Chapter 05: Formulas and Equations
Formulas and Equations
- 1. Write the formulas of ions, starting with cation the anion.
- 2. Bring the number from the charge of the ions diagonally down to obtain ratio of cation to anion.
- Simplify when necessary.
- 3. Write formula according to the number of ions needed.
- If only 1 oin is needed, do not put the number 1 as subscript.
What about ionic compounds containing polyatomic ions? Eg. Calcium phosphate. Calcium being a group II metal will lose 2 valence electrons to form Ca2+ ions and will experience electrostatic forces of attraction with phosphate (PO43-) ions. The common multiple of 2 and 3 is 6. Thus, 3 Ca2+ ions are needed to provide a total charge of 6+ to balance out the total charge of 2 PO43- ions which is 6-. Thus, the formula will be Ca3(PO4)2.
The diagram below shows how the cross method is done to find the formula of ionic compounds.
- 1. Write the formulas of ions, starting with cation the anion.
- 2. Bring the number from the charge of the ions diagonally down to obtain ratio of cation to anion.
- 3. Write formula according to the number of ions needed.
- NOTE: Bracket is needed if more than one polyatomic ion is needed in formula.
Definition
Acid: Substance that dissociates in water to produce hydrogen (H⁺) ions.
Base: Substance that reacts with acid to form salt and water.
Alkali: A soluble base that will produce hydroxide (OH⁻) ions when dissolved in water.
Salt: A salt is an ionic compound formed when a metallic ion or an ammonium ion (NH₄⁺) replaces one or more hydrogen ions of an acid.
State symbol
This is the state that the compounds or elements are in at room temperature and pressure.
4 state symbols:
- 1. Solid, denoted by (s)
- 2. Liquid or molten, denoted by (l)
- 3. Gas, denoted by (g)
- 4. Aqueous or dissolved in water, denoted by (aq)
3 acid reactions to remember:
- 1. Metal + Acid → Salt + Hydrogen gas
- Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)
- 2. Carbonate + Acid → Salt + Water + Carbon dioxide gas
- ZnCO₃(s) + 2HNO₃(aq) → Zn(NO₃)₂(aq) + H₂O(l) + CO₂(g)
- 3. Oxide or Hydroxide + Acid → Salt + Water
- ZnO(s) + 2HCl(aq) → ZnCl₂(aq) + H₂O(l)
2 alkali reactions to remember:
- 1. Alkali + Acid → Salt + Water (same as acid reaction #3)
- 2. Alkali + Ammonium Salt → Salt + Water + Ammonia gas
- KOH(aq) + NH₄Cl(aq) → KCl(aq) + H₂O(l) + NH₃(g)
Balancing chemical equations (including ionic equations) will have two requirements:
- 1. The mass of reactants and the mass of products must be the same OR the number of each type of atoms must be the same before and after the reaction.
- 2. The total charge of reactants must be the same as the total charge of products OR the charges before and after the reaction must be the same.
In order to construct ionic equations, we need to first know of the solubilities of the compounds by looking at the solubility table.
| Salt | Soluble | Insoluble |
|---|---|---|
| Group I salt | All are soluble | NIL |
| Ammonium (NH₄⁺) salt | All are soluble | NIL |
| Nitrates (NO₃⁻) | All nitrates are soluble | NIL |
| Chlorides (Cl⁻) and Iodides (l⁻) | Most are soluble | AgCl, AgI AgCl₂, PbI₂ |
| Carbonates (CO₃²⁻) | Group I Carbonates, (NH₄)₂CO₃ |
Most carbonates are insoluble |
| Sulfates (SO₄²⁻) | Most sulfates are soluble | BaSO₄, CaSO₄, PbSO₄ |
| Hydroxides (OH⁻) and Oxides (O²⁻) | Group I Hyroxides, NH₄OH, Ca(OH)₂, Ba(OH)₂, Sr(OH)₂ |
Most hydroxides and oxides are insoluble. |
| Salt |
|---|
| Group I salt |
| Ammonium (NH₄⁺) salt |
| Nitrates (NO₃⁻) |
| Chlorides (Cl⁻) and Iodides (l⁻) |
| Carbonates (CO₃²⁻) |
| Sulfates (SO₄²⁻) |
| Hydroxides (OH⁻) and Oxides (O²⁻) |
| Soluble |
|---|
| All are soluble |
| All are soluble |
| All nitrates are soluble |
| Most are soluble |
| Group I Carbonates, (NH₄)₂CO₃ |
| Most sulfates are soluble |
| Group I Hyroxides, NH₄OH, CA(OH)₂, BA(OH)₂, SR(OH)₂ |
| Insoluble |
|---|
| NIL |
| NIL |
| NIL |
| AgCl, AgI AgCl₂, PbI₂ |
| Most carbonates are insoluble |
| BaSO₄, CaSO₄, PbSO₄ |
| Most hydroxides and oxides are insoluble. |
To construct an ionic equation from a balanced chemical equation, refer to 5C – Concentrate, Cut, Cancel, Copy, Check.
For example, using the reaction between ZnO and HCl, this is what you need to do after constructing the balanced chemical equation.
For step 2, when separating ionic compounds into its individual ions,
- 1. The number in front of the compound will need to be applied to both cation and anion.
- 2. The number as subscript needs to only be applied to the ion before it.
