Molecular Equations - Online Tutor, Practice Problems & Exam Prep

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.

Molecular Equations

A molecular equation shows the intact compounds instead of their dissociated ionic forms. Now when we talk about a molecular equation, we're reacting aqueous reactant one with aqueous reactant two to produce product one plus product two. A good example of this is we have 2 HClO 4 (aq) + 1 Ba(OH)2 (aq) → 1 Ba(ClO)4 (aq) + 2 H2O (l) creating or producing 1 Ba(ClO)4 (aq) + 2 H2O (l).

Now when it comes to a molecular equation, we can define different types of molecular equations based on the products they form. In a neutralization equation, we have an aqueous acid reacting with an aqueous base just like in the example we provided. And what we tend to form as products are some type of ionic compound plus water. Now when we get to later chapters where we talk about more advanced acid and base reactions, we'll see that sometimes we don't form water, so we won't have to worry about that. For now, just realize that when we're talking about a basic acid-base neutralization equation, we will produce water and ionic salt as products.

Now in a gas evolution equation, from the name we know that a gas is involved. Here we still have aqueous reactant one and two reacting, and at least one of the products produced will be a gas, so we'll have a gas plus product two. Now product two of course could be a gas too, but that's not always a guarantee. Finally, we have a precipitation equation, and from the name precipitation we know that a precipitate is involved. And if you've watched my solubility rules videos, you'll know that precipitate represents a solid.

So in a precipitation equation, it's if at least one of the products formed is a solid ionic compound. So just remember we have our molecular equation which tends to be aqueous reactant one and aqueous reactant two reacting to create products one and products two. Based on the identities of those products, we can have different types of molecular equations.

Molecular Equations Example 1

In this example question, it says which of the following is a precipitation reaction. So remember in a precipitation reaction we have aqueous reactant 1 reacting with aqueous reactant 2 to produce products, and at least one of those products has to be a solid ionic compact.

So if we take a look here in the first one we have two aqueous reactants. That's cool. But here we're creating water as a liquid and an ionic compound. Here HCL is an acid, Koh is a base. So this actually is an acid base reaction or equation.

For the next one. We have a solid reactor reacting with an aqueous reactant to produce a solid product and an aqueous product. Now remember 4 to be precipitation reaction, both of my reactants have to be in the aqueous form. One of them is a solid, so this will not do.

For the last one. We have two aqueous reactants and we created at least one solid product. Because of this option, C would have to represent our precipitation reaction.

Molecular Equations Example 2

Here we can say that a molecular equation can be written when given a reactant 1 and react in two. If we take a look at this example question, it says predict whether a chemical reaction occurs and write the balanced molecular equation. All right, so write the molecular equation. We're going to follow the given steps.

So step one says we have to break up reaction one and react in two into their ionic forms. If we look at reactant one, it's composed of lithium and hydroxide. Remember, lithium is in Group 1A, so it's plus one. Hydroxide is a polyatomic ion, so it's -1. Magnesium sulfate magnesiums in Group two way, so it's two plus for its charge. Sulfate is 2 minus. Now if you don't quite remember how I'm coming up with these charges, make sure you go back and take a look at my videos on periodic table and charges and then as well as my polyatomic ions concept videos.

So we've done that first part. Step two, we swamp ionic partners by remembering that opposite charges attract. Apply the rules for combining ions based on the numerical values of their charges. All right, so opposite charges attract. This positive will be attracted to this new negative. This negative will be attracted to this new positive. So they're switching partners now. Remember, if the numbers within the charges are different, they don't cancel out. They're going to crisscross. So the one from here would come here, the two from here would come here. That would give me Li2O4 plus. Then we're going to have the one from here come over here, the two from here come over here, here. That would give me MG. And then the two is for oh wait, so O2, this you can find when we talked about naming ionic compounds and writing ionic compounds.

So again, everything we've learned about ionic compounds up to this point is necessary to effectively write a molecular equation. All right. So we've written out what the products are. Now step three is important. We're going to say a reaction occurs only occurs if a solid gas or liquid water is formed as a product. If both of the products formed are aqueous, meaning they're soluble, then no reaction has occurred and we stop. We just say no reaction. Here we have to use the solubility rules to determine if the product's form will be soluble or insoluble.

So let's come back up here. One product that we made is Lithium Sulfate. Remember Ghana Cash and Cops? Lithium is a group 1A element, so that's part of Ghana, right? So anything connected to a Group One AI is automatically soluble. So this would be aqueous magnesium hydroxide. Hydroxide is part of cops. We say that hydroxides are insoluble unless they're connected to CBS, calcium, barium or sulfur. Also, if they're connected to things from Ghana, Ghana, cache, they could also be soluble. But here hydroxide is not connected to calcium, barium or sulfur, and magnesium is not part of gonocaching anyway. So here this would be solid.

So we just created a solid. So we know a reaction has occurred because remember you have to create a solid gas or liquid water. So Step 4, if necessary balance your molecular equation by placing the correct coefficients in front of each, in front of each molecule. So we're going to do that now. We're going to place the right coefficient in front of each molecule. So coming back U, if we look at our balanced equation or unbalanced equation, we have two lithium's here, but only one here O we're going to put a 2 here. Now we have 2 lithiums, but it also gives us 2 hydroxides. That's OK, because look, we have two hydroxides here. Then what do we see? We see that we have one magnesium, one magnesium, one sulfate, 1 sulfate. So our equation is balanced. So if we write all the coefficients in front it would be 2:1:1 and 1:00. So this would represent our balanced molecular equation.

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

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

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)

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.