Solubility and Double Replacement Reactions

Vocabulary

• Double Replacement Reaction
• Soluble/Insoluble
• Molecular Equation

“Two Couples” by Zule - Brazil

AP Topics

Topic 4.7 Types of Chemical Reactions

• Identify a reaction as acid-base, oxidation-reduction, or precipitation.
• Precipitation reactions frequently involve mixing ions in aqueous solution to produce an insoluble or sparingly soluble ionic compound. All sodium, potassium, ammonium, and nitrate salts are soluble in water.

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What happens in a double replacement reaction?

AB + CD 🡪 AD + CB

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​

In aqueous solution, ions from two different ionic compounds switch places.

+

🡪

+

What happens in a double replacement reaction?

• Q: What happens in a double replacement reaction?
• A: Ions from two different ionic compounds switch places

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• Q: In what state must the ionic compounds be in order to participate in a double replacement reaction?
• A: aqueous

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Dissociation

Ionic compounds dissolve by dissociation where ions are pulled apart due to their attraction to the opposite poles of water molecules.

• Solute: Substance that is dissolved
• Solvent: Substance doing the dissolving

Watch

Why do some of the substances dissolved in water conduct electricity and some do not?

Dissociation

When an ionic compound like sodium chloride dissolves in water, the ions dissociate. Take a look:

If the video to the

left does not work,

click on this link.

The ions are more attracted to the water molecules than they are to each other.

Dissociation

Not all ionic compounds dissociate

Cu²⁺

Cu²⁺

Cu²⁺

NO₃^-

NO₃^-

NO₃^-

NO₃^-

NO₃^-

NO₃^-

Cu(NO₃)₂

PbI₂

How Can You Tell?��Solubility Rules!

Insoluble

= does not dissolve in water

= solid

Soluble� = dissolves in water

= aqueous

Cu²⁺

Cu²⁺

Cu²⁺

NO₃^-

NO₃^-

NO₃^-

NO₃^-

NO₃^-

NO₃^-

Cu(NO₃)₂

PbI₂

Cu²⁺

Cu²⁺

Cu²⁺

NO₃^-

NO₃^-

NO₃^-

NO₃^-

NO₃^-

NO₃^-

Cu(NO₃)₂

PbI₂

Solubility...what do you need to know?

• In class, you will use the solubility chart to determine solubility for now.
• For the AP test, you need to memorize that compounds containing nitrate (NO₃^-), ammonium (NH₄⁺), potassium (K⁺), or sodium (Na⁺) ions are soluble and all others are assumed insoluble.
• Remember NAPS if it helps.

Equation Practice

Aqueous potassium iodide reacts with aqueous lead(II) nitrate.

Step 1

Write the formulas for each of the reactants.

KI (aq)

+

Pb(NO₃)₂ (aq)

Aqueous potassium iodide reacts with aqueous lead(II) nitrate.

Step 2

• Swap cations and write formulas for each product.

KI (aq)

+

Pb(NO₃)₂ (aq)

→

PbI₂

+

KNO₃

Aqueous potassium iodide reacts with aqueous lead(II) nitrate.

Reminder

• You get the formulas for the products by swapping the charges of the ions.

KI (aq)

+

Pb(NO₃)₂ (aq)

→

PbI₂

+

KNO₃

2+ 1-

1+ 1-

Aqueous potassium iodide reacts with aqueous lead(II) nitrate.

Step 3

• Balance the equation.

KI (aq)

+

Pb(NO₃)₂ (aq)

→

PbI₂

+

KNO₃

2

2

Aqueous potassium iodide reacts with aqueous lead(II) nitrate.

Now what?

An important part of writing double replacement reactions is determining if the reaction will occur or not. To do this, we need to identify the states of the products using the solubility chart.

KI (aq)

+

Pb(NO₃)₂ (aq)

→

PbI₂ (?)

+

KNO₃ (?)

2

2

Predicting solubility

Let’s first look at the lead(II) iodide. According to the solubility chart, is it soluble or insoluble in water?

PbI₂ (?)

Predicting solubility

It looks like it’s insoluble meaning that it is a solid when in water. The ions remain bonded and are not free to move around the solution and you will see it as a precipitate.

PbI₂ (s)

Predicting solubility

Let’s now look at the solubility of the other product in this reaction, potassium nitrate.

KNO₃ (?)

Predicting solubility

This one looks like it’s soluble. That means that the ions remain dissociated in the solution. You will not see this compound form in this reaction.

KNO₃ (aq)

Predicting solubility

Because at least one of the products is a solid, this reaction WILL occur. You will see the lead(II) iodide precipitate which is evidence of a chemical reaction.

KI (aq)

+

Pb(NO₃)₂ (aq)

→

PbI₂ (s)

+

KNO₃ (aq)

2

2

This format is called the molecular equation, an equation showing the complete neutral formulas for each compound in the reaction as if they existed as molecules.

• At least one product is insoluble = reaction occurs
• All products are soluble = no reaction

Practice

Aqueous copper(II) sulfate and aqueous sodium carbonate

1. Write the balanced molecular chemical equation
2. predict if the reaction occurs or not.

Step 1: Write reactant formulas

CuSO₄(aq) + Na₂CO₃(aq) →

Step 2: Swap cations and write formulas for products

CuSO₄(aq) + Na₂CO₃(aq) →

Na₂SO₄(?) + CuCO₃(?)

Step 3: Balance the equation

CuSO₄(aq) + Na₂CO₃(aq) →

Na₂SO₄(?) + CuCO₃(?)

The equation is already balanced.

Step 4: Predict states of products

CuSO₄(aq) + Na₂CO₃(aq) →

Na₂SO₄(?) + CuCO₃(?)

Step 4: Predict states of products

CuSO₄(aq) + Na₂CO₃(aq) →

Na₂SO₄ + CuCO₃(?)

(aq)

Step 4: Predict states of products

CuSO₄(aq) + Na₂CO₃(aq) →

Na₂SO₄(aq) + CuCO₃(?)

Step 4: Predict states of products

CuSO₄(aq) + Na₂CO₃(aq) →

Na₂SO₄(aq) + CuCO₃

(s)

Conclusion: Does the reaction occur?

CuSO₄(aq) + Na₂CO₃(aq) →

Na₂SO₄(aq) + CuCO₃(s)

The reaction does occur because at least one of the products is a solid.

Last Practice: On Your Own

When a solution of sodium hydroxide is mixed with a solution of lead(II) nitrate, will a reaction occur? Write the balanced molecular chemical equation for the reaction.

Last Practice: On Your Own

NaOH (aq) + Pb(NO₃)₂ (aq) →

NaNO₃ (?) + Pb(OH)₂ (?)

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Last Practice: On Your Own

2 NaOH (aq) + Pb(NO₃)₂ (aq) →

2 NaNO₃ (?) + Pb(OH)₂ (?)

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Last Practice: On Your Own

2 NaOH (aq) + Pb(NO₃)₂ (aq) →

2 NaNO₃ (aq) + Pb(OH)₂ (s)

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Yes, a reaction occurs because lead(II) hydroxide is a precipitate