The Chemistry Book

Definition: Chemical Reactions

Chemical reactions are a process in which one or more substances are changed into others. ^[1]

Evidence of a Chemical Reaction

Chemical reactions often give visual clues such as color changes, a solid forms, bubbles are produced, and a flame occurs. ^[2] Chemical reactions are not always visible but they sometimes give you signals by a change of temperature. If heat or a flame is produce or the heat is absorbed, that is a signal that a chemical reaction has occurred. There are many physical changes that can be confused with chemical changes. For example, boiling water or freezing water results in many changes in the properties of water, but those are physical changes, not chemical ones. But whether water is liquid, solid or a gas, it is still water, and that is why it is a physical rather than chemical change. The most common method is to use analytical chemical technique to verify the presence of the product molecules, or the disappearance of reactant molecules. ^[3]

 Ex. Rusty Nails: A chemical reaction between the iron in steel and oxygen in  the atmosphere causes rust.

Chemical Equations

A chemical equation is a representation of a chemical reaction in which the chemicals present before the reaction called reactants, shown on the left of an arrow and the chemicals formed by the reaction called the product, shown on the right of the arrow. ^[4]. The arrow indicates the direction of change

  Ex.  Al + O  →  AlO 
      Reactants  Product

The “+” is read as “reacts with” and the arrow means “produces”. ^[5]. Chemical equations also contain coefficients which are the whole numbers to the left of the element

Types of Chemical Reactions

• Single Dispalcement Reaction

In a single replacement reaction a single uncombined element replaces another in a compound. Two reactants yield two products. ^[6].

A + BC → AC + B

• Double Displacement Reaction

In a double replacement reaction parts of two compounds switch places to form two new compounds. Two reactants yield two products. ^[7].

AB + CD → AD + CB  

• Synthesis Reaction

In a synthesis reaction two or more simple substances combine to form a more complex substance. Two or more reactants yielding one product is another way to identify a synthesis reaction. ^[8]

A + B → AB

• Decomposition Reaction

In a decomposition reaction a more complex substance breaks down into its more simple parts. One reactant yields 2 or more products. Basically, synthesis and decomposition reactions are opposites. ^[9]

AB → A + B

• Combustion

A combustion reaction involves oxygen and produces heat so rapidly that a flame results. This reaction occurs between certain compounds, especially organic compounds, oxygen, and hydrocarbons, which are carbon and hydrogen attached. The products of this reaction include carbon dioxide and water. ^[10]

C(x)H(x)+O → 2H2O(g)+CO2(g)

Balancing Equations

The principle to understanding the balancing process of a chemical equation is that atoms are conserved in a chemical reaction. ^[11]. This principle means that an atom can neither be created or destroyed. When balancing chemical equations, the same number of the atom should be found in the reactants and the product. Subscripts in a chemical equation cannot be changed to make the equation equal on both sides, the coefficient is the only number that can be changed to make the reactants and product equal.

   Ex.  SnO₂ + H₂ → Sn + H₂O

See also

Physical changes are the changes in which the original substance hasn't been chemically altered. Physical changes pertain to changes of energy and states of matter; the substance has only changed form. ^[12] This change does not produce a new substance; it merely changes the distance between particles.

References

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