Background
Before getting started, it is important you know the following:
Name
Sodium ion
Chloride ion
Hydroxide ion
Sulfate ion
Nitrate ion
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Formula
Na+
Cl-
OH-
SO42-
NO3-
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Name
Hydrogen carbonate ion
Ethanoate ion
Thyiocyanate ion
Carbonate ion
Ammonium ion
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Formula
HCO3-
CH3COO-
SCN-
CO32-
NH4+
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To write the formula of a compound, you need to know the charges of the ions involved.
Let's look at silver carbonate - first we need to know the symbols and charges of the silver ion and the carbonate ion
Let's look at silver carbonate - first we need to know the symbols and charges of the silver ion and the carbonate ion
Let's take a look at Iron (III) nitrate
Soluble ionic compounds
Some ionic compounds can dissolve in water and separate to form aqueous (aq) ions, we call this dissociation.
Example: Sodium chloride is a soluble ionic compound and dissolves in water to form a solution containing Na+(aq) and Cl-(aq) ions in a large amount of H2O(l).
This can be seen in the following diagram
Example: Sodium chloride is a soluble ionic compound and dissolves in water to form a solution containing Na+(aq) and Cl-(aq) ions in a large amount of H2O(l).
This can be seen in the following diagram
Insoluble ionic coumpounds
Insoluble simply means that the substance will not dissolve in water. However, most insoluble compounds will dissolve to some extent in water if left long enough. Whether a substance is classified as being soluble or insoluble comes down to the filed of science in which it is used, for example, to a geologist, calcium carbonate is soluble as it dissolves over a very long time to form caves and rock formations. To a chemist, calcium carbonate is regarded as being insoluble because so little would dissolve over an experimental time of interest.
Soluble and insoluble compunds are the extremes of a range of solubilities. Compounds inbetween may be described as slightly or sparingly soluble
Precipitation
When two soluble solutions are mixed, an insoluble ionic solid (a precipitate) can form. The cations from one solution and anions from the other react to form an insoluble solid, we call this precipitation.
Precipitation: Lead iodide
When colourless potassium iodide (KI) is poured into the colourless lead nitrate ( Pb(NO3)2 ), the yellow precipitate, lead iodide (PbI2) is formed. The cation Pb2+ from the potassium iodide, reacts with the anion I- from the lead nitrate and forms PbI2, lead iodide. |
What is soluble and what is not?
These rules describe which compounds are soluble and which are insoluble. They are used to predict the formation of precipitates when solutions are mixed.
Solubility grids can be helpful when predicting if a precipitate will form or not.
To predict if a precipitate will form when NaOH solution is mixed with BaSO4 solution, we arrange the ions in a grid and look at the possible cation-anion combinations. We then refer to the solubility rules to determine if a precipitate forms or not.
The possible combinations are sodium sulfate and barrium hydroxide. The above rules allow us to determine that sodium sulfate (NaSO4) is soluble and barrium hydroxide ( Ba(OH)2 ) is insoluble, therefore, barrium hydroxide is our precipitate.
To predict if a precipitate will form when NaOH solution is mixed with BaSO4 solution, we arrange the ions in a grid and look at the possible cation-anion combinations. We then refer to the solubility rules to determine if a precipitate forms or not.
The possible combinations are sodium sulfate and barrium hydroxide. The above rules allow us to determine that sodium sulfate (NaSO4) is soluble and barrium hydroxide ( Ba(OH)2 ) is insoluble, therefore, barrium hydroxide is our precipitate.
Writing ionic equations
Equations for precipitation reactions show only the ions involved in precipitation. Spectator (soluble) ions are not shown.
Example 1: As shown above, when the colourless sodium hydroxide solution, H2SO4(aq), is added to the colourless barrium sulfate solution BaSO4(aq), the barrium ions combine with sulfate ions to form a white precipitate of BaOH2.
Example 1: As shown above, when the colourless sodium hydroxide solution, H2SO4(aq), is added to the colourless barrium sulfate solution BaSO4(aq), the barrium ions combine with sulfate ions to form a white precipitate of BaOH2.
Example 2: When the pale green solution of iron (II) sulfate is added to the colourless sodium hydroxide solution, a pale green precipitate of iron (II) hydroxide forms.
Example 3: When the colourless solution of sodium hydroxide is added to the silver nitrate solution, the precipitate silver oxide, Ag2O(s) forms. Some AgOH forms but very little because it is not as stable as Ag2O.
Complex ions
Complex ions consist of a metal ion and a ligand. The ligand is the ion or molecule that is bonded to the metal.
A complex ion can form when:
The overall charge:
Zinc has a charge of 2+, each OH has a charge of 1-, therefore, there are two more negative charges than positive, resulting in tetrahydroxyzincate having an overall charge of 2-. |
Complex ions may be formed when a precipitate dissolves in excess of a solution.
When excess NaOH solution is added, it is possible that:
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Adding ammonia solution
Ammonia is a weak base which means that when it is in solution, it can exist in two forms:
When excess ammonia solution is added, it is possible that:
The overall charge:
Zinc has a charge of 2+, NH3 has no charge, therefore, the overall charge must be 2+. |