What do skeleton equations not show?

Shrimp gumbo is one of many dishes that are part of the Cajun culture in Louisiana. It's a spicy dish that needs careful control of all the ingredients so that it has a "kick", but is not overwhelming. Recipes convey not only what the preparation entails, but also describe how much of each ingredient is needed, and the details of how to cook the meal. Similarly, we need this type of information in order to carry out chemical reactions successfully and safely.

Word equations are time-consuming to write and do not prove to be convenient for many of the things that chemists need to do with equations. A chemical equation is a representation of a chemical reaction that displays the reactants and products with chemical formulas. The chemical equation for the reaction of methane with oxygen is shown:

\[\ce{CH_4} + \ce{O_2} \rightarrow \ce{CO_2} + \ce{H_2O}\nonumber \]

The equation above, called a skeleton equation, is an equation that shows only the formulas of the reactants and products with nothing to indicate the relative amounts. The first step in writing an accurate chemical equation is to write the skeleton equation, making sure that the formulas of all substances involved are written correctly. All reactants are written to the left of the yield arrow, separated from one another by a plus sign. Likewise, products are written to the right of the yield arrow, also separated with a plus sign.

It is often important to know the physical states of the reactants and products taking part in a reaction. To do this, put the appropriate symbol in parentheses after each formula: \(\left( s \right)\) for solid, \(\left( l \right)\) for liquid, \(\left( g \right)\) for gas, and \(\left( aq \right)\) for an aqueous (water-based) solution. The previous reaction becomes:

\[\ce{CH_4} \left( g \right) + \ce{O_2} \left( g \right) \rightarrow \ce{CO_2} \left( g \right) + \ce{H_2O} \left( l \right)\nonumber \]

The table below shows a listing of symbols used in chemical equations. Some, such as the double arrow which represents equilibrium, and the use of a catalyst in a reaction, will be detailed in other concepts.

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.

Note:
Oxygen is a diatomic element which is always found as O2 in nature. Other diatomic elements are H2, F2, Cl2, Br2, I2, O2, and N2―all of which are gases. Therefore, one must take care to use proper formula for these elements while substituting them in skeleton equations. The rest of the elements can be represented in the mono-atomic form.

We ourselves and indeed the whole world are made of the 118 identified elements in the periodic table. These basic forms of matter react with each other chemically to form all the complex living and non-living things that we see around us. Needless to say, therefore, it is of utmost importance that we study how these important chemical reactions take place.

But to be able to study the literally thousands of possible chemical reactions, there needs to be a proper way of representing them in writing, which is in accordance to the laws of nature, and which is mathematically correct. For this purpose, scientists around the world make use of chemical equations.

Skeleton equations are a form of chemical equations which are a quick and convenient way of writing a mathematical equation. They are viewed in chemistry as an important step which ultimately leads to the development of a complete chemical equation.

In the following lines, we shall find out how to write a skeleton equation. But before that, we shall have a quick overview at the fundamentals of writing a chemical reaction.

Basics of a Chemical Reaction

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.

Word 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.

Skeleton Equations

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.

Balanced Equations

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.

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)