Question

Hydroiodic acid reduces chlorine to hydrochloric acid and iodine. Write a balanced net ionic equation for the reaction.

Short answer

The balanced net ionic equation for the reaction is: 2 I- + Cl2 → I2 + 2 Cl-.

Step-by-step solution

Write the Unbalanced Reaction

First, identify the reactants and products for the chemical reaction. Hydroiodic acid (HI) reduces chlorine (Cl2) to hydrochloric acid (HCl) and iodine (I2). The unbalanced reaction is HI + Cl2 → HCl + I2.

Write the Half Reactions

Split the unbalanced reaction into two half reactions. One for the oxidation: 2 I- → I2 + 2 e-, and one for the reduction: Cl2 + 2 e- → 2 Cl-.

Balance the Atoms Other Than Oxygen and Hydrogen

Balance the iodine and chlorine atoms in both half-reactions. The oxidation half-reaction is already balanced with 2 iodide ions producing 1 iodine molecule. The reduction half-reaction is also balanced.

Balance the Charges

Make sure that each half-reaction has the same number of electrons on both sides to balance the charges. The oxidation half-reaction gives away 2 electrons and the reduction half-reaction gains 2 electrons, so they are already balanced.

Combine the Half Reactions

Combine the two half-reactions to get the net ionic equation. The electrons should cancel out, which means the overall reaction is balanced. 2 I- + Cl2 → I2 + 2 Cl-.

Simplify the Net Ionic Equation

Check if there are spectator ions or other substances that don't change during the reaction. Since this is a net ionic equation, only ions and molecules that undergo a change are shown, so the equation is already simplified.

Key concepts

Oxidation-Reduction Reactions

An oxidation-reduction or redox reaction is a type of chemical process that involves the transfer of electrons between two species. It's fundamental to understanding how certain substances interact to gain or lose electrons, transforming their chemical nature. Redox reactions are essential in various fields, including energy production, metallurgy, and biology.

In our exercise scenario, the reaction between hydroiodic acid and chlorine is a prime example of a redox process. Here, chlorine is reduced, meaning it gains electrons, and hydroiodic acid is oxidized, meaning it loses electrons. Redox reactions are always made up of two parts – oxidation where a substance loses electrons, and reduction where a substance gains electrons. A useful mnemonic to remember this is 'LEO the lion says GER' - Loss of Electrons is Oxidation, Gain of Electrons is Reduction.

Identifying the substances that get oxidized and reduced, as well as how electrons are transferred in a reaction, can give us deeper insight into the chemical behavior of the elements involved.

Chemical Reaction Balancing

Balancing chemical reactions is a crucial step in the study of chemistry. It ensures that the law of conservation of mass is upheld, meaning the mass of reactants equals the mass of products. In a balanced equation, the number of atoms for each element and the total charge must be the same on both sides of the reaction.

When writing the unbalanced equation for the reaction between hydroiodic acid and chlorine, as shown in the exercise, the initial step is simply to write down the reactants and expected products. After this, you follow a systematic approach to balance the chemical equation by ensuring the number of atoms of each element are equal on both sides.

For complex reactions, balancing can be a challenge because you may have multiple atoms and charges to consider. This is where advanced methods, such as the half-reaction method, become incredibly valuable to avoid errors and ensure accuracy in the balancing process.

Half-Reaction Method

The half-reaction method is particularly effective for balancing oxidation-reduction reactions. It involves separating the overall reaction into two simpler equations, one for oxidation and one for reduction. This technique allows you to balance the number of electrons transferred in each half of the reaction and then combine them to form a balanced net ionic equation.

This method ensures each element and charge is balanced separately, which is especially useful when dealing with reactions in acidic or basic solutions where hydrogen ions (H+) or hydroxide ions (OH-) may need to be included to achieve balance.

Steps to Use the Half-Reaction Method

• Split the reaction into two half-reactions.
• Balance the atoms of each element except for oxygen and hydrogen.
• Balance the oxygen atoms by adding water molecules.
• Balance the hydrogen atoms by adding H+ or OH- ions, depending on the reaction environment.
• Balance the charges by adding electrons.
• Combine the half-reactions and ensure that the electrons cancel out.

In our example, the half-reaction process was straightforward because there was no need to account for oxygen or hydrogen, and the charges were already balanced. However, in more complicated scenarios, executing each step with precision is key to achieving a correct balanced net ionic equation.