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

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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.

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.

Chemical Properties of Matter

In addition to physical properties like hardness, malleability, and melting point, matter has properties related to how they are arranged at the molecular level. Chemical properties can only be observed when a substance interacts with other substances and changes.

Chemical Properties:

• any ability to produce change in the composition of matter at the molecular level.
• can only be observed when the substances in a sample of matter change into different substances.

A chemical property is any of a material's properties that becomes evident during or after a chemical reaction; that is, any quality that can be established only by changing a substance's chemical identity. 

Simply speaking, chemical properties cannot be determined just by viewing or touching the substance; the substance's internal structure must be affected greatly for its chemical properties to be investigated. When a substance goes under a chemical reaction, the properties will change drastically, resulting in chemical change. (https://en.wikipedia.org/wiki/Chemical_property)

Two chemical properties are flammability and reactivity.

Flammability

Flammability is a material's ability to burn in the presence of oxygen.

Burning is a chemical reaction in which molecules of the flammable substance combine with oxygen and give off energy that causes the air nearby to glow and give off light (flame).

Reactivity

Reactivity is the property that describes how readily a substance combines chemically with other substances.

Substances react with other substances in different ways. Some things are highly reactive and others are not. Oxygen is an example of something that reacts easily. Nitrogen is an example of something that does not.

https://www.youtube.com/watch?v=-cGueiLfVDU

Many chemical properties require a more advanced understanding of chemistry, but here are a some of them:

• Toxicity

• Types of chemical bonds that can be formed

• Heat of combustion: how much energy is given of when it burns

• Enthalpy of formation

• Acidity or basicity

• Radioactivity

Chemical Changes

Chemical changes (which rely on chemical properties) occur when one substance reacts with another substance and forms one or more NEW substances that are different in molecular composition compared to the original substances. For example…

Many people know that water is chemically represented by H2O. This notation means that two molecules of hydrogen chemically combine with one molecule of oxygen to make up a molecule of water. So, to form water, two substances, hydrogen and oxygen, chemically react, and the NEW substance is water. Water is different from both the hydrogen and oxygen that form it.

Evidence of Chemical Change

When chemical changes occur, there are usually physical changes and often other signs of the change.

A Change in Color

Many chemical changes result in the new substance showing a different color than the original.

Production of Gas

Some reactions release gas, and can be observed as bubbles or an odor.

Formation of a Precipitate

In liquid mixtures, chemical reactions will often cause the newly formed substances to take on a solid form and suspend more obviously in the liquid. This process is called formation of a precipitate.

The solid that forms in a liquid mixture is called a precipitate. The precipitate may remain suspended in the liquid, may settle to the bottom, or may float to the top.

Temperature Change

It is not uncommon for chemical changes to involve a noticeable temperature change. When a chemical change gives off heat (the system gets warmer), it is said to be exothermic. When a chemical change takes in heat (the system cools off), it is said to be endothermic.

It is not always easy to tell if an observed change is physical or chemical. A heated piece of iron will change color, but still be iron. Gas bubbles will form when water's temperature approaches its boiling point, but it is still water.

For a chemical change to take place, two or more substances must change at the molecular level to become one or more NEW substances.

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Definitions and content from:

New Oxford American Dictionary

Physical Science Concepts in Action, Pearson

http://en.wikipedia.org
https://www.thoughtco.com/definition-of-chemical-property-and-examples-604908