USING ENGLISH TO PREDICT RENDEMENT OF PRODUCT A REACTION

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.

Classifying Reactions to help with Predicting Reactions

There are five different types of reactions possible in the reactions section :

1. Double Replacement Reactions (Metathesis)

2. Redox (Oxidation - Reduction) Reactions

3. Organic Reactions

4. Complex Ion Formation

5. Lewis Acid-Base Reactions

1) Double Replacement or (Metathesis) Reactions

When you see two binary ionic compounds (including acids), the compounds switch partners to form two new compounds. The driving force and product is either a gas, a precipitate, or a weak electrolyte.

a gas	memorize the double replacement products that form gases
a precipitate	memorize the solubility rules
a weak electrolytes	memorize the strong acids so you recognize a weak acid; memorize H2O and NH4OH as weak electrolytes

Watchout for:

Important stoichiometry...key words "equimolar", etc. for the formation of acid salts like HPO₄^2-

Complex ion formation through double replacement does not seem to form new compounds. For example: Zn(OH)₂ + excess (or concentrated) NaOH ------> Zn(OH)₄^2- + Na⁺.

2) RedOx (Oxidation - Reduction) Reactions

Memorize the common strong oxidizers, generally ions with lots of oxygen, MnO₄^-, Cr₂O₇^2-, IO₃^-, etc. Memorize what they turn into, and look for something to oxidize.

Memorize the common strong reducers (on the handout mentioned above), memorize what they turn into, and look for something to reduce.

Memorize the equations for the oxidation and the reduction reactions of water during the electrolysis of water.

Reduction: 4H₂O + 4e^- --------> 2H₂ + 4OH^-

Oxidation: 2H₂O -------> O₂ + 4H⁺ + 4e^-

Watchout for:

Keywords "acidified solution" or an acid included in the reactants. The H⁺ ions form H₂O with the oxygens.

Anytimes you see a neutral element, Cu°, O₂, H₂, etc. it must be redox.

Tips & Tricks:

1. When reactions occur between a metal like Fe°, Cu°, Sn°, etc. (multiple ions possible metals: Fe²⁺ and Fe³⁺) reacts with reactive gases like O₂, F₂, Cl₂ with the addition of heat, Fe° will ionize itself to maximum positive charge (-ic metal ions). Example:

2Fe°(s) + 3Cl₂(g) + heat --------> 2FeCl₃(s)

2. When applying the rule of "Free Halogens + Dilute OH^- -----> Hypohalites ions," the addition of halide ions (such as Cl^-) to the equation are required to obtain full credit on the reaction equations. Example:

Cl₂(g) + 2OH^- -------> ClO^- + Cl^- + H₂O

3. When the oxides of an alkali metal (Family 1), Ca, Ba, or Sr dissolve in water, hydroxides will form, but no gases will be released. Example:

K₂O(s) + H₂O ----------> 2K⁺ + 2OH^-

4. When the hydrides of an alkali metal (Family 1), Ca, Ba, or Sr dissolve in water, hydroxides will form and H₂ gas is released. Example:

LiH(s) + H₂O --------> Li⁺ + OH^- + H₂(g)

5. Look for "battery" reactions (activity series of metals). When you are not sure which one will undergo changes, look at the reduction potential chart given in the AP Test (the metal with greatest potential will reduce). Examples:

Mg°(s) + 2Ag⁺ ---------> Mg²⁺ + 2Ag°(s)

6. Be aware of Disproportionation Redox Reactions. These are the reactions when a portion substance is oxidized while the rest is reduced. The same chemical substance undergoes both oxidation and reduction. NO₂ and H₂O₂ are classic chemicals that have this ability. Example:

3NO₂(g) + H₂O --------> 2H⁺(aq) + 2NO₃^-(aq) + NO(g)

7. (Trick #1) When you see electrolysis such as KI in water, and you are stuck on whether H₂ gas will form from which side. The following is the solution that always works. You know potassium is always going to the negative electrode. Let say K° forms at the negative electrode (cathode) and immediately undergoes reaction with water:

2K° + 2H₂O -------> 2KOH + H₂

Since all these happens on the negative electrode. So, H₂ gas forms from the negative electrode and that's exactly what happens when water reduces at the cathode.

8. (Trick #2) When CuSO₄(aq) is electrolyzed, you know that Cu° metal is going to form because copper's potential is higher than water. So, positive side will attract SO₄^2- ions. Nevertheless, SO₄^2- can't further oxidize (full of oxygen and no more unshared pair of electrons possible for further oxidation). As the result, you should use the other side of the hydrolysis in Trick #1: instead of OH^-, put H⁺; instead of H₂, put O₂. This makes sense because when you electrolyze H₂O, you get H₂, and O₂; and OH^- and H⁺ from each electrode will neutralize and become water again.

