Molecular Equations: Videos & Practice Problems

Understanding molecular equations is crucial for grasping how substances react in aqueous solutions. A molecular equation represents the reactants and products as intact compounds, which can be categorized based on the products they form. In a neutralization reaction, an acid and a base react to produce an ionic compound and water. However, not all acid-base reactions yield water, which will be explored in advanced topics. A gas evolution reaction involves the formation of a gas as one of the products, while a precipitation reaction results in the formation of a solid ionic compound, known as a precipitate. The formation of these products is influenced by the solubility rules, which determine whether a compound will remain dissolved or form a precipitate. Remember, the identity of the products—whether they are aqueous, a gas, or a solid—helps classify the type of molecular equation at hand.

Types of Molecular Equations

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) in liquid form.

There are various types of molecular equations based on the products formed. A neutralization equation occurs when an aqueous acid reacts with an aqueous base, typically producing an ionic compound and water. In the example provided, the reaction between perchloric acid and barium hydroxide exemplifies this, resulting in water and an ionic salt.

In contrast, a gas evolution equation involves the production of a gas as one of the products. Here, two aqueous reactants react to form at least one gaseous product, while the other product may be in a different state, such as aqueous or solid.

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

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

Precipitation Reaction Example

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 reaction is classified as 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 state, this does not qualify as a precipitation reaction.

3. The final scenario presents two aqueous reactants that yield at least one solid product. This meets the criteria for a precipitation reaction, confirming that this option represents a true precipitation reaction.

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

Molecular Equation Example

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 (SO₄^2-) 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. For instance, the +1 from lithium will pair with sulfate, resulting in lithium sulfate (Li₂SO₄), and the +2 from magnesium will pair with hydroxide, yielding magnesium hydroxide (Mg(OH)₂).

To determine if a reaction occurs, 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 (aqueous) because it is associated with a Group 1A ion. However, magnesium hydroxide is insoluble (solid) since it does not pair with calcium, barium, or sulfur, which would make it soluble. Since a solid is formed, 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.

Predict whether a chemical reaction occurs and write the balanced molecular equation.
Ag₂SO₄ (aq) + KCl (aq) →

• A. Ag₂SO₄ (aq) + KCl (aq) → AgCl (s) + K₂SO₄ (aq)
• B. Ag₂SO₄ (aq) + 2 KCl (aq) → 2 AgCl (s) + K₂SO₄ (aq)
• C. Ag₂SO₄ (aq) + 2 KCl (aq) → 2 AgCl (aq) + K₂SO₄ (aq)
• D. No reaction

Answer: (B)

Predict whether a chemical reaction occurs and write the balanced molecular equation.
MgBr₂ (aq) + NaC₂H₃O₂ (aq) →

• A. MgBr₂ (aq) + NaC₂H₃O₂ (aq) → NaBr (aq) + Mg(C₂H₃O₂)₂ (aq)
• B. MgBr₂ (aq) + NaC₂H₃O₂ (aq) → 2 NaBr (aq) + Mg(C₂H₃O₂)₂ (aq)
• C. MgBr₂ (aq) + 2 NaC₂H₃O₂ (aq) → 2 NaBr (s) + Mg(C₂H₃O₂)₂ (aq)
• D. No reaction

Answer: (D)

Determine the balanced equation for the neutralization equation
Ca(OH)₂ (aq) + HCN (aq) →

• A. Ca(OH)₂ (aq) + 2 HCN (aq) → 2 Ca(CN)₂ (aq) + 2 H₂O (l)
• B. Ca(OH)₂ (aq) + HCN (aq) → Ca(CN)₂ (aq) + 2 H₂O (l)
• C. Ca(OH)₂ (aq) + 2 HCN (aq) → Ca(CN)₂ (aq) + H₂O (l)
• D. Ca(OH)₂ (aq) + 2 HCN (aq) → Ca(CN)₂ (aq) + 2 H₂O (l)

Answer: (D)

Here’s what students ask on this topic:

What is a molecular equation?

