Question

When aqueous solutions of potassium chromate and barium chloride mix, a yellow solid forms. (a) Identify the solid. (b) Write a balanced molecular equation for the reaction. (c) Write a balanced ionic equation for the reaction. (d) Identify any spectator ions in the reaction. (c) Write a balanced net ionic equation for the reaction.

Short answer

The yellow solid formed is Barium Chromate. The balanced molecular equation is K2CrO4 + BaCl2 → BaCrO4 + 2 KCl. The balanced ionic equation is 2 K+ + CrO4(2-) + Ba(2+) + 2 Cl- → BaCrO4 + 2 K+ + 2 Cl-. The spectator ions are K+ and Cl-. The balanced net ionic equation is CrO4(2-) + Ba(2+) → BaCrO4.

Step-by-step solution

- Identify the Solid

When the aqueous solutions of potassium chromate (K2CrO4) and barium chloride (BaCl2) mix, a yellow solid forms. This yellow solid is barium chromate (BaCrO4).

- Balance the Molecular Equation

The unbalanced molecular equation for this reaction would be K2CrO4 + BaCl2 → BaCrO4 + KCl. To balance this equation, it becomes: K2CrO4 + BaCl2 → BaCrO4 + 2 KCl.

- Write the Balanced Ionic Equation

The balanced ionic equation includes all of the particles in the solution. It is: 2 K+ + CrO4(2-) + Ba(2+) + 2 Cl− → BaCrO4 + 2 K+ + 2 Cl−.

- Identify Spectator Ions

Spectator ions are ions that do not participate in the reaction. They are present on both the reactant and product sides of the equation. For this equation, the spectator ions are K+ and Cl-.

- Write the Balanced Net Ionic Equation

The net ionic equation is the chemical equation that shows only those particles, compounds, and ions involved in the chemical reaction. By removing the spectator ions from the ionic equation, we get CrO4(2-) + Ba(2+) → BaCrO4.

Key concepts

Reaction Equation

In a chemical reaction, the reaction equation is the starting point for understanding what takes place when substances interact. When aqueous solutions of potassium chromate (K\(_2\)CrO\(_4\)) and barium chloride (BaCl\(_2\)) mix, an exchange occurs between ions in solution leading to a solid forming. Hence, we say a precipitation reaction takes place.

For this example, the initial unbalanced reaction equation is:

• K\(_2\)CrO\(_4\)(aq) + BaCl\(_2\)(aq) → BaCrO\(_4\)(s) + KCl(aq)

This equation represents the actual reactants used and products formed during the chemical reaction.
To ensure the conservation of mass, we need to balance this equation. This means having an equal number of each type of atom on both sides of the equation. In the balanced form, the reaction equation becomes:

• K\(_2\)CrO\(_4\)(aq) + BaCl\(_2\)(aq) → BaCrO\(_4\)(s) + 2KCl(aq)

Balancing is crucial because it adheres to the law of conservation of matter, ensuring that no atoms are lost or gained during the reaction.

Spectator Ions

In a chemical reaction occurring in solution, spectator ions are those ions that do not partake in the actual chemical change. They remain unchanged, floating in the solution before and after the reaction. They are present on both sides of a complete ionic equation, illustrating their non-participatory role.

In the mixing of potassium chromate and barium chloride, the complete ionic equation is:

• 2K\(^+\)(aq) + CrO\(_4^{2-}\)(aq) + Ba\(^{2+}\)(aq) + 2Cl\(^-\)(aq) → BaCrO\(_4\)(s) + 2K\(^+\)(aq) + 2Cl\(^-\)(aq)

Upon inspection, it's clear that the potassium (K\(^+\)) and chloride (Cl\(^-\)) ions remain intact and unaltered irrespective of the reaction conditions. These ions are spectators.

• Potassium ions (K\(^+\))
• Chloride ions (Cl\(^-\))

Thus, they don't need to be included in the net ionic equation, simplifying the representation of the actual reaction occurring.

Net Ionic Equation

The net ionic equation represents only the substances that undergo a chemical change in the reaction. It excludes the spectator ions, providing a more direct view of the process by focusing only on the reacting species.

For students, understanding the net ionic equation helps emphasize what truly happens chemically within the reaction.

From the equation:

• CrO\(_4^{2-}\)(aq) + Ba\(^{2+}\)(aq) → BaCrO\(_4\)(s)

It shows that the chromate ions (CrO\(_4^{2-}\)) and barium ions (Ba\(^{2+}\)) are the ions actually forming the solid precipitate barium chromate (BaCrO\(_4\)).
Through writing net ionic equations, the essential details of a reaction become clear, spotlighting the interaction that gives rise to a new substance, in this case, the yellow solid barium chromate.