Reactions and Stoichiometry

Reactions

With our knowledge of atomic structure and, since elements are known to bond in regular, predictable patterns in whole number ratios, there are a number of quantitative relationships that can be determined using information from the periodic table.

A conveniently measured amount of any element that equals the mass in grams of that element is called a mole. One mole of hydrogen weighs 2 grams. One mole of chlorine weighs 70 grams. Just like a dozen equals twelve, a mole equals Avogadro’s number of atoms or 6.02 × 10²³. 6.02 × 10²³ molecules of hydrogen would weigh 2 grams. An equal number of chlorine molecules would weigh 70 grams. Chlorine is much heavier than hydrogen because it has more protons and neutrons. A similar analogy is that a dozen bowling balls would weigh more than a dozen golf balls. A mole is simply a very large number of atoms. You need a very large number of atoms to be able to measure something you can see, like a teaspoon of salt. The coefficient is given to determine the number of moles in the balanced equation. Using these mole ratios, you can substitute the grams for atomic mass units and calculate relative masses for reactants and products.

Here are some simple examples of the gram atomic weight for one mole of different common compounds.

Carbon dioxide: CO₂

One mole (C) 12 grams + One mole (O₂) 32 grams = 44 grams

Water: H₂O

One-half mole (O₂) 16 grams + One mole (H₂) 2 grams = 18 grams

Balanced equations and chemical reactions

There are four basic types of chemical reactions. The first is the simplest and is known as synthesis, or a composition reaction. This type of reaction combines two or more substances to form at least one new compound. One example has already been given which is when sodium metal, a solid, combines with chlorine gas to form sodium chloride. The reaction can be symbolized in the following way:

Na (s) + Cl₂ (g) → NaCl (s)

The reactants are on the left hand side of the arrow and the product(s) is (are) on the right. It can be read to mean that the reactants, sodium and chlorine, will react to form a single product, sodium chloride. In the chemical equation above, the (s) stands for solid, and the (g) stands for gas which indicates which state of matter the substance is in. Sometimes the arrow is read as “yields.” As you look at this equation, the natural question to ask is why are there three atoms on one side of the equation and only two atoms on the other? What happened to the extra chlorine atom? Good question. This equation is not a balanced equation, and matter doesn’t just disappear and appear. A more accurate way to correctly represent this reaction is as follows:

2Na (s) + Cl₂ (g) a 2NaCl (s)

This can be understood by saying that two sodium atoms combine with one diatomic chlorine molecule to form two ionic units of sodium chloride.

Another aspect of chemical reactions is the physical properties of compounds, or how atoms and molecules fit together. If we look at the crystal structure of NaCl, we notice that it is a closely packed cube.

The sodium chloride crystal structure has each atom with six of its nearest neighbors in an octahedral geometric pattern. Na ions are in light blue and Cl ions are darker green for contrast.

All reactions are simply rearrangements of matter in more stable ways. New matter is never created, and no matter is ever destroyed. This is known as the law of conservation of matter; matter can neither be created nor destroyed, but matter simply changes form.

sodium + chlorine → sodium chloride

2 Na (s) + Cl₂ (g) a 2 NaCl (s)

2 moles + 1 mole = 2 moles

2(23 amu) + 2(35 amu) = 2(58 amu)

46 amu + 70 amu = 116 amu

116 amu = 116 amu

Using the atomic weighs from the periodic table, you can see that two sodium atoms weigh 46 atomic mass units. One chlorine molecule weighs 70 atomic mass units. Two units of a sodium chloride crystal weigh 116 atomic mass units. The total mass of the reactants must equal the total mass of the product.

A second type of chemical reaction is known as a decomposition reaction. This is where a compound is broken down into two or more simpler parts. One example is the electrolysis of water. Water can be broken down in to its constituent elements, hydrogen and oxygen with the addition of electricity. The equation looks like this:

water → hydrogen + oxygen

2 H₂O (l) → 2 H₂ (g) + 2 O₂(g)

2 moles → 2 moles + 1 mole

2(18 grams) = 2(2 grams) + 32 grams)

36 grams = 4 grams + 32 grams

Based on these quantitative relationships you can predict that 36 grams of water will yield 4 grams of hydrogen gas and 32 grams of oxygen gas. The lowercase (l) stands for liquid, which is what the state of water is in the reaction.

