A Review of Reactions
We started our look at chemical reactions with the combustion of magnesium… to make a white, powdery substance which students described as "paint chips." The balanced equation is
2Mg(s) + O2(g) ® 2MgO(s)
This reaction is a synthesis reaction. We can tell it’s a synthesis reaction because there is one product. Because of the high temperature, magnesium will combine with nitrogen in a synthesis reaction.
3Mg(s) + N2(g) ® Mg3N2(s)
Magnesium oxide will react with water in a synthesis reaction to make magnesium hydroxide.
MgO(s) + H2O(l) ® Mg(OH)2(s)
A compound that contains a metal and the hydroxide ion is called a base. A base is the "opposite" of an acid. Acids and bases neutralize each other if in the right proportions. Only the alkali metal hydroxides are soluble in water. Mg(OH)2 is only sparingly soluble in water, and only a relatively small amount of Mg(OH)2 dissociates into ions. But it is enough to make the solution basic as indicated by the pink color of phenolphthalein.
When we burned steel wool in oxygen we got Fe2O3 in another synthesis reaction.
Fe(s) + O2(g) ® Fe2O3(s)
Another important synthesis reaction is the formation of rust. The most common form of rust is FeO(OH). To form "rust", iron must always react with water, along with oxygen.
4Fe(s) + 3O2(g) + 2H2O(l) ® 4FeO(OH)(s)
Underwater, rust takes the form of Fe(OH)3 and only when the rust is severely dehydrated does it have the formula Fe2O3.
A second kind of chemical reaction is the decomposition reaction. In this reaction there is a single reactant, but two or more products. Many decomposition reactions are the reverse of synthesis reactions.
For instance, water can be broken down by passing electricity through it (electrolysis), and hydrogen gas and oxygen gas will be produced. This is a decomposition reaction.
2H2O(l) ® 2H2(g) + O2(g)
When hydrogen gas is ignited in oxygen, water is the product in a synthesis reaction.
2H2(g) + O2(g) ® 2H2O(l)
Some other example of decomposition reactions. A common way to make small amounts of oxygen gas.
2KClO3(s) ® 2KCl(s) + 3O2(g)
The process of starting with limestone and making lime in a lime kiln as at the paper mill.
CaCO3(s) ® CaO(s) + CO2(g)
The “bubbles” which form on your skin or wound when using hydrogen peroxide are oxygen gas.
The hydrogen peroxide is decomposed into water and oxygen gas, hence the bubbles. Treating a cut or scrape isn’t all you can do with hydrogen peroxide.
H2O2(l) ® 2H2O(l) + O2(g)
A related reaction is the combustion of a hydrocarbon. A hydrocarbon, as the name implies, contains both hydrogen and carbon. Hydrocarbons are the simplest of the organic compounds. Methane, CH4, is a hydrocarbon. The first reaction is the combustion of methane. The second is the combustion of paraffin wax.
CH4(g) + 2O2(g) ® CO2(g) + 2H2O(l)
C35H72(s) + 53O2(g) ® 35CO2(g) + 36H2O(l)
The combustion of a hydrocarbon always results in the formation carbon dioxide and water.
Double replacement reactions involve the reaction of two compounds, usually in aqueous solution, and the formation of two compounds as products. In a double replacement reaction there is at least one of the products which is not soluble in water. Some double replacement reactions:
AgNO3(aq) + NaCl(aq) ® AgCl(s) + NaNO3(aq)
2KOH(aq) + Cu(NO3)2(aq) ® Cu(OH)2(s) + 2KNO3(aq)
3Cu(NO3)2(aq) +2Na3PO4(aq) ® Cu3(PO4)2(s) + 6NaNO3(aq)
In a double replacement reaction there is either a solid precipitate or the formation of undissociated water molecules. Many acid-base reactions are examples of double replacement reactions. In the case of acid-base reactions, the products are a “salt” and water. A salt is the combination of the cation (positive ion) of a base and the anion (negative ion) of an acid. The main reaction in an acid-base reaction is the formation of undissociated water molecules.
HCl(aq) + NaOH(aq) ® NaCl(aq) + H2O(l)
3H2SO4(aq) + 2Al(OH)3(s) ® Al2(SO4)3(aq) + 6H2O(l)
For a more comprehensive review of double replacement reactions, refer to the handout on double replacement reactions. (CW double replacement reactions 030812.doc)
And this brings us to our last type of reaction, the single replacement reaction. In a single replacement reaction, many of which occur in aqueous solution, an element in the elemental state reacts with a compound, an a compound and a different element are produced.
There are two types of single replacement reactions, one where a metal in the elemental state replaces a metal in a compound. The other is the case where an elemental halogen replaces a halide in a compound.
Mg(s) + ZnCl2(aq) ® MgCl2(aq) + Zn(s)
Cu(s) + AgNO3(aq) ® Cu(NO3)2(aq) + Ag(s)
3F2(g) + 2AlCl3(aq) ® 2AlF3(aq) + 3Cl2(g)