Chemistry

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.

The Mole Says

The tips in this section, while helping you to figure out what reaction might occur, aren't infallible in correctly predicting the reaction that will take place. However, if you're not sure what will happen, these tips will be useful in suggesting some possibilities.

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.

Bad Reactions

There are two common mistakes when predicting the products of a chemical reaction. The first is predicting the formation of a theoretically impossible product such as NaCO₃ or Ag₄Cl. The second is failing to balance the equation once the products have been accurately predicted

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.

You've Got Problems

Problem 4: Write balanced chemical equations for the reactions that might occur when the following reactants are combined:
a) NaOH + H₂ SO₄ ⇔ ?
b) NH₃ + I₂ ⇔ ?
c) C₃H₈O + O₂ ⇔ ?

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.

Excerpted from The Complete Idiot's Guide to Chemistry © 2003 by Ian Guch. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.

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