Types of Reactions

A great deal of information about reactions has gone forth up to this point. By now, a solid familiarity with how substances react should exist.

One additional concept related to the reaction of substances is being able to describe them as one of several types of reactions. Based on how the elements combine, separate, or reorganize, reactions can be differentiated into one of five reaction types.

Synthesis

When two (or more) substances combine to form a single new substance, the type of reactions is called synthesis.

What this looks like:

A + B --> AB 

"Two smaller things join together to become one bigger thing." 

In synthesis, the substances that combine (the reactants) can be either elements or compounds.

The product will always be a compound.

There are many examples of synthesis. For example, 2H2 + O2 --> 2H2O represents the synthesis of water from hydrogen and oxygen. Likewise, Na + Cl --> NaCL represents the synthesis of salt.

Decomposition

Decomposition occurs when a compound breaks down into two or more substances.

What this looks like:

AB --> A + B

"One bigger thing breaks up into two (or more) smaller things."

In decomposition, the substance that comes apart (the reactants) must be a compound.

The product can be compounds or elements.

An example of decomposition can be seen in the reaction CaCO3 --> CaO + CO2 where the reactant breaks down into two different products—in this case a compound decomposes into two different compounds.

Single Replacement

Single replacement occurs when there are multiple reactants that combine in a way that something in one compound is replaced by the other reactant.

"A single thing replaces something in a compound." 

What this looks like:

AB + C --> A + BC

A + BC -> AB + C

In single replacement, at least one of the reactants and one of the products has to be a compound.

An example of single replacement can be seen in the reaction

2K + 2H2O --> H2 + 2KOH

where the K takes the place of one H. Here is another example: 

2AG + H2S --> Ag2S + H2 

where the Ag takes the place of the H. 

Double Replacement

Double replacement occurs when there are multiple reactants that combine in a way that something in each reactant is replaced by something from the other reactant. In essence, parts each compound swap places.

"Two things in different compounds swap places."

What this looks like:

AB + CD --> AC + BD

In double replacement, the reactants and products are compounds.

An example of double replacement can be seen in the reaction:

BaCl2 + Na2SO4 --> BaSO4 + 2NaCl

where the Ba and Na switch places (as demonstrated by the colors).

Combustion

Combustion exists whenever a substance reacts with oxygen and often produces heat and light. This is usually called burning. The key is that it is a reaction wherein something combines with oxygen. 

"Rapid chemical combination of a substance with oxygen involving the production of heat and light."

There are many, many examples.

See also: https://en.wikipedia.org/wiki/Combustion

****

These five ways of classifying reactions provide a convenient way to describe reactions between substances. It is a very useful way to get an idea of what is happening when considering chemical reactions.

Periodic Table Revisited—Columns and Valences

Previously, much has been learned about the periodic table and what it tells about elements. Each listing in the table contains information about the number of protons (Atomic Number), its mass (Atomic Mass), and other information can be derived, such as number of protons and electrons.

It is worth repeating that the arrangement in the table is ascending from left to right, top to bottom by Atomic Number. Furthermore, each row of elements represents all of the elements filling a set of electron orbitals, and are called periods. Each column, therefore, represents all of the elements whose outer set of electrons are similarly filled, and are called groups or families.

Also, it should be remembered that the set of electrons being filled are called the valence electrons. The unfilled set of valence electrons result in the atoms of elements either being prone to give up or take in electrons. This tendency is expressed as a plus or minus, where the plus means the atom has more than it wants and will be happy to give it up, and where the minus means the atom wants to collect other electrons from other atoms. This tendency to either give up or take in electrons is called the valence charge.

In many of the element groups, all of the elements share the same number of valence electrons. This is true for columns 1, 2, 13, 14, 15, 16, 17, and 18. Columns 3 through 12 do not follow the pattern.

How the elements react with each other is based on their electron configuration. While it would be nice if each element in each family behaved exactly the same way, there is actually a great deal of variation, and elements can combine in many different ways. Another way to look at valence is a related concept called oxidation state, and this periodic table reveals that many of the groups/families can have a wide range of values:

http://sciencenotes.org/downloadable-periodic-table-oxidation-states/

However, to get a sense of what is going on in general, further discussion will focus on the simple, most common behaviors of each family.

Elements in column 1 tend to give up their single valence electron and have a valence charge of +1 (plus meaning they have the tendency to give it away).

Elements in column 2 have will give up both of their valence electrons and have a valence charge of + 2

Columns 3 through 12 reflect a great deal of variation with the number of valence electrons being 1 or 2 and valence charges ranging from +8 to -4

Elements in column 13 have 3 valence electrons and usually have a valance charge of +3.

