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Chapter 4 - reactions of ionic compounds
Skeletons and Hydroxyapatite
Our skeletons are primarily composed of hydroxyapatite, which has the chemical formula Ca₁₀(PO₄)₆(OH)₂
Ionic Bonding Model
Metal Atoms and Electron Donation:
Metal atoms donate their valence electrons to non-metal atoms.
This donation results in the formation of cations (positively charged ions) and anions (negatively charged ions).
Electrostatic Attraction:
Cations are electrostatically attracted to anions.
This attraction is the basis of ionic bonding.
Overall Charge:
Ionic compounds, also known as salts, have no overall charge; they are neutral.
Ion Definition:
An ion is an atom that has either lost or gained electrons, thus acquiring a charge.
Cation Definition:
A cation is a positively charged ion formed when an atom loses one or more valence electrons.
Anion Definition:
An anion is a negatively charged ion formed when an atom gains one or more valence electrons.
Crystal Lattice Formation:
When cations and anions combine as solids, they form a three-dimensional crystal lattice.
An ionic bond is notably strong.
Example: Sodium Chloride (NaCl)
In sodium chloride, each sodium ion (Na+) is surrounded by 6 chloride ions (Cl-).
Conversely, each chloride ion is surrounded by 6 sodium ions.
The coordination number for sodium chloride is 6.
Ionic Crystal Lattice:
Definition: A symmetrical, three-dimensional arrangement of ions.
Coordination Number:
Definition: The number of atoms, molecules, or ions bonded to a central chemical species.
Properties of Ionic Compounds
There are four common properties:
Brittleness
Hardness
High melting point
No electrical conductivity as a solid
Brittleness
Definition: The tendency of a material to fracture (break) when subjected to pressure.
Hardness
Definition: Resistance to deformation when subjected to pressure.
Melting Point
Definition: The temperature at which a substance changes its state from solid to liquid.
Electrical Conductivity
Definition: A measurement of how easily a material allows an electrical current to flow through it.
Brittleness Explained: When a force is applied to an ionic compound, like charges line up and repel, causing the crystal to fracture. Ionic compounds are hard and brittle, unlike malleable metals.
High Melting Point: A lot of energy is required to melt a solid ionic crystal.
When a solid melts, the ionic compound becomes molten (liquefied by heat).
Bunsen burner flames reach 700°C, which is insufficient to melt many ionic compounds.
Examples of melting points:
Sodium chloride ($\text{NaCl}$): 801 \, ^\circ\text{C}
Aluminum oxide ($\text{Al}2\text{O}3): 2072 \, ^\circ\text{C}
Magnesium oxide ($\text{MgO}$): 2852 \, ^\circ\text{C}
Electrical Conductivity: Solid ionic compounds do not conduct electricity because the anions and cations cannot move. Electrical conductivity requires the flow of charge, which can be the flow of electrons or ions.
If an ionic compound is melted (molten state), the ions can move, allowing electrical conductivity.
Similarly, if an ionic compound is dissolved in water, the ions can move to oppositely charged electrodes, facilitating conductivity.
Identifying Bonding in Solids
Chalk Example: Chalk is made of calcium carbonate.
In its solid state, chalk exhibits ionic bonding.
Evidence: Chalk is hard and brittle and does not conduct electricity in solid form, but it will conduct electricity when dissolved in water.
Writing and Naming Ionic Formulas
The group number in the s and p blocks signifies the number of valence electrons for an atom.
Metal atoms lose electrons to become cations, while non-metal atoms gain electrons to become anions.
When writing the charge on an ion that gains or loses more than one electron, the number is written before the + or – sign (e.g., \text{Ca}^{2+}).
Rules for Elemental Ions
For elemental ions, look at the Group number.
Add the word ‘ion’ after the name of the element to show it is a different species from an atom.
Charge on Elemental Ions: The charge relates to the number of valence electrons. See table below:
Group # | Number of valence electrons | Charge on elemental ion |
|---|---|---|
1 | 1 | +1 |
2 | 2 | +2 |
13 | 3 | +3 |
14 | 4 | Not usually ionic |
15 | 5 | –3 |
16 | 6 | –2 |
17 | 7 | –1 |
Transition Metals
Transition metals (groups 3-12) have variable electrovalencies except for \,\text{Zn}^{2+} and \text{Ag}^{+}.
Roman numerals are used with the name to signify the charge on the ion.
Transition metal ion | Formula | Charge on ion |
|---|---|---|
iron(II) ion | \text{Fe}^{2+} | +2 |
iron(III) ion | \text{Fe}^{3+} | +3 |
copper(I) ion | \text{Cu}^{+} | +1 |
copper(II) ion | \text{Cu}^{2+} | +2 |
tin(II) ion | \text{Sn}^{2+} | +2 |
tin(IV) ion | \text{Sn}^{4+} | +4 |
Roman numeral | Number |
|---|---|
I | 1 |
II | 2 |
III | 3 |
IV | 4 |
V | 5 |
VI | 6 |
VII | 7 |
Naming Ions
Cations have the same name as the metal element, with the word ‘ion’ following.
Examples: \text{Na}^{+} is sodium ion, \text{Mg}^{2+} is magnesium ion, \text{Al}^{3+} is aluminum ion, \text{Cu}^{2+} is copper(II) ion.
For anions, change the ending of the element to ‘ide’.
