Representing Chemical Reactions

Types of Chemical Reactions

Chemical reactions are happening all around us at all times. They are so common, that we often don't even notice when they happen.

Since chemical reactions are so common, it is important to distinguish them from physical reactions. A physical change is merely a substance that goes through a change in state. Like ice melting, or gaseous water condensing, these changes are water remaining water. While still important, and very common, they are not to be confused with a chemical change.

Interactions of Matter

The interaction of all things, whether they be atoms, molecules, living creatures, or even stars, cause reactions. Let's just focus on the interaction of atoms and molecules. If you were to evacuate a box of all of its air and create a vacuum, you would have a box with absolutely no matter in it. It would be completely empty. Now, say you released pure H₂ gas into it, followed by pure O₂ gas. Would anything happen?

Well, for thermodynamic and kinetic reasons, they would react. More specifically, they would react to form water according to the following reaction:

$$ 2H_2(g) + O_2(g) \rightarrow 2H_2O(g) $$

When molecules are in the gaseous phase, they are full of energy. These molecules vibrate very quickly, which causes them to diffuse quickly. When they bump into another molecule, they transfer their vibrational energy into another molecule. This happens thousands of times per second, causing these two gases to mix very quickly.

When two molecules bump into each other, sometimes they transfer enough energy to react. If a reaction is both thermodynamically and kinetically allowed, then a collision may produce enough energy to cause a reaction.

In the gas state, molecules have the most energy and move the quickest. Chemical reactions often happen in this state, but they can also happen in the liquid or solid state. Whether something will actually occur is based on energy and proximity. We will take a brief look at:

• Combination and Decomposition Reactions
• Single-Replacement and Double-Replacement Reactions
• Combustion Reactions
• Acid-Base Reactions
• Redox Reactions

Combination and Decomposition Reactions

Combination and Decomposition reactions can be viewed as reactions that move in the opposite direction.

• In Combination reactions, multiple reactants combine to form a single product.

$$ 2Na(s) + Cl_2 (g) \rightarrow 1NaCl (s) $$

In this example, two separate reactant molecules, sodium and chlorine, combine to form one product molecule, table salt.

• Decomposition reactions are the counter to combination reactions. One reactant will decompose to form multiple products.

This can be seen when any salt is dissolved in water:

$$ NaCl (s) \rightarrow Na^+(aq) + Cl^-(aq) $$

Salts will dissociate in water to form the ions which make up the solid form. In this reaction, one single molecule will decompose into multiple ions, forming an aqueous solution.

Single-Replacement and Double-Replacement Reactions

• Single-Replacement and Double-Replacement reactions both involve replacing a reactant that is made up of different components.

To visualize this, we'll highlight an example of a single-replacement reaction.

$$ HCl(aq) + H_2O(l) \rightarrow H_3O^+(aq) + Cl^-(aq) $$

In this example, hydrogen is transferred from hydrogen chloride to water. Hydrogen is the only atom which transfers.

Double-Replacement reactions behave as other replacement reactions, but two exchanges take place. Check out this example:

$$ AgNO_3 (s) + NaCl(s) + H_2O(l) \rightarrow AgCl (s) + NaNO_3 (s) + H_2O(l) $$

In this example, silver is exchanging with sodium as the cations. This results in 2 reactant molecules making 2 product molecules. Note that water is a spectator, and is not participating in the reaction.

Combustion Reactions

• Combustion reactions occur when some reactant is burned. This process requires oxygen and results in the release of energy (in the form of heat and light) as a byproduct.

Combustion reactions power vehicles, buildings, and many other facets of life. The simplest combustion reaction is the combustion of methane:

$$ CH_4 (g) + 2O_2 (g) \rightarrow CO_2 (g) + 2H_2O (g) $$

Combustion reactions will always burn organic material and oxygen to form carbon dioxide and water as products. These reactions can be very complicated, but they will always produce carbon dioxide water.¹

Acid-Base Reactions

Another set of reactions, known as acid-base reactions—or neutralization reactions—typically take place in water.

• Acid-Base reactions involve an acid and a base reacting to form water and a salt.

$$ HCl (aq) + NaOH (s) \rightarrow H_2O (l) + NaCl (aq) $$

In this reaction, hydrochloric acid—a strong acid—reacts with sodium hydroxide—a strong base—to form water and sodium chloride salt. The acid gives up its hydrogen, also known as a "proton", to the base, which takes the proton. This reaction just shown as the full reaction, but to help visualize what the acid and the base look like, let's try writing it a bit differently.

$$H_3O^+(aq)+OH^-(aq) \rightarrow H_2O\,(l) $$

The hydronium ion, H₃O⁺, is the acid molecule. The hydroxide base, OH^-, is negatively charged and likes to interact with positively charged species. These two highly reactive species come together to form water. They neutralize each other's charges and reactivity.

