http://www.ck12.org Chapter 3. Chemical Interactions
Single Replacement Reactions
A single replacement reaction occurs when one element replaces another in a single compound. This type of reaction
has the general equation:
A + BC→B + ACIn this equation, A represents a more reactive element and BC represents the original compound. During the reaction,
A replaces B, forming the product compound AC and releasing the less reactive element B.
An example of a single replacement reaction occurs when potassium (K) reacts with water (H 2 O). A colorless solid
compound named potassium hydroxide (KOH) forms, and hydrogen gas (H 2 ) is set free. The equation for the
reaction is:
2K + 2H 2 O→2KOH + H 2In this reaction, a potassium ion replaces one of the hydrogen atoms in each molecule of water. Potassium is a highly
reactive group 1 alkali metal, so its reaction with water is explosive. You can watch the reaction occurring at this
URL: http://commons.wikimedia.org/wiki/File:Potassium_water_20.theora.ogv
Q:Find potassium in the periodic table of the elements. What other element might replace hydrogen in water in a
similar replacement reaction?
A:Another group 1 element, such as lithium or sodium, might be involved in a similar replacement reaction with
water.
Double Replacement Reactions
A double replacement reaction occurs when two ionic compounds exchange ions. This produces two new ionic
compounds. A double replacement reaction can be represented by the general equation:
AB + CD→AD + CBAB and CD are the two reactant compounds, and AD and CB are the two product compounds that result from the
reaction. During the reaction, the ions B and D change places.
Q:Could the product compounds be DA and BC?
A:No, they could not. In an ionic compound, the positive metal ion is always written first, followed by the negative
nonmetal ion. Therefore, A and C must always come first, followed by D or B.
An example of a double replacement reaction is sodium chloride (NaCl) reacting with silver fluoride (AgF). This
reaction is represented by the equation:
NaCl + AgF→NaF + AgCl
During the reaction, chloride and fluoride ions change places, so two new compounds are formed in the products:
sodium fluoride (NaF) and silver chloride (AgCl). To better understand how and why double replacement reactions
occur, watch the video at this URL: http://www.youtube.com/watch?v=eck_s_J6nKo
Q:When iron sulfide (FeS) and hydrogen chloride (HCl) react together, a double replacement reaction occurs. What
are the products of this reaction? What is the chemical equation for this reaction?
A:The products of the reaction are iron chloride (FeCl 2 ) and hydrogen sulfide (H 2 S). The chemical equation for this
reaction is:
FeS + 2HCl→H 2 S + FeCl 2