A third type of reaction is called a single replacement reaction. This is where a more active element will replace a less active element in a compound. One example is when the metal zinc is dropped into a solution of hydrogen chloride to generate hydrogen gas. The equation would look like this:

zinc + hydrogen chloride → zinc chloride + hydrogen

Zn (s) + HCl (aq) → ZnCl₂ (aq) + H₂ (g)

The (aq) stands for aqueous, which means that the substance is dissolved in a water solution. Can you determine what coefficients are missing to make the equation a balanced one?

If you thought a 2 needed to be in front of the reactant HCl, you would be right. How many moles of zinc would be needed to react with 2 moles of hydrogen chloride to form 1 mole of aqueous zinc chloride?

You are correct if you thought the answer was one. One mole of zinc will react with 2 moles of aqueous hydrogen chloride to form one mole of zinc chloride and one mole of hydrogen gas.

A fourth type of reaction is known as a double replacement reaction. These often occur when two soluble reactants are dissolved in water and one of the products is not soluble. These insoluble products drop out of solution as a precipitant and can make the solution appear cloudy. One example is when lead chloride and sodium sulfide are dissolved in water; a precipitant forms known as lead sulfide. This substance is believed to be the “snow” that precipitates on the surface of Venus. The reaction can be written as follows:

Lead chloride + sodium sulfide → lead sulfide + sodium chloride

PbCl₂ (aq) + Na₂S (aq) → PbS (s) + NaCl (aq)

To summarize the four basic types of reactions:

• A + B → AB composition
• AB → A + B decomposition
• A + BC → AC + B single replacement
• AB + CD → AD + CB double replacement

Oxidation reactions

Oxygen is a common component of the Earth’s atmosphere and therefore is readily available to react with most elements and compounds on Earth. With its relatively small size and high electronegativity, oxygen participates in a large number of chemical reactions. One special type of reaction that is a combination of two basic types of reactions (composition and single replacement) is called an oxidation reaction, first named because oxygen was the most common element to be observed in this type of reaction. Other electronegative elements are observed to react in the same manner.

Let’s take a common example of an oxidation reaction with a molecule of methane, CH₄. Methane is also known as natural gas, or swamp gas. This gas is flammable because it reacts in the presence of a spark with oxygen and produces a flame. Natural gas stoves make use of this reaction for heating and cooking.

methane + oxygen → carbon dioxide + water

CH4 (g) + O₂ → (g) → CO₂ (g) + H₂O (g)

Oxidation reactions are the basis for respiration in all living things because most organic molecules can be oxidized to produce energy. Proteins, lipids, and carbohydrates can all be used as fuels similar to the methane in this example. Oxidation reactions are essential for life on Earth to exist.

Neutralization reactions

One very common type of a double replacement reaction occurs when an acid and a base react to form a salt and water. Examples are given below:

hydrochloric acid + sodium hydroxide → sodium chloride + water

HCl (aq) + NaOH (aq) → NaCl (aq) + H₂O (l)

sulfuric acid + magnesium hydroxide → magnesium sulfate + water

H₂SO₄ (aq) + Mg(OH)₂ (aq) → MgSO₄ (aq) + 2H₂O (l)

An equally strong acid and base will produce a neutral solution of salt water. The acidity of solutions can be of varying strengths and can be measured by what is called the pH. A solution will be labeled an acid if the pH is less than 7, a base when the pH is greater than 7, and neutral when the pH equals 7.

The pH is defined as the negative log of the hydrogen ion ([H⁺]) or hydronium ion ([H₃O⁺]) concentration depending on what source you use. Remember that hydrogen ions are really just bare protons (positively charged nuclei) that are attracted to the lone pair of electrons in water molecules. The nature of this relationship determines the strength of the acid in water because some substances tend to give up more [H⁺] in water than others.