E2.5 Write net ionic equations for precipitation and neutralization

Ontario Grade 11 Chemistry

Solutions and Solubility

Ontario Grade 11 Chemistry — study-guide label E2.5

When two clear solutions are mixed, a cloudy solid may appear. In another mixture, no solid forms, but acid and base particles react to make water. Net ionic equations describe the particles that actually change in these reactions. To write one, identify the products, show the dissolved ions, and remove particles that remain unchanged on both sides.

What you will learn

Explain how particle-level changes produce precipitation and neutralization.
Use state symbols and solubility information to identify aqueous ions and solids.
Write and check net ionic equations that conserve atoms and total charge.

1. From visible change to particles

A solution is a uniform mixture. An aqueous substance is dissolved in water and is marked with the state symbol

(aq)

. A solid is marked

(s)

. When a solid forms from substances that were dissolved, it is called a precipitate. Cloudiness or a visible solid can be evidence that a precipitate formed.

Many soluble ionic compounds separate into positive and negative ions when dissolved. For example, sodium chloride in water is represented by dissolved sodium ions and chloride ions. The state symbol

(aq)

tells you that the ions are dissolved in water; it does not mean water is a reactant.

In a precipitation reaction, ions from solutions can join to make a solid ionic compound. The ions that become part of the solid have changed. Other ions may stay dissolved before and after mixing. These unchanged ions are called spectator ions.

A net ionic equation shows only the particles that take part in the change. It leaves out spectator ions. This makes the equation a shorter description of what happens among the particles.

A precipitate is an insoluble solid formed from dissolved substances.
A spectator ion remains unchanged and appears on both sides of the complete ionic equation.
A net ionic equation includes only the particles that change.

2. Write a net ionic equation for precipitation

Begin with a balanced molecular equation. A molecular equation writes substances as formulas. Use the course solubility table to decide which products are aqueous and which are solids. The state matters: aqueous ionic compounds are written as ions in the complete ionic equation, while a solid precipitate stays together as a formula.

Next, write the complete ionic equation. This equation shows the ions in aqueous ionic compounds. Keep each formula's subscripts and charges. Do not split solids, liquids, or gases into ions. A coefficient applies to every ion represented by the formula.

Compare the complete ionic equation on both sides. Cancel spectator ions only when the ion, including its charge, is identical on each side. Cancel equal amounts from each side. What remains is the net ionic equation.

For example, lead(II) ions and iodide ions can form solid lead(II) iodide. A solubility table identifies the product as insoluble. Potassium ions and nitrate ions, if present in the mixture, remain dissolved and are spectators. After they are cancelled, only the lead and iodide ions that form the solid remain.

\mathrm{Pb^{2+}(aq) + 2I^-(aq) \rightarrow PbI_2(s)}

Use the solubility table to assign product states.
Split aqueous ionic compounds, but keep a precipitate together as a solid formula.
Cancel unchanged ions and check atoms and total charge.

3. Write a net ionic equation for neutralization

In the course-level model used here, an acid produces hydrogen ions in aqueous solution. A base produces hydroxide ions in aqueous solution. In neutralization, hydrogen ions and hydroxide ions combine to form water. This particle-level change is the same for the aqueous acids and soluble hydroxide bases used in these examples.

Write and balance the molecular equation first. Then separate aqueous ionic compounds into ions to make the complete ionic equation. Keep water as a liquid molecule. Cancel ions that appear unchanged on both sides. The remaining equation shows the particles that form water.

Do not cancel an ion just because the same element appears on both sides. The ion must have the same formula and charge. Also, do not split water into ions in this net ionic equation.

Check atoms as well as charge. In the neutralization equation, the left side has two hydrogen atoms and one oxygen atom in total. The water molecule on the right has two hydrogen atoms and one oxygen atom. The total charge is zero on each side.

\mathrm{H^+(aq) + OH^-(aq) \rightarrow H_2O(l)}

In this course-level model, neutralization includes hydrogen ions and hydroxide ions forming water.
Keep water as a molecule rather than splitting it into ions.
Cancel only identical ions that remain unchanged.

4. A reliable checking routine

Use the same sequence each time: write the molecular equation, balance it, assign states, and form the complete ionic equation. Then cancel spectator ions and write the net ionic equation. This sequence helps prevent missed coefficients and incorrect splitting.

Balance equations with coefficients, not by changing subscripts. A coefficient changes the number of particles represented. A subscript is part of a compound's formula, so changing it would change the substance.

There are no measured quantities or units to calculate in this task. The checks are chemical: formulas must be correct, states must agree with solubility information, atoms must balance, and total charge must balance.

Ask which particles have actually changed. An ion that remains unchanged does not belong in the net ionic equation. An ion that becomes part of a precipitate or water does belong.

Balance with coefficients, not altered subscripts.
Use states to decide what splits into ions.
Check atoms and total charge after cancellation.

Worked example

Precipitation and neutralization

Write the net ionic equation for mixing aqueous lead(II) nitrate with aqueous potassium iodide. Then write the net ionic equation for aqueous hydrochloric acid reacting with aqueous sodium hydroxide.

