Question

Consider the unbalanced chemical equation. $$ \mathrm{H}_{2} \mathrm{O}(l) \underset{\text { electrical current }}{\longrightarrow} \mathrm{H}_{2}(g)+\mathrm{O}_{2}(g) $$ A chemistry student tries to balance the equation by placing the subscript 2 after the oxygen atom in \(\mathrm{H}_{2} \mathrm{O}\). Explain why this is not correct. What is the correct balanced equation?

Short answer

The correct balanced equation is \[2\mathrm{H}_2\mathrm{O}(l) \longrightarrow 2\mathrm{H}_2(g) + \mathrm{O}_2(g)\]. The student's approach was not correct because it changed the chemical formula of water, rather than applying coefficients to balance the equation.

Step-by-step solution

Identify Incorrect Subscript Change

The student attempted to balance the equation by changing the formula of water from \(\mathrm{H}_2\mathrm{O}\) to \(\mathrm{H}_2\mathrm{O}_2\), which is incorrect. The chemical formula of a compound should not be altered to balance a chemical equation, as the formula is determined by the compound’s chemical structure and properties. Water’s formula, \(\mathrm{H}_2\mathrm{O}\), has two hydrogen atoms and one oxygen atom, and should remain unchanged.

Balance Oxygen Atoms

Begin by balancing the oxygen atoms. Since there are two oxygen atoms on the product side (\(\mathrm{O}_2\)) and only one on the reactant side (\(\mathrm{H}_2\mathrm{O}\)), place a coefficient of 2 in front of \(\mathrm{H}_2\mathrm{O}\) on the reactant side. This gives us 2 oxygen atoms on both sides of the equation.

Balance Hydrogen Atoms

Now, look at balancing the hydrogen atoms. After balancing the oxygen atoms, we have four hydrogen atoms on the reactant side (from 2 molecules of \(\mathrm{H}_2\mathrm{O}\)). To balance the hydrogen atoms on the product side, we place a coefficient of 2 in front of the \(\mathrm{H}_2\) to give us four hydrogen atoms as well.

Verify Balanced Equation

Check to make sure the equation is balanced. There should be the same number of each type of atom on both the reactant and product sides of the chemical equation. The balanced chemical equation is \[2\mathrm{H}_2\mathrm{O}(l) \longrightarrow 2\mathrm{H}_2(g) + \mathrm{O}_2(g)\].

Key concepts

Chemical Equation

A chemical equation is a symbolic representation of a chemical reaction, where the reactants are shown on the left side and the products on the right, separated by an arrow indicating the direction of the reaction. Each molecule is noted by its chemical formula, which denotes the number of atoms of each element present.

For instance, in the electrolysis of water, the unbalanced chemical equation is written as \(\mathrm{H}_2\mathrm{O}(l) \longrightarrow \mathrm{H}_2(g) + \mathrm{O}_2(g)\). Chemical equations must balance because they follow the law of conservation of mass, which states that atoms are neither created nor destroyed during a chemical reaction. Balancing an equation involves finding the coefficients that provide the same number of each type of atom on both sides.

Stoichiometry

Stoichiometry is the calculation of reactants and products in chemical reactions. It is a fundamental concept in chemistry that deals with the quantitative relationships between the substances involved in a reaction, based on the balanced chemical equation.

To ensure the accuracy of these calculations, the chemical equation must be perfectly balanced. Stoichiometry allows chemists to predict how much product will form from a certain amount of reactants, and to determine the proportion of multiple reactants that will be consumed in a reaction.

Subscript and Coefficient in Chemistry

The subscripts and coefficients in chemical formulas and equations are vital for understanding the quantities of substances involved. A subscript is a small number located slightly below and to the right of a chemical symbol, indicating the number of atoms of the element in the molecule. For example, in water (\(\mathrm{H}_2\mathrm{O}\)), the subscript 2 indicates there are two hydrogen atoms for every one oxygen atom.

On the other hand, coefficients are numbers placed in front of chemical formulas to indicate how many molecules (or moles) of that substance are involved. In the balanced equation for the electrolysis of water, the coefficient 2 in front of \(\mathrm{H}_2\mathrm{O}\) specifies that two water molecules are required for the reaction. Altering subscripts changes the identity of the substance, which is incorrect when balancing equations. Instead, altering coefficients is the correct method to achieve a balanced equation.