Writing Net Ionic Equations for Aqueous Reactions: A Step-by-Step Guide | Lecture notes Chemistry

09. Net Ionic Equations tutorial.doc

Daley 1 10/9/09

Introduction to Net Ionic Equations

This tutorial will give you an algorithm for writing net ionic equations for aqueous reactions in

general chemistry. Knowledge of the solubility rules is necessary to complete this tutorial. A list of

solubility rules is provided at the end of the tutorial for review. Net ionic equations are necessary in

chemistry for several reasons:

1. They show only those species undergoing chemical change – “spectator” ions are removed.

2. They must be used in any equilibrium calculation.

3. They allow chemical reactions to be combined properly.

4. Used when balancing half-reactions in oxidation-reduction chemistry.

To take a chemical reaction in molecular form and rewrite as a net ionic equation is straight forward

if you follow the steps given below. We will cover each step in detail using a common gas forming

reaction from chemistry 1A.

1. Write the correct reactant and products – DO NOT BALANCE!

The first step is to write the correct chemical formulas for the reactants and products, DO NOT

BALANCE. The reaction will be balanced once the net-ionic equation is complete. As an example

reaction, we will look at the aqueous reaction of sodium carbonate with acetic acid.

Na2CO3 + HC2H3O2 ! NaC2H3O2 + H2O + CO2 (skeleton equation)

The above reaction is called the skeleton equation; it contains only the correct chemical formulas of

each reactant and product. Do not balance this equation!

2. Add phase symbols to EVERY reactant and product.

This next step is crucial! You need to add the correct phase symbols to each reactant and product.

You must rely on the solubility rules to assign the phase symbols. The phase symbols are

• (s) solid - for insoluble salts. Check the solubility rules!

• (l) liquid - for pure liquids like water.

• (g) gas – for gaseous reactants and products.

• (aq) aqueous – for any compound, ionic or covalent, that is soluble in water. Check the

solubility rules!

For the above reaction lets assign phases for each compound.

1. Na2CO3: sodium salt. All sodium salts are soluble. Phase is (aq).

2. HC2H3O2: acetic acid. Low molar mass organic acid. Should be soluble. Phase is (aq).

3. NaC2H3O2: sodium salt. All sodium salts are soluble. Phase is (aq).

4. H2O: water as a product or reactant. We will treat it as pure. Phase is (l).

5. CO2: Low molar mass covalent compound. Gas at room temperature. Phase is (g).

Rewrite the equation with the phase symbols:

Na2CO3(aq) + HC2H3O2(aq) ! NaC2H3O2(aq) + H2O(l) + CO2(g) (molecular equation)

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Introduction to Net Ionic Equations

This tutorial will give you an algorithm for writing net ionic equations for aqueous reactions in

general chemistry. Knowledge of the solubility rules is necessary to complete this tutorial. A list of

solubility rules is provided at the end of the tutorial for review. Net ionic equations are necessary in

chemistry for several reasons:

1. They show only those species undergoing chemical change – “spectator” ions are removed.

2. They must be used in any equilibrium calculation.

3. They allow chemical reactions to be combined properly.

4. Used when balancing half-reactions in oxidation-reduction chemistry.

To take a chemical reaction in molecular form and rewrite as a net ionic equation is straight forward

if you follow the steps given below. We will cover each step in detail using a common gas forming

reaction from chemistry 1A.

1. Write the correct reactant and products – DO NOT BALANCE!

The first step is to write the correct chemical formulas for the reactants and products, DO NOT

BALANCE. The reaction will be balanced once the net-ionic equation is complete. As an example

reaction, we will look at the aqueous reaction of sodium carbonate with acetic acid.

Na 2 CO 3 + HC 2 H 3 O 2! NaC 2 H 3 O 2 + H 2 O + CO 2 ( skeleton equation )

The above reaction is called the skeleton equation; it contains only the correct chemical formulas of

each reactant and product. Do not balance this equation!

2. Add phase symbols to EVERY reactant and product.

This next step is crucial! You need to add the correct phase symbols to each reactant and product.

You must rely on the solubility rules to assign the phase symbols. The phase symbols are

• (s) solid - for insoluble salts. Check the solubility rules!

• (l) liquid - for pure liquids like water.

• (g) gas – for gaseous reactants and products.

• (aq) aqueous – for any compound, ionic or covalent, that is soluble in water. Check the

solubility rules!

For the above reaction lets assign phases for each compound.

1. Na 2 CO 3 : sodium salt. All sodium salts are soluble. Phase is (aq).

2. HC 2 H 3 O 2 : acetic acid. Low molar mass organic acid. Should be soluble. Phase is (aq).

3. NaC 2 H 3 O 2 : sodium salt. All sodium salts are soluble. Phase is (aq).

4. H 2 O: water as a product or reactant. We will treat it as pure. Phase is (l).

5. CO 2 : Low molar mass covalent compound. Gas at room temperature. Phase is (g).

Writing Net Ionic Equations for Aqueous Reactions: A Step-by-Step Guide, Lecture notes of Chemistry

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Partial preview of the text

Download Writing Net Ionic Equations for Aqueous Reactions: A Step-by-Step Guide and more Lecture notes Chemistry in PDF only on Docsity!

