Molecular Equations: Videos & Practice Problems

A molecular equation represents the intact compounds involved in a chemical reaction, rather than their dissociated ionic forms. In a typical molecular equation, two aqueous reactants combine to yield products, which can also be in aqueous form or other states. For instance, when 2 moles of perchloric acid (HClO₄) react with 1 mole of barium hydroxide (Ba(OH)₂), the products formed are 1 mole of barium perchlorate (Ba(ClO₄)₂) and 2 moles of water (H₂O).

There are various types of molecular equations based on the nature of the products formed. In a neutralization reaction, an aqueous acid reacts with an aqueous base, typically producing an ionic compound and water. This is exemplified by the reaction of an acid with a base, resulting in water and a salt as products. However, in more advanced acid-base reactions, water may not always be a product.

Another type is the gas evolution reaction, where at least one of the products is a gas. In this scenario, two aqueous reactants react to produce a gas along with another product, which may also be a gas but is not guaranteed.

Lastly, a precipitation reaction involves the formation of a solid, known as a precipitate, from the reaction of aqueous reactants. This occurs when at least one of the products is a solid ionic compound, indicating that a reaction has taken place that results in the formation of an insoluble substance.

In summary, molecular equations illustrate the interaction of aqueous reactants to form various products, and the type of reaction—whether neutralization, gas evolution, or precipitation—depends on the identities of these products.

In chemistry, a precipitation reaction occurs when two aqueous reactants combine to form at least one solid ionic compound as a product. This solid product, known as a precipitate, separates from the solution. To identify a precipitation reaction, it is essential to ensure that both reactants are in the aqueous state.

For example, consider the following scenarios:

1. In the first reaction, we have two aqueous reactants that produce water (a liquid) and an ionic compound. This scenario represents an acid-base reaction, specifically between hydrochloric acid (HCl) and potassium hydroxide (KOH), rather than a precipitation reaction.

2. The second reaction involves a solid reactant and an aqueous reactant, resulting in a solid product and an aqueous product. Since one of the reactants is not in the aqueous form, this does not qualify as a precipitation reaction.

3. The final reaction features two aqueous reactants that yield at least one solid product. This aligns with the definition of a precipitation reaction, confirming that this option represents a true precipitation reaction.

In summary, to classify a reaction as a precipitation reaction, ensure both reactants are aqueous and that at least one product is a solid ionic compound.

A molecular equation can be constructed by following a systematic approach when given two reactants. To illustrate this, let’s consider an example where we need to predict whether a chemical reaction occurs and write the balanced molecular equation.

First, we break down the reactants into their ionic forms. For instance, if reactant 1 is lithium hydroxide (LiOH), lithium (Li) is in Group 1A and has a +1 charge, while hydroxide (OH) is a polyatomic ion with a -1 charge. Reactant 2, magnesium sulfate (MgSO₄), consists of magnesium (Mg) from Group 2A with a +2 charge and sulfate (SO₄^2-) with a -2 charge. Understanding these charges is crucial, and reviewing the periodic table and polyatomic ions can be beneficial.

Next, we swap the ionic partners based on the principle that opposite charges attract. This means the +1 charge from lithium will pair with the sulfate, and the +2 charge from magnesium will pair with hydroxide. The resulting products are lithium sulfate (Li₂SO₄) and magnesium hydroxide (Mg(OH)₂). This step relies on the rules for combining ions, where the charges crisscross if they differ.

To determine if a reaction occurs, we check the solubility of the products. A reaction is confirmed if a solid, gas, or liquid water is produced. Using solubility rules, lithium sulfate is soluble due to lithium being a Group 1A element, making it aqueous. However, magnesium hydroxide is insoluble because it does not pair with calcium, barium, or sulfur, thus forming a solid. Since a solid is produced, we conclude that a reaction has occurred.

Finally, we balance the molecular equation. Initially, we have two lithium ions and one magnesium ion, which requires us to place a coefficient of 2 in front of lithium hydroxide. This adjustment balances the equation, resulting in the final balanced molecular equation: 2 LiOH + MgSO₄ → Mg(OH)₂ + Li₂SO₄.

In summary, constructing a balanced molecular equation involves identifying ionic forms, swapping partners based on charge attraction, checking solubility to confirm a reaction, and ensuring the equation is balanced through appropriate coefficients.