The example given is an example of strong acids reacting with strong bases. This is not always the case; think of the reaction between vinegar and baking soda, which is a weak acid and weak base.

Redox Reactions

Redox reactions are the final type of reactions we will mention.

• Redox reactions, known as reduction-oxidation reactions, are ionic reactions.
• These reactions involve the exchange of electrons between two different atoms or molecules.
• One molecule will give up electrons, which is referred to as oxidation.
• The other molecule will take up the electrons, which is referred to as reduction.

Consider:

$$ Zn (s) + Cu^ {2+} (aq) \rightarrow Zn^ {2+} (aq) + Cu (s) $$

In this reaction, zinc metal is giving two of its electrons to the copper cation. This results in a zinc cation and copper metal.

Chemical Reaction Equations

It is important to understand how to read and write equations. The first thing we should discuss are reaction arrows. Although reactions may proceed in only one direction, this is not always the case.

$$ 2Na (s) + Cl_2 (g) \rightarrow 2NaCl (s) $$

In this example, sodium and chlorine are irreversibly reacting together to form a salt. The full arrow means that the reaction does not proceed in the opposite direction. When the reactants react together, they will become something else, and they will not go back to their original form.

This is not always the case in reactions. In fact, often, reactions form a dynamic equilibrium. Essentially, there is movement from reactants to products, and from products to reactants.

$$ H_3O^+ (aq) + OH^ - (aq) \rightleftharpoons H_2O (l) $$

The acid and base will react together to form water, neutralizing each other. However, water will also split apart and go back to the reactants. The arrows in this reaction are called equilibrium arrows because this reaction will level out until it reaches a steady state. Now, that doesn't mean that the reactants and products are in equal amounts. Water is present in amounts millions of times more than its reactants. However, there will always be some hydronium, and some hydroxide, in a solution. Since this reaction is reversible, it can also be written in the other direction.

$$ H_2O (l) \rightleftharpoons H_3O^+ (aq) + OH^ - (aq) $$

Another important thing to consider when looking at chemical equations is the physical states of the substances. You have probably noticed the following states:

• Solid, (s)
• Liquid, (l)
• Gas, (g)
• Aqueous, (aq)

Aqueous just means that the substance is dissolved in water. This may include ions, or full molecules, which are contained within water. If a liquid is added to water, but doesn't mix, then it would not be aqueous.

Short Representation of a Chemical Reaction

In the lab, you are asked to prepare a stock solution of concentrated sodium hydroxide, NaOH. Later you will be using it to neutralize an acidic solution of concentrated sulfuric acid, H₂SO₄. You note that sodium hydroxide is a strong base and sulfuric acid is a strong acid. Then you write down everything you do and record the reaction equation. After you're done, your lab journal looks something like this:

• Added 40 g of sodium hydroxide, NaOH, to a 1 L solution of water and stirred it until it all dissolved.
• Prepared a reaction using concentrated sulfuric acid, H₂SO₄.
• Neutralized H₂SO₄ solution with NaOH. Added NaOH one drop at a time. The neutralization reaction was very exothermic.
• Tested the pH of the solution every few dozen drops until the solution was pH neutral (pH = 7).
• \( NaOH (aq) + H_2SO_4 (aq) \rightarrow H_2O (l) + NaHSO_4 (aq) \)

This may represent something you would do in a real lab. Knowing what you are doing is essential if you are to avoid doing anything potentially catastrophic. For example, in this reaction, the neutralization of sulfuric acid produces a lot of heat. By doing it too quickly, a serious accident could happen. Knowing the reaction equation and the properties of both the reactants and products is absolutely necessary when conducting chemical reactions.

Visual Representation of a Chemical Reaction

Chemical reactions can be seen any time you light a flame, or bake a cake, or create a volcano using baking soda and vinegar. Some can't be seen, but it is still important to recognize that they are happening, and how they look on a molecular scale.

When methane, a flammable gas, is ignited in the presence of oxygen, it breaks down into its atoms. The bonds in CH₄ will split and carbon will react with one equivalent of oxygen to form CO₂. The four hydrogens will react with another equivalent of oxygen to form two equivalents of water.

On a microscopic scale, bonds are breaking and bonds are forming. On the macroscopic scale, methane is ignited with a large ball of flame. It is easy to tell that a chemical reaction occurred because there was a visual change. Visual changes are a great indicator of whether a chemical reaction happened.

Representation of a Chemical Reaction That Uses Symbols

We have seen many different examples of chemical reactions. A representation of a chemical reaction that uses symbols is one that combines all the tools we have learned so far. By combining states of molecules, reaction arrows, and ion species, we can properly construct chemical equations.