Non-metal element | Anion |
|---|---|
nitrogen (N) | nitride ($\text{N}^{3-}$ ) |
oxygen (O) | oxide ($\text{O}^{2-}$ ) |
fluorine (F) | fluoride ($\text{F}^{-}$ ) |
chlorine (Cl) | chloride ($\text{Cl}^{-}$) |
sulfur (S) | sulfide ($\text{S}^{2-}$ ) |
phosphorus (P) | phosphide ($\text{P}^{3-}$ ) |
iodine (I) | iodide ($\text{I}^{-}$ ) |
bromine (Br) | bromide ($\text{Br}^{-}$) |
Polyatomic Ions
Polyatomic ions contain more than one atom.
You are expected to know the formula and charge of common polyatomic ions.
Be careful with the spelling and formula of these ions. In year 12 you will only gain marks if you spell the chemical names correctly.
Name of polyatomic ion | Formula | Commonly confused with |
|---|---|---|
ammonium ion | NH4+ | ammonia ($\text{NH}_3) |
hydroxide ion | \text{OH}^- | oxide ($\text{O}^{2-}) |
nitrate ion | \text{NO}_3^{-}$ | nitrite ($\text{NO}_2^{-}$), nitride ($\text{N}^{3-}) |
hydrogen carbonate ion | \text{HCO}_3^{-}$ | carbonate ion ($\text{CO}_3^{2-}) |
sulfate ion | \text{SO}_4^{2-}$ | |
phosphate ion | \text{PO}_4^{3-}$ | phosphide ($\text{P}^{3-}) |
Balancing Ionic Compounds
The number of cations must balance the charge for the number of anions.
The ionic compound must have no overall charge.
For ionic compounds, name the cation first, followed by the anion.
Cation formula | Anion formula | Ionic compound formula | Ionic compound name |
|---|---|---|---|
\text{Na}^{+} | \text{Cl}^{-} | \text{NaCl} | sodium chloride |
\text{Li}^{+} | \text{OH}^{-} | \text{LiOH} | lithium hydroxide |
\text{Mg}^{2+} | \text{NO}_3^{-} | \text{Mg(NO}3)2 | magnesium nitrate |
\text{Cu}^{2+} | \text{S}^{2-} | \text{CuS} | copper(II) sulfide |
\text{Al}^{3+} | \text{CO}_3^{2-} | \text{Al}2\text{(CO}3)_3 | aluminum carbonate |
\text{NH}_4^{+} | \text{SO}_4^{2-} | \text{(NH}4)2\text{SO}_4 | ammonium sulfate |
\text{Ca}^{2+} | \text{P}^{3-} | \text{Ca}3\text{P}2 | calcium phosphide |
Writing Balanced Equations for Ionic Compounds
Metal atoms donate one or more electrons to non-metal atoms.
Metal atoms become cations.
Non-metal atoms become anions.
The overall ionic compound is neutral.
Balance equations by changing the coefficients of reactants and products. Never change the subscripts of formulas!
Reactions Between Ionic Compounds
Sometimes two ionic solutions will swap partners to form new ionic compounds.
If an insoluble product is made, this is called a precipitate.
Example: A solution of sodium chloride reacts with a solution of silver nitrate to form silver chloride and sodium nitrate.
\text{NaCl} + \text{AgNO}3 \rightarrow \text{AgCl} + \text{NaNO}3$$
Precipitate definition: An insoluble product that is made when substances in solution are mixed.
Precipitation and Ionic Equations
Formation of Precipitates
Sometimes two ionic solutions will swap partners to form new ionic compounds.
One of the ionic compounds can form a precipitate, which is a solid.
The other ionic compound remains in solution.
Key Qualitative Indicators (COBALT mnemonic):
Colour
Odour
Bubbles
Appearance or disappearance of a solid
Light or sound
Temperature change
Precipitate definition: A solid that forms when two ionic solutions are mixed.
Qualitative definition: A factor or variable that is non-numeric (e.g., color).
Writing Full Balanced Equations for Precipitation Reactions
Write the formula for the two ionic solutions.
Swap partners to predict what the products would be in a precipitation reaction.
Balance the equation.
Full balanced equation definition: Balanced equation showing all reactants and products in a reaction, including non-reacting species.
Solubility Tables
Solubility tables help predict and identify if a precipitate will form when two ionic solutions are mixed.
The precipitate is assigned a (s) state (solid).
The other ionic compound remains aqueous (aq).
Solubility table definition: A table showing which ionic compounds are likely to be soluble or insoluble in water at 25°C.
States definition: Physical form of species: solid (s), liquid (l), gas (g), or aqueous (aq).
Ions that are highly soluble in water (PANES mnemonic):
Potassium ($\text{K}^+$)
Ammonium ($\text{NH}_4^+$)
Nitrate ($\text{NO}_3^-$)
Ethanoate ($\text{CH}_3\text{COO}^-$)
Sodium ($\text{Na}^+$)
Ions that are highly insoluble in water (CHOPS mnemonic):
Carbonate ($\text{CO}_3^{2-}$)
Hydroxide ($\text{OH}^-$)
Oxide ($\text{O}^{2-}$)
Phosphate ($\text{PO}_4^{3-}$)
Sulfide ($\text{S}^{2-}$)
Writing Balanced Ionic Equations for Precipitation Reactions
Write the formula for the two ionic solutions.
Swap partners to predict what the products would be in a precipitation reaction.
Identify the spectator ions.
Exclude the spectator ions.
Summarize the equation as a balanced ionic equation, including states.
Spectator ions definition: Ions that do not participate and do not change state in a chemical reaction.
Ionic equation definition: Equation showing only the species that react. Contains one or more ions.
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