Chemical Reactions
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Chemical Reactions. By: Nishant Sahoo. Chemical Reaction Basics. What Happens in a Chemical Reaction?. In a chemical reaction: The electrons are the only part of the atom that is affected by the reaction.

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Chemical Reactions

By: Nishant Sahoo




What Happens in a Chemical


  • In a chemical reaction:

  • The electrons are the only part of the atom that is affected by the reaction.

  • Electrons are either bonded to other atoms or their bonds are broken and the atoms are separated.

Example of a Chemical


Reactants: Also known as substrates, are the starting materials for a chemical reaction.

Product: The result of the reaction between the reactants.



How Does the Law of

Conservation of Mass Apply?

The Law of Conservation of Mass states that matter cannot be created/destroyed,

although it may be rearranged. This implies that for any chemical process in a closed system,

the mass of the reactants must equal the mass of the products.

4 Hydrogen atoms in both

the reactants and product

2 Oxygen atoms in both

the reactants and product

Signs of a Chemical Reaction:

The formation of a precipitate, a precipitate is formed in a chemical reaction and is a solid that is different from the reactants.

A change in the color of the reactants

A formation of gases, usually in the form of bubbles or foam

A change in temperature









The Role of Energy

In Chemical Reactions

  • In every chemical reaction there is energy

  • This energy is needed to either break bonds in the reaction or create them

  • As the bonds break energy is released, but when bonds are made energy is absorbed

  • Chemical reactions are classified as exothermic or endothermic

Exothermic Reactions:

Exothermic reactions are reactions in which energy is released. The energy that is released is called chemical potential energy. This energy is the energy stored in the bonds between atoms. It is called potential energy because it has the potential to be converted to other forms of energy. Some of the time this energy is turned into heat which causes the product to feel hot. This would be called combustion (a.k.a. burning) and is always a exothermic reaction.

Endothermic Reactions:

Endothermic reactions are reactions in which energy is absorbed. This energy is needed for the reaction to occur. Mostly this energy is either heat of electrical energy. When you add electrical energy to metal oxides, it can separate them into just metal and oxygen. If you add electrical energy to sodium chloride it may cause the table salt to break into sodium and chlorine.


Exothermic Reaction:

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In the first picture a heated metal rod in placed into a mixture of iron powder and sulfur (iron sulfide). This heated metal rod starts a exothermic reaction. Then as you can see in the second picture the product of the reaction is that the iron sulfite is fused to the metal rod.

Endothermic Reaction:

Barium hydroxide and an ammonium salt. When the two solids are mixed and then combined with water. When mixed, the temperature of the solution drops below the freezing point of water.

Starting a Reaction

(Activation Energy)

Activation Energy is the least amount of energy needed for a chemical reaction to start. Some elements and compounds react to each other by just being near each other. This makes their activation energy zero. Most other elements and compounds will only react to each other if a certain amount of energy is added to both of them. For example: when you slide a match on the side of a matchbox, the friction provides activation energy (in the heat form) for the match to light. Activation energy is measured in the number of joules per mole of reactants.

Types of



Synthesis Reactions

A synthesis reaction occurs when two or more simple substances (reactants) combine to form a bigger, more complex substance (product). In a chemical formula: if two or more reactants produce one product, it is a synthesis reaction.

General Form:

A + B ---> AB

In this example, hydrogen and oxygen bond together in a synthesis reaction to produce water. This reaction is easy to perform. Just mix hydrogen and oxygen and the reaction does the rest.

Liquid OXYGEN in a bowl

The Hindenburg balloon blowing up after the HYDROGEN gas inside was ignited

WATER droplet

Decomposition Reaction:

A decomposition reaction occurs when a complex substance containing more than one simple substance breaks down into its original elements. Basically in a chemical formula a reactant makes two or more products as in the example below. Yes, synthesis and decomposition reactions are exact opposites.

General Form

AB ---> A + B

In this example, water breaks down into hydrogen and oxygen as separate substances. This reaction is not as easy as bonding hydrogen and oxygen. It takes a more complex procedure.

The reaction of calcium metal and water. The bubbles you see is a HYDROGEN gas

A tank of compressed OXYGEN gas

A glass of WATER

Combustion Reactions

Combustion (also know as burning) is a sequence of exothermic reactions . These reactions include a fuel and an oxidant. When this reaction takes place, heat is produced. This heat is shown as just a rise in temperature or by a rise in temperature and the production of light. The light is either shown in the form of flames (fire) or a glow.

C10H8 + 12 O2 ---> 10 CO2 + 4 H2O

A combustion reaction is when oxygen combines with another compound to form water and carbon dioxide. This means the chemical formula must have a CO2 and a H2O in the product as seen above in the formula for the burning of naphthalene.


Naphthalene is know as the traditional, primary ingredient in moth balls.

Single Replacement


A single replacement reaction occurs when one uncombined element and one compound are the reactants in a chemical reaction. In this type of reaction the one single element takes the place of one of the elements in the compound.








In this example zinc and a combination of hydrogen and chlorine , called hydrochloric acid, are the reactants. But, as you can see, in the product, zinc took the place of hydrogen in the compound and now hydrogen is the single element. So, in a chemical formula if a lone element takes the place of an element in a compound you know it is a single replacement reaction.




General Form

A + BC ---> AC + B