Chemistry

Adding Variety to Our Menu

To be an excellent and accomplished cook, knowing how to prepare many different dishes using many different methods is essential. For example, you can't be a good cook if you only know how to work a fryer. To be a good cook, you must also know how to broil, sauté, bake, braise, poach, etc.

Many textbooks and chemistry teachers describe reactions as being one of six or so different types. Though I would argue that chemical reactions are all basically the same (they all make products from reactants), it's usually not taught that way, so I'll very briefly describe the six types of reaction so you'll know what other people are talking about. However, I hope that in your own mind you realize that these types of reaction are arbitrarily assigned and don't really mean much.

  • Combustion reaction: Combustion reactions occur when organic molecules combine with oxygen to form carbon dioxide, water vapor, and a large quantity of heat. A simple example of such a reaction is the combustion of methane, a constituent of natural gas:
    CH4(g) + 2 O2(g)Δ CO2(g) + 2 H2O(g)
  • Synthesis reaction: Synthesis reactions are when small molecules combine to form larger ones. A commercially important example of a synthesis reaction occurs during the Haber process, which results in the formation of ammonia from nitrogen and hydrogen:
    N2(g) + 3 H2(g)Δ 2 NH3(g)
  • Decomposition reaction: Decomposition reactions are the opposite of synthesis reactions, and take place when large molecules break apart to form smaller molecules. An example of a decomposition reaction is when carbon dioxide bubbles are formed by the decomposition of carbonic acid in a bottle of soda:
    H2CO3(aq) ⇔ H2O(l) + CO2(g)
  • Single displacement reactions (also called single replacement reactions): Single displacement reactions occur when a pure element switches places with one of the elements in a chemical compound. An example of this type of reaction occurs when zinc reacts with acetic acid to form hydrogen gas and zinc acetate:
    Zn8 + H2CO3(l) ⇔ H2(g) ↑+ ZnCO3(aq)
  • Double displacement reaction: These reactions occur when the cations of two ionic compounds switch places. A double displacement reaction takes place when dissolved magnesium sulfate is added to sodium hydroxide:
    MgSO4(aq) + 2 NaOH(aq) ⇔ Mg(OH)2(s)↓ + Na2SO4(aq)
  • Acid-base reactions; Acid-base reactions take place when an OH- and H+ ion combine to form water. An acid-base reaction takes place when household ammonium hydroxide is added to household vinegar, which contains acetic acid:
    NH4OH(aq) + HC2H3O2(aq) ⇔ NH4C2H3O + H2O(l)
You've Got Problems

Problem 3: Identify the type of reaction taking place in each of the equations below:
a) AgNO3(aq) + HCl(aq) ⇔ AgCl(s)↓ + HNO3(aq)
b) Cu(s) + AgNO3(aq) ⇔ CuNO3(aq) + Ag(s)

book cover

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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