Problem 72 - Chemistry The Central Science In Si Units

Question

Name the following ionic compounds: \((\mathbf{a}) \mathrm{KCN},(\mathbf{b}) \mathrm{NaBrO}_{2},\) (c) \(\mathrm{Sr}(\mathrm{OH})_{2}\), (d) CoTe, (e) \(\mathrm{Fe}_{2}\left(\mathrm{CO}_{3}\right)_{3},\), (f) \(\mathrm{Cr}\left(\mathrm{NO}_{3}\right)_{3}\), (h) \(\mathrm{NaH}_{2} \mathrm{PO}_{4}\), (i) \(\mathrm{KMnO}_{4}\), (g) \(\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{3},\), , (j) \(\mathrm{Ag}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\).

Step-by-Step Solution

Answer

(a) Potassium cyanide, (b) Sodium bromite, (c) Strontium hydroxide, (d) Cobalt(II) telluride, (e) Iron(III) carbonate, (f) Chromium(III) nitrate, (g) Ammonium sulfite, (h) Sodium dihydrogen phosphate, (i) Potassium permanganate, (j) Silver dichromate.

Step 1: Identify cations and anions

Start by identifying the cation (positive ion) and anion (negative ion) in each compound. This will help in determining the appropriate name for each compound based on the constituent ions.

Step 2: Naming simple ionic compounds

For simple ionic compounds like KCN and NaBrO2, the name of the compound is made by first stating the name of the cation (elemental name) followed by the name of the anion. KCN is potassium cyanide, and NaBrO2 is sodium bromite.

Step 3: Naming hydroxides

For compounds with hydroxide ions, use the name of the metal followed by 'hydroxide'. Strontium hydroxide is derived from Sr(OH)2.

Step 4: Naming transition metal compounds with simple anions

For compounds involving transition metals like CoTe and Fe2(CO3)3, consider the charge of the metal and state it using Roman numerals. CoTe is cobalt(II) telluride, and Fe2(CO3)3 is iron(III) carbonate.

Step 5: Naming complex ionic compounds

For complex compounds containing polyatomic ions, use the name of the metal followed by the name of the polyatomic ion. Cr(NO3)3 is chromium(III) nitrate.

Step 6: Naming compounds with hydrogen-containing ions

For compounds like NaH2PO4, the name includes 'hydrogen' as sodium dihydrogen phosphate.

Step 7: Naming manganate compounds

For compounds like KMnO4, identify the anion MnO4 which is 'permanganate'. Hence, it is potassium permanganate.

Step 8: Naming ammonium compounds

For compounds containing ammonium ions, use 'ammonium' followed by the anion name. (NH4)2SO3 is ammonium sulfite.

Step 9: Naming chromate compounds

For compounds like Ag2Cr2O7, recognize Cr2O7 as 'dichromate'. Hence, it is silver dichromate.

Key Concepts

Identifying Cations and Anions

When naming ionic compounds, the first step is always to identify the cations and anions. Cations are positively charged ions, while anions carry a negative charge. In a compound, cations are typically metals or ammonium ions, whereas anions can be nonmetals or polyatomic ions. Recognizing these ions is crucial as each behaves differently in forming ionic compounds.
For instance, in (a) KCN, potassium (K) is the cation, and cyanide (CN) is the anion.
Once identified, naming the compound becomes straightforward: the cation name followed by the anion name. Understanding the nature of ions involved allows us to correctly assign charges, simplifying the naming process.

Transition Metal Naming

Transition metals often possess multiple oxidation states, meaning they can have various possible charges. This necessitates including Roman numerals in their names to indicate which charge is present in a compound. Example: In (d) CoTe, cobalt can have different charges. By assuming the telluride ion (Te) has a charge of -2, cobalt must have a +2 charge to balance the compound. Thus, it is cobalt(II) telluride. Both clarity and correctness require paying careful attention when naming these compounds. Always check the anion charges to ensure you're representing the correct oxidation state of the transition metal involved.

Polyatomic Ions

Polyatomic ions are groups of atoms that carry an overall charge, which can make naming ionic compounds more complex. These ions have specific names that must be memorized as they do not change regardless of the metal they combine with. Some common polyatomic ions are sulfate ( ext{SO}_4^{2-}), nitrate ( ext{NO}_3^-), and phosphate ( ext{PO}_4^{3-}).
For example, in (f), Cr(NO extsubscript{3}) extsubscript{3}, the nitrate ion is NO_3^- and carries a -1 charge. Chromium, being a transition metal, has a charge of +3 to balance the three nitrate ions, hence the compound name is chromium(III) nitrate. Naming such compounds involves naming the metal first, then the polyatomic ion.

Naming Hydroxides

Hydroxides are a unique category of ionic compounds characterized by the presence of the hydroxide ion ext{OH}^-. When naming these compounds, the metal's name is followed by 'hydroxide.'
For instance, in (c) Sr(OH)₂, the hydroxide ions balance the strontium ion's +2 charge, thus forming strontium hydroxide.
Despite the simplicity, one must be careful when the metal is a transition metal as this might require indicating the metal's oxidation state. In general, naming hydroxides provides a straightforward framework because the structure and names remain consistent across the board.