A molecular equation is a chemical equation in which the reactants and products are written as if they were molecular substances, even if they exist in solution as ions. This type of equation shows the complete chemical formulas of the compounds in the reaction without indicating the ionic nature of the substances involved. For example, when aqueous solutions of silver nitrate (AgNO₃) and sodium chloride (NaCl) are mixed, the molecular equation would be written as:

${\mathrm{AgNO}}_3(aq)+{\mathrm{NaCl}}_{}(aq)\to {\mathrm{AgCl}}_{}(s)+{\mathrm{NaNO}}_3(aq)$

Here, the silver chloride (AgCl) is shown as a solid because it precipitates out of the solution, while the other compounds are shown in their aqueous state, implying they are dissolved in water. The molecular equation provides a straightforward representation of the overall reaction but does not reveal the actual ionic species that are present in the reaction mixture or the complete ionic interactions that occur.

How do you write a molecular equation?

To write a molecular equation, you first need to know the reactants and products involved in the chemical reaction. A molecular equation represents the chemicals in their molecular form, not showing the ions that may be present in a solution. Here's a step-by-step process:

1. Write the Chemical Formulas: Begin by writing the correct chemical formulas for all the reactants on the left side of the equation and all the products on the right side. Use the periodic table and chemical nomenclature rules to determine the correct formulas.
2. Balance the Equation: Ensure that the number of atoms of each element is the same on both sides of the equation. Adjust coefficients (the numbers before the chemical formulas) as necessary to balance the equation. Remember, you can only change the coefficients, not the subscripts in the formulas.
3. Indicate Physical States: Include the physical state of each component next to its formula. Use (s) for solids, (l) for liquids, (g) for gases, and (aq) for aqueous solutions, which are substances dissolved in water.
4. Check for Accuracy: Review the equation to ensure that the chemical formulas are correct, the equation is balanced, and the physical states are appropriately indicated.

An example of a balanced molecular equation is:

$2H_2\left(g\right)+O_2\left(g\right)\to 2H_2{O}_{\left(}$

What is a neutralization reaction?

To find the volume of an object when you have its density and mass, you can use the formula:

$\mathrm{Volume}=\frac{\mathrm{Mass}}{\mathrm{Density}}$

Density is typically expressed in units such as grams per cubic centimeter (g/cm³) or kilograms per cubic meter (kg/m³), while mass is usually given in grams (g) or kilograms (kg). Volume will be in cubic centimeters (cm³) or cubic meters (m³), depending on the units of density.

Here's how to apply the formula:

1. Ensure that the mass and density are in compatible units. If they are not, convert them to compatible units (e.g., if mass is in grams, density should be in g/cm³).
2. Divide the mass of the object by its density using the formula.
3. The result will give you the volume of the object in cubic units corresponding to the density units used.

For example, if an object has a mass of 200 grams and a density of 10 g/cm³, the volume would be:

$\frac{200}{10 \text{g/cm³}}=20 \text{cm³}$

What is a precipitation reaction?

A precipitation reaction is a type of chemical reaction that occurs when two soluble salts in aqueous solution are combined and form an insoluble product, known as a precipitate. This precipitate is a solid that will settle out of the liquid solution or can be filtered out. The reaction takes place when the cations (positively charged ions) of one reactant and the anions (negatively charged ions) of another reactant exchange partners and produce at least one combination that is not soluble in water.

The general form of a precipitation reaction can be represented as:

$\mathrm{AB}\left(\mathrm{aq}\right)+\mathrm{CD}\left(\mathrm{aq}\right)\to \mathrm{AD}\left(s\right)+\mathrm{CB}\left(\mathrm{aq}\right)$

Here, AB and CD are the aqueous reactants, AD is the solid precipitate, and CB remains in solution. To predict whether a precipitation reaction will occur, you can refer to the solubility rules, which provide guidelines on the solubility of various ionic compounds in water. Precipitation reactions are important in many areas, including chemical analysis, environmental science, and industrial processes.