Elements in column 14 have 4 valence electrons. They will either give up 4 or take 4 (usually) so they have a valence of ±4.

Elements in column 15 have 5 valence electrons. They wish to complete their orbital set, so they want 3 electrons giving them a usual valence charge of +3 (though they will behave differently in some cases, even giving away all five of their electrons).

Elements in column 16 have 6 valence electrons and need 2 more to complete their orbital set, so they have a usual valence charge of -2.

Elements in column 17 have 7 valence electrons and need one more to fill their orbital sets, so they have a usual valence charge of -1

Elements in column 18 have complete sets of electrons in all of their orbitals, wanting neither to take or give. Thus, they have a valence of 0.

In a general, over-simplified way, elements in columns 1-13 will give away electrons in order to empty their incomplete set of orbitals, while elements in columns 15 to 17 are trying to collect electrons to fill their incomplete set of orbitals. Thus, in an equally over-simplified way, valence charges can be thought of as:

Column	1	2	3-12	13	14	15	16	17	18
	+	++	??	+++	++++ or ----	---	--	-	0
Usual Valence Charge	+1	+2	Unsure	+3	±4	-3	-2	-1	0

In many, many cases, these valence charges will allow the prediction of the ratio in which elements will combine. When combined, as a rule, the sum of the valence charges will be 0.

Thus, as a rule, and for example:

elements from column 1 and column 17 will combine in a 1:1 ratio (such as NaCl).

elements from column 1 and column 16 will combine in a 2:1 ratio (such as H2O).

elements from column 13 and column 17 will combine in a 1:3 ratio (such as AlCl3).

This is NOT all of the normal, set combinations. It is only three of the many, many examples showing how the column position determines the ratio in which elements will combine.

So, based on the electron configuration, 1 carbon will give up its 4 electrons to 2 oxygen so that both of the 2 oxygen can complete their sets of orbitals by adding 2 electrons each.

For many, many compounds, this over-simplified table will provide insight into why the elements are combined as they are. While it is not true for EVERY case, it is useful for gaining a basic understanding of how element combine.

More About Coefficients in Chemical Equations

In any given chemical reaction, the equation that describes it is made up of numbers and symbols that represent the molecules that are combining. Any molecule is represented by the atomic symbols and subscripts telling how many of each type of atom make up a single molecule.

When the equation is balanced, coefficients are added to the molecules of the reactants and to the molecules of the products until the number of atoms of each type are equal on each side of the reaction. Thus, if a reaction begins with 4 atoms of "X" and 3 of "Y", it must end up with the same number of each atom.

To balance an equation, coefficients are added in front of the molecules. The coefficient tells how many of which molecule is needed in order to come up with the right number of atoms on each side. The coefficient is a multiplier for the numbers of each atom in each molecule of the equation.

As has been explained previously (for example), 3 C6H12O6 means 3 molecules of C6H12O6, which results in 18 C, 36 H, and 18 O atoms.

However, the coefficients also give more information. They serve additionally as a ratio of molecules that will react with each other. The coefficients can be seen as the how many of any numeric units are needed to properly react.

Using water as an example, the equation is this:

2H2 + O2 --> 2H2O

At the simplest level, this means 2 molecules of H2 reacted with 1 molecule of O2 results in 2 molecules of H2O.

HOWEVER, it ALSO means that any number of molecules can be combined, so long as the ration of 2:1 --> is preserved. Thus:

2 molecules of H2 reacted with 1 molecule of O2 results in 2 molecules of H2O

2 dozen molecules of H2 reacted with 1 dozen molecules of O2 results in 2 dozen molecules of H2O

2 score molecules of H2 reacted with 1 score molecules of O2 results in 2 score molecules of H2O

2 bazillion molecules of H2 reacted with 1 bazillion molecules of O2 results in 2 bazillion molecules of H2O

And for the most relevant example…

2 moles of H2 reacted with 1 mole of O2 results in 2 moles of H2O

It is when we relate the coefficients to moles that we tap a great deal of power! While we cannot count moles, we can use the atomic mass of the atoms to find the right amounts of elements or compounds to use in reactions.

For the reaction of sodium (Na) and Chlorine (Cl) we can use the balanced equation to determine the masses of the two elements that would be needed in a reaction. Here is the reaction:

2 Na + Cl2 → 2 NaCl

According the the formula above, we need molecules of Na and Cl2 in a ration of 2:1. Thus, if we have 2 moles of Na and 1 mole of Cl2 we will have the right ratio for a complete reaction with no left-overs.