Introduction to Net Ionic Equations

This tutorial will give you an algorithm for writing net ionic equations for aqueous reactions in

general chemistry. Knowledge of the solubility rules is necessary to complete this tutorial. A list of

solubility rules is provided at the end of the tutorial for review. Net ionic equations are necessary in

chemistry for several reasons:

1. They show only those species undergoing chemical change – “spectator” ions are removed.

2. They must be used in any equilibrium calculation.

3. They allow chemical reactions to be combined properly.

4. Used when balancing half-reactions in oxidation-reduction chemistry.

To take a chemical reaction in molecular form and rewrite as a net ionic equation is straight forward

if you follow the steps given below. We will cover each step in detail using a common gas forming

reaction from chemistry 1A.

1. Write the correct reactant and products – DO NOT BALANCE!

The first step is to write the correct chemical formulas for the reactants and products, DO NOT

BALANCE. The reaction will be balanced once the net-ionic equation is complete. As an example

reaction, we will look at the aqueous reaction of sodium carbonate with acetic acid.

Na 2 CO 3 + HC 2 H 3 O 2! NaC 2 H 3 O 2 + H 2 O + CO 2 ( skeleton equation )

The above reaction is called the skeleton equation; it contains only the correct chemical formulas of

each reactant and product. Do not balance this equation!

2. Add phase symbols to EVERY reactant and product.

This next step is crucial! You need to add the correct phase symbols to each reactant and product.

You must rely on the solubility rules to assign the phase symbols. The phase symbols are

• (s) solid - for insoluble salts. Check the solubility rules!

• (l) liquid - for pure liquids like water.

• (g) gas – for gaseous reactants and products.

• (aq) aqueous – for any compound, ionic or covalent, that is soluble in water. Check the

solubility rules!

For the above reaction lets assign phases for each compound.

1. Na 2 CO 3 : sodium salt. All sodium salts are soluble. Phase is (aq).

2. HC 2 H 3 O 2 : acetic acid. Low molar mass organic acid. Should be soluble. Phase is (aq).

3. NaC 2 H 3 O 2 : sodium salt. All sodium salts are soluble. Phase is (aq).

4. H 2 O: water as a product or reactant. We will treat it as pure. Phase is (l).

5. CO 2 : Low molar mass covalent compound. Gas at room temperature. Phase is (g).

Rewrite the equation with the phase symbols:

Na 2 CO 3 (aq) + HC 2 H 3 O 2 (aq)! NaC 2 H 3 O 2 (aq) + H 2 O(l) + CO 2 (g) ( molecular equation )

The above reaction is called the molecular equation. All reactant and products are present with the

correct phases.

3. Find the STRONG ELECTROLYTES in the molecular equation

In this step you highlight only the strong electrolytes in the molecular equation. Only the strong

electrolytes will be rewritten as ions in solution. Weak or non-electrolytes will be ignored and not

written as ions. To find the strong electrolytes LOOK ONLY at (AQ) REACTANTS and

PRODUCTS. Why? To be a strong electrolyte a compound must be soluble! What (aq) species are

strong electrolytes in water? ONLY TWO: 1) SOLUBLE SALTS (including the strong bases) AND

2) STRONG ACIDS.

The strong acids are: HCl(aq), HBr(aq), HI(aq), HNO 3 (aq), HClO 4 (aq), HClO 3 (aq), H 2 SO 4 (aq)

Note: for H 2 SO 4 (aq) write as H+(aq) and HSO 4 – (aq) NOT H+(aq) and SO 42 – (aq)

STRONG ELECTROLYTES

Yes

STRONG ELECTROLYTE

Yes

STRONG ELECTROLYTE

No

NOT a Strong Electrolyte

No

Is Chemical a Strong Acid?

Is Chemical a Soluble Salt?

Yes

Possible Strong Electrolyte

No

NOT a Strong Electrolyte

Phase is (aq)?

Lets apply the flowchart to each of our aqueous chemicals in our example reaction.

1. Na 2 CO 3 (aq): Soluble salt = STRONG ELECTROLYTE

2. HC 2 H 3 O 2 (aq): Weak acid = NOT a STRONG ELECTROLYTE

3. NaC 2 H 3 O 2 (aq): Soluble salt = STRONG ELECTROLYTE

We have two strong electrolytes in the molecular equation, Na 2 CO 3 (aq) and NaC 2 H 3 O 2 (aq).

4. Write the STRONG ELECTROLYTES as ions

Now we rewrite the strong electrolytes as ions in solution. Each ion retains the (aq) phase. All other

species are written as they were! When writing the strong electrolytes as ions, DO NOT ADD A

COEFFIENT FOR THE NUMBER OF IONS FROM THE COMPOUND, ONLY ONE IS

NEEDED FOR EACH ION. We will balance latter.

Summary

In this tutorial we went over the 6 steps necessary to write a net ionic equation for any aqueous

reaction. You must practice and pay close attention to the solubility rules to gain mastery of this

subject. When balancing, make sure the charge is balanced as well!

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