl is insoluble and will form a precipitate.

Verify that the reaction is balanced by adding coefficients in front of the reactants and products as necessary to ensure that each type of atom is present on each side of the reaction arrow in equal numbers. In the example from Step 2, the left side of the equation contains 1 Na, 1 Cl, 1 Ag, 1 N, and 3 O; the right side contains 1 Na, 1 Cl, 1 Ag, 1 N, and 3 O. Thus, the reaction is balanced.

Acid-Base Reactions

Identify the acidic compound (containing H? in its formula) and the basic compound (usually a hydroxide, OH?).

Determine the products according to the general reaction:

acid + base ? salt + water

For example, the reaction of hydrochloric acid (HCl) with sodium hydroxide (NaOH) produces sodium chloride and water:

HCl + NaOH ? NaCl + H?O

Determine if the salt is soluble or insoluble by referring to the solubility rules.

Balance the reaction. In this case, the reaction from Step 2 is already balanced.

Combustion Reactions

Determine the fuel (the source of carbon and/or hydrogen) and the oxidant (the source of oxygen) (see Resources). If the combustion is carried out in air, the oxidant is assumed to be molecular oxygen (O?). Other oxidants, such as nitrous oxide (N?O), are possible, but this would require special reaction conditions.

Predict the products by assuming this general reaction:

Fuel + oxidant ? CO? + H?O

For example, propane (C?H?) combines with O? during combustion as:

C?H? + O? ? CO? + H?O

Balance the reaction. For the example in Step 2:

C?H? + 5 O? ? 3 CO? + 4 H?O

Predicting Reaction Products

When cooking, it's frequently handy to predict what will happen when we mix a bunch of ingredients together. For example, if we're interested in making a delicious new salad dressing, we would have a very small chance of making anything edible if we had no way of knowing which ingredients would have the greatest chance of succeeding. Likewise, it's often necessary for chemists to predict the chemical reactions that will take place when two chemicals are combined. For example, if we're adding a chemical to a tank of toxic waste to stabilize it, we'd be very unhappy if we failed to predict an explosive reaction. An easy way to predict what reaction will take place when two chemicals are mixed is to identify the type of reaction that's likely to occur when the chemicals are combined. Of course, we mentioned before that these types of reaction are arbitrary, but they do sometimes have a useful purpose.

           Here are some tips you may find handy in helping to predict the type of reaction that will occur if you know only the reactants. Keep in mind that not all combiations of chemicals will result in a chemical reaction—these tips are handy only for helping to predict what would happen should they happen to react. If two ionic compounds are combined, it's usually safe to predict that a double displacement reaction will occur.

If the chemicals mixed are oxygen and something containing carbon, it's usually a combustion reaction.

If we start with only one reactant, the reaction taking place is probably a decomposition reaction. To predict the products of such a reaction, see what happens if the chemical breaks into smaller, familiar products such as water, carbon dioxide, or any of the gaseous elements.

When pure elements are combined, synthesis reactions are the frequent result.

If a pure element combines with an ionic compound, a single displacement reaction may take place.

If a compound containing the hydroxide ion is involved, check the other compound to see if it contains hydrogen. If it does, it may be an acid-base reaction.

PREDICTING PRODUCT OF CHEMICAL REACTIONS

Type of Reactions	Reactants	Probable Products	Generic Equations
Synthesis	Two or more substances	One compound	A+ B → AB
Combution	√ a metal and oxygen  √ a nonmetal and   oxygen  √ a compound and oxygen	√ the oxide of the metal  √ the oxide of the nonmetal √two or more oxsides	A + O2 → AO
Decomposition	One compound	Two or more elements and/or compounds	AB→ A+ B
Single Replacement	√ a metal and a compound √ a nonmetal and a compound	√ a new compound and the replace metal  √ a new compound and replace nonmetal	A + BX → AX + B
Double Replacement	Two compounds	Two different compounds, one of which is a solid, water, or gas	AX + BY → AY + BX