Understanding some forms of chemical reactions

Introduction

In the last few courses, I discussed electrovalent and covalent bonding as examples of chemical bonding or reactions. One important thing about chemical reaction is that it usually leads to the formation of a new substance. The various forms of chemical combinations discussed above are just examples of the many forms of reactions. There are many others and that is what I will cover in this discussion

So let us see some of the different types of reactions and how they are all unique.

Types of chemical reactions

• Combination reaction: This type of reaction involves the uniting of one element or compound with another. A compound could be combined with another compound to form a more complex compound. Elements also combine with each other during chemical reactions to form a compound.

Examples of combination reaction

2Na(s) + Cl₂(g) → 2NaCl(s)

The above combination reaction illustrates the formation of common salt or Sodium chloride by 2 molecules of sodium and 2 molecules of chlorine gas. These are two elements combined to form a compound.

CO₂(g) + H₂O(l) → H₂CO₃(aq)

This above is another example of combination reaction. Here we see two compounds reacting to form another compound. Water combines with carbon dioxide to form carbonic acid. Unlike the previous example, a bigger compound is formed by the combination of two simpler compounds.

• Reversible reaction: A reversible reaction is one in which the chemical change could be reversed after the reactants formed the product. So it means that if certain conditions are maintained, the reactants could be recovered from the products.

A reversible reaction can thus proceed in both directions - forward and backward. The reactants can form the products. The products could still produce the reactants.

A normal irreversible reactions is denoted by a single arrow pointing from the reactants to the products. But in the case of a reversible reaction, two arrows are used. One arrow is pointing from the reactants to the products. The second arrow points from the products to the reactants. Check out the illustration of a reversible reaction:

The forward reactions see the products Y and Z formed from the reaction between X and Y. In the reverse reaction, Y and Z could combine if the right conditions are met to form the original reactants X and Y. One important thing to note is that the conditions for the forward reaction always differ from the conditions for the reverse reaction

Examples of reversible reaction

Example 1: CaCO₃(s) + H₂O(l) + CO₂(g) ⇌ Ca(HCO₃)₂(aq)

In the above reaction, carbon (iv) oxide combines with lime water to produce hydrogentrioxocarbonate (iv). The milky lime water reappears if this new compound is slightly heated. The conditions for the two reactions are thus different and must be met for a reversible reaction to take place.

The forward reaction takes place under a cold environment. But for the reverse reaction to happen, the liquid must be heated. This supports the fact that most reversible reactions happen under different conditions. These conditions must be met, otherwise only the forward reaction happens.

Example 2: I₂(g) + H₂(g) ⇌ 2HI(g)

The above example shows the formation of Hydrogen iodide by the combination of hydrogen and iodine in the forward reaction. This happens under cool conditions. For the reverse reaction, heating hydrogen iodide will lead to thermal decomposition of the compound into its original constituents -iodine and hydrogen.

• 3. Decomposition: Another type of reaction is called decomposition. In this reaction, a compound can split into its constituent elements under the right conditions. In most decomposition reactions, heating is involved. When that happens, it is often referred to as thermal decomposition.

Lets look at some examples of thermal decomposition.

Example 1: Calcium carbonate Direct heating of calcium carbonate leads to its splitting into constituent elements carbon dioxide and calcium oxide. See the equation of reaction below:

CaCO₃(s) → CO₂(g) + CaO(s)

Example 2: Hydrogen iodide: Hydrogen iodide could be split into iodine and hydrogen when slightly heated. That is another example of thermal decomposition. Here is the reaction equation.

2HI(g) → H₂(g) + I₂(g)

Example 3: Lead (II) Nitrate: This compound can undergo thermal decomposition to split into constituent elements. The constituent elements are oxygen, Nitrogen (iv) oxide, and lead (II) nitrate. Check out the equation of reaction below:

2Pb(NO₃)₂(s) → 2PbO(s) + 4NO₂

Conclusion

In this presentation, we have seen three different types of chemical reactions with examples - combination reaction, reversible reaction and decomposition. There are many other types of reactions which we would learn about in out next presentation

Reference material

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