Question

Explain how hydrates are named.

Short answer

Hydrates are named by stating the name of the ionic compound followed by a numerical prefix to indicate the number of water molecules per formula unit and the word "hydrate." The general form is: \( \text{Compound name}\cdot n \text{H}_2\text{O}\), where \(n\) is the number of water molecules. For example, \( \text{CuSO}_4\cdot 5\text{H}_2\text{O} \) is named Copper(II) sulfate pentahydrate. To determine the formula of a hydrate from its name or vice versa, identify the ionic compound, the number of water molecules, and write the formula or name accordingly.

Step-by-step solution

Define Hydrates

A hydrate is a compound that contains a defined ratio of water molecules (H2O) bound to it. These water molecules are called "waters of hydration." Hydrates are formed when ionic compounds interact with water and the water molecules become part of the crystal structure of the ionic compound.

General Form of Hydrate Names

Hydrates are named by stating the name of the ionic compound followed by a numerical prefix to indicate the number of water molecules per formula unit of the compound. The prefix is followed by the word "hydrate." The general form is: \[ \text{Compound name}\cdot n \text{H}_2\text{O}\] where \(n\) is the number of water molecules.

Provide Examples

We will now provide examples illustrating how hydrates are named. 1. \( \text{CuSO}_4\cdot 5\text{H}_2\text{O} \) is named Copper(II) sulfate pentahydrate. 2. \( \text{Na}_2\text{CO}_3\cdot 10\text{H}_2\text{O} \) is named Sodium carbonate decahydrate. 3. \( \text{MgSO}_4\cdot 7\text{H}_2\text{O} \) is named Magnesium sulfate heptahydrate.

Determining the Formula of a Hydrate from Its Name

To determine the formula of a hydrate from its name, we will follow these steps: 1. Identify the ionic compound in the name. 2. Determine the numerical prefix representing the number of water molecules. 3. Write the formula for the hydrate by placing the numeral after the compound formula, followed by "H2O." For example, if we have the name "Calcium chloride dihydrate": 1. The ionic compound is Calcium chloride (\(\text{CaCl}_2\)). 2. The numerical prefix is "di," which represents 2 water molecules. 3. The formula is \( \text{CaCl}_2\cdot 2\text{H}_2\text{O} \).

Naming a Hydrate from Its Formula

To name a hydrate from its formula, we will follow these steps: 1. Identify the ionic compound in the formula. 2. Determine the number of water molecules represented in the formula. 3. Write the name of the hydrate by stating the compound's name, the numerical prefix for the number of water molecules, and the word "hydrate." For example, for the formula \( \text{Fe(NO}_3\text{)}_3\cdot 9\text{H}_2\text{O} \): 1. The ionic compound is Iron(III) nitrate (\(\text{Fe(NO}_3\text{)}_3\)). 2. The number of water molecules is 9. 3. The name of the hydrate is Iron(III) nitrate nonahydrate.

Key concepts

Hydrate Chemical Formula

When a crystal structure incorporates water molecules directly into its lattice, we are dealing with a special type of compound called a hydrate. The hydrate chemical formula reveals this unique composition by indicating the exact number of water molecules tethered to the compound. This is traditionally written as the compound’s formula followed by a dot and then the number of water molecules, each represented by H₂O. For instance, the hydrate of copper(II) sulfate with five water molecules is expressed as CuSO₄·5H₂O.

Waters of Hydration

The term waters of hydration specifically points to the water molecules that are an integral part of a hydrate's structure. Each of these molecules is held in place within the crystalline framework of the parent ionic compound. These aren't loosely associated or surface-bound; they are a stoichiometric part of the compound, meaning they are present in a fixed ratio. This ratio is crucial for the proper nomenclature and understanding of the physical properties of hydrates. It's general practice to heat the hydrate in order to evaporate the waters of hydration to determine this ratio, which then dictates the numeral we use in the hydrate chemical formula.

Ionic Compounds

To understand the basis of naming hydrates, one must be familiar with ionic compounds. These are substances formed by the electrostatic attraction between ions of opposite charges. Ionic compounds, like sodium chloride (NaCl), are made up of a metal and a non-metal that exchange electrons, becoming positively and negatively charged ions respectively. The waters of hydration that contribute to the makeup of hydrates will bind to these ionic compounds, often at the time of their crystallization. Knowledge of the ionic component is pivotal when naming a hydrate, as it constitutes the first part of the name before we specify the degree of hydration using prefixes like 'mono,' 'di,' 'tri,' and so on, followed by the word 'hydrate' to indicate the integration of water molecules.