Skeleton equations are one of the basic means of representing a chemical reaction. Click here to find out what skeleton equations are, and how they are written.
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In chemistry, a chemical reaction is said to have taken place when two or more elements react with each other to produce a new element/s. It is important to note that, chemical reactions are different from processes like melting or freezing, which are essentially physical processes wherein only the state of the matter changes.
In chemical reactions, atomic/molecular bonds are formed, broken or rearranged, so that a chemically different product is formed. Thus, for example, while freezing water to ice is merely a physical process, the chemical combination of sodium and chlorine gas which results in the formation of sodium chloride (salt) is a chemical reaction.
The most basic elements of any chemical reaction are the reactants and the products. The reactants are the original elements (atoms and molecules), which, under suitable conditions, react chemically with each other. The products are the molecules that are obtained as a result of a chemical reaction. Thus, a chemical reaction can be expressed as:
Reactants ——-> Products
Note that, as shown above, typically, reactants are written on the left, while products on the right side. The arrow indicates that reactants ‘give rise to’ or ‘produce’ the products.
Writing a Chemical Reaction
The basic process of writing a chemical reaction involves first writing the word equation, then the skeleton equation, and finally the balanced equation. The skeleton equation is the intermediate step, which enables the conversion of an equation expressed in words to an actual balanced chemical equation. Let’s learn more about all these three types of equations.
In chemistry, an equation is the most basic way of expressing a chemical reaction. In it, the reaction is expressed by writing down the names of the reactants and the product in plain English. The following is an example of a word equation:
Sodium + Chlorine ——-> Sodium Chloride
Note that, in a word equation, the ‘+’ symbol means ‘reacts with’, and the arrow means ‘to give’. Thus, the above word equation literally means sodium reacts with chlorine to give sodium chloride.
The next step towards writing a complete chemical equation is writing a skeleton equation. A skeleton equation, by definition, is a way of using formulas to indicate the chemicals that are a part of the chemical reaction. In essence, it is identical to a word equation, except that the names of the reactants and the products are substituted by their chemical symbols.
Further, in skeleton equations, the physical states of the reactants and products are mentioned as subscripts on the right-hand side next to each element. The symbol (s) is used to represent a solid, (l) a liquid, (aq) an aqueous, and (g) a gas. This increases the utility of the skeleton equation. The following is an equation for the reaction that produces salt:
Na(s) + Cl2(g) ——-> NaCl(s)
The skeleton equation thus provides us with a better means of expressing a chemical reaction, as compared to a word equation. However, if you carefully observe the above example equation, you will notice that, even though there are two chlorine atoms on the reaction side, there is but a single one on the product side. One chlorine atom isn’t accounted for by the skeleton equation.
Thus, the skeleton equation shows only the elements which are involved in a chemical reaction, but does not express the quantities in which they react or are produced. Since the quantity of reactants on the left-hand side (LHS) mostly differs from the quantity of the product/s on the right-hand side (RHS), a skeleton equations is said to be an unbalanced equation.
Skeleton equations as such aren’t wrong. They are just the unbalanced representation of a chemical reaction. For obtaining the final complete chemical reaction, the skeleton equation needs to be balanced. The balancing procedure is carried out by making a skeleton equation compliant with the law of conservation of mass. It is stated as following:
Law of Conservation of Mass
In any chemical reaction, the mass of the product is always equal to the mass of the reactants.
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Thus, to obtain a balanced equation, the LHS of a skeleton equation has to be made equal to the RHS, by accounting for each and every atom, and making appropriate adjustments on both sides. In case of sodium chloride, the balanced equation can be obtained as:
2Na(s) + Cl2(g) ——-> 2NaCl(s)
The ‘2’ added to both sides of the equation are known as coefficients. They represent the number of each reactant or product (atom or molecule) that must take part in a balanced reaction. The final balanced equation for sodium chloride thus states that, two atoms of sodium (solid) combine with one molecule of chlorine (gas), producing sodium chloride.
Skeleton Equation Examples and Balancing
The following are a few examples of skeleton equations, followed by their balanced versions:
For Water:Skeleton Equation: H2(g) + O2(g) ——-> H2O(l)Balanced Equation: 2H2(g) + O2(g) ——-> 2H2O(l)
For Sulfur Dioxide:Skeleton Equation: S(s) + O2(g) ——-> SO2(g)Balanced Equation: S(s) + O2(g) ——-> SO2(g)
For Hydrochloric Acid:Skeleton Equation: H2(g) + Cl2(g) ——-> 2HCl(g)Balanced Equation: H2(g) + Cl2(g) ——-> 2HCl(g)
For Iron Chloride:Skeleton Equation: Fe(s) + Cl2(g) ——-> FeCl3(s)Balanced Equation: 2Fe(s) + 3Cl2(g) ——-> 2FeCl3(s)