Question

When a hydrochloric acid solution is combined with a potassium hydroxide solution, an acid-base reaction occurs. Write a balanced molecular equation and a net ionic equation for this reaction.

Short answer

Balanced molecular equation: HCl(aq) + KOH(aq) -> KCl(aq) + H2O(l). Net ionic equation: H+(aq) + OH-(aq) -> H2O(l).

Step-by-step solution

Write the Unbalanced Molecular Equation

First, write down the reactants and products in their molecular form. Hydrochloric acid (HCl) reacts with potassium hydroxide (KOH) to produce potassium chloride (KCl) and water (H2O). The unbalanced molecular equation is: HCl + KOH -> KCl + H2O.

Balance the Molecular Equation

Balance the equation by making sure that there are equal numbers of each type of atom on both sides of the equation. In this case, the equation is already balanced with one of each type of atom on both sides. The balanced molecular equation is: HCl + KOH -> KCl + H2O.

Write the Total Ionic Equation

Split the soluble compounds into their respective ions. HCl, KOH, and KCl are all strong electrolytes and will dissociate completely in solution. The total ionic equation is: H+ + Cl- + K+ + OH- -> K+ + Cl- + H2O.

Cancel Spectator Ions

Identify and remove the spectator ions; these are ions that appear unchanged on both sides of the reaction. The spectator ions are K+ and Cl-. After removing them, the net ionic equation is left with: H+ + OH- -> H2O.

Key concepts

Balancing Chemical Equations

Balancing chemical equations is like solving a puzzle where each side of the equation must have the same number of atoms for each element. Starting with the equation unbalanced shows us which atoms are participating in the reaction. In the case of the hydrochloric acid reacting with potassium hydroxide, the equation begins as HCl + KOH -> KCl + H2O. To balance it, we compare the number of atoms of each element on each side of the arrow. Luckily, each atom has a one-to-one correspondence in this reaction, so the equation is already balanced.

When equations are not balanced, it's essential to find the lowest whole-number coefficients that will equalize the number of atoms on each side. The key to mastering this process is practice and an understanding of the conservation of mass, meaning matter is neither created nor destroyed during a chemical reaction, which is why the atoms must balance out.

Writing Net Ionic Equations

Writing net ionic equations allows us to focus on the particles that actually participate in the reaction. The process begins by writing the total ionic equation, which breaks down aqueous compounds into their ions. From the reaction between HCl and KOH, we get H+ + Cl- + K+ + OH- -> K+ + Cl- + H2O. Here, some ions do not change and are present on both sides; these are called spectator ions. By removing the spectator ions, we are left with the net ionic equation, showing the essential chemical change: H+ + OH- -> H2O.

This abbreviated equation tells us that the hydrogen ion from the acid combines with the hydroxide ion from the base to form water -- the essence of an acid-base reaction. Understanding net ionic equations is useful because it simplifies complex reactions and reveals the actual chemical changes taking place.

Acid-Base Reactions

Acid-base reactions involve an acid and a base reacting to form a salt and usually water, as reflected in the classic Arrhenius definition. These reactions are also known as neutralization reactions. In our example, hydrochloric acid, which is an acid, is reacting with potassium hydroxide, a base, to produce potassium chloride, a salt, and water.

The general form of an acid-base reaction can be summarized as: acid + base -> salt + water. In the molecular equation we have HCl + KOH -> KCl + H2O, following this pattern. Understanding acid-base reactions is fundamental in chemistry as it covers a broad spectrum of applications, including biological systems, industrial processes, and environmental science.

Spectator Ions

Spectator ions are ions that do not take part in the chemical reaction and remain unchanged in both the reactants and products. They can be thought of as 'bystanders' watching the reaction happen without getting involved. Identifying these ions is crucial when writing net ionic equations. In the given acid-base reaction, K+ and Cl- ions are spectator ions. After writing the total ionic equation, we strike them out, as they are present on both sides of the reaction and do not affect the outcome.

Understanding spectator ions helps simplify complex chemical equations and allows chemists to predict the outcomes of reactions in solution. It also highlights the more significant changes occurring during the reaction, providing a clearer picture of the process at a molecular level.