From the atomic masses, we know that one mole of NA weighs 22.989 grams. Thus, we begin with 45.979 grams of sodium, we will have the right amount.

Because chlorine is always Cl2, one mole of it will weigh 70.90 grams. Finding the mass of a gas is not as easy as finding the mass of a solid, but it can be done, so starting with 70.90 grams of chlorine is the right amount as well.

It is the coefficients of the balanced equation that guide us to these masses. Understanding that coefficients give us the correct ratio of molecules allows us to use the relationship between atomic mass and numbers of moles to come up with the correct amounts of reactants for any reaction.

Balancing Chemical Equations: Summary and Recap

The following is a list of basic principles related to reading chemical notation and balancing chemical reactions. Mastering this list and fully understanding it is a great stride in mastering the ability to balance equations in chemistry.

1. When balanced, the number of each atom is the same on each side of the reaction—the reactant side and the product side.
2. You cannot change subscripts to balance a reaction.
3. You can ONLY change coefficients.
4. The subscript applies ONLY to the element that immediately comes before it and tells how many atoms of that type are in one molecule.
5. The coefficient comes before the molecule and tells how many molecules are present. Multiplying the coefficient by the number of any atom in the molecule tells the total number of that atom present.

IMPORTANT: Correctly balanced, the coefficients will always be the SMALLEST possible numbers—not multiples—that results in having the same number of atoms on each side of the reaction. It is like finding least common denominators and reducing fractions. For instance, if the equation balances with the coefficients of 4, 6, and 2, then it would be proper to "reduce" that to 2, 3, and 1.

Balancing Equations and Conservation of Matter

General Chemistry Index

The action of balancing chemical equations primarily sets out to find the ratios of compounds that result in there being the same number of atoms in the product as there were in the reactants. This is the most fundamental concept that must be understood. It is so important, it bears being repeated and emphasized.

At the most basic level, balancing chemical equations is the process of finding coefficients which result in the number of atoms in the reactants being equal to the number of atoms in the product.

If there are 8 atoms total in all of the molecules of the reactants, then there MUST be 8 atoms total in all of the molecules of the products.

It does not matter how many different compounds are involved. It does not matter how many molecules of any of the compounds are involved. The driving factor in balancing equations is that the number of atoms on each side of the reaction are equal.

Read that last sentence ten times!

Conservation of Matter

The reason that the number of atoms must be the same stems from the Law of Conservation of Matter. In basic terms, the Law of Conservation states that matter cannot be created or destroyed. It really, if thought about, is logical. If you have 15 atoms of something, no matter how you combine or arrange them, you end up with 15 of something.

The CRINGE has NO LIMITS

https://youtu.be/84MUkdZyxUQ

This law drives chemistry and chemical reactions. This is why that the number of atoms cannot change. 

One way to look at this principle as it applies to chemical reactions is to compare the mass of the reactants to the mass of the products. Because the number of atoms CANNOT change, and because each atom has a certain mass, there should be no difference between the mass of the products and the mass of the reactants.

Consider the reaction of hydrogen and oxygen as an example:

2H2 + O2 = 2H2O

This reaction begins with 2 compounds on the left and ends up with one compound on the right.

This reaction begins with 2 molecules of hydrogen and one molecule of oxygen on the left and ends up with 2 molecules of water on the right.

Neither the number of compounds nor the number of molecules remain the same.

However, it begins with 6 atoms on the left (4 hydrogen and 2 oxygen) and ends up with 6 atoms (the same) on the right.

Look at it as a diagram:

2H2 + O2 = 2H2O

HH          OO  HH

=

HOH      HOH

Though the compounds (elements in this case) recombine and rearrange, the number of (and indeed, the ratio of) atoms stays the same. The reaction begins with 4 hydrogen and 2 oxygen and ends up with 4 hydrogen and 2 oxygen.

Since the number of atoms stays the same, and since atoms have a particular mass, it logically and naturally follows that the mass on both sides of the reaction stays equal.

