While studying chemistry, you will come across several types reactions, and it is often necessary to predict the products of a certain reaction or simply be able to identify any of these types of reactions.
Direct Combination/Synthesis Reactions
This type of reaction is where two or more substances combine to form a single new substance (product). This is represented by the general equation:
A + B → AB
Metals react with oxygen to produce the metal oxide (either when heated or at room temperature):
metal + oxygen → metal oxide
2Zn(s) + O2(g) → 2ZnO(s)
When a non metal is heated in oxygen or air, the non-metal oxide is produced:
non-metal + oxygen → non-metal oxide
S(s) + O2(g) → SO2(g)
Some non-metals can react with metals to form a salt:
metal + non-metal → salt
2Na(s) + Cl2(g) → 2NaCl(s)
In decomposition reactions, one compound breaks down into two or more simpler substances. This usually requires conditions such as light, heat or electricity. The general form of the equation is this:
AB → A + B
If a compound decomposes in the presence in heat, it is known as thermal decomposition. The products of a decomposition reaction can be elements or compounds.
Metal hydroxides decompose on heating to yield metal oxides and water.
metal hydroxide → metal oxide + water
2NaOH → Na2O + H2O
Water splits into hydrogen gas and oxygen gas when an electric current is run through it.
2H2O → 2H2 + O2
In a substitution or single displacement reaction, one element replaces a similar one in another compound. This can be shown in the general equation:
A + BX → AX + B
In this case, A is a metal replacing metal B in a compound. However, if a non-metal is replacing another, it looks like this:
Y + XZ → XY + Z
There are three basic types of single displacement reactions:
1) a more reactive metal displaces a less reactive metal from an aqueous solution of its salt:
Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)
Zinc is more reactive than Copper, so it replaces Copper in its salt. Think of it as both of the metals being magnets, but Zinc is stronger than Copper. So, zinc will attract the sulphate ion more strongly. This really occurs because Zinc is easier to ionize than copper.
2) a reactive metal displaces hydrogen from an acid or water:
Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)
2Na(s) + H2O(l) → 2NaOH(aq) + H2(g)
3) a more reactive non-metal displaces a less reactive non-metal from an aqueous solution of its salt.
F2(g) + 2NaCl(aq) → 2NaF(aq) + Cl2(g)
Fluorine is the most electronegative element on the periodic table, so it will displace any other non-metal from its salt.
Ionic Precipitation/Double Displacement Reactions
Here, two soluble ionic compounds react to form one soluble compound another insoluble precipitate. (In order for a reaction to occur, one of the products must be either an insoluble precipitate, a gas, or a molecular compound like water). The positive and negative ions of both ionic compounds switch places. This can be shown with the general equation:
AB + CD → AD + CB
A and C are positively charged ions (cations) while B and D are negatively charged ions (anions).
AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq)
These reactions occur when a substance reacts with oxygen gas to release energy as light or heat.