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Lesson 1: A Model of Solutions
Part e: Dissociation of Ionic Compounds
Part a:
What is a Solution?
Part b:
Solubility and Structure
Part c:
The Dissolving Process
Part d:
Solubility, Temperature, and Pressure
Part e: Dissociation of Ionic Compounds
What is Dissociation?
The word dissociate, in nearly all contexts in which it is used, means to separate from or to split apart. Dissociation is used in the context of Chemistry to describe what occurs when an ionic compound dissolves in water. The ions of the compounds separate from each other and split apart. In the undissolved state, the ions are held together in a crystal lattice by strong attractions known as ionic bonds. But in the dissolved state, the ions have been separated from each other and attracted to and surrounded by water molecules.
Dissociation Equations
The dissolving of a salt in water is often represented by a chemical equation known as a dissociation equation. Like any chemical equation, a dissociation equation includes the formulas of reactants and products, the charges on the ions, state symbols, and coefficients. As an example, the equation for the dissociation of calcium chloride (CaCl2) when dissolved in water is:
CaCl2(s) → Ca2+(aq) + 2 Cl-(aq)
Writing dissociation equations requires that you are able to:
- Write accurate formulae for ionic compounds. Review Formula Writing.
- Identify the symbols (with charges) of the ions in the salt. (Next section)
- Use coefficients to balance the equation. Review Balancing Equations.
Identifying the Ions
For our purposes,
the names of ionic compounds will consist of two words. The first word is the cation name and the second word is the anion name. These ions could be
main group elements. Examples of salts composed of main group elements include lithium fluoride (LiF), calcium chloride (CaCl
2), aluminum oxide (Al
2O
3), magnesium sulfide (MgS), etc. All
main group nonmetals form ions whose name ends with the -ide suffix. For ions of
main group elements, the ion charge can be determined from the element’s location in the periodic table.
The cation could be an ion of
a transition metal. These ions are multivalent; that is, they can have more than one possible charge. For instance, iron can be a 2+ ion or a 3+ ion. A Roman numeral in the name identifies the charge on the
transition metal ion. For instance, the cation of iron(III) chloride is the Fe
3+ ion. Though not transition metals, lead and tin are also multivalent. The cation of lead(II) sulfate is Pb
2+.
One or both of the ions can be
a polyatomic ion. These contain more than one atom and most often two or more elements. Use
our polyatomic ion list to match the formula and charge of
polyatomic ions to the name. With very rare exceptions (e.g., hydroxide, cyanide, and peroxide), polyatomic anions do not end with the -ide suffix. If the second word of the name ends -ate or -ite, then the anion is a polyatomic ion. Examples include sulfate (SO
42-), sulfite (SO
32-), nitrate (NO
3-), nitrite (NO
2-), acetate (C
2H
3O
2-), phosphate (PO
43-), etc. The most commonly encountered polyatomic cation will be ammonium (NH
4+).
The following examples illustrate how to determine the ion formula from the name of the ionic compound:
| |
Name |
Cation |
Anion |
Cation Formula |
Anion Formula |
| a. |
aluminum fluoride |
Main group |
Main group
(-ide ending) |
Al3+ |
F- |
| b. |
calcium nitrate |
Main group |
Polyatomic
(no -ide ending) |
Ca2+ |
NO3- |
| c. |
iron(III) oxide |
Transition metal element |
Main group
(-ide ending) |
Fe3+ |
O2- |
| d. |
ammonium nitrite |
Polyatomic |
Polyatomic
(no -ide ending) |
NH4+ |
NO2- |
| e. |
magnesium sulfate |
Main group |
Polyatomic
(no -ide ending) |
Mg2+ |
SO42- |
The examples above illustrate how to determine the cation and anion formulae if given the name of the ionic compound. You will also need to know how to determine the ion formulae if given the formula of the ionic compound. To do so, you need to recognize that the cation symbol is listed first followed by the anion symbol. If there are only two elemental symbols in the compound’s formula, then both the cation and the anion are monatomic – like Mg
2+, Zn
2+, P
3-, and Cl
-. The location of the element in the periodic table is all that is needed to determine the ion’s charge. See
the Periodic Table graphic above.
If the first element listed is a transition metal, tin (Sn), or lead (Pb), then the charge must be determined by inspecting the subscripts in the formula. The overall charge of the ionic compound is 0. Thus, the total cation charge must equal the total anion charge. Use the subscript for the anion and the charge of the anion to determine the total anion charge. Then use the subscript on the cation to determine the cation charge.
If there are three or more elemental symbols in the compound’s formula, then there is at least one
polyatomic ion. The most common polyatomic cation is ammonium (NH
4+). If ammonium is not the cation, then the cation is most likely the ion of an element and the anion would be a polyatomic anion. Always have
a list of polyatomic ions available to quickly identify the formula with its charge.
The following examples illustrate how to determine the ion formulae from the formula of the ionic compound:
Practice – Ion Identification
Identify the cation and the anion formula (with appropriate charge) for the following salts. Try to complete the table on your own with the help of
a periodic table and
a list of polyatomic ions. Then tap the
Check Answers button to check your work.
Writing Dissociation Equations
Now it’s time to learn how to write dissociation equations. Our example equation was
CaCl2(s) → Ca2+(aq) + 2 Cl-(aq)
This equation, like any dissociation equation, includes:
- a reactant formula – for the salt being dissolved
- two product formulae – one for the cation and one for the anion
- appropriate charges on both ions
- state symbols for the reactant – solid or (s)
- state symbols for both products – aqueous or (aq)
- coefficients to balance atoms (and charge)
A balanced dissociation equation will always have the following general format:
To write and balance the equation, a student will need to determine the formula of the ionic compound and the formulae of the cation and anion. The coefficients in front of the ions will match the number of each ion that is shown in the formula of the compound.