Of course, proving this involves math and a periodic table!

http://www.ptable.com/

Consider the table below for this reaction:

2H2 + O2 = 2H2O

______________________________________________________________________________

Reactants	Products
hydrogen: 2 molecules, 2 atoms each	water: 2 molecules with 2 hydrogen and 1 oxygen each
atomic mass of 1 hydrogen: 1.0008	atomic mass of 1 water molecule: 2.0016 + 15.999 = 18.0006
total atomic mass of 4 hydrogen: 4.0032 oxygen: 1 molecule of 2 atoms
atomic mass of 1 oxygen: 15.999
total atomic mass of 2 oxygen: 31.998
TOTAL NUMBER OF ATOMS: 4H and 2O	TOTAL NUMBER OF ATOMS: 4H and 2O
TOTAL ATOMIC MASS OF REACTANTS: 36.0012	TOTAL ATOMIC MASS OF PRODUCTS: 36.0012

______________________________________________________________________________

In the above, the sum of the atomic masses of the reactants is equal to the sum of the masses of the products. Just in the same way, the total number of each atom is the same.

This confirms the Law of Conservation of Matter. Because matter cannot be created or destroyed, it has to be equal, both in the number of atoms as well as in the masses.

Balancing Equations and Reactions with Polyatomic Ions

General Chemistry Index

When working with chemical notation, it has been established that the subscripts (or numbers FOLLOWING the elements) represent the number of those elements present. The coefficient tells how many of the molecule is present.

Thus,

3 H20 or sometimes 3 H2O

means 3 molecules of water in which are 2 atoms of hydrogen and 1 atom of oxygen.

Understanding Polyatomic Nomenclature

There is another variation of this notation that is applied in certain cases with some compounds. Because of how the compounds form, there is sometimes a value in keeping some of the elements as a unit and subscripting the whole unit to show how many of that unit are present.

Look at the reaction below:

CaC2   +   2H2O   --->   Ca(OH)2   +   C2H2

Notice on the product side, the OH is inside parenthesis. This represents that that is a unit of molecules that are being kept together based on how the compound is formed. The subscript indicates that there are two of these units present.

Examples:

3Ca(OH)2 has in it, 3 Ca, 6 O, and 6 H. (The subscripted 2 applies to both atoms inside the parenthesis, and the coefficient of 3 applies to the whole molecule.)

Ca3(PO4)2 has in it, 3 Ca, 2 P, and 8 O. (The subscripted 2 applies to the PO4, so there are 2 P and 8 0)

2Cu(NO3)2 has—to begin with, there are 2 molecules of Cu(NO3)2 as indicated by the coefficient.

• EACH molecule has 1 Cu and 2 (NO3). Since there are 2 (NO3) (The subscript 2 applies to everything inside the parenthesis.), that means there are 2 N and 6 O in each Cu(NO3)2 molecule.
• Since there are 2 molecules of Cu(NO3)2, there are in TOTAL:
• 2 Cu
• 4 N
• 12 O

Summary:

• Coefficients, the numbers in front, apply to the whole molecule and tell how many molecules or "sets" of molecules are present.
• Subscripts (or number FOLLOWING the atom symbols) tell how many of that atom are in the molecule.
• If a group of atoms are inside parenthesis:
• They are to be kept together as a unit.
• Any subscripts to the closing parenthesis means that there are that many units of the atoms inside the parenthesis are present.

Balancing Polyatomic Reactions

Keeping in mind that the process of balancing ANY equation means finding coefficients that result in the same numbers of the same types of atoms appearing on both sides of the reaction…

…then to do so with polyatomic reactions means doing a little more work.

The first thing you need to do is get the reaction written out unbalanced…

Fe(NO3)3 + (NH4)2CO3 → Fe2(CO3)3 + NH4NO3   <<<--- Unbalanced reaction

The second step is to do a little inspection. This is vital. And tedious. The goal is to identify the polyatomic "chunks" that move across the reaction unchanged. The parenthesis, if present, will help!

The chunk has to move across UNCHANGED. If for instance a chunk of PO4 becomes PO3, then it changed. If it does move unchanged, then we can balance the equation looking at the "chunks."

In the above example, there are three polyatomic chunks (ions) that move across:

NO3

CO3

NH4

Since they move unchanged, they can be treated chunk by chunk. Optionally, if it makes it easier on the eyes and brain, you can even use an abbreviation for or color code the chunk.

Once you have identified the chunks you can treat in whole, the third step is to do the balancing:

Fe(NO3)3 + (NH4)2CO3 → Fe2(CO3)3 + NH4NO3   <<<--- Unbalanced reaction

            Reactant Side                    Product Side

Fe                 1                                           2

NO3              3                                            1

NH4              2                                            1

CO3              1                                            3

2Fe(NO3)3 + 3(NH4)2CO3 → Fe2(CO3)3 + 6NH4NO3   <<<--- Balanced reaction

            Reactant Side                    Product Side

Fe                 2                                           2

NO3              6                                           6

NH4               6                                           6

CO3              3                                           3

Balancing Chemical Equations and Reactions

General Chemistry Index

"Chemical reactions must be balanced." What does that mean?

