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 products formed. In a neutralization reaction, an aqueous acid reacts with an aqueous base, resulting in the formation of an ionic compound and water. This is a fundamental type of acid-base reaction where the products typically include water and a salt.

In contrast, gas evolution reactions involve the production of a gas as one of the products. Here, two aqueous reactants react to produce at least one gaseous product, which may be accompanied by another product that could also be a gas, but this is not guaranteed.

Lastly, precipitation reactions are characterized by the formation of a solid, known as a precipitate, from the reaction of two aqueous reactants. This solid ionic compound forms when the solubility rules indicate that at least one of the products is insoluble in water.

In summary, molecular equations illustrate the interaction of aqueous reactants to form various products, which can be categorized into neutralization, gas evolution, and precipitation reactions based on the nature of the products formed.

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 aqueous, this does not qualify as a precipitation reaction.

3. The final reaction features two aqueous reactants that yield at least one solid product. This scenario meets the criteria for a precipitation reaction, confirming that it is indeed a precipitation reaction.

In summary, to determine if a reaction is a precipitation reaction, check that both reactants are in the aqueous state and that at least one product is a solid ionic compound.

A molecular equation represents the reactants and products of a chemical reaction in their molecular forms. To predict whether a chemical reaction occurs and to write the balanced molecular equation, follow these steps:

First, identify the ionic forms of the reactants. For example, if reactant 1 is lithium hydroxide (LiOH), lithium (Li) has a +1 charge as it belongs to Group 1A, while hydroxide (OH) is a polyatomic ion with a -1 charge. Reactant 2, magnesium sulfate (MgSO₄), consists of magnesium (Mg) with a +2 charge from Group 2A and sulfate (SO₄) with a -2 charge. Understanding these charges is crucial, so reviewing the periodic table and polyatomic ions is recommended.

Next, swap the ionic partners based on the principle that opposite charges attract. This means that the positive ion from one reactant will pair with the negative ion from the other. When combining ions, if the charges differ, they crisscross to form the new compounds. For instance, lithium will combine with sulfate to form lithium sulfate (Li₂SO₄), and magnesium will combine with hydroxide to form magnesium hydroxide (Mg(OH)₂).

After determining the products, assess whether a reaction occurs. A reaction is confirmed if a solid, gas, or liquid water is produced. If both products are aqueous (soluble), then no reaction takes place. Using solubility rules, lithium sulfate is soluble due to lithium being a Group 1A element, while magnesium hydroxide is insoluble because hydroxides are generally insoluble unless paired with calcium, barium, or strontium. Since magnesium hydroxide is solid, a reaction has occurred.

Finally, balance the molecular equation by adjusting coefficients. In the unbalanced equation, there are 2 lithiums in the products but only 1 in the reactants. Placing a coefficient of 2 in front of lithium hydroxide balances the equation, resulting in 2 LiOH + MgSO₄ → Li₂SO₄ + Mg(OH)₂. The final balanced molecular equation is 2 LiOH + 1 MgSO₄ → 1 Li₂SO₄ + 1 Mg(OH)₂.