Let’s try three examples.
Example 1
Write the dissociation equation for the dissolving of calcium nitrite in water.
Solution
Calcium is a Group 2 element; it forms the Ca
2+ ion. The nitrite ion is a polyatomic ion; the -ite ending gives this away. Nitrite is the NO
2- ion (see
polyatomic ion list). The formula of the ionic compound (the reactant) is Ca(NO
2)
2. This formula adheres to
the law of electrical neutrality – with a subscript of 2 after the parenthesis, the two nitrite ions have a total charge of -2. This is equal to the total positive charge on the Ca
2+ ion. Thus, Ca(NO
2)
2 is overall neutral.
Now we know the reactant formula and the two ion formulae with their charges. We can write and balance the dissociation equation using
the general format shown above:
Ca(NO2)2(s) → Ca2+(aq) + 2 NO2-(aq)
The balancing of these equations involves an understanding of the formula for the ionic compound. Inspecting Ca(NO
2)
2, we observe that there are two nitrite ions; the subscript 2 after the parenthesis indicates that there are two nitrite ions in a formula unit. The lack of a subscript after the Ca in the same formula indicates that there is only one Ca
2+ ion in a formula unit. Understanding this about the reactant, we put a coefficient of 2 in front of NO
2- and a “1” in front of Ca
2+.
Example 2
Write the dissociation equation for the dissolving of iron(III) sulfate in water.
Solution
Iron is a transition metal. The Roman numeral indicates that the formula is Fe
3+. Sulfate is a polyatomic anion; the -ite ending gives this away. Sulfate is the SO
42- ion (see
polyatomic ion list). The formula of the ionic compound (the reactant) is Fe
2(SO
4)
3. This formula adheres to
the law of electrical neutrality. The total positive charge from the two cations is 2*(+3) or +6. And the total negative charge from the three sulfate ions is 3*(-2) or -6. Thus, Fe
2(SO
4)
3 is overall neutral.
With all formulae known, we can write and balance the dissociation equation using
the general format shown above:
Fe2(SO4)3(s) → 2 Fe2+(aq) + 3 SO42-(aq)
Because of the subscript 2 after Fe in the reactant formula, we use a coefficient of 2 in front of the Fe
2+ ion. And because of the subscript 3 after the parenthesis of the reactant formula, we use a coefficient of 3 in front of the SO
42- ion.
Example 3
Write the dissociation equation for the dissolving of (NH
4)
3PO
4 in water.
Solution
Both ions are polyatomic ions. A
polyatomic ion list can be used to determine the formulae of each with their charges. The cation is ammonium - NH
4+. The anion is phosphate - PO
43-.
With all formulae known, we can write and balance the dissociation equation using
the general format shown above:
(NH4)3PO4(s) → 3 NH4+(aq) + PO43-(aq)
The subscript after the parenthesis in the reactant formula indicates there are three of NH
4+ ions in a formula unit of this salt. Thus, a coefficient of 3 is placed in front of the NH
4+ ion on the product side. There are no parenthesis around PO
4 in the reactant formula; this indicates that there is only one PO
43- ion in a formula unit. A coefficient of “1” (implied, not written) is used in front of the PO
43- ion.
When done correctly, using coefficients to balance atoms in your equation will also balance the charge. There will be a charge of 0 on the reactant side; this means there must be a total charge of 0 on the product side. For this to be true, the total positive charge from all the cations should equal the total negative charge from all the anions.
Writing dissociation equations is far from a spectator sport. You have to practice to perfect it. Use one of the following suggestions in the
Before You Leave section to ensure that you have acquired the skill.
Before You Leave
- Download our Study Card on Dissociation Equations. Save it to a safe location and use it as a review tool. (Coming Soon.)
- We highly recommend the first two activities of our Dissociation Concept Builder.
- The Check Your Understanding section below include questions with answers and explanations. It provides a great chance to self-assess your understanding.
Check Your Understanding
Use the following questions to assess your understanding. Tap the
Check Answer buttons when ready.
1. For the dissociation equation in Example 2:
Fe2(SO4)3(s) → 2 Fe2+(aq) + 3 SO42-(aq)
show some calculations that demonstrate that charge is balanced (the same total on reactant and product side).
2. Aaron Agin is working hard on writing dissociation equations for salts dissolved in water. Unfortunately, it is not going to well. For the following attempts, tell Aaron what he has done wrong.
- CaCl2(aq) → Ca2+ + 2 Cl-
- KNO3(s) → K+(aq) + 3 NO-(aq)
- Al(NO3)3(s) → 3 Al+(aq) + NO33-(aq)
- NaC2H3O2(s) → NaC2+(aq) + H3O2-(aq)
3. For the following solutes dissolving in water, do the following:
- Identify the formula of the solid.
- Identify the number of each ion in a formula unit of the ionic compound
- Identify the formulae (including charges) of the two ions.
- Write the balanced dissociation equation for the dissociation of the salt in water.
- Magnesium nitrate
- Iron(II) carbonate
- Aluminum sulfate
- Calcium nitrate
- Magnesium phosphide
4. Write and balance the dissociation equations for the dissolving of the following salts in water:
- Al(C2H3O2)3
- Na2C2O4
- Fe(NO2)3
- NH4NO3
- MgSO4