The number of atoms that begin the reaction must end up somewhere. If there are ten atoms at the beginning, when the reaction is done, there will still be ten atoms. However, how the atoms are combined into compounds changes!

The action of balancing chemical equations primarily sets out to find the ratios of compounds that result in there being the same number of atoms in the product as there were in the reactants. This is the most fundamental concept that must be understood. It is so important, it bears being repeated and emphasized.

At the most basic level, balancing chemical equations is the process of finding coefficients which result in the number of atoms in the reactants being equal to the number of atoms in the product.

If there are 8 atoms total in all of the molecules of the reactants, then there MUST be 8 atoms total in all of the molecules of the products.

It does not matter how many different compounds are involved. It does not matter how many molecules of any of the compounds are involved. The driving factor in balancing equations is that the number of atoms on each side of the reaction are equal—and the mass is the same on both sides.

Read that last sentence ten times!

Conservation of Matter

The reason that the number of atoms must be the same stems from the Law of Conservation of Matter. In basic terms, the Law of Conservation states that matter cannot be created or destroyed. It really, if thought about, is logical. If you have 15 atoms of something, no matter how you combine or arrange them, you end up with 15 of something.

This law drives chemistry and chemical reactions. This is why that the number of atoms cannot change. 

One way to look at this principle as it applies to chemical reactions is to compare the mass of the reactants to the mass of the products. Because the number of atoms CANNOT change, and because each atom has a certain mass, there should be no difference between the mass of the products and the mass of the reactants.

METHOD

The process of balancing an equation begins by writing out all of the reactants and the products, putting them on opposite sides of either an equal sign or an arrow. Water and aluminum chloride will be used as examples.

Suppose it is desirable to combine oxygen and hydrogen to make water. The formula for those two elements are O2 and H2, meaning that each molecule of oxygen has 2 oxygen atoms and each molecule of hydrogen has two hydrogen atoms. The formula for water is H2O, meaning there are two hydrogen atoms and one oxygen atom in each molecule.

EXAMPLE AS INSTRUCTION

So the reaction looks like this, to begin with:

H2 + O2 = H2O

However, this equation is NOT in balance. On the left side (reactant side), there are two hydrogen and two oxygen, but on the right side (product side), there is only ONE oxygen. To correct this, the following can be done:

H2 + O2 = 2H2O

Adding the coefficient of 2 in front of the water molecule results in there being two oxygens present, so the number of oxygens balance. HOWEVER, adding the coefficient means that there are, now, 4 hydrogen present. Another adjustment needs to take place. 

The equation can be brought into balance if 4 hydrogens can be on the left side of the equation. This can happen by adding yet another coefficient:

2H2 + O2 = 2H2O

At last, everything balances—on the left are 4 hydrogen and 2 oxygen, and on the right are the same numbers of the same atoms.

LET'S DO THAT AGAIN!

Look at the unbalanced formula for making aluminum chloride:

Al + HCl = AlCl3 + H2

To balance this, several coefficients are needed. That HCL has one of each hydrogen and chlorine on the left, but there are 3 and 2 of them on the right, results in some "tricky" math. 

Trying a coefficient of 2 on the left results in:

Al + 2HCl = ?AlCl3 + H2

That brought the hydrogen into balance, but the chlorine still did not work out. Where 3 chlorines are needed, only two were available. The balanced equation requires this:

2Al + 6HCl = 2AlCl3 + 3H2

Thus, there are on BOTH sides of the equal sign: 2 Al, 6 H, and 6 Cl.

Summary Thoughts:

To balance a chemical equation:

• Begin with the atomic symbols for each element or compound involved.
• Place the reactants on the left of the yield symbol (or equal sign) and the products on the right.
• Change the coefficients as needed until—VITAL CONCEPT—there are the same number of each atom on both sides of the equal sign.

• The coefficients must be lowest common factor and it must be a whole number. Thus, if you have 2, 4, 2 then you need to reduce that to 1,2,1.

• You CANNOT change subscripts. You can ONLY change coefficients
• You CANNOT change subscripts. You can ONLY change coefficients
• You CANNOT change subscripts. You can ONLY change coefficients
• You CANNOT change subscripts. You can ONLY change coefficients
• You CANNOT change subscripts. You can ONLY change coefficients

MORE INFO

In the following video, a pseudo-hands-on method for balancing equations is presented. It shows how to "draw out" the reaction and make sure it is balanced.

https://youtu.be/9BlQmfE9cfQ