CHM 1032C Learning Tools

  Predictions of Chemical Reactions

Types of Reactions

1. Synthesis (or Combination) A + B ® C

    Common situations: 2 elements combining to form a single compound. The formula of the compound should be based on the common oxidation numbers of the elements. The driving force for these reactions is always the transfer of electrons from the less electronegative element to the more electronegative element.

    Example 1: Al(s) + N2(g) ® ?

    The product, solid aluminum nitride, is the result of Al losing three electrons to form a +3 ion and nitrogen gaining three electrons to form a –3 ion. This is the only possible product, since aluminum only forms a +3 ion.

    2Al(s) + N2(g) ® 2 AlN(s)

    Example 2: Co(s) + F2(g) ® ?

    Either cobalt(II) fluoride or cobalt(III) fluoride is possible, depending on the conditions of the reaction. Do you want to memorize all of the common charges of metal ions (Co forms a +2 and a +3 ion)??? I don’t think so! However, most metals have more than one possible oxidation number and you should know at least one common one. Most of the first row transition elements have 2 valence electrons, so they commonly will form a +2 ion. If you have to hypothesize a possible product for these metals, predict that they will form a +2 ion.

    Co(s) + F2(g) ® CoF2(s)

    If two nonmetals are being combined, the bonding will be covalent so you can’t use charges to write your hypothesis. Instead, you use oxidation numbers. The most electronegative element is Group Number-8. The less electronegative element can be the positive group number, or the positive group number –2.

    Example 3: P4(s) + 5O2(g) ® 2P2O5(s) (likely if there is an excess of oxygen)

    P4(s) + 3O2(g) ® 2P2O3(s) (likely if there is less oxygen)

    In P2O5 phosphorus has a +5 oxidation number, which is equal to the group number (V). In P2O3 phosphorus has a+3 oxidation number, which is equal to the group number (V) – 2.

    Another common synthesis reaction is the formation of a hydrate. Note that this process involves no changes in oxidation numbers; unlike the other example, this is not a redox reaction. The driving force here is the formation of a new solid.

    Generally: Salt + XH2O ® Salt· XH2O

    Example: CuSO4(s) + 5H2O(l) ® CuSO4· 5H2O(s)

    Without additional information, it is impossible to predict the numbers of waters of hydration. I knew that copper(II) sulfate pentahydrate was the most common hydrate formed by copper, so I chose this as the product. I do not expect you to know this.

    A third common synthesis reaction is the formation of a ternary acid from the reaction of a nonmetal oxide with water. It is reactions like this that allows us to classify nonmetal oxides as acids. This reaction does not involve any changes in oxidation numbers, so you determine the formula of the product using the oxidation number of the nonmetal (in the nonmetal oxide). Compare that oxidation number with the oxidation numbers of the polyatomic ions containing that nonmetal. The correct ternary acid will contain that polyatomic ion bonded to hydrogen in an appropriate ratio to make the compound neutral. The driving force is the formation of a new covalent compound.

    Example: SO3(g) + H2O(g) ® H2SO4(aq)

    S in sulfur trioxide has an oxidation number of +6. The oxidation number of S in the sulfate ion is also +6, so the formation of sulfuric acid is predicted.

    Along the same lines, a water-soluble metal oxide will react with water to form a metal hydroxide. This is one example of a metal oxide behaving as a base.

    Example: Na2O(s) + H2O(l) ® 2NaOH(aq)

2. Decomposition A + heat ® B + C A common example is the dehydration of a hydrate.

CuSO4· 5H2O(s) + heat ® CuSO4(s) + 5H2O(g)

Another common example is the thermal decomposition of metal carbonates.

Metal carbonate + heat ® metal oxide + carbon dioxide gas

Metal bicarbonate + a little heat ® metal carbonate + water vapor + carbon dioxide gas

Metal bicarbonate + a lot of heat ® metal oxide + water + carbon dioxide gas

Notice that none of these involve changes in oxidation numbers. These are not redox reactions. The formulas of the products are based on the reactant metal ion charge and the charge of the oxide ion. The driving force is the formation of a new solid and gas.

Examples: K2CO3(s) + heat ® K2O(s) + CO2(g)

2Fe(HCO3)3 + a little heat ® Fe2(CO3)3(s) + 3H2O(g) + 3CO2(g)

2NaHCO3(s) + a lot of heat ® Na2O(s) + 2CO2(g) + H2O(g)
 

 3. Double Replacement                           AB + CD ® AD + CB Double replacement reactions generally involve two compounds, each contributing a cation and anion to the reaction. The cations exchange anions, with no change in oxidation number.

Precipitation Reactions involve the formation of an insoluble product as the driving force of chemical change. To determine the presence of this driving force, predict the products of the reaction, being careful to maintain the oxidation numbers of the cations and anions as you write the formulas of the products. Then check the solubility rules to determine if you formed an insoluble product. If both products are soluble, look for another type of driving force. It was not a precipitation reaction.

Example: NiCl2(aq) + (NH4)2S(aq) ® NiS(s) + 2NH4Cl(aq)

Acid-Base Reactions involve the formation of the covalent bond in water or in a weak acid as the driving force for chemical change.

Example 1: 2NaOH(aq) + H2SO4(aq) ® Na2SO4(aq) + 2H2O(l)

Examination of the net ionic equation tells the story: The H+ of the acid combines with the OH- of the base to form water molecule. This is the classic net ionic equation for any strong acid and strong base combination.

Example 2: Na3PO4(s) + 3HCl(aq) ® H3PO4(aq) + 3NaCl(aq)

The formation of a weak acid from a strong acid will go. The reverse will not.

Gas Forming Reactions involve a double replacement reaction that produce carbonic acid which is a thermally unstable product. When it is formed, it spontaneously decomposes into carbon dioxide and water. Notice that this type is a combination double replacement and decomposition.

The patterns: metal carbonate + acid ® salt + water + carbon dioxide

metal bicarbonate + acid ® salt + water + carbon dioxide

Example: CaCO3(s) + 2HCl(aq) ® CaCl2(aq) + H2O(l) + CO2(g)


4. Single Replacement Reactions A + BC ® B + AC if A is a metal

A + BC ® C + BA if A is a nonmetal

The driving force for these redox reactions is the transfer of electrons from the more reactive element to the less reactive element. The reactions will only go in one direction – always the electrons will flow from more reactive species to the less reactive species. To know which metal is the more reactive element, you need an activity series. (Attached.) To know which nonmetal is the more reactive element, you have only to look at the periodic table. Using the halogens as an example, the more reactive nonmetals are at the top of the group. For instance, F2 can replace Cl in NaCl, but Cl2 cannot replace F in NaF.

Gas Forming Reactions Metal + acid ® salt + hydrogen gas

Metals that are more reactive than hydrogen on the activity series can replace hydrogen in the acid.

Example: 2Sc(s) + 6HCl(aq) ® 2ScCl3(aq) + 3H2(g)

Remember to base the formula of the salt on the common charges of the metal and the anion. Scandium always is +3 (the group number).
 

 5. Combustion                              hydrocarbon + oxygen gas ® carbon dioxide and water and heat Combustion reactions are reactions with oxygen that always generate heat and light. The oxygen is the oxidizing agent and the other reactant serves as the reducing agent. The driving force is the transfer of electrons to oxygen.

Example: CH4(g) + 2O2 ® CO2(g) + 2H2O(g) + heat
 


 
Last modified December 28, 1998                Top        For more information, contact:  ksanchez@fccj.org