Identify the precipitation products
The solubility table shows that lead(II) iodide is a solid. Potassium nitrate is aqueous. Balance the molecular equation before splitting any aqueous compounds into ions.
$\mathrm{Pb(NO_3)_2(aq) + 2KI(aq) \rightarrow PbI_2(s) + 2KNO_3(aq)}$
Separate aqueous compounds into ions
Split each aqueous ionic compound into ions. Keep solid lead(II) iodide together. The coefficients apply to each ion produced.
$\mathrm{Pb^{2+}(aq) + 2NO_3^-(aq) + 2K^+(aq) + 2I^-(aq) \rightarrow PbI_2(s) + 2K^+(aq) + 2NO_3^-(aq)}$
Cancel spectator ions
Potassium ions and nitrate ions are unchanged on both sides. Cancel equal amounts of these ions. The remaining lead and iodide ions form the precipitate.
$\mathrm{Pb^{2+}(aq) + 2I^-(aq) \rightarrow PbI_2(s)}$
Write the neutralization equation
Hydrochloric acid, sodium hydroxide, and sodium chloride are aqueous here. Water remains a liquid molecule. Balance the molecular equation before writing the ions.
$\mathrm{HCl(aq) + NaOH(aq) \rightarrow NaCl(aq) + H_2O(l)}$
Keep only the changing particles
Separate the aqueous substances into ions. Sodium ions and chloride ions appear unchanged on both sides, so cancel them. Hydrogen ions and hydroxide ions form water. There are two hydrogen atoms and one oxygen atom on each side overall, and the total charge is zero on both sides.
$\mathrm{H^+(aq) + OH^-(aq) \rightarrow H_2O(l)}$

Answer: The precipitation net ionic equation is

\mathrm{Pb^{2+}(aq) + 2I^-(aq) \rightarrow PbI_2(s)}

. The neutralization net ionic equation is

\mathrm{H^+(aq) + OH^-(aq) \rightarrow H_2O(l)}

.

Check: For precipitation, there is one lead atom and two iodine atoms on each side. The charge on the left is

+2 + 2(-1)=0

, matching the neutral solid on the right. For neutralization, there are two hydrogen atoms and one oxygen atom on each side. The total charge is zero on each side.

Common mistakes and how to avoid them

Splitting a solid precipitate into ions in the complete ionic equation.

Correction: Split aqueous ionic compounds only. Keep the precipitate together with state symbol

(s)

.

Leaving spectator ions in the net ionic equation.

Correction: Compare the complete ionic equation on both sides and cancel identical ions in equal amounts.

Changing a formula subscript to balance an equation.

Correction: Use coefficients to balance atoms. Changing a subscript changes the chemical substance.

Checking atoms but not charge.

Correction: Add the ionic charges on each side. The total charge must be equal.

Lesson summary

A precipitation reaction forms an insoluble solid from dissolved ions.
A neutralization net ionic equation shows hydrogen ions and hydroxide ions forming water.
Write a balanced molecular equation, assign states, split aqueous ionic compounds, cancel spectators, and check atoms and charge.

Check your understanding

Question 1

Which expression is the net ionic equation for a precipitation reaction that forms solid silver chloride?

$\mathrm{Ag^+(aq) + Cl^-(aq) \rightarrow AgCl(s)}$
$\mathrm{Ag^+(aq) + Cl^-(aq) \rightarrow Ag^+(aq) + Cl^-(aq)}$
$\mathrm{AgCl(s) \rightarrow Ag^+(aq) + Cl^-(aq)}$
$\mathrm{Ag^+(aq) + Cl^-(aq) \rightarrow AgCl(aq)}$

Show answer and explanation

\mathrm{Ag^+(aq) + Cl^-(aq) \rightarrow AgCl(s)}

Silver ions and chloride ions combine to form solid silver chloride. The second choice shows unchanged ions, the third reverses solid formation, and the fourth gives the product the wrong state.

Question 2

Which ions combine in the net ionic equation for a neutralization that forms water?

$\mathrm{Na^+(aq)}$ and $\mathrm{Cl^-(aq)}$
$\mathrm{H^+(aq)}$ and $\mathrm{OH^-(aq)}$
$\mathrm{K^+(aq)}$ and $\mathrm{NO_3^-(aq)}$
$\mathrm{H^+(aq)}$ and $\mathrm{Cl^-(aq)}$

Show answer and explanation

\mathrm{H^+(aq)}

and

\mathrm{OH^-(aq)}

Hydrogen ions and hydroxide ions form water. Sodium, chloride, potassium, and nitrate ions may be spectators in suitable reactions.

Question 3

A complete ionic equation contains identical potassium ions on both sides. What should you do when writing the net ionic equation?

Cancel equal amounts of potassium ions from both sides.
Keep all potassium ions because they are charged.
Change potassium ions into neutral potassium atoms.
correctIndex`: 0

Show answer and explanation

Cancel equal amounts of potassium ions from both sides.

Potassium ions that remain unchanged are spectator ions. Cancel equal amounts from both sides; do not change their identity or charge.

Key terms

Aqueous: Dissolved in water; shown by the state symbol $(aq)$ .
Precipitate: An insoluble solid formed when dissolved substances react.
Spectator ion: An ion that remains unchanged in a reaction and appears on both sides of the complete ionic equation.
Net ionic equation: An equation showing only the particles that change during a reaction.
Neutralization: A reaction in which hydrogen ions and hydroxide ions form water in the course-level model.

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Published by DoAssignment. This AI-assisted lesson follows Ontario Grade 11 Chemistry (SCH3U), expectation E2.5. It is a study resource, not an official curriculum publication.

Before publication, the draft is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability, and then requires administrator approval. Errors can still occur, so corrections are welcomed.

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E2.5 · Write net ionic equations for precipitation and neutralization

Ontario Grade 11 Chemistry

Ontario Grade 11 Chemistry — study-guide label E2.5

What you will learn

1. From visible change to particles

2. Write a net ionic equation for precipitation

3. Write a net ionic equation for neutralization

4. A reliable checking routine

Precipitation and neutralization

Common mistakes and how to avoid them

Lesson summary

Check your understanding

Question 1

Question 2

Question 3

Key terms

Continue through SCH3U

About this lesson and its review