2Cu²⁺ + 2H₂O --------> O₂ + 4H⁺ + 2Cu°

9. (Trick #3) When you recognize great oxidizers like Cr₂O₇^2-, MnO₄^-, and MnO₂, with acidified solutions in the test, but you may not remember what they turn into. So, remember the definition of an oxidizer: the ability to give off its oxygen components. So, acidified Cr₂O₇^2-, turn into Cr³⁺and MnO₂, and MnO₄^- will turn to Mn²⁺. As you can see, the oxygen components are "ripped off" and form water with H⁺ ions.

3) Organic Reactions

These are especially useful for the older tests. The "acorn book" states that organic compounds may show up as examples, but organic reactions are pretty restricted.

Oxidation - complete combustion of hydrocarbons results in CO₂ and H₂O. Milder oxidation may change a primary alcohol into an aldehyde, a secondary alcohol into a ketone, etc.

Substitution - example: Br₂ + a hydrocarbon results in HBr + a Br substitute for one of the hydorgens on a carbon.

Esterfication - organic acid + alcohol results in an ester and H₂O

Addtion - a double bond is borken and atoms are attached to two carbons. If H₂O is added, an H and OH add to the carbons. If Br₂ is added, a Br atom is added to each of the carbons.

4) Complex Ion Formation

These are usually formed from a transition metal surrounded by ligands (polar molecules or negative ions). As a "rule of thumb" you place twice the number of ligands around an ion as the charge on the ion... example: the dark blue Cu(NH₃)₄²⁺ (ammonia is used as a test for Cu²⁺ ions), and Ag(NH₃)²⁺.

Memorize the common ligands.

Ligands	Names used in the ion
H2O	aqua
NH3	ammine
OH-	hydoxy
Cl-	chloro
Br-	bromo
CN-	cyano
SCN-	thiocyanato (bonded through sulphur) isothiocyanato (bonded through nitrogen)

Watchout for:

Alumninum also forms complex ions as do some post transitions metals. Ex: Al(H₂O)₆³⁺

The names are very impressive, but easy..the ions above are the tetraamminecopper(II) ion, the diamminesilver(I) ion, and hexaaquoaluminum(III) ion. Zn(OH)₄^2- is the tetrahydroxyzinc(II) ion, the charge is the sum of the parts (2+)+4(-1)= -2.

Acid-base reactions may change NH₃ into NH₄⁺ (or vice versa) which will alter its ability to act as a ligand.

Visually, a precipitate may go back into solution as a complex ion is formed. For example, Cu²⁺ + a little NH₄OH will form the light blue precipitate, Cu(OH)₂. With access ammonia, the complex, Cu(NH₃)₄²⁺, forms.

Keywords such as "excess" and "concentrated" of any solution may indicate complex ions. AgNO₃ + HCl forms the white precipitate, AgCl. With excess, concentrated HCl, the complex ion, AgCl₂^-, forms and the solution clears.

The odd complex ion, FeSCN²⁺, shows up once in a while simply because it is commonly used in the CHEMStudy first-year equilibrium lab.

Transitional metals, such as Iron, Zinc and Chromium, can form complex ions. Aluminum can form complex ions as well.

5) Lewis Acid-Base Reactions

"HAVE PAIR WILL SHARE" -- Lewis Base. The formation of a coordinate covalent bond between NH₃ and BF₃ to form H₃N:BF₃ is a classic example of a Lewis acid-base reaction.

Oxides of metals and nonmetals are also examples:

CO₂ + H₂O --------> H₂CO₃

CaO + CO₂ ----------> CaCO₃

Watchout for:

Some reactions come from industrial pocesses such as the formation of bleach (ClO^- ion) from Cl₂ and dilute NaOH). Note these as you come across them and add them to your "bag of tricks".

You can write (or think about) chemicals in different ways for different reactions. Ammonia may be NH₃ (aq) for complex ions, NH₄OH for double replacement or acid-base reactions; water may be H₂O or you might think of it as H⁺ and OH^- for hydrolysis or redox reactions; HNO₃may be an acid (donating H⁺) or it may be an oxidizer (forming NO or NO₂ + H₂O).

Tips & Tricks:

1. When you see NO₃^-, NO₂^-, CO₃^2-, SO₃^2-, SO₄^2- ions in solids and are heated in vacuum, NO₂(g), NO(g), CO₂(g), SO₂(g), SO₃(g) are evolved. Example:

CaCO₃(s) + heat ---------> CaO(s) + CO₂(g)

2. (Trick #4) When you see a compounded gas bubble through any solution, they should be changed to "The Dissolved" state which is actually a Lewis Acid-Base Reaction:

CO₂(g) + H₂O -------->  H₂CO₃

NH₃(g) + H₂O --------> NH₄OH

SO₂(g) + H₂O